<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019b0f23-5b75-7d24-a03f-6c078f92340f,g:518ed3b7-5b40-4f3b-af92-043ca93454af-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://powersint.com/role/CONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="simple" xlink:href="psix-20251231.xsd#CONSOLIDATEDBALANCESHEETSParenthetical"/>
  <link:definitionLink xlink:role="http://powersint.com/role/CONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019b0f23-5d0d-7805-bade-5f40eee21c8c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019b0f23-5d0d-77d5-96ba-4338f8e4d422" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019b0f23-5d0d-7805-bade-5f40eee21c8c" xlink:to="loc_us-gaap_StatementTable_019b0f23-5d0d-77d5-96ba-4338f8e4d422" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019b0f23-5d0d-73c1-b0ca-c032299ab7bd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019b0f23-5d0d-77d5-96ba-4338f8e4d422" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019b0f23-5d0d-73c1-b0ca-c032299ab7bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-73c1-b0ca-c032299ab7bd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019b0f23-5d0d-73c1-b0ca-c032299ab7bd" xlink:to="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-73c1-b0ca-c032299ab7bd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-74b2-bc84-17930018550b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019b0f23-5d0d-73c1-b0ca-c032299ab7bd" xlink:to="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-74b2-bc84-17930018550b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019b0f23-5d0d-7586-8baa-d401ef4a19c2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-74b2-bc84-17930018550b" xlink:to="loc_us-gaap_RelatedPartyMember_019b0f23-5d0d-7586-8baa-d401ef4a19c2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019b0f23-5d0d-72ba-a28c-5049015aadc6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019b0f23-5d0d-7805-bade-5f40eee21c8c" xlink:to="loc_us-gaap_AssetsAbstract_019b0f23-5d0d-72ba-a28c-5049015aadc6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_019b0f23-5d0d-7234-a6c3-449f648bf39d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019b0f23-5d0d-72ba-a28c-5049015aadc6" xlink:to="loc_us-gaap_AssetsCurrentAbstract_019b0f23-5d0d-7234-a6c3-449f648bf39d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_019b0f23-5d0d-7701-99c6-24c219cbc2c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019b0f23-5d0d-7234-a6c3-449f648bf39d" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_019b0f23-5d0d-7701-99c6-24c219cbc2c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019b0f23-5d0d-70ae-9c82-77b77f9704d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019b0f23-5d0d-7234-a6c3-449f648bf39d" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019b0f23-5d0d-70ae-9c82-77b77f9704d9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019b0f23-5d0d-7fc3-83ae-5d5cc344bcc8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019b0f23-5d0d-7805-bade-5f40eee21c8c" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019b0f23-5d0d-7fc3-83ae-5d5cc344bcc8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_019b0f23-5d0d-7115-a4a7-9c4e26afe365" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019b0f23-5d0d-7fc3-83ae-5d5cc344bcc8" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_019b0f23-5d0d-7115-a4a7-9c4e26afe365" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_019b0f23-5d0d-7376-9cfd-c845b9f98252" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019b0f23-5d0d-7115-a4a7-9c4e26afe365" xlink:to="loc_us-gaap_AccountsPayableCurrent_019b0f23-5d0d-7376-9cfd-c845b9f98252" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherShortTermBorrowings_019b0f23-5d0d-7cbb-afc8-83a856671a94" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherShortTermBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019b0f23-5d0d-7115-a4a7-9c4e26afe365" xlink:to="loc_us-gaap_OtherShortTermBorrowings_019b0f23-5d0d-7cbb-afc8-83a856671a94" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAccruedLiabilitiesCurrent_019b0f23-5d0d-70c5-bc1b-c6ef172ed2ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019b0f23-5d0d-7115-a4a7-9c4e26afe365" xlink:to="loc_us-gaap_OtherAccruedLiabilitiesCurrent_019b0f23-5d0d-70c5-bc1b-c6ef172ed2ab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_019b0f23-5d0d-7aef-b863-b6b7f35c098a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019b0f23-5d0d-7fc3-83ae-5d5cc344bcc8" xlink:to="loc_us-gaap_StockholdersEquityAbstract_019b0f23-5d0d-7aef-b863-b6b7f35c098a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019b0f23-5d0d-7f29-92c6-583d864a45d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019b0f23-5d0d-7aef-b863-b6b7f35c098a" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019b0f23-5d0d-7f29-92c6-583d864a45d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019b0f23-5d0d-7b29-913c-cae37da79f7a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019b0f23-5d0d-7aef-b863-b6b7f35c098a" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019b0f23-5d0d-7b29-913c-cae37da79f7a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019b0f23-5d0d-74e0-a895-2265eafae602" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019b0f23-5d0d-7aef-b863-b6b7f35c098a" xlink:to="loc_us-gaap_CommonStockSharesIssued_019b0f23-5d0d-74e0-a895-2265eafae602" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019b0f23-5d0d-7e4a-80f5-fa3707f9b12d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019b0f23-5d0d-7aef-b863-b6b7f35c098a" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019b0f23-5d0d-7e4a-80f5-fa3707f9b12d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_019b0f23-5d0d-7938-a9d5-b3d1a55ec8db" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019b0f23-5d0d-7aef-b863-b6b7f35c098a" xlink:to="loc_us-gaap_TreasuryStockCommonShares_019b0f23-5d0d-7938-a9d5-b3d1a55ec8db" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/CONSOLIDATEDSTATEMENTSOFINCOMEParenthetical" xlink:type="simple" xlink:href="psix-20251231.xsd#CONSOLIDATEDSTATEMENTSOFINCOMEParenthetical"/>
  <link:definitionLink xlink:role="http://powersint.com/role/CONSOLIDATEDSTATEMENTSOFINCOMEParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019b0f23-5d0d-7b92-9a4b-88b6658d0738" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019b0f23-5d0d-7697-ac2e-c4091f6d469c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019b0f23-5d0d-7b92-9a4b-88b6658d0738" xlink:to="loc_us-gaap_StatementTable_019b0f23-5d0d-7697-ac2e-c4091f6d469c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019b0f23-5d0d-70aa-941b-c2a9c2d0cb63" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019b0f23-5d0d-7697-ac2e-c4091f6d469c" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019b0f23-5d0d-70aa-941b-c2a9c2d0cb63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-70aa-941b-c2a9c2d0cb63_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019b0f23-5d0d-70aa-941b-c2a9c2d0cb63" xlink:to="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-70aa-941b-c2a9c2d0cb63_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-7f87-b4b1-bc4f8d4de02a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019b0f23-5d0d-70aa-941b-c2a9c2d0cb63" xlink:to="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-7f87-b4b1-bc4f8d4de02a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019b0f23-5d0d-797d-9457-f87eb53686cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-7f87-b4b1-bc4f8d4de02a" xlink:to="loc_us-gaap_RelatedPartyMember_019b0f23-5d0d-797d-9457-f87eb53686cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019b0f23-5d0d-736a-bcf0-c80c35e0dcc0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019b0f23-5d0d-7b92-9a4b-88b6658d0738" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019b0f23-5d0d-736a-bcf0-c80c35e0dcc0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartiesAmountInCostOfSales_019b0f23-5d0d-72d5-bd5a-b9726ddaa605" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartiesAmountInCostOfSales"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019b0f23-5d0d-7b92-9a4b-88b6658d0738" xlink:to="loc_us-gaap_RelatedPartiesAmountInCostOfSales_019b0f23-5d0d-72d5-bd5a-b9726ddaa605" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseNonoperating_019b0f23-5d0d-78e1-b787-41392020eebc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseNonoperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019b0f23-5d0d-7b92-9a4b-88b6658d0738" xlink:to="loc_us-gaap_InterestExpenseNonoperating_019b0f23-5d0d-78e1-b787-41392020eebc" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/CONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="psix-20251231.xsd#CONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://powersint.com/role/CONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019b0f23-5d0d-7f14-aa65-cc3d37aa1f9a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019b0f23-5d0d-764e-ae1d-d5cf60068652" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019b0f23-5d0d-7f14-aa65-cc3d37aa1f9a" xlink:to="loc_us-gaap_StatementTable_019b0f23-5d0d-764e-ae1d-d5cf60068652" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019b0f23-5d0d-752b-a3d6-a8108536810d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019b0f23-5d0d-764e-ae1d-d5cf60068652" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019b0f23-5d0d-752b-a3d6-a8108536810d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019b0f23-5d0d-752b-a3d6-a8108536810d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019b0f23-5d0d-752b-a3d6-a8108536810d" xlink:to="loc_us-gaap_EquityComponentDomain_019b0f23-5d0d-752b-a3d6-a8108536810d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019b0f23-5d0d-7bd5-bbf8-02223ae2f0b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019b0f23-5d0d-752b-a3d6-a8108536810d" xlink:to="loc_us-gaap_EquityComponentDomain_019b0f23-5d0d-7bd5-bbf8-02223ae2f0b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019b0f23-5d0d-74ee-a5ee-6defb6ba0404" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019b0f23-5d0d-7bd5-bbf8-02223ae2f0b9" xlink:to="loc_us-gaap_CommonStockMember_019b0f23-5d0d-74ee-a5ee-6defb6ba0404" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019b0f23-5d0d-7abf-b74a-5905b851e2a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019b0f23-5d0d-7bd5-bbf8-02223ae2f0b9" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019b0f23-5d0d-7abf-b74a-5905b851e2a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019b0f23-5d0d-7a74-a8e5-09f197863073" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019b0f23-5d0d-7bd5-bbf8-02223ae2f0b9" xlink:to="loc_us-gaap_RetainedEarningsMember_019b0f23-5d0d-7a74-a8e5-09f197863073" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_019b0f23-5d0d-781b-b700-ddf1ba63c9d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019b0f23-5d0d-7bd5-bbf8-02223ae2f0b9" xlink:to="loc_us-gaap_TreasuryStockCommonMember_019b0f23-5d0d-781b-b700-ddf1ba63c9d4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5d0d-7f91-8b31-d1a3b149e0a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019b0f23-5d0d-7f14-aa65-cc3d37aa1f9a" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5d0d-7f91-8b31-d1a3b149e0a7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019b0f23-5d0d-7c42-847c-9f5e1c5342d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5d0d-7f91-8b31-d1a3b149e0a7" xlink:to="loc_us-gaap_StockholdersEquity_019b0f23-5d0d-7c42-847c-9f5e1c5342d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019b0f23-5d0d-7581-9af3-60cae505384f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5d0d-7f91-8b31-d1a3b149e0a7" xlink:to="loc_us-gaap_NetIncomeLoss_019b0f23-5d0d-7581-9af3-60cae505384f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019b0f23-5d0d-7033-a83b-528b4e8877c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5d0d-7f91-8b31-d1a3b149e0a7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019b0f23-5d0d-7033-a83b-528b4e8877c7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019b0f23-5d0d-7aef-b2b6-d3af90a1b036" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5d0d-7f91-8b31-d1a3b149e0a7" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019b0f23-5d0d-7aef-b2b6-d3af90a1b036" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_TaxBenefitFromExerciseOfShareBasedCompensation_019bd9aa-d6cf-7ca7-8f87-01cd220e1868" xlink:href="psix-20251231.xsd#psix_TaxBenefitFromExerciseOfShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5d0d-7f91-8b31-d1a3b149e0a7" xlink:to="loc_psix_TaxBenefitFromExerciseOfShareBasedCompensation_019bd9aa-d6cf-7ca7-8f87-01cd220e1868" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019b0f23-5d0d-7c1e-9f4c-56c9d4706e9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5d0d-7f91-8b31-d1a3b149e0a7" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019b0f23-5d0d-7c1e-9f4c-56c9d4706e9d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation_019b0f23-5d0d-74b7-93db-4a2a4449a891" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5d0d-7f91-8b31-d1a3b149e0a7" xlink:to="loc_us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation_019b0f23-5d0d-74b7-93db-4a2a4449a891" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019b0f23-5d0d-7734-9a98-4dd93221db21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/SummaryofSignificantAccountingPoliciesandOtherInformationNarrativeDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#SummaryofSignificantAccountingPoliciesandOtherInformationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/SummaryofSignificantAccountingPoliciesandOtherInformationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-711f-b4b7-e92678f02845" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7243-8f23-84fb33e99f4b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-711f-b4b7-e92678f02845" xlink:to="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7243-8f23-84fb33e99f4b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis_019b0f23-5d0d-7597-a982-5dc799431bad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7243-8f23-84fb33e99f4b" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis_019b0f23-5d0d-7597-a982-5dc799431bad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019b0f23-5d0d-7597-a982-5dc799431bad_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis_019b0f23-5d0d-7597-a982-5dc799431bad" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019b0f23-5d0d-7597-a982-5dc799431bad_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019b0f23-5d0d-7774-abce-5955ad4381b2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis_019b0f23-5d0d-7597-a982-5dc799431bad" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019b0f23-5d0d-7774-abce-5955ad4381b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ContractWithCustomerMember_019b0f23-5d0d-7d8a-bce9-564e8afce02c" xlink:href="psix-20251231.xsd#psix_ContractWithCustomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019b0f23-5d0d-7774-abce-5955ad4381b2" xlink:to="loc_psix_ContractWithCustomerMember_019b0f23-5d0d-7d8a-bce9-564e8afce02c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019b0f23-5d0d-7c8e-ab73-e61e4b13c75a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7243-8f23-84fb33e99f4b" xlink:to="loc_srt_RangeAxis_019b0f23-5d0d-7c8e-ab73-e61e4b13c75a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019b0f23-5d0d-7c8e-ab73-e61e4b13c75a_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019b0f23-5d0d-7c8e-ab73-e61e4b13c75a" xlink:to="loc_srt_RangeMember_019b0f23-5d0d-7c8e-ab73-e61e4b13c75a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019b0f23-5d0d-74e1-aaf8-8c20133ce0f3" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019b0f23-5d0d-7c8e-ab73-e61e4b13c75a" xlink:to="loc_srt_RangeMember_019b0f23-5d0d-74e1-aaf8-8c20133ce0f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019b0f23-5d0d-7685-a488-ffcf09848ca2" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019b0f23-5d0d-74e1-aaf8-8c20133ce0f3" xlink:to="loc_srt_MinimumMember_019b0f23-5d0d-7685-a488-ffcf09848ca2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019b0f23-5d0d-7be8-99bd-5c716823b5ec" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019b0f23-5d0d-74e1-aaf8-8c20133ce0f3" xlink:to="loc_srt_MaximumMember_019b0f23-5d0d-7be8-99bd-5c716823b5ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019b0f23-5d0d-7fa9-b30e-93b6b01b1b59" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7243-8f23-84fb33e99f4b" xlink:to="loc_us-gaap_CreditFacilityAxis_019b0f23-5d0d-7fa9-b30e-93b6b01b1b59" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019b0f23-5d0d-7fa9-b30e-93b6b01b1b59_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019b0f23-5d0d-7fa9-b30e-93b6b01b1b59" xlink:to="loc_us-gaap_CreditFacilityDomain_019b0f23-5d0d-7fa9-b30e-93b6b01b1b59_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019b0f23-5d0d-75f4-94c8-0b0a36ab0645" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019b0f23-5d0d-7fa9-b30e-93b6b01b1b59" xlink:to="loc_us-gaap_CreditFacilityDomain_019b0f23-5d0d-75f4-94c8-0b0a36ab0645" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019b0f23-5d0d-76d7-82a1-ebf6f653f05f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019b0f23-5d0d-75f4-94c8-0b0a36ab0645" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019b0f23-5d0d-76d7-82a1-ebf6f653f05f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019b0f23-5d0d-7132-8718-7ab476c4ac9a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7243-8f23-84fb33e99f4b" xlink:to="loc_us-gaap_DebtInstrumentAxis_019b0f23-5d0d-7132-8718-7ab476c4ac9a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-7132-8718-7ab476c4ac9a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019b0f23-5d0d-7132-8718-7ab476c4ac9a" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-7132-8718-7ab476c4ac9a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-7f5a-b789-3890793e6f4f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019b0f23-5d0d-7132-8718-7ab476c4ac9a" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-7f5a-b789-3890793e6f4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_RevolvingCreditAgreementMember_019ba14d-70fe-74a8-bd57-78ab4c33a7cb" xlink:href="psix-20251231.xsd#psix_RevolvingCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-7f5a-b789-3890793e6f4f" xlink:to="loc_psix_RevolvingCreditAgreementMember_019ba14d-70fe-74a8-bd57-78ab4c33a7cb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0d-742e-a014-915dcff1b5a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7243-8f23-84fb33e99f4b" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0d-742e-a014-915dcff1b5a6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-742e-a014-915dcff1b5a6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0d-742e-a014-915dcff1b5a6" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-742e-a014-915dcff1b5a6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-7a79-a49d-8eb4ea62ea47" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0d-742e-a014-915dcff1b5a6" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-7a79-a49d-8eb4ea62ea47" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019b0f23-5d0d-710f-86c5-60bc6401f426" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-7a79-a49d-8eb4ea62ea47" xlink:to="loc_us-gaap_LineOfCreditMember_019b0f23-5d0d-710f-86c5-60bc6401f426" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019bd9b2-f4dd-7d58-b10f-3c87c1739237" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7243-8f23-84fb33e99f4b" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019bd9b2-f4dd-7d58-b10f-3c87c1739237" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019bd9b2-f4dd-7d58-b10f-3c87c1739237_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019bd9b2-f4dd-7d58-b10f-3c87c1739237" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019bd9b2-f4dd-7d58-b10f-3c87c1739237_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019bd9b2-f4dd-7cd7-a7b1-f275437910e6" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019bd9b2-f4dd-7d58-b10f-3c87c1739237" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019bd9b2-f4dd-7cd7-a7b1-f275437910e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_PowerSolutionsInternationalInc.Member_019bd9b2-f4dd-75e1-82cd-79dde2108fa1" xlink:href="psix-20251231.xsd#psix_PowerSolutionsInternationalInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_019bd9b2-f4dd-7cd7-a7b1-f275437910e6" xlink:to="loc_psix_PowerSolutionsInternationalInc.Member_019bd9b2-f4dd-75e1-82cd-79dde2108fa1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019bd9b2-f4dd-7f62-9231-73f257523f4a" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7243-8f23-84fb33e99f4b" xlink:to="loc_dei_LegalEntityAxis_019bd9b2-f4dd-7f62-9231-73f257523f4a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019bd9b2-f4dd-7f62-9231-73f257523f4a_default" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019bd9b2-f4dd-7f62-9231-73f257523f4a" xlink:to="loc_dei_EntityDomain_019bd9b2-f4dd-7f62-9231-73f257523f4a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019bd9b2-f4dd-70b0-8766-2437e585de4e" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019bd9b2-f4dd-7f62-9231-73f257523f4a" xlink:to="loc_dei_EntityDomain_019bd9b2-f4dd-70b0-8766-2437e585de4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_WeichaiAmericaCorpMember_019bd9b2-f4dd-7292-9abb-44798d16e989" xlink:href="psix-20251231.xsd#psix_WeichaiAmericaCorpMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019bd9b2-f4dd-70b0-8766-2437e585de4e" xlink:to="loc_psix_WeichaiAmericaCorpMember_019bd9b2-f4dd-7292-9abb-44798d16e989" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipAxis_019bd9b2-f4dd-7d3f-9927-97adf9f0c53c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7243-8f23-84fb33e99f4b" xlink:to="loc_srt_OwnershipAxis_019bd9b2-f4dd-7d3f-9927-97adf9f0c53c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_019bd9b2-f4dd-7d3f-9927-97adf9f0c53c_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_019bd9b2-f4dd-7d3f-9927-97adf9f0c53c" xlink:to="loc_srt_OwnershipDomain_019bd9b2-f4dd-7d3f-9927-97adf9f0c53c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_019bd9b2-f4dd-7212-96f1-c2c80b635404" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_019bd9b2-f4dd-7d3f-9927-97adf9f0c53c" xlink:to="loc_srt_OwnershipDomain_019bd9b2-f4dd-7212-96f1-c2c80b635404" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_PowerSolutionsInternationalInc.Member_019bd9b2-f4dd-7e07-9bfc-7c3e147ab4a4" xlink:href="psix-20251231.xsd#psix_PowerSolutionsInternationalInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_019bd9b2-f4dd-7212-96f1-c2c80b635404" xlink:to="loc_psix_PowerSolutionsInternationalInc.Member_019bd9b2-f4dd-7e07-9bfc-7c3e147ab4a4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019bd9b2-f4dd-7dee-8f67-f45cfd207f19" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-711f-b4b7-e92678f02845" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019bd9b2-f4dd-7dee-8f67-f45cfd207f19" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_NumberOfBoardMembers_019b0f23-5d0d-7d42-8843-001d760e4616" xlink:href="psix-20251231.xsd#psix_NumberOfBoardMembers"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-711f-b4b7-e92678f02845" xlink:to="loc_psix_NumberOfBoardMembers_019b0f23-5d0d-7d42-8843-001d760e4616" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019b0f23-5d0d-7a41-bddb-4a8ac51fc81f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-711f-b4b7-e92678f02845" xlink:to="loc_us-gaap_NetIncomeLoss_019b0f23-5d0d-7a41-bddb-4a8ac51fc81f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019b0f23-5d0d-7649-91f8-b4f3d383b3f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-711f-b4b7-e92678f02845" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019b0f23-5d0d-7649-91f8-b4f3d383b3f5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019b0f23-5d0d-7321-9542-e45168604911" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-711f-b4b7-e92678f02845" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019b0f23-5d0d-7321-9542-e45168604911" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashCurrent_019b0f23-5d0d-7bf3-94cf-41534fcc0f35" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-711f-b4b7-e92678f02845" xlink:to="loc_us-gaap_RestrictedCashCurrent_019b0f23-5d0d-7bf3-94cf-41534fcc0f35" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProgressPaymentsNettedAgainstInventoryForLongTermContractsOrPrograms_019b0f23-5d0d-744e-a93a-38b2bbb54d65" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProgressPaymentsNettedAgainstInventoryForLongTermContractsOrPrograms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-711f-b4b7-e92678f02845" xlink:to="loc_us-gaap_ProgressPaymentsNettedAgainstInventoryForLongTermContractsOrPrograms_019b0f23-5d0d-744e-a93a-38b2bbb54d65" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019b0f23-5d0d-720d-b840-8d2d9b01ad71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-711f-b4b7-e92678f02845" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019b0f23-5d0d-720d-b840-8d2d9b01ad71" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillImpairmentLoss_019c0e25-6504-71c5-a9ed-c3a081458923" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-711f-b4b7-e92678f02845" xlink:to="loc_us-gaap_GoodwillImpairmentLoss_019c0e25-6504-71c5-a9ed-c3a081458923" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_019b0f23-5d0d-7573-947c-c187bc36298e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairmentOfLongLivedAssetsHeldForUse"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-711f-b4b7-e92678f02845" xlink:to="loc_us-gaap_ImpairmentOfLongLivedAssetsHeldForUse_019b0f23-5d0d-7573-947c-c187bc36298e" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ProductWarrantyReimbursements_019b0f23-5d0d-7b63-92a0-1c0cc7189aa5" xlink:href="psix-20251231.xsd#psix_ProductWarrantyReimbursements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-711f-b4b7-e92678f02845" xlink:to="loc_psix_ProductWarrantyReimbursements_019b0f23-5d0d-7b63-92a0-1c0cc7189aa5" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/SummaryofSignificantAccountingPoliciesandOtherInformationScheduleofConcentrationofRiskbyRiskFactorDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#SummaryofSignificantAccountingPoliciesandOtherInformationScheduleofConcentrationofRiskbyRiskFactorDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/SummaryofSignificantAccountingPoliciesandOtherInformationScheduleofConcentrationofRiskbyRiskFactorDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-7d54-a4e2-c5e178e6985c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7336-aad7-f66d2df833af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-7d54-a4e2-c5e178e6985c" xlink:to="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7336-aad7-f66d2df833af" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_019b0f23-5d0d-7e06-ad0f-2ed23135477d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7336-aad7-f66d2df833af" xlink:to="loc_srt_MajorCustomersAxis_019b0f23-5d0d-7e06-ad0f-2ed23135477d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019b0f23-5d0d-7e06-ad0f-2ed23135477d_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_019b0f23-5d0d-7e06-ad0f-2ed23135477d" xlink:to="loc_srt_NameOfMajorCustomerDomain_019b0f23-5d0d-7e06-ad0f-2ed23135477d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019b0f23-5d0d-7980-af79-2330ec035d05" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_019b0f23-5d0d-7e06-ad0f-2ed23135477d" xlink:to="loc_srt_NameOfMajorCustomerDomain_019b0f23-5d0d-7980-af79-2330ec035d05" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_CustomerBMember_019b0f23-5d0d-7cad-bba3-ca699077c598" xlink:href="psix-20251231.xsd#psix_CustomerBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019b0f23-5d0d-7980-af79-2330ec035d05" xlink:to="loc_psix_CustomerBMember_019b0f23-5d0d-7cad-bba3-ca699077c598" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_CustomerCMember_019b0f23-5d0d-78ff-9af6-12a930fd586a" xlink:href="psix-20251231.xsd#psix_CustomerCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019b0f23-5d0d-7980-af79-2330ec035d05" xlink:to="loc_psix_CustomerCMember_019b0f23-5d0d-78ff-9af6-12a930fd586a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_CustomerDMember_019b0f23-5d0d-7c04-ba92-052199ecd5e1" xlink:href="psix-20251231.xsd#psix_CustomerDMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019b0f23-5d0d-7980-af79-2330ec035d05" xlink:to="loc_psix_CustomerDMember_019b0f23-5d0d-7c04-ba92-052199ecd5e1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_CustomerEMember_019bd9b5-29d0-772c-b32f-ca796f5717d8" xlink:href="psix-20251231.xsd#psix_CustomerEMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019b0f23-5d0d-7980-af79-2330ec035d05" xlink:to="loc_psix_CustomerEMember_019bd9b5-29d0-772c-b32f-ca796f5717d8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_CustomerFMember_019bd9b5-29d0-756b-aa9c-019192c0004f" xlink:href="psix-20251231.xsd#psix_CustomerFMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019b0f23-5d0d-7980-af79-2330ec035d05" xlink:to="loc_psix_CustomerFMember_019bd9b5-29d0-756b-aa9c-019192c0004f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_CustomerGMember_019bd9b8-3113-71fb-815e-ad7cec0629d1" xlink:href="psix-20251231.xsd#psix_CustomerGMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019b0f23-5d0d-7980-af79-2330ec035d05" xlink:to="loc_psix_CustomerGMember_019bd9b8-3113-71fb-815e-ad7cec0629d1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019b0f23-5d0d-7458-b52e-fcab600969a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7336-aad7-f66d2df833af" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019b0f23-5d0d-7458-b52e-fcab600969a1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019b0f23-5d0d-7458-b52e-fcab600969a1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019b0f23-5d0d-7458-b52e-fcab600969a1" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019b0f23-5d0d-7458-b52e-fcab600969a1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019b0f23-5d0d-7ee6-bcbe-1b27b385d594" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019b0f23-5d0d-7458-b52e-fcab600969a1" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019b0f23-5d0d-7ee6-bcbe-1b27b385d594" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerMember_019b0f23-5d0d-7861-9751-c08a0816c338" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019b0f23-5d0d-7ee6-bcbe-1b27b385d594" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerMember_019b0f23-5d0d-7861-9751-c08a0816c338" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_019b0f23-5d0d-777a-a2c3-c8424d7eea2d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019b0f23-5d0d-7ee6-bcbe-1b27b385d594" xlink:to="loc_us-gaap_AccountsReceivableMember_019b0f23-5d0d-777a-a2c3-c8424d7eea2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsTotalMember_019b0f23-5d0d-742a-be17-d5960a3a6c05" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfGoodsTotalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019b0f23-5d0d-7ee6-bcbe-1b27b385d594" xlink:to="loc_us-gaap_CostOfGoodsTotalMember_019b0f23-5d0d-742a-be17-d5960a3a6c05" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019b0f23-5d0d-7905-b023-e339d20c1f83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7336-aad7-f66d2df833af" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019b0f23-5d0d-7905-b023-e339d20c1f83" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019b0f23-5d0d-7905-b023-e339d20c1f83_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019b0f23-5d0d-7905-b023-e339d20c1f83" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019b0f23-5d0d-7905-b023-e339d20c1f83_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019b0f23-5d0d-77ca-887d-e08297a40273" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019b0f23-5d0d-7905-b023-e339d20c1f83" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019b0f23-5d0d-77ca-887d-e08297a40273" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_019b0f23-5d0d-7161-8542-87a121dcd0af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019b0f23-5d0d-77ca-887d-e08297a40273" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_019b0f23-5d0d-7161-8542-87a121dcd0af" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplierConcentrationRiskMember_019b0f23-5d0d-73ca-8237-d2f98732b26f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SupplierConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019b0f23-5d0d-77ca-887d-e08297a40273" xlink:to="loc_us-gaap_SupplierConcentrationRiskMember_019b0f23-5d0d-73ca-8237-d2f98732b26f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019b0f23-5d0d-7de4-a6fa-c941eec715ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019b0f23-5d0d-7336-aad7-f66d2df833af" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019b0f23-5d0d-7de4-a6fa-c941eec715ad" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019b0f23-5d0d-7de4-a6fa-c941eec715ad_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019b0f23-5d0d-7de4-a6fa-c941eec715ad" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019b0f23-5d0d-7de4-a6fa-c941eec715ad_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019b0f23-5d0d-718a-bb48-f09487936039" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019b0f23-5d0d-7de4-a6fa-c941eec715ad" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019b0f23-5d0d-718a-bb48-f09487936039" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_SupplierAMember_019bd9b6-e3d2-7e46-9ee1-69535661875a" xlink:href="psix-20251231.xsd#psix_SupplierAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019b0f23-5d0d-718a-bb48-f09487936039" xlink:to="loc_psix_SupplierAMember_019bd9b6-e3d2-7e46-9ee1-69535661875a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019b0f23-5d0d-7a63-8faa-e1f268db91bb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019b0f23-5d0d-7d54-a4e2-c5e178e6985c" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019b0f23-5d0d-7a63-8faa-e1f268db91bb" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/SummaryofSignificantAccountingPoliciesandOtherInformationScheduleofEstimatedUsefulLivesbyTypeofAssetCategoryDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#SummaryofSignificantAccountingPoliciesandOtherInformationScheduleofEstimatedUsefulLivesbyTypeofAssetCategoryDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/SummaryofSignificantAccountingPoliciesandOtherInformationScheduleofEstimatedUsefulLivesbyTypeofAssetCategoryDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019b0f23-5d0d-7f29-ada6-ad3541bab143" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019b0f23-5d0d-7b24-a733-62e28ac6f552" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019b0f23-5d0d-7f29-ada6-ad3541bab143" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019b0f23-5d0d-7b24-a733-62e28ac6f552" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019b0f23-5d0d-7f96-a067-d16b168178ae" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019b0f23-5d0d-7b24-a733-62e28ac6f552" xlink:to="loc_srt_RangeAxis_019b0f23-5d0d-7f96-a067-d16b168178ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019b0f23-5d0d-7f96-a067-d16b168178ae_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019b0f23-5d0d-7f96-a067-d16b168178ae" xlink:to="loc_srt_RangeMember_019b0f23-5d0d-7f96-a067-d16b168178ae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019b0f23-5d0d-73d4-a34b-2635e019b1fd" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019b0f23-5d0d-7f96-a067-d16b168178ae" xlink:to="loc_srt_RangeMember_019b0f23-5d0d-73d4-a34b-2635e019b1fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019b0f23-5d0d-7bfe-89e6-83027ecd1b62" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019b0f23-5d0d-73d4-a34b-2635e019b1fd" xlink:to="loc_srt_MaximumMember_019b0f23-5d0d-7bfe-89e6-83027ecd1b62" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019b0f23-5d0d-7d33-9529-ec4340c56d63" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019b0f23-5d0d-73d4-a34b-2635e019b1fd" xlink:to="loc_srt_MinimumMember_019b0f23-5d0d-7d33-9529-ec4340c56d63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019b0f23-5d0d-770e-b54f-e03f4ccb23a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019b0f23-5d0d-7b24-a733-62e28ac6f552" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019b0f23-5d0d-770e-b54f-e03f4ccb23a9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019b0f23-5d0d-770e-b54f-e03f4ccb23a9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019b0f23-5d0d-770e-b54f-e03f4ccb23a9" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019b0f23-5d0d-770e-b54f-e03f4ccb23a9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019b0f23-5d0d-7490-bdfc-fec0e5a99c34" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019b0f23-5d0d-770e-b54f-e03f4ccb23a9" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019b0f23-5d0d-7490-bdfc-fec0e5a99c34" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BuildingAndBuildingImprovementsMember_019b0f23-5d0d-74ef-a4f3-c45afeecff7a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BuildingAndBuildingImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019b0f23-5d0d-7490-bdfc-fec0e5a99c34" xlink:to="loc_us-gaap_BuildingAndBuildingImprovementsMember_019b0f23-5d0d-74ef-a4f3-c45afeecff7a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_WarehouseEquipmentMember_019b0f23-5d0d-76d4-aa9d-d28dc9f0f11e" xlink:href="psix-20251231.xsd#psix_WarehouseEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019b0f23-5d0d-7490-bdfc-fec0e5a99c34" xlink:to="loc_psix_WarehouseEquipmentMember_019b0f23-5d0d-76d4-aa9d-d28dc9f0f11e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_019b0f23-5d0d-7c87-935c-70af7ca26dc3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019b0f23-5d0d-7f29-ada6-ad3541bab143" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_019b0f23-5d0d-7c87-935c-70af7ca26dc3" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/RevenueNarrativeDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#RevenueNarrativeDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/RevenueNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019b0f23-5d0d-729d-891a-7d1a0015b1a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019b0f23-5d0d-7a2a-b312-1710c1f204de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019b0f23-5d0d-729d-891a-7d1a0015b1a8" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019b0f23-5d0d-7a2a-b312-1710c1f204de" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019b0f23-5d0d-7788-8a98-dbeb6f3a7fc1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019b0f23-5d0d-7a2a-b312-1710c1f204de" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019b0f23-5d0d-7788-8a98-dbeb6f3a7fc1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019b0f23-5d0d-7788-8a98-dbeb6f3a7fc1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019b0f23-5d0d-7788-8a98-dbeb6f3a7fc1" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019b0f23-5d0d-7788-8a98-dbeb6f3a7fc1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019b0f23-5d0d-776d-9632-c6d61eb7f29c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019b0f23-5d0d-7788-8a98-dbeb6f3a7fc1" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019b0f23-5d0d-776d-9632-c6d61eb7f29c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredAtPointInTimeMember_019b0f23-5d0d-7dfd-a4b9-e9523ea92261" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferredAtPointInTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019b0f23-5d0d-776d-9632-c6d61eb7f29c" xlink:to="loc_us-gaap_TransferredAtPointInTimeMember_019b0f23-5d0d-7dfd-a4b9-e9523ea92261" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredOverTimeMember_019b0f23-5d0d-7ea6-98eb-019c3e6d8936" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferredOverTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019b0f23-5d0d-776d-9632-c6d61eb7f29c" xlink:to="loc_us-gaap_TransferredOverTimeMember_019b0f23-5d0d-7ea6-98eb-019c3e6d8936" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019b0f23-5d0d-7d13-bdf2-cf6f6109ebef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019b0f23-5d0d-7a2a-b312-1710c1f204de" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019b0f23-5d0d-7d13-bdf2-cf6f6109ebef" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019b0f23-5d0d-7169-aabd-ef884585a696" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019b0f23-5d0d-729d-891a-7d1a0015b1a8" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019b0f23-5d0d-7169-aabd-ef884585a696" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019b0f23-5d0d-7c46-9c96-45ed5eaa0784" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019b0f23-5d0d-729d-891a-7d1a0015b1a8" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019b0f23-5d0d-7c46-9c96-45ed5eaa0784" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_019b0f23-5d0d-7b8a-b47f-106bdf7746e8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019b0f23-5d0d-729d-891a-7d1a0015b1a8" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_019b0f23-5d0d-7b8a-b47f-106bdf7746e8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019b0f23-5d0d-7c63-8f04-51287aed947d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019b0f23-5d0d-729d-891a-7d1a0015b1a8" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019b0f23-5d0d-7c63-8f04-51287aed947d" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/RevenueNarrativeDetails_1" xlink:type="simple" xlink:href="psix-20251231.xsd#RevenueNarrativeDetails_1"/>
  <link:definitionLink xlink:role="http://powersint.com/role/RevenueNarrativeDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019b0f23-5d0d-729d-891a-7d1a0015b1a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019b0f23-5d0d-7a2a-b312-1710c1f204de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019b0f23-5d0d-729d-891a-7d1a0015b1a8" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019b0f23-5d0d-7a2a-b312-1710c1f204de" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019b0f23-5d0d-7788-8a98-dbeb6f3a7fc1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019b0f23-5d0d-7a2a-b312-1710c1f204de" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019b0f23-5d0d-7788-8a98-dbeb6f3a7fc1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019b0f23-5d0d-7788-8a98-dbeb6f3a7fc1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019b0f23-5d0d-7788-8a98-dbeb6f3a7fc1" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019b0f23-5d0d-7788-8a98-dbeb6f3a7fc1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019b0f23-5d0d-776d-9632-c6d61eb7f29c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019b0f23-5d0d-7788-8a98-dbeb6f3a7fc1" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019b0f23-5d0d-776d-9632-c6d61eb7f29c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredAtPointInTimeMember_019b0f23-5d0d-7dfd-a4b9-e9523ea92261" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferredAtPointInTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019b0f23-5d0d-776d-9632-c6d61eb7f29c" xlink:to="loc_us-gaap_TransferredAtPointInTimeMember_019b0f23-5d0d-7dfd-a4b9-e9523ea92261" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredOverTimeMember_019b0f23-5d0d-7ea6-98eb-019c3e6d8936" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferredOverTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019b0f23-5d0d-776d-9632-c6d61eb7f29c" xlink:to="loc_us-gaap_TransferredOverTimeMember_019b0f23-5d0d-7ea6-98eb-019c3e6d8936" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019b0f23-5d0d-7169-aabd-ef884585a696" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019b0f23-5d0d-729d-891a-7d1a0015b1a8" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019b0f23-5d0d-7169-aabd-ef884585a696" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019b0f23-5d0d-7c46-9c96-45ed5eaa0784" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019b0f23-5d0d-729d-891a-7d1a0015b1a8" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019b0f23-5d0d-7c46-9c96-45ed5eaa0784" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_019b0f23-5d0d-7b8a-b47f-106bdf7746e8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019b0f23-5d0d-729d-891a-7d1a0015b1a8" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_019b0f23-5d0d-7b8a-b47f-106bdf7746e8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019b0f23-5d0d-7c63-8f04-51287aed947d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019b0f23-5d0d-729d-891a-7d1a0015b1a8" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019b0f23-5d0d-7c63-8f04-51287aed947d" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/RevenueScheduleofDisaggregationofRevenueandScheduleofRevenuefromExternalCustomersbyGeographicAreasDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#RevenueScheduleofDisaggregationofRevenueandScheduleofRevenuefromExternalCustomersbyGeographicAreasDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/RevenueScheduleofDisaggregationofRevenueandScheduleofRevenuefromExternalCustomersbyGeographicAreasDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019b0f23-5d0d-7a6c-b0ce-a9f35e0381aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019b0f23-5d0d-73b4-9d28-d67a1c4eacf3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019b0f23-5d0d-7a6c-b0ce-a9f35e0381aa" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019b0f23-5d0d-73b4-9d28-d67a1c4eacf3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019b0f23-5d0d-75ad-a8ca-2f874be3bfcc" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019b0f23-5d0d-73b4-9d28-d67a1c4eacf3" xlink:to="loc_srt_ProductOrServiceAxis_019b0f23-5d0d-75ad-a8ca-2f874be3bfcc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019b0f23-5d0d-75ad-a8ca-2f874be3bfcc_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019b0f23-5d0d-75ad-a8ca-2f874be3bfcc" xlink:to="loc_srt_ProductsAndServicesDomain_019b0f23-5d0d-75ad-a8ca-2f874be3bfcc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019b0f23-5d0d-70f4-b0a2-ba7e0f6a8d11" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019b0f23-5d0d-75ad-a8ca-2f874be3bfcc" xlink:to="loc_srt_ProductsAndServicesDomain_019b0f23-5d0d-70f4-b0a2-ba7e0f6a8d11" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_EnergyEndMarketMember_019b0f23-5d0d-769d-a912-1975d1183cf3" xlink:href="psix-20251231.xsd#psix_EnergyEndMarketMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019b0f23-5d0d-70f4-b0a2-ba7e0f6a8d11" xlink:to="loc_psix_EnergyEndMarketMember_019b0f23-5d0d-769d-a912-1975d1183cf3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_IndustrialEndMarketMember_019b0f23-5d0d-75b6-9d08-8a20267b73dc" xlink:href="psix-20251231.xsd#psix_IndustrialEndMarketMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019b0f23-5d0d-70f4-b0a2-ba7e0f6a8d11" xlink:to="loc_psix_IndustrialEndMarketMember_019b0f23-5d0d-75b6-9d08-8a20267b73dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_TransportationEndMarketMember_019b0f23-5d0d-75ec-bb99-f105ae8980f5" xlink:href="psix-20251231.xsd#psix_TransportationEndMarketMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019b0f23-5d0d-70f4-b0a2-ba7e0f6a8d11" xlink:to="loc_psix_TransportationEndMarketMember_019b0f23-5d0d-75ec-bb99-f105ae8980f5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019b0f23-5d0d-7b1c-9e83-d9a4c138193b" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019b0f23-5d0d-73b4-9d28-d67a1c4eacf3" xlink:to="loc_srt_StatementGeographicalAxis_019b0f23-5d0d-7b1c-9e83-d9a4c138193b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019b0f23-5d0d-7b1c-9e83-d9a4c138193b_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019b0f23-5d0d-7b1c-9e83-d9a4c138193b" xlink:to="loc_srt_SegmentGeographicalDomain_019b0f23-5d0d-7b1c-9e83-d9a4c138193b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019b0f23-5d0d-7e5e-b817-c108946d3312" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019b0f23-5d0d-7b1c-9e83-d9a4c138193b" xlink:to="loc_srt_SegmentGeographicalDomain_019b0f23-5d0d-7e5e-b817-c108946d3312" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019b0f23-5d0d-7bd3-9ed1-63bd31b479db" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019b0f23-5d0d-7e5e-b817-c108946d3312" xlink:to="loc_country_US_019b0f23-5d0d-7bd3-9ed1-63bd31b479db" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_NorthAmericaExcludingUnitedStatesMember_019b0f23-5d0d-714a-81ba-e8cc641ee15e" xlink:href="psix-20251231.xsd#psix_NorthAmericaExcludingUnitedStatesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019b0f23-5d0d-7e5e-b817-c108946d3312" xlink:to="loc_psix_NorthAmericaExcludingUnitedStatesMember_019b0f23-5d0d-714a-81ba-e8cc641ee15e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_PacificRimMember_019b0f23-5d0d-7573-a196-8dcc41557567" xlink:href="psix-20251231.xsd#psix_PacificRimMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019b0f23-5d0d-7e5e-b817-c108946d3312" xlink:to="loc_psix_PacificRimMember_019b0f23-5d0d-7573-a196-8dcc41557567" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EuropeMember_019b0f23-5d0d-74ec-a354-09821515ca77" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019b0f23-5d0d-7e5e-b817-c108946d3312" xlink:to="loc_srt_EuropeMember_019b0f23-5d0d-74ec-a354-09821515ca77" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_OtherGeographicalAreasMember_019b0f23-5d0d-779c-b8f1-ff5077c2d562" xlink:href="psix-20251231.xsd#psix_OtherGeographicalAreasMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019b0f23-5d0d-7e5e-b817-c108946d3312" xlink:to="loc_psix_OtherGeographicalAreasMember_019b0f23-5d0d-779c-b8f1-ff5077c2d562" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019b0f23-5d0d-756d-86f5-5af2027f6fa1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019b0f23-5d0d-7a6c-b0ce-a9f35e0381aa" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019b0f23-5d0d-756d-86f5-5af2027f6fa1" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/WeichaiTransactionsDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#WeichaiTransactionsDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/WeichaiTransactionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockLineItems_019b0f23-5d0d-7171-a80e-cd271f9c295a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_019b0f23-5d0d-797c-9a8c-5ed40c26f15b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_019b0f23-5d0d-7171-a80e-cd271f9c295a" xlink:to="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_019b0f23-5d0d-797c-9a8c-5ed40c26f15b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019b0f23-5d0d-7a61-b07e-03c04ada1a2d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_019b0f23-5d0d-797c-9a8c-5ed40c26f15b" xlink:to="loc_us-gaap_DebtInstrumentAxis_019b0f23-5d0d-7a61-b07e-03c04ada1a2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-7a61-b07e-03c04ada1a2d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019b0f23-5d0d-7a61-b07e-03c04ada1a2d" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-7a61-b07e-03c04ada1a2d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-7980-86eb-6af7dddd9b4f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019b0f23-5d0d-7a61-b07e-03c04ada1a2d" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-7980-86eb-6af7dddd9b4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ShareholdersLoanAgreementWeichaiMember_019b0f23-5d0d-72bb-a77d-3329d34fce34" xlink:href="psix-20251231.xsd#psix_ShareholdersLoanAgreementWeichaiMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-7980-86eb-6af7dddd9b4f" xlink:to="loc_psix_ShareholdersLoanAgreementWeichaiMember_019b0f23-5d0d-72bb-a77d-3329d34fce34" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019b0f23-5d0d-70a9-9327-497c54aa0a5b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_019b0f23-5d0d-797c-9a8c-5ed40c26f15b" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019b0f23-5d0d-70a9-9327-497c54aa0a5b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-70a9-9327-497c54aa0a5b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019b0f23-5d0d-70a9-9327-497c54aa0a5b" xlink:to="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-70a9-9327-497c54aa0a5b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-7dfa-977f-5cfc30111aec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019b0f23-5d0d-70a9-9327-497c54aa0a5b" xlink:to="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-7dfa-977f-5cfc30111aec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019b0f23-5d0d-70f7-9a83-e178e1663270" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019b0f23-5d0d-7dfa-977f-5cfc30111aec" xlink:to="loc_us-gaap_RelatedPartyMember_019b0f23-5d0d-70f7-9a83-e178e1663270" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0d-7067-b1ec-65c0ddaddf4c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_019b0f23-5d0d-797c-9a8c-5ed40c26f15b" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0d-7067-b1ec-65c0ddaddf4c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-7067-b1ec-65c0ddaddf4c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0d-7067-b1ec-65c0ddaddf4c" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-7067-b1ec-65c0ddaddf4c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-7d43-95b2-2aad79179419" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0d-7067-b1ec-65c0ddaddf4c" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-7d43-95b2-2aad79179419" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedDebtMember_019b0f23-5d0d-701b-8452-084bc521f868" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-7d43-95b2-2aad79179419" xlink:to="loc_us-gaap_SubordinatedDebtMember_019b0f23-5d0d-701b-8452-084bc521f868" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019b0f23-5d0d-729c-a715-df7372f532be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_019b0f23-5d0d-7171-a80e-cd271f9c295a" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019b0f23-5d0d-729c-a715-df7372f532be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_CollaborativeArrangementTerm_019b0f23-5d0d-7c53-be2e-44289ff7c94c" xlink:href="psix-20251231.xsd#psix_CollaborativeArrangementTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_019b0f23-5d0d-7171-a80e-cd271f9c295a" xlink:to="loc_psix_CollaborativeArrangementTerm_019b0f23-5d0d-7c53-be2e-44289ff7c94c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_CollaborativeArrangementInventoryPurchasedGoods_019b0f23-5d0d-7cd2-90e9-a8bddd676310" xlink:href="psix-20251231.xsd#psix_CollaborativeArrangementInventoryPurchasedGoods"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_019b0f23-5d0d-7171-a80e-cd271f9c295a" xlink:to="loc_psix_CollaborativeArrangementInventoryPurchasedGoods_019b0f23-5d0d-7cd2-90e9-a8bddd676310" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/PropertyPlantandEquipmentDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#PropertyPlantandEquipmentDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/PropertyPlantandEquipmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019b0f23-5d0d-751c-9c02-271913bd167f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019b0f23-5d0d-7249-86ac-cdb3d7c360eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019b0f23-5d0d-751c-9c02-271913bd167f" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019b0f23-5d0d-7249-86ac-cdb3d7c360eb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019b0f23-5d0d-729c-bee6-10e33af6c757" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019b0f23-5d0d-7249-86ac-cdb3d7c360eb" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019b0f23-5d0d-729c-bee6-10e33af6c757" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019b0f23-5d0d-729c-bee6-10e33af6c757_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019b0f23-5d0d-729c-bee6-10e33af6c757" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019b0f23-5d0d-729c-bee6-10e33af6c757_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019b0f23-5d0d-701f-80ee-0bf22ca327d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019b0f23-5d0d-729c-bee6-10e33af6c757" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019b0f23-5d0d-701f-80ee-0bf22ca327d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_019b0f23-5d0d-7f0e-977a-37178826f27a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MachineryAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019b0f23-5d0d-701f-80ee-0bf22ca327d0" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_019b0f23-5d0d-7f0e-977a-37178826f27a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransportationEquipmentMember_019b0f23-5d0d-77a2-98e4-dbad895bfb52" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransportationEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019b0f23-5d0d-701f-80ee-0bf22ca327d0" xlink:to="loc_us-gaap_TransportationEquipmentMember_019b0f23-5d0d-77a2-98e4-dbad895bfb52" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_019b0f23-5d0d-7fed-8041-93990a9d9107" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019b0f23-5d0d-701f-80ee-0bf22ca327d0" xlink:to="loc_us-gaap_ConstructionInProgressMember_019b0f23-5d0d-7fed-8041-93990a9d9107" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization_019b0f23-5d0d-7f5c-ab96-1ac8483d28e1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019b0f23-5d0d-751c-9c02-271913bd167f" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization_019b0f23-5d0d-7f5c-ab96-1ac8483d28e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_019b0f23-5d0d-7db8-b9cc-71e4e664a823" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019b0f23-5d0d-751c-9c02-271913bd167f" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_019b0f23-5d0d-7db8-b9cc-71e4e664a823" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019b0f23-5d0d-72d2-9656-d93c94474f3a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019b0f23-5d0d-751c-9c02-271913bd167f" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019b0f23-5d0d-72d2-9656-d93c94474f3a" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/GoodwillandOtherIntangiblesScheduleofComponentsofIntangibleAssetsDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#GoodwillandOtherIntangiblesScheduleofComponentsofIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/GoodwillandOtherIntangiblesScheduleofComponentsofIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019b0f23-5d0d-7bc4-a178-2335e893ed4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019b0f23-5d0d-7c60-9be2-7d621b7cbbc8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019b0f23-5d0d-7bc4-a178-2335e893ed4a" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019b0f23-5d0d-7c60-9be2-7d621b7cbbc8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019b0f23-5d0d-7d1f-ab0c-d0682119cd08" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019b0f23-5d0d-7c60-9be2-7d621b7cbbc8" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019b0f23-5d0d-7d1f-ab0c-d0682119cd08" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019b0f23-5d0d-7d1f-ab0c-d0682119cd08_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019b0f23-5d0d-7d1f-ab0c-d0682119cd08" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019b0f23-5d0d-7d1f-ab0c-d0682119cd08_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019b0f23-5d0d-7a8c-a718-e3fc96908c6e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019b0f23-5d0d-7d1f-ab0c-d0682119cd08" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019b0f23-5d0d-7a8c-a718-e3fc96908c6e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019b0f23-5d0d-7069-9ee3-b3ed8fc9bc58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019b0f23-5d0d-7a8c-a718-e3fc96908c6e" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019b0f23-5d0d-7069-9ee3-b3ed8fc9bc58" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_019b0f23-5d0d-7727-b861-ee55169e98b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019b0f23-5d0d-7a8c-a718-e3fc96908c6e" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_019b0f23-5d0d-7727-b861-ee55169e98b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksAndTradeNamesMember_019b0f23-5d0d-7fd6-9a10-74c717320402" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TrademarksAndTradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019b0f23-5d0d-7a8c-a718-e3fc96908c6e" xlink:to="loc_us-gaap_TrademarksAndTradeNamesMember_019b0f23-5d0d-7fd6-9a10-74c717320402" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019b0f23-5d0d-794d-8b9a-0ce33c361536" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019b0f23-5d0d-7bc4-a178-2335e893ed4a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019b0f23-5d0d-794d-8b9a-0ce33c361536" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019b0f23-5d0d-7dcd-b16b-0e0e1452d4a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019b0f23-5d0d-7bc4-a178-2335e893ed4a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019b0f23-5d0d-7dcd-b16b-0e0e1452d4a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019b0f23-5d0d-77e7-b33e-5d81f47c9b72" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019b0f23-5d0d-7bc4-a178-2335e893ed4a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019b0f23-5d0d-77e7-b33e-5d81f47c9b72" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/DebtScheduleofOutstandingDebtDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#DebtScheduleofOutstandingDebtDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/DebtScheduleofOutstandingDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_psix_ScheduleOfLongTermDebtAndShortTermDebtInstrumentsLineItems_019b0f23-5d0d-729a-8524-4c91e3fe554a" xlink:href="psix-20251231.xsd#psix_ScheduleOfLongTermDebtAndShortTermDebtInstrumentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ScheduleOfLongTermDebtAndShortTermDebtInstrumentsTable_019b0f23-5d0d-742a-b577-0d782de5d699" xlink:href="psix-20251231.xsd#psix_ScheduleOfLongTermDebtAndShortTermDebtInstrumentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_psix_ScheduleOfLongTermDebtAndShortTermDebtInstrumentsLineItems_019b0f23-5d0d-729a-8524-4c91e3fe554a" xlink:to="loc_psix_ScheduleOfLongTermDebtAndShortTermDebtInstrumentsTable_019b0f23-5d0d-742a-b577-0d782de5d699" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019b0f23-5d0d-70a8-87bb-7ba29e7e753c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_psix_ScheduleOfLongTermDebtAndShortTermDebtInstrumentsTable_019b0f23-5d0d-742a-b577-0d782de5d699" xlink:to="loc_us-gaap_CreditFacilityAxis_019b0f23-5d0d-70a8-87bb-7ba29e7e753c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019b0f23-5d0d-70a8-87bb-7ba29e7e753c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019b0f23-5d0d-70a8-87bb-7ba29e7e753c" xlink:to="loc_us-gaap_CreditFacilityDomain_019b0f23-5d0d-70a8-87bb-7ba29e7e753c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019b0f23-5d0d-776d-b952-0918ab53ab37" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019b0f23-5d0d-70a8-87bb-7ba29e7e753c" xlink:to="loc_us-gaap_CreditFacilityDomain_019b0f23-5d0d-776d-b952-0918ab53ab37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019b0f23-5d0d-77ae-b320-f6bfa92b800d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019b0f23-5d0d-776d-b952-0918ab53ab37" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019b0f23-5d0d-77ae-b320-f6bfa92b800d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019b0f23-5d0d-7c00-b323-24722c1c70b1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_psix_ScheduleOfLongTermDebtAndShortTermDebtInstrumentsTable_019b0f23-5d0d-742a-b577-0d782de5d699" xlink:to="loc_us-gaap_DebtInstrumentAxis_019b0f23-5d0d-7c00-b323-24722c1c70b1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-7c00-b323-24722c1c70b1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019b0f23-5d0d-7c00-b323-24722c1c70b1" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-7c00-b323-24722c1c70b1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-75b9-a155-97f8c3ba865a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019b0f23-5d0d-7c00-b323-24722c1c70b1" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-75b9-a155-97f8c3ba865a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_RevolvingCreditAgreementMember_019b5a53-b157-76f4-8bca-b751a097053e" xlink:href="psix-20251231.xsd#psix_RevolvingCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-75b9-a155-97f8c3ba865a" xlink:to="loc_psix_RevolvingCreditAgreementMember_019b5a53-b157-76f4-8bca-b751a097053e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ShareholdersLoanAgreementWeichaiMember_019b0f23-5d0d-7832-a189-35b6ba65394d" xlink:href="psix-20251231.xsd#psix_ShareholdersLoanAgreementWeichaiMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5d0d-75b9-a155-97f8c3ba865a" xlink:to="loc_psix_ShareholdersLoanAgreementWeichaiMember_019b0f23-5d0d-7832-a189-35b6ba65394d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0d-70ad-acb1-ed07ec784fdb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_psix_ScheduleOfLongTermDebtAndShortTermDebtInstrumentsTable_019b0f23-5d0d-742a-b577-0d782de5d699" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0d-70ad-acb1-ed07ec784fdb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-70ad-acb1-ed07ec784fdb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0d-70ad-acb1-ed07ec784fdb" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-70ad-acb1-ed07ec784fdb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-710d-88a1-103dde4b776f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0d-70ad-acb1-ed07ec784fdb" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-710d-88a1-103dde4b776f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019b0f23-5d0d-7b19-9499-ad6bc83a9b9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-710d-88a1-103dde4b776f" xlink:to="loc_us-gaap_LineOfCreditMember_019b0f23-5d0d-7b19-9499-ad6bc83a9b9d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedDebtMember_019b0f23-5d0d-77bb-b30f-32bc21aeefe1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0d-710d-88a1-103dde4b776f" xlink:to="loc_us-gaap_SubordinatedDebtMember_019b0f23-5d0d-77bb-b30f-32bc21aeefe1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019b5a53-b157-7520-b47e-2a1e01cd9b19" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_psix_ScheduleOfLongTermDebtAndShortTermDebtInstrumentsTable_019b0f23-5d0d-742a-b577-0d782de5d699" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019b5a53-b157-7520-b47e-2a1e01cd9b19" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b5a53-b157-7520-b47e-2a1e01cd9b19_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b5a53-b157-7520-b47e-2a1e01cd9b19" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b5a53-b157-7520-b47e-2a1e01cd9b19_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b5a53-b157-7e80-ab62-b6bbaf650a72" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b5a53-b157-7520-b47e-2a1e01cd9b19" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b5a53-b157-7e80-ab62-b6bbaf650a72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019b5a53-b157-7509-a189-e6e58acfedae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019b5a53-b157-7e80-ab62-b6bbaf650a72" xlink:to="loc_us-gaap_LineOfCreditMember_019b5a53-b157-7509-a189-e6e58acfedae" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtOtherDisclosuresAbstract_019b0f23-5d0d-7b7f-aed0-4f90bfac3cba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtOtherDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_psix_ScheduleOfLongTermDebtAndShortTermDebtInstrumentsLineItems_019b0f23-5d0d-729a-8524-4c91e3fe554a" xlink:to="loc_us-gaap_ShortTermDebtOtherDisclosuresAbstract_019b0f23-5d0d-7b7f-aed0-4f90bfac3cba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermBorrowings_019b0f23-5d0d-73a2-90ec-1a36df155281" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtOtherDisclosuresAbstract_019b0f23-5d0d-7b7f-aed0-4f90bfac3cba" xlink:to="loc_us-gaap_ShortTermBorrowings_019b0f23-5d0d-73a2-90ec-1a36df155281" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtWeightedAverageInterestRate_019b0f23-5d0d-70a3-83cb-f52d58688e87" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtOtherDisclosuresAbstract_019b0f23-5d0d-7b7f-aed0-4f90bfac3cba" xlink:to="loc_us-gaap_ShortTermDebtWeightedAverageInterestRate_019b0f23-5d0d-70a3-83cb-f52d58688e87" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtOtherDisclosuresAbstract_019b0f23-5d0d-76c0-a882-2c330520ef3c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtOtherDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_psix_ScheduleOfLongTermDebtAndShortTermDebtInstrumentsLineItems_019b0f23-5d0d-729a-8524-4c91e3fe554a" xlink:to="loc_us-gaap_LongTermDebtOtherDisclosuresAbstract_019b0f23-5d0d-76c0-a882-2c330520ef3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019b5a53-b157-70ec-a753-6639a473a51f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtOtherDisclosuresAbstract_019b0f23-5d0d-76c0-a882-2c330520ef3c" xlink:to="loc_us-gaap_LongTermDebt_019b5a53-b157-70ec-a753-6639a473a51f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtWeightedAverageInterestRate_019b5a53-b157-754f-b786-272b9f2dc3a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtOtherDisclosuresAbstract_019b0f23-5d0d-76c0-a882-2c330520ef3c" xlink:to="loc_us-gaap_LongtermDebtWeightedAverageInterestRate_019b5a53-b157-754f-b786-272b9f2dc3a1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_FinanceLeaseLiabilityAndOtherLongTermDebt_019b0f23-5d0d-78e3-b30f-43ba2c102160" xlink:href="psix-20251231.xsd#psix_FinanceLeaseLiabilityAndOtherLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtOtherDisclosuresAbstract_019b0f23-5d0d-76c0-a882-2c330520ef3c" xlink:to="loc_psix_FinanceLeaseLiabilityAndOtherLongTermDebt_019b0f23-5d0d-78e3-b30f-43ba2c102160" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019b0f23-5d0d-73f4-b33b-1b8e1b61f0da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtOtherDisclosuresAbstract_019b0f23-5d0d-76c0-a882-2c330520ef3c" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019b0f23-5d0d-73f4-b33b-1b8e1b61f0da" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_019b0f23-5d0d-7ced-9fdd-a1ad7181555f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtOtherDisclosuresAbstract_019b0f23-5d0d-76c0-a882-2c330520ef3c" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_019b0f23-5d0d-7ced-9fdd-a1ad7181555f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_LongTermDebtExcludingNotesPayable_019b0f23-5d0d-7361-bc28-9b3bf0167fac" xlink:href="psix-20251231.xsd#psix_LongTermDebtExcludingNotesPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtOtherDisclosuresAbstract_019b0f23-5d0d-76c0-a882-2c330520ef3c" xlink:to="loc_psix_LongTermDebtExcludingNotesPayable_019b0f23-5d0d-7361-bc28-9b3bf0167fac" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_019b0f23-5d0d-7645-b970-28c503e469ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_psix_ScheduleOfLongTermDebtAndShortTermDebtInstrumentsLineItems_019b0f23-5d0d-729a-8524-4c91e3fe554a" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_019b0f23-5d0d-7645-b970-28c503e469ee" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/DebtNarrativeDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#DebtNarrativeDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/DebtNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0d-7d2d-beed-db256acf47d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0d-7d2d-beed-db256acf47d0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019ba156-7cc6-7e90-a972-634561090bcd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0d-7d2d-beed-db256acf47d0" xlink:to="loc_us-gaap_CreditFacilityAxis_019ba156-7cc6-7e90-a972-634561090bcd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019ba156-7cc6-7e90-a972-634561090bcd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019ba156-7cc6-7e90-a972-634561090bcd" xlink:to="loc_us-gaap_CreditFacilityDomain_019ba156-7cc6-7e90-a972-634561090bcd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019ba156-7cc6-786b-ac82-a7086fbf9e19" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019ba156-7cc6-7e90-a972-634561090bcd" xlink:to="loc_us-gaap_CreditFacilityDomain_019ba156-7cc6-786b-ac82-a7086fbf9e19" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019ba156-7cc6-7946-acbb-4250114c67ff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019ba156-7cc6-786b-ac82-a7086fbf9e19" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019ba156-7cc6-7946-acbb-4250114c67ff" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019ba156-7cc6-70a4-91aa-c419af3aa9c2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0d-7d2d-beed-db256acf47d0" xlink:to="loc_us-gaap_DebtInstrumentAxis_019ba156-7cc6-70a4-91aa-c419af3aa9c2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ba156-7cc6-70a4-91aa-c419af3aa9c2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ba156-7cc6-70a4-91aa-c419af3aa9c2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ba156-7cc6-70a4-91aa-c419af3aa9c2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ba156-7cc6-72cf-8384-86cc0dab339a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ba156-7cc6-70a4-91aa-c419af3aa9c2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ba156-7cc6-72cf-8384-86cc0dab339a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_RevolvingCreditAgreementMember_019ba156-7cc6-72e7-b707-3b4f0639025d" xlink:href="psix-20251231.xsd#psix_RevolvingCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ba156-7cc6-72cf-8384-86cc0dab339a" xlink:to="loc_psix_RevolvingCreditAgreementMember_019ba156-7cc6-72e7-b707-3b4f0639025d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_RevolvingCreditAgreementInEventMajorityShareholderHoldsLessThan50OfEquityMember_019ba156-b044-77a4-8a0f-8b73fae0e165" xlink:href="psix-20251231.xsd#psix_RevolvingCreditAgreementInEventMajorityShareholderHoldsLessThan50OfEquityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_psix_RevolvingCreditAgreementMember_019ba156-7cc6-72e7-b707-3b4f0639025d" xlink:to="loc_psix_RevolvingCreditAgreementInEventMajorityShareholderHoldsLessThan50OfEquityMember_019ba156-b044-77a4-8a0f-8b73fae0e165" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_FourthAmendedAndRestatedCreditAgreementMember_019ba156-7cc6-7b6a-b5d3-8db12f66eb64" xlink:href="psix-20251231.xsd#psix_FourthAmendedAndRestatedCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ba156-7cc6-72cf-8384-86cc0dab339a" xlink:to="loc_psix_FourthAmendedAndRestatedCreditAgreementMember_019ba156-7cc6-7b6a-b5d3-8db12f66eb64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_PreviousShareholderLoanAgreementsWeichaiMember_019ba156-7cc6-743d-b7c2-fd83cd2e8a6c" xlink:href="psix-20251231.xsd#psix_PreviousShareholderLoanAgreementsWeichaiMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ba156-7cc6-72cf-8384-86cc0dab339a" xlink:to="loc_psix_PreviousShareholderLoanAgreementsWeichaiMember_019ba156-7cc6-743d-b7c2-fd83cd2e8a6c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ShareholdersLoanAgreementWeichaiMember_019ba156-7cc6-7b01-9e9b-2344adcca0d2" xlink:href="psix-20251231.xsd#psix_ShareholdersLoanAgreementWeichaiMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ba156-7cc6-72cf-8384-86cc0dab339a" xlink:to="loc_psix_ShareholdersLoanAgreementWeichaiMember_019ba156-7cc6-7b01-9e9b-2344adcca0d2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_019ba156-7cc6-7270-aa80-0eebcf6d9b78" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0d-7d2d-beed-db256acf47d0" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_019ba156-7cc6-7270-aa80-0eebcf6d9b78" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019ba156-7cc6-7270-aa80-0eebcf6d9b78_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019ba156-7cc6-7270-aa80-0eebcf6d9b78" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019ba156-7cc6-7270-aa80-0eebcf6d9b78_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019ba156-7cc6-7e8c-833a-2db745e56f71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019ba156-7cc6-7270-aa80-0eebcf6d9b78" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019ba156-7cc6-7e8c-833a-2db745e56f71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019ba156-7cc6-7a09-b9df-357d9b7b1cf6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_019ba156-7cc6-7e8c-833a-2db745e56f71" xlink:to="loc_us-gaap_LineOfCreditMember_019ba156-7cc6-7a09-b9df-357d9b7b1cf6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedDebtMember_019ba156-7cc6-7d21-a3d2-083734970ac7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_019ba156-7cc6-7e8c-833a-2db745e56f71" xlink:to="loc_us-gaap_SubordinatedDebtMember_019ba156-7cc6-7d21-a3d2-083734970ac7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019ba156-b044-78f4-9174-6d90af631f3b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0d-7d2d-beed-db256acf47d0" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019ba156-b044-78f4-9174-6d90af631f3b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ba156-b044-78f4-9174-6d90af631f3b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ba156-b044-78f4-9174-6d90af631f3b" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ba156-b044-78f4-9174-6d90af631f3b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ba156-b044-7e35-a7f0-202fc9650e06" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ba156-b044-78f4-9174-6d90af631f3b" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ba156-b044-7e35-a7f0-202fc9650e06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019ba17a-6988-73d6-a7bb-5bb9ad1279b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ba156-b044-7e35-a7f0-202fc9650e06" xlink:to="loc_us-gaap_LineOfCreditMember_019ba17a-6988-73d6-a7bb-5bb9ad1279b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_019ba156-7cc6-7816-8882-f14e76588cb9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_InterestExpenseDebt_019ba156-7cc6-7816-8882-f14e76588cb9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019ba156-7cc6-77e4-89fa-aa03dcb0571f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019ba156-7cc6-77e4-89fa-aa03dcb0571f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ba156-7cc6-765e-87ae-1cda21c391f2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ba156-7cc6-765e-87ae-1cda21c391f2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsGross_019ba156-7cc6-70c2-a259-d9f3e00faba9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_DeferredFinanceCostsGross_019ba156-7cc6-70c2-a259-d9f3e00faba9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromShortTermDebt_019ba156-7cc6-78e1-b565-c0c6f14083e3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromShortTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_ProceedsFromShortTermDebt_019ba156-7cc6-78e1-b565-c0c6f14083e3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfShortTermDebt_019ba156-7cc6-768c-ac2b-cdf2e08b64b1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfShortTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_RepaymentsOfShortTermDebt_019ba156-7cc6-768c-ac2b-cdf2e08b64b1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDebtExtinguishmentCosts_019ba156-7cc6-774d-9a8f-d58afd12a67f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDebtExtinguishmentCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_PaymentsOfDebtExtinguishmentCosts_019ba156-7cc6-774d-9a8f-d58afd12a67f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019ba156-7cc6-7603-b481-5b9be55570f6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019ba156-7cc6-7603-b481-5b9be55570f6" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_DebtInstrumentIncrementalBorrowingCost_019ba156-7cc6-7a98-92b8-69366b527406" xlink:href="psix-20251231.xsd#psix_DebtInstrumentIncrementalBorrowingCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_psix_DebtInstrumentIncrementalBorrowingCost_019ba156-7cc6-7a98-92b8-69366b527406" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_DebtInstrumentAmountNotSubjectToSubordination_019ba156-7cc6-7cca-a2c9-5fbd471a8a00" xlink:href="psix-20251231.xsd#psix_DebtInstrumentAmountNotSubjectToSubordination"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_psix_DebtInstrumentAmountNotSubjectToSubordination_019ba156-7cc6-7cca-a2c9-5fbd471a8a00" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermBorrowings_019ba156-7cc6-7e39-a23b-742bee852241" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_ShortTermBorrowings_019ba156-7cc6-7e39-a23b-742bee852241" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_DebtInstrumentMajorityShareholderOwnershipThresholdForVariableRateIncrease_019ba156-b044-7839-89e4-3d539585c3f2" xlink:href="psix-20251231.xsd#psix_DebtInstrumentMajorityShareholderOwnershipThresholdForVariableRateIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_psix_DebtInstrumentMajorityShareholderOwnershipThresholdForVariableRateIncrease_019ba156-b044-7839-89e4-3d539585c3f2" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtLongtermAndShorttermCombinedAmount_019ba156-b044-7676-bc5f-4a34ffa149a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtLongtermAndShorttermCombinedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_DebtLongtermAndShorttermCombinedAmount_019ba156-b044-7676-bc5f-4a34ffa149a0" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_019c0e2b-f6c3-7dbc-9653-9a0440489f97" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_LineOfCredit_019c0e2b-f6c3-7dbc-9653-9a0440489f97" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019ba156-b044-70f2-bc1a-be9d9e9d98c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019ba156-b044-70f2-bc1a-be9d9e9d98c8" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPayableCurrentAndNoncurrent_019ba156-b044-76ad-a21e-a86367ca8a99" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPayableCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ba156-7cc6-7634-a429-55a7a00c01f3" xlink:to="loc_us-gaap_InterestPayableCurrentAndNoncurrent_019ba156-b044-76ad-a21e-a86367ca8a99" xlink:type="arc" order="15"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/DebtScheduleofRemainingMaturitiesofLongTermDebtDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#DebtScheduleofRemainingMaturitiesofLongTermDebtDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/DebtScheduleofRemainingMaturitiesofLongTermDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0d-74aa-9cdc-6daaa9d548ff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0d-79ed-8c76-b946516fc64f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0d-74aa-9cdc-6daaa9d548ff" xlink:to="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0d-79ed-8c76-b946516fc64f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0d-7a1f-a0f8-92079a46dddf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0d-79ed-8c76-b946516fc64f" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0d-7a1f-a0f8-92079a46dddf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0d-7a1f-a0f8-92079a46dddf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0d-7a1f-a0f8-92079a46dddf" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0d-7a1f-a0f8-92079a46dddf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0d-75ef-8ce4-9daa891a4083" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0d-7a1f-a0f8-92079a46dddf" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0d-75ef-8ce4-9daa891a4083" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_LongTermDebtExcludingNotesPayableMember_019b0f23-5d0d-72fa-b5b4-7e9021c5de9b" xlink:href="psix-20251231.xsd#psix_LongTermDebtExcludingNotesPayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0d-75ef-8ce4-9daa891a4083" xlink:to="loc_psix_LongTermDebtExcludingNotesPayableMember_019b0f23-5d0d-72fa-b5b4-7e9021c5de9b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths_019b0f23-5d0d-72b2-8bf9-fbf37db41e27" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0d-74aa-9cdc-6daaa9d548ff" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths_019b0f23-5d0d-72b2-8bf9-fbf37db41e27" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo_019b0f23-5d0d-7c32-9037-1915923b936d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0d-74aa-9cdc-6daaa9d548ff" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo_019b0f23-5d0d-7c32-9037-1915923b936d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019b0f23-5d0d-7200-9397-ae1132c1c33e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0d-74aa-9cdc-6daaa9d548ff" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019b0f23-5d0d-7200-9397-ae1132c1c33e" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/OtherNonCurrentLiabilitiesNarrativeDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#OtherNonCurrentLiabilitiesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/OtherNonCurrentLiabilitiesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-774f-aabe-987548fb9a15" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0e-7d9d-b24e-fc81431e5fb7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-774f-aabe-987548fb9a15" xlink:to="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0e-7d9d-b24e-fc81431e5fb7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0e-7a5d-a768-449b98e98c37" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0e-7d9d-b24e-fc81431e5fb7" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0e-7a5d-a768-449b98e98c37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-7a5d-a768-449b98e98c37_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0e-7a5d-a768-449b98e98c37" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-7a5d-a768-449b98e98c37_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-73f8-bade-fc78e3b88a93" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0e-7a5d-a768-449b98e98c37" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-73f8-bade-fc78e3b88a93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayableOtherPayablesMember_019b0f23-5d0e-7aee-8450-5ddc5fba65d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesPayableOtherPayablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-73f8-bade-fc78e3b88a93" xlink:to="loc_us-gaap_NotesPayableOtherPayablesMember_019b0f23-5d0e-7aee-8450-5ddc5fba65d3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0e-7e05-9940-f6416b9c2a5f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0e-7d9d-b24e-fc81431e5fb7" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0e-7e05-9940-f6416b9c2a5f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0e-7e05-9940-f6416b9c2a5f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0e-7e05-9940-f6416b9c2a5f" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0e-7e05-9940-f6416b9c2a5f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0e-768f-8c1e-5e6bac52f25a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019b0f23-5d0e-7e05-9940-f6416b9c2a5f" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0e-768f-8c1e-5e6bac52f25a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayableOtherPayablesMember_019b0f23-5d0e-7103-8fff-68eb14883304" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesPayableOtherPayablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_019b0f23-5d0e-768f-8c1e-5e6bac52f25a" xlink:to="loc_us-gaap_NotesPayableOtherPayablesMember_019b0f23-5d0e-7103-8fff-68eb14883304" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019b0f23-5d0e-7731-9756-0aa39823f5f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-774f-aabe-987548fb9a15" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019b0f23-5d0e-7731-9756-0aa39823f5f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019b0f23-5d0e-7050-b19a-bb26dbda21c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-774f-aabe-987548fb9a15" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019b0f23-5d0e-7050-b19a-bb26dbda21c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_DebtInstrumentOptionalFaceAmount_019b0f23-5d0e-7264-a963-390f67538610" xlink:href="psix-20251231.xsd#psix_DebtInstrumentOptionalFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-774f-aabe-987548fb9a15" xlink:to="loc_psix_DebtInstrumentOptionalFaceAmount_019b0f23-5d0e-7264-a963-390f67538610" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermBorrowings_019b0f23-5d0e-723d-8b3e-0c5691c5941a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-774f-aabe-987548fb9a15" xlink:to="loc_us-gaap_ShortTermBorrowings_019b0f23-5d0e-723d-8b3e-0c5691c5941a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfDebtDiscountPremium_019b0f23-5d0e-7989-ac7a-b638c04b34e7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfDebtDiscountPremium"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-774f-aabe-987548fb9a15" xlink:to="loc_us-gaap_AmortizationOfDebtDiscountPremium_019b0f23-5d0e-7989-ac7a-b638c04b34e7" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/OtherNonCurrentLiabilitiesScheduleofNotePayableDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#OtherNonCurrentLiabilitiesScheduleofNotePayableDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/OtherNonCurrentLiabilitiesScheduleofNotePayableDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-7690-ac4a-2cbfdab671c2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0e-7af5-9b12-1047668381ca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-7690-ac4a-2cbfdab671c2" xlink:to="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0e-7af5-9b12-1047668381ca" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0e-7c09-ab04-97d583cdd903" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0e-7af5-9b12-1047668381ca" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0e-7c09-ab04-97d583cdd903" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-7c09-ab04-97d583cdd903_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0e-7c09-ab04-97d583cdd903" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-7c09-ab04-97d583cdd903_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-7c26-a764-127ebd6bcd0d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0e-7c09-ab04-97d583cdd903" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-7c26-a764-127ebd6bcd0d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayableOtherPayablesMember_019b0f23-5d0e-7855-9d2a-19ec7b71dbbc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesPayableOtherPayablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-7c26-a764-127ebd6bcd0d" xlink:to="loc_us-gaap_NotesPayableOtherPayablesMember_019b0f23-5d0e-7855-9d2a-19ec7b71dbbc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019b0f23-5d0e-7af8-82de-ba47c70a9f61" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-7690-ac4a-2cbfdab671c2" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019b0f23-5d0e-7af8-82de-ba47c70a9f61" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019b0f23-5d0e-7940-9078-453f1c00879c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-7690-ac4a-2cbfdab671c2" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019b0f23-5d0e-7940-9078-453f1c00879c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019b0f23-5d0e-78ab-8513-8b96be627c9b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-7690-ac4a-2cbfdab671c2" xlink:to="loc_us-gaap_LongTermDebt_019b0f23-5d0e-78ab-8513-8b96be627c9b" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/OtherNonCurrentLiabilitiesScheduleofRemainingMaturitiesofLongTermDebtDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#OtherNonCurrentLiabilitiesScheduleofRemainingMaturitiesofLongTermDebtDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/OtherNonCurrentLiabilitiesScheduleofRemainingMaturitiesofLongTermDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-79d5-87d0-78ca75c0614b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0e-70e1-9751-dee5fb9dc9fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-79d5-87d0-78ca75c0614b" xlink:to="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0e-70e1-9751-dee5fb9dc9fe" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0e-7aa5-9d44-807f8f44147f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019b0f23-5d0e-70e1-9751-dee5fb9dc9fe" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0e-7aa5-9d44-807f8f44147f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-7aa5-9d44-807f8f44147f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0e-7aa5-9d44-807f8f44147f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-7aa5-9d44-807f8f44147f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-7e51-9ce8-64ec9ae3f847" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b0f23-5d0e-7aa5-9d44-807f8f44147f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-7e51-9ce8-64ec9ae3f847" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayableOtherPayablesMember_019b0f23-5d0e-791e-bcfb-dbc2d185b2ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesPayableOtherPayablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019b0f23-5d0e-7e51-9ce8-64ec9ae3f847" xlink:to="loc_us-gaap_NotesPayableOtherPayablesMember_019b0f23-5d0e-791e-bcfb-dbc2d185b2ab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MaturitiesOfLongTermDebtAbstract_019b0f23-5d0e-704a-b587-9970232f296a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MaturitiesOfLongTermDebtAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-79d5-87d0-78ca75c0614b" xlink:to="loc_us-gaap_MaturitiesOfLongTermDebtAbstract_019b0f23-5d0e-704a-b587-9970232f296a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths_019b0f23-5d0e-769b-95da-fbed0a783a2a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MaturitiesOfLongTermDebtAbstract_019b0f23-5d0e-704a-b587-9970232f296a" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths_019b0f23-5d0e-769b-95da-fbed0a783a2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo_019b0f23-5d0e-783d-b916-1f0800e155ff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MaturitiesOfLongTermDebtAbstract_019b0f23-5d0e-704a-b587-9970232f296a" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo_019b0f23-5d0e-783d-b916-1f0800e155ff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree_019b0f23-5d0e-7ce9-94aa-4de96ba5f017" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MaturitiesOfLongTermDebtAbstract_019b0f23-5d0e-704a-b587-9970232f296a" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree_019b0f23-5d0e-7ce9-94aa-4de96ba5f017" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019b0f23-5d0e-753c-af7c-6c74987ce607" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MaturitiesOfLongTermDebtAbstract_019b0f23-5d0e-704a-b587-9970232f296a" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019b0f23-5d0e-753c-af7c-6c74987ce607" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_MaturitiesOfLongTermDebtDiscountAmortizationAbstract_019b0f23-5d0e-714b-a4df-1302c21bb7d8" xlink:href="psix-20251231.xsd#psix_MaturitiesOfLongTermDebtDiscountAmortizationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b0f23-5d0e-79d5-87d0-78ca75c0614b" xlink:to="loc_psix_MaturitiesOfLongTermDebtDiscountAmortizationAbstract_019b0f23-5d0e-714b-a4df-1302c21bb7d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_AmortizationOfDebtDiscountPremiumYearOne_019b0f23-5d0e-71cc-b70b-a4282b39f6a1" xlink:href="psix-20251231.xsd#psix_AmortizationOfDebtDiscountPremiumYearOne"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_psix_MaturitiesOfLongTermDebtDiscountAmortizationAbstract_019b0f23-5d0e-714b-a4df-1302c21bb7d8" xlink:to="loc_psix_AmortizationOfDebtDiscountPremiumYearOne_019b0f23-5d0e-71cc-b70b-a4282b39f6a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_AmortizationOfDebtDiscountPremiumYearTwo_019b0f23-5d0e-78bf-abc8-acc499470651" xlink:href="psix-20251231.xsd#psix_AmortizationOfDebtDiscountPremiumYearTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_psix_MaturitiesOfLongTermDebtDiscountAmortizationAbstract_019b0f23-5d0e-714b-a4df-1302c21bb7d8" xlink:to="loc_psix_AmortizationOfDebtDiscountPremiumYearTwo_019b0f23-5d0e-78bf-abc8-acc499470651" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_AmortizationOfDebtDiscountPremiumYearThree_019b0f23-5d0e-7dfa-840c-d3c8fbc26176" xlink:href="psix-20251231.xsd#psix_AmortizationOfDebtDiscountPremiumYearThree"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_psix_MaturitiesOfLongTermDebtDiscountAmortizationAbstract_019b0f23-5d0e-714b-a4df-1302c21bb7d8" xlink:to="loc_psix_AmortizationOfDebtDiscountPremiumYearThree_019b0f23-5d0e-7dfa-840c-d3c8fbc26176" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019b0f23-5d0e-78cb-bcc1-a5c443a1a361" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_psix_MaturitiesOfLongTermDebtDiscountAmortizationAbstract_019b0f23-5d0e-714b-a4df-1302c21bb7d8" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019b0f23-5d0e-78cb-bcc1-a5c443a1a361" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/LeasesScheduleofLeaseExpensebyCategoryinConsolidatedStatementofIncomeDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#LeasesScheduleofLeaseExpensebyCategoryinConsolidatedStatementofIncomeDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/LeasesScheduleofLeaseExpensebyCategoryinConsolidatedStatementofIncomeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019b0f23-5d0e-7b4f-8a26-bcbde5c1f16d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_019b0f23-5d0e-7272-9adb-e6d0555b9ee1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019b0f23-5d0e-7b4f-8a26-bcbde5c1f16d" xlink:to="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_019b0f23-5d0e-7272-9adb-e6d0555b9ee1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_019b0f23-5d0e-71cd-862f-1f5bde88bbef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_019b0f23-5d0e-7272-9adb-e6d0555b9ee1" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_019b0f23-5d0e-71cd-862f-1f5bde88bbef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_019b0f23-5d0e-71cd-862f-1f5bde88bbef_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_019b0f23-5d0e-71cd-862f-1f5bde88bbef" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_019b0f23-5d0e-71cd-862f-1f5bde88bbef_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_019b0f23-5d0e-780a-b99e-7fb2bece5a10" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_019b0f23-5d0e-71cd-862f-1f5bde88bbef" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_019b0f23-5d0e-780a-b99e-7fb2bece5a10" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_019b0f23-5d0e-73dd-b39a-47e7a0d1e5d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019b0f23-5d0e-780a-b99e-7fb2bece5a10" xlink:to="loc_us-gaap_CostOfSalesMember_019b0f23-5d0e-73dd-b39a-47e7a0d1e5d4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_019b0f23-5d0e-7393-9af1-0fcd1ff16f15" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019b0f23-5d0e-780a-b99e-7fb2bece5a10" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_019b0f23-5d0e-7393-9af1-0fcd1ff16f15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_019b0f23-5d0e-7eaa-b1c2-54fb1da63f70" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019b0f23-5d0e-780a-b99e-7fb2bece5a10" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_019b0f23-5d0e-7eaa-b1c2-54fb1da63f70" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseMember_019b0f23-5d0e-7aef-8b57-0783fbca8fd5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019b0f23-5d0e-780a-b99e-7fb2bece5a10" xlink:to="loc_us-gaap_InterestExpenseMember_019b0f23-5d0e-7aef-8b57-0783fbca8fd5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseCost_019b0f23-5d0e-7d8f-979c-f2c6c2ac7f89" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_019b0f23-5d0e-7b4f-8a26-bcbde5c1f16d" xlink:to="loc_us-gaap_LeaseCost_019b0f23-5d0e-7d8f-979c-f2c6c2ac7f89" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/FairValueofFinancialInstrumentsDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#FairValueofFinancialInstrumentsDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/FairValueofFinancialInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019b0f23-5dbb-7417-beb0-221edcc971ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019b0f23-5dbb-7a5e-bb17-c80216f25341" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019b0f23-5dbb-7417-beb0-221edcc971ab" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019b0f23-5dbb-7a5e-bb17-c80216f25341" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019b0f23-5dbb-7ae2-8ac6-c63383bb9f37" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019b0f23-5dbb-7a5e-bb17-c80216f25341" xlink:to="loc_us-gaap_DebtInstrumentAxis_019b0f23-5dbb-7ae2-8ac6-c63383bb9f37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5dbb-7ae2-8ac6-c63383bb9f37_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019b0f23-5dbb-7ae2-8ac6-c63383bb9f37" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5dbb-7ae2-8ac6-c63383bb9f37_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5dbb-754f-b87e-f3bf1b6a7d9b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019b0f23-5dbb-7ae2-8ac6-c63383bb9f37" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5dbb-754f-b87e-f3bf1b6a7d9b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_RevolvingCreditAgreementMember_019bb339-efd4-7084-adfe-687c82f4e226" xlink:href="psix-20251231.xsd#psix_RevolvingCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019b0f23-5dbb-754f-b87e-f3bf1b6a7d9b" xlink:to="loc_psix_RevolvingCreditAgreementMember_019bb339-efd4-7084-adfe-687c82f4e226" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019b0f23-5dbb-76b3-955a-32a5b872e6b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019b0f23-5dbb-7a5e-bb17-c80216f25341" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019b0f23-5dbb-76b3-955a-32a5b872e6b5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019b0f23-5dbb-76b3-955a-32a5b872e6b5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019b0f23-5dbb-76b3-955a-32a5b872e6b5" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019b0f23-5dbb-76b3-955a-32a5b872e6b5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019b0f23-5dbb-79d0-9142-ff1e48403aa8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019b0f23-5dbb-76b3-955a-32a5b872e6b5" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019b0f23-5dbb-79d0-9142-ff1e48403aa8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019b0f23-5dbb-7404-8d12-10e2077b873c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019b0f23-5dbb-79d0-9142-ff1e48403aa8" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019b0f23-5dbb-7404-8d12-10e2077b873c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019b0f23-5dbb-73f9-9743-1cfc117563b6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019b0f23-5dbb-79d0-9142-ff1e48403aa8" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019b0f23-5dbb-73f9-9743-1cfc117563b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019b0f23-5dbb-707e-8b3f-8437ae4e59c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019b0f23-5dbb-79d0-9142-ff1e48403aa8" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019b0f23-5dbb-707e-8b3f-8437ae4e59c4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019c06af-170e-7c1b-985a-3c595f736634" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019b0f23-5dbb-7417-beb0-221edcc971ab" xlink:to="loc_us-gaap_LongTermDebt_019c06af-170e-7c1b-985a-3c595f736634" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LinesOfCreditFairValueDisclosure_019b0f23-5dbb-77dd-88b7-3590e77e9187" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LinesOfCreditFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019b0f23-5dbb-7417-beb0-221edcc971ab" xlink:to="loc_us-gaap_LinesOfCreditFairValueDisclosure_019b0f23-5dbb-77dd-88b7-3590e77e9187" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayable_019b0f23-5dbb-74cf-94ec-7dffe3d09b7d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019b0f23-5dbb-7417-beb0-221edcc971ab" xlink:to="loc_us-gaap_NotesPayable_019b0f23-5dbb-74cf-94ec-7dffe3d09b7d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayableFairValueDisclosure_019b0f23-5dbb-749f-88c0-f6699fc7ad5a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesPayableFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019b0f23-5dbb-7417-beb0-221edcc971ab" xlink:to="loc_us-gaap_NotesPayableFairValueDisclosure_019b0f23-5dbb-749f-88c0-f6699fc7ad5a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherBorrowings_019b0f23-5dbb-7750-a902-3dfe037560a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019b0f23-5dbb-7417-beb0-221edcc971ab" xlink:to="loc_us-gaap_OtherBorrowings_019b0f23-5dbb-7750-a902-3dfe037560a6" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/CommitmentsandContingenciesDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#CommitmentsandContingenciesDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/CommitmentsandContingenciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_019b0f23-5dbb-72b1-b68e-66f14021bea6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_us-gaap_LossContingenciesTable_019b0f23-5dbb-72b1-b68e-66f14021bea6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseAxis_019b0f23-5dbb-7e4e-b699-198500a129c4" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019b0f23-5dbb-72b1-b68e-66f14021bea6" xlink:to="loc_srt_LitigationCaseAxis_019b0f23-5dbb-7e4e-b699-198500a129c4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019b0f23-5dbb-7e4e-b699-198500a129c4_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_LitigationCaseAxis_019b0f23-5dbb-7e4e-b699-198500a129c4" xlink:to="loc_srt_LitigationCaseTypeDomain_019b0f23-5dbb-7e4e-b699-198500a129c4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019b0f23-5dbb-76fe-a90f-ca6adc85ff91" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_LitigationCaseAxis_019b0f23-5dbb-7e4e-b699-198500a129c4" xlink:to="loc_srt_LitigationCaseTypeDomain_019b0f23-5dbb-76fe-a90f-ca6adc85ff91" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_JeromeTreadwellv.TheCompanyMember_019b0f23-5dbb-7c13-b319-14f61b86edd0" xlink:href="psix-20251231.xsd#psix_JeromeTreadwellv.TheCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_019b0f23-5dbb-76fe-a90f-ca6adc85ff91" xlink:to="loc_psix_JeromeTreadwellv.TheCompanyMember_019b0f23-5dbb-7c13-b319-14f61b86edd0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_MastPowertrainV.TheCompanyMember_019b0f23-5dbb-7b79-b7d2-c181207ed155" xlink:href="psix-20251231.xsd#psix_MastPowertrainV.TheCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_019b0f23-5dbb-76fe-a90f-ca6adc85ff91" xlink:to="loc_psix_MastPowertrainV.TheCompanyMember_019b0f23-5dbb-7b79-b7d2-c181207ed155" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_GaryWinemasterVersusTheCompanyMember_019b0f23-5dbb-7b78-9798-f557c352c3ce" xlink:href="psix-20251231.xsd#psix_GaryWinemasterVersusTheCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_019b0f23-5dbb-76fe-a90f-ca6adc85ff91" xlink:to="loc_psix_GaryWinemasterVersusTheCompanyMember_019b0f23-5dbb-7b78-9798-f557c352c3ce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_019b0f23-5dbb-7e2a-bb68-2e155e38673a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019b0f23-5dbb-72b1-b68e-66f14021bea6" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_019b0f23-5dbb-7e2a-bb68-2e155e38673a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_019b0f23-5dbb-7e2a-bb68-2e155e38673a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_019b0f23-5dbb-7e2a-bb68-2e155e38673a" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_019b0f23-5dbb-7e2a-bb68-2e155e38673a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_019b0f23-5dbb-7bb2-bb27-b6195eee0bab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_019b0f23-5dbb-7e2a-bb68-2e155e38673a" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_019b0f23-5dbb-7bb2-bb27-b6195eee0bab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_019b0f23-5dbb-7b33-aa88-4d8105d799be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019b0f23-5dbb-7bb2-bb27-b6195eee0bab" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_019b0f23-5dbb-7b33-aa88-4d8105d799be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019c2bf7-7993-71db-a591-8ac8070f8c54" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019b0f23-5dbb-72b1-b68e-66f14021bea6" xlink:to="loc_dei_LegalEntityAxis_019c2bf7-7993-71db-a591-8ac8070f8c54" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019c2bf7-7993-71db-a591-8ac8070f8c54_default" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019c2bf7-7993-71db-a591-8ac8070f8c54" xlink:to="loc_dei_EntityDomain_019c2bf7-7993-71db-a591-8ac8070f8c54_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019c2bf7-7993-71f0-962d-cd94c25f94eb" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019c2bf7-7993-71db-a591-8ac8070f8c54" xlink:to="loc_dei_EntityDomain_019c2bf7-7993-71f0-962d-cd94c25f94eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_InsuranceCarrierMember_019c2bf7-7993-7b42-a508-b376994f3bfc" xlink:href="psix-20251231.xsd#psix_InsuranceCarrierMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019c2bf7-7993-71f0-962d-cd94c25f94eb" xlink:to="loc_psix_InsuranceCarrierMember_019c2bf7-7993-7b42-a508-b376994f3bfc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_LossContingencyDamagesSoughtValuePerNegligentViolation_019b0f23-5dbb-7676-9bdf-a75ffe633594" xlink:href="psix-20251231.xsd#psix_LossContingencyDamagesSoughtValuePerNegligentViolation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_psix_LossContingencyDamagesSoughtValuePerNegligentViolation_019b0f23-5dbb-7676-9bdf-a75ffe633594" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_LossContingencyDamagesSoughtValuePerIntentionalViolation_019b0f23-5dbb-7d7e-aab0-39df059f0559" xlink:href="psix-20251231.xsd#psix_LossContingencyDamagesSoughtValuePerIntentionalViolation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_psix_LossContingencyDamagesSoughtValuePerIntentionalViolation_019b0f23-5dbb-7d7e-aab0-39df059f0559" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_LossContingencyStayCaseTerm_019b0f23-5dbb-7a6e-b88c-d0c8159c2641" xlink:href="psix-20251231.xsd#psix_LossContingencyStayCaseTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_psix_LossContingencyStayCaseTerm_019b0f23-5dbb-7a6e-b88c-d0c8159c2641" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyAccrualAtCarryingValue_019b0f23-5dbb-7818-974d-d200b2024626" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyAccrualAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_us-gaap_LossContingencyAccrualAtCarryingValue_019b0f23-5dbb-7818-974d-d200b2024626" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationSettlementAmountAwardedToOtherParty_019b0f23-5dbb-7c3c-aff3-fe0de44ca5b1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LitigationSettlementAmountAwardedToOtherParty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_us-gaap_LitigationSettlementAmountAwardedToOtherParty_019b0f23-5dbb-7c3c-aff3-fe0de44ca5b1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForLegalSettlements_019c2bf7-44bc-7570-b59c-1584d22029e8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForLegalSettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_us-gaap_PaymentsForLegalSettlements_019c2bf7-44bc-7570-b59c-1584d22029e8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyDamagesSoughtValue_019b0f23-5dbb-7500-8109-c7a779bc9e25" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyDamagesSoughtValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_us-gaap_LossContingencyDamagesSoughtValue_019b0f23-5dbb-7500-8109-c7a779bc9e25" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationReserve_019b0f23-5dbb-7a0d-94f2-47134a9564f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LitigationReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_us-gaap_LitigationReserve_019b0f23-5dbb-7a0d-94f2-47134a9564f3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationSettlementExpense_019b0f23-5dbb-787f-a6f0-23c36e935ad4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LitigationSettlementExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_us-gaap_LitigationSettlementExpense_019b0f23-5dbb-787f-a6f0-23c36e935ad4" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationSettlementGain_019b0f23-5dbb-7ea5-a5aa-2ae17df281b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LitigationSettlementGain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_us-gaap_LitigationSettlementGain_019b0f23-5dbb-7ea5-a5aa-2ae17df281b3" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_NumberOfOutstandingLettersOfCredit_019b0f23-5dbb-7c7f-810f-4ecc3fa894e1" xlink:href="psix-20251231.xsd#psix_NumberOfOutstandingLettersOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_psix_NumberOfOutstandingLettersOfCredit_019b0f23-5dbb-7c7f-810f-4ecc3fa894e1" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_019b0f23-5dbb-7b1a-bca8-7d4b7635c4eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_019b0f23-5dbb-7b1a-bca8-7d4b7635c4eb" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashCurrent_019b0f23-5dbb-7372-8af4-7e55e5d71ba5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019b0f23-5dbb-7ef1-95c4-becb9a33abe5" xlink:to="loc_us-gaap_RestrictedCashCurrent_019b0f23-5dbb-7372-8af4-7e55e5d71ba5" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/IncomeTaxesNarrativeDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#IncomeTaxesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/IncomeTaxesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsLineItems_019c8b57-3447-7915-b4be-f40fe8fce1fa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwardsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsTable_019c8b57-3447-7474-a0c5-f94b9967e51a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwardsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019c8b57-3447-7915-b4be-f40fe8fce1fa" xlink:to="loc_us-gaap_OperatingLossCarryforwardsTable_019c8b57-3447-7474-a0c5-f94b9967e51a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardAxis_019c8b57-3447-7612-86ec-dc215a1a69d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_019c8b57-3447-7474-a0c5-f94b9967e51a" xlink:to="loc_us-gaap_TaxCreditCarryforwardAxis_019c8b57-3447-7612-86ec-dc215a1a69d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardNameDomain_019c8b57-3447-7612-86ec-dc215a1a69d0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TaxCreditCarryforwardAxis_019c8b57-3447-7612-86ec-dc215a1a69d0" xlink:to="loc_us-gaap_TaxCreditCarryforwardNameDomain_019c8b57-3447-7612-86ec-dc215a1a69d0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardNameDomain_019c8b57-3447-7ed5-9abd-20fb7bed6ba6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TaxCreditCarryforwardAxis_019c8b57-3447-7612-86ec-dc215a1a69d0" xlink:to="loc_us-gaap_TaxCreditCarryforwardNameDomain_019c8b57-3447-7ed5-9abd-20fb7bed6ba6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ResearchAndDevelopmentAndStateTaxCreditCarryforwardMember_019c8b57-a8f4-7129-8012-6a6bf89833cf" xlink:href="psix-20251231.xsd#psix_ResearchAndDevelopmentAndStateTaxCreditCarryforwardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardNameDomain_019c8b57-3447-7ed5-9abd-20fb7bed6ba6" xlink:to="loc_psix_ResearchAndDevelopmentAndStateTaxCreditCarryforwardMember_019c8b57-a8f4-7129-8012-6a6bf89833cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_019c8b57-de52-7ce7-bb18-86a078a2a8c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_019c8b57-3447-7474-a0c5-f94b9967e51a" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_019c8b57-de52-7ce7-bb18-86a078a2a8c4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019c8b57-de52-7ce7-bb18-86a078a2a8c4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019c8b57-de52-7ce7-bb18-86a078a2a8c4" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019c8b57-de52-7ce7-bb18-86a078a2a8c4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019c8b57-de52-70c2-9471-8be8e96aa33d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019c8b57-de52-7ce7-bb18-86a078a2a8c4" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019c8b57-de52-70c2-9471-8be8e96aa33d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticCountryMember_019caf37-906c-78a4-9b69-f764f2c12bb5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DomesticCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019c8b57-de52-70c2-9471-8be8e96aa33d" xlink:to="loc_us-gaap_DomesticCountryMember_019caf37-906c-78a4-9b69-f764f2c12bb5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019caf37-555c-7dcd-ad1d-63769aafc659" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticCountryMember_019caf37-906c-78a4-9b69-f764f2c12bb5" xlink:to="loc_country_US_019caf37-555c-7dcd-ad1d-63769aafc659" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_019c8b58-18a2-7b34-ad89-c5f894441e9e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StateAndLocalJurisdictionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticCountryMember_019caf37-906c-78a4-9b69-f764f2c12bb5" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_019c8b58-18a2-7b34-ad89-c5f894441e9e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019c656c-8cc6-74c8-85f9-731ea0eb4d38" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019c8b57-3447-7915-b4be-f40fe8fce1fa" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019c656c-8cc6-74c8-85f9-731ea0eb4d38" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_019c6568-8d7a-72e8-9cde-4b1e03356eb8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019c8b57-3447-7915-b4be-f40fe8fce1fa" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_019c6568-8d7a-72e8-9cde-4b1e03356eb8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_019c685b-44c5-7f36-81cf-676caf2093ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019c8b57-3447-7915-b4be-f40fe8fce1fa" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_019c685b-44c5-7f36-81cf-676caf2093ae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsValuationAllowance_019b0f23-5dbc-799b-bfa9-ef3c7b13d577" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsValuationAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019c8b57-3447-7915-b4be-f40fe8fce1fa" xlink:to="loc_us-gaap_DeferredTaxAssetsValuationAllowance_019b0f23-5dbc-799b-bfa9-ef3c7b13d577" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_019cad34-2216-725b-bfc5-ca2d6db69bd3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019c8b57-3447-7915-b4be-f40fe8fce1fa" xlink:to="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_019cad34-2216-725b-bfc5-ca2d6db69bd3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance_019cad35-f069-73ba-b095-8b17816ac2b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019c8b57-3447-7915-b4be-f40fe8fce1fa" xlink:to="loc_us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance_019cad35-f069-73ba-b095-8b17816ac2b4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardAmount_019c8b55-91a4-7d28-a8d4-71a3dd68940e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019c8b57-3447-7915-b4be-f40fe8fce1fa" xlink:to="loc_us-gaap_TaxCreditCarryforwardAmount_019c8b55-91a4-7d28-a8d4-71a3dd68940e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwards_019c8b56-1377-797d-bd6a-8194519fb88b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019c8b57-3447-7915-b4be-f40fe8fce1fa" xlink:to="loc_us-gaap_OperatingLossCarryforwards_019c8b56-1377-797d-bd6a-8194519fb88b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_019c8b58-c70b-740c-a797-588cbf4c3b1a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_019c8b57-3447-7915-b4be-f40fe8fce1fa" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_019c8b58-c70b-740c-a797-588cbf4c3b1a" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/IncomeTaxesScheduleofCashIncomeTaxesPaidDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#IncomeTaxesScheduleofCashIncomeTaxesPaidDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/IncomeTaxesScheduleofCashIncomeTaxesPaidDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionLineItems_019c8869-8c24-7b89-863f-1bce88d15664" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidByIndividualJurisdictionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionTable_019c8869-8c24-7ee0-b7d5-ee441a16d05a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidByIndividualJurisdictionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionLineItems_019c8869-8c24-7b89-863f-1bce88d15664" xlink:to="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionTable_019c8869-8c24-7ee0-b7d5-ee441a16d05a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_019c8869-8c24-7f7e-95a3-bb2bbd3dda4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionTable_019c8869-8c24-7ee0-b7d5-ee441a16d05a" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_019c8869-8c24-7f7e-95a3-bb2bbd3dda4a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019c8869-8c24-7f7e-95a3-bb2bbd3dda4a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019c8869-8c24-7f7e-95a3-bb2bbd3dda4a" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019c8869-8c24-7f7e-95a3-bb2bbd3dda4a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_019c8869-8c24-799a-961b-7491209abc16" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_019c8869-8c24-7f7e-95a3-bb2bbd3dda4a" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_019c8869-8c24-799a-961b-7491209abc16" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_019c886a-ba38-7332-99cc-b414c9f2daa2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StateAndLocalJurisdictionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_019c8869-8c24-799a-961b-7491209abc16" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_019c886a-ba38-7332-99cc-b414c9f2daa2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_WI_019c886c-2725-7084-85f3-043ab118ceb2" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_WI"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StateAndLocalJurisdictionMember_019c886a-ba38-7332-99cc-b414c9f2daa2" xlink:to="loc_stpr_WI_019c886c-2725-7084-85f3-043ab118ceb2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_PA_019c886d-e86a-79ec-bd72-d3fad67a9326" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_PA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StateAndLocalJurisdictionMember_019c886a-ba38-7332-99cc-b414c9f2daa2" xlink:to="loc_stpr_PA_019c886d-e86a-79ec-bd72-d3fad67a9326" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_IL_019c886e-2bed-76db-b6ff-1f960a4b9579" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_IL"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StateAndLocalJurisdictionMember_019c886a-ba38-7332-99cc-b414c9f2daa2" xlink:to="loc_stpr_IL_019c886e-2bed-76db-b6ff-1f960a4b9579" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_MN_019c886e-6b5f-76d9-8929-9eaae8cbfe2c" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_MN"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StateAndLocalJurisdictionMember_019c886a-ba38-7332-99cc-b414c9f2daa2" xlink:to="loc_stpr_MN_019c886e-6b5f-76d9-8929-9eaae8cbfe2c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidFederalAfterRefundReceived_019c7af7-daeb-786e-b9fa-8b7eb12e9b53" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidFederalAfterRefundReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionLineItems_019c8869-8c24-7b89-863f-1bce88d15664" xlink:to="loc_us-gaap_IncomeTaxPaidFederalAfterRefundReceived_019c7af7-daeb-786e-b9fa-8b7eb12e9b53" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived_019c7af7-daeb-7b23-b79c-f36794252cf4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionLineItems_019c8869-8c24-7b89-863f-1bce88d15664" xlink:to="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived_019c7af7-daeb-7b23-b79c-f36794252cf4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaidNet_019c7af7-daeb-7944-94b6-ee4539256a50" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxesPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxPaidByIndividualJurisdictionLineItems_019c8869-8c24-7b89-863f-1bce88d15664" xlink:to="loc_us-gaap_IncomeTaxesPaidNet_019c7af7-daeb-7944-94b6-ee4539256a50" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/StockholdersEquityScheduleofChangesinSharesofCommonandTreasuryStockDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#StockholdersEquityScheduleofChangesinSharesofCommonandTreasuryStockDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/StockholdersEquityScheduleofChangesinSharesofCommonandTreasuryStockDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019b0f23-5dbc-7a6a-9aca-bc2f6db89fc7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019b0f23-5dbc-7a04-a9b7-a9189eaa42ed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019b0f23-5dbc-7a6a-9aca-bc2f6db89fc7" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019b0f23-5dbc-7a04-a9b7-a9189eaa42ed" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019b0f23-5dbc-7afa-9a98-862611714f9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019b0f23-5dbc-7a04-a9b7-a9189eaa42ed" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019b0f23-5dbc-7afa-9a98-862611714f9d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019b0f23-5dbc-7afa-9a98-862611714f9d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019b0f23-5dbc-7afa-9a98-862611714f9d" xlink:to="loc_us-gaap_EquityComponentDomain_019b0f23-5dbc-7afa-9a98-862611714f9d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019b0f23-5dbc-7462-92ad-b252b614df64" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019b0f23-5dbc-7afa-9a98-862611714f9d" xlink:to="loc_us-gaap_EquityComponentDomain_019b0f23-5dbc-7462-92ad-b252b614df64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019b0f23-5dbc-79fb-951e-6195438d6123" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019b0f23-5dbc-7462-92ad-b252b614df64" xlink:to="loc_us-gaap_CommonStockMember_019b0f23-5dbc-79fb-951e-6195438d6123" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_019b0f23-5dbc-79ee-b87f-496378139d17" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019b0f23-5dbc-7462-92ad-b252b614df64" xlink:to="loc_us-gaap_TreasuryStockCommonMember_019b0f23-5dbc-79ee-b87f-496378139d17" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5dbc-7b53-aba4-6c143f5babc7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019b0f23-5dbc-7a6a-9aca-bc2f6db89fc7" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5dbc-7b53-aba4-6c143f5babc7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract_019b0f23-5dbc-78c4-8fba-852cc3ca51b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5dbc-7b53-aba4-6c143f5babc7" xlink:to="loc_us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract_019b0f23-5dbc-78c4-8fba-852cc3ca51b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019b0f23-5dbc-7adc-9ba0-eaaf0ff9660b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract_019b0f23-5dbc-78c4-8fba-852cc3ca51b3" xlink:to="loc_us-gaap_CommonStockSharesIssued_019b0f23-5dbc-7adc-9ba0-eaaf0ff9660b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019b0f23-5dbc-7e3f-857c-2945ae2fbd3c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAbstract_019b0f23-5dbc-779f-a4f3-779698b845b2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5dbc-7b53-aba4-6c143f5babc7" xlink:to="loc_us-gaap_TreasuryStockValueAbstract_019b0f23-5dbc-779f-a4f3-779698b845b2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_019b0f23-5dbc-78ba-ad10-2cf96ed53ec3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TreasuryStockValueAbstract_019b0f23-5dbc-779f-a4f3-779698b845b2" xlink:to="loc_us-gaap_TreasuryStockCommonShares_019b0f23-5dbc-78ba-ad10-2cf96ed53ec3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019b0f23-5dbc-7133-b26e-fd039f6a3c20" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TreasuryStockValueAbstract_019b0f23-5dbc-779f-a4f3-779698b845b2" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019b0f23-5dbc-7133-b26e-fd039f6a3c20" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_019b0f23-5dbc-7c62-8764-e44634f148d2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockIncludingAdditionalPaidInCapitalNetOfDiscountAbstract_019b0f23-5dbc-70ea-864a-c1aa8789971b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockIncludingAdditionalPaidInCapitalNetOfDiscountAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019b0f23-5dbc-7b53-aba4-6c143f5babc7" xlink:to="loc_us-gaap_CommonStockIncludingAdditionalPaidInCapitalNetOfDiscountAbstract_019b0f23-5dbc-70ea-864a-c1aa8789971b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019b0f23-5dbc-776e-9437-a66352c00182" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CommonStockIncludingAdditionalPaidInCapitalNetOfDiscountAbstract_019b0f23-5dbc-70ea-864a-c1aa8789971b" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019b0f23-5dbc-776e-9437-a66352c00182" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019b0f23-5dbc-76e2-b9d9-fa8fc477e75d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019b0f23-5dbc-70d0-a939-eb9a5933e732" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/StockholdersEquityNarrativeDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#StockholdersEquityNarrativeDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/StockholdersEquityNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019b0f23-5dbc-7748-9c26-39741b927637" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019b0f23-5dbc-7cc6-b0c4-0b6c512b23d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019b0f23-5dbc-7748-9c26-39741b927637" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019b0f23-5dbc-7cc6-b0c4-0b6c512b23d3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019b0f23-5dbc-73a2-b55f-c3ac978edfe7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019b0f23-5dbc-7cc6-b0c4-0b6c512b23d3" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019b0f23-5dbc-73a2-b55f-c3ac978edfe7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019b0f23-5dbc-73a2-b55f-c3ac978edfe7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019b0f23-5dbc-73a2-b55f-c3ac978edfe7" xlink:to="loc_us-gaap_ClassOfStockDomain_019b0f23-5dbc-73a2-b55f-c3ac978edfe7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019b0f23-5dbc-7dc6-aa21-04f68f8d9f9c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019b0f23-5dbc-73a2-b55f-c3ac978edfe7" xlink:to="loc_us-gaap_ClassOfStockDomain_019b0f23-5dbc-7dc6-aa21-04f68f8d9f9c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesBPreferredStockMember_019b0f23-5dbc-74ef-81e5-fcc651e64197" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019b0f23-5dbc-7dc6-aa21-04f68f8d9f9c" xlink:to="loc_us-gaap_SeriesBPreferredStockMember_019b0f23-5dbc-74ef-81e5-fcc651e64197" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_019b0f23-5dbc-7ad4-8cca-70329c2bac42" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019b0f23-5dbc-7748-9c26-39741b927637" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_019b0f23-5dbc-7ad4-8cca-70329c2bac42" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_019b0f23-5dbc-77c9-b500-9c6980fb6ff3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019b0f23-5dbc-7748-9c26-39741b927637" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_019b0f23-5dbc-77c9-b500-9c6980fb6ff3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_PreferredStockSeriesAuthorized_019b0f23-5dbc-7962-bbb1-c1afaa25183f" xlink:href="psix-20251231.xsd#psix_PreferredStockSeriesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019b0f23-5dbc-7748-9c26-39741b927637" xlink:to="loc_psix_PreferredStockSeriesAuthorized_019b0f23-5dbc-7962-bbb1-c1afaa25183f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_019b0f23-5dbc-7a17-886c-a64365c2e080" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019b0f23-5dbc-7748-9c26-39741b927637" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_019b0f23-5dbc-7a17-886c-a64365c2e080" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/StockBasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#StockBasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/StockBasedCompensationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-744a-9270-36013e59f150" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-76b0-b05d-b87a26d0cec0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-744a-9270-36013e59f150" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-76b0-b05d-b87a26d0cec0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7821-8d8e-5aa66ca85e36" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-76b0-b05d-b87a26d0cec0" xlink:to="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7821-8d8e-5aa66ca85e36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7821-8d8e-5aa66ca85e36_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7821-8d8e-5aa66ca85e36" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7821-8d8e-5aa66ca85e36_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-70ba-aea0-55413cf979e8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7821-8d8e-5aa66ca85e36" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-70ba-aea0-55413cf979e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockAppreciationRightsSARSMember_019b0f23-5dbc-7c7c-b4c6-93805d162dda" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockAppreciationRightsSARSMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-70ba-aea0-55413cf979e8" xlink:to="loc_us-gaap_StockAppreciationRightsSARSMember_019b0f23-5dbc-7c7c-b4c6-93805d162dda" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019b0f23-5dbc-7204-a885-6f4b38dbdfb5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-76b0-b05d-b87a26d0cec0" xlink:to="loc_srt_RangeAxis_019b0f23-5dbc-7204-a885-6f4b38dbdfb5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019b0f23-5dbc-7204-a885-6f4b38dbdfb5_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019b0f23-5dbc-7204-a885-6f4b38dbdfb5" xlink:to="loc_srt_RangeMember_019b0f23-5dbc-7204-a885-6f4b38dbdfb5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019b0f23-5dbc-7cc6-ac57-4bd07372eae6" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019b0f23-5dbc-7204-a885-6f4b38dbdfb5" xlink:to="loc_srt_RangeMember_019b0f23-5dbc-7cc6-ac57-4bd07372eae6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019b0f23-5dbc-723e-ba08-e47cb46fda09" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019b0f23-5dbc-7cc6-ac57-4bd07372eae6" xlink:to="loc_srt_MinimumMember_019b0f23-5dbc-723e-ba08-e47cb46fda09" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019b0f23-5dbc-79cd-87b7-2ea4689c3059" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019b0f23-5dbc-7cc6-ac57-4bd07372eae6" xlink:to="loc_srt_MaximumMember_019b0f23-5dbc-79cd-87b7-2ea4689c3059" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusAxis_019b0f23-5dbc-7ee5-acdb-e7086399d397" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GranteeStatusAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-76b0-b05d-b87a26d0cec0" xlink:to="loc_us-gaap_GranteeStatusAxis_019b0f23-5dbc-7ee5-acdb-e7086399d397" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_019b0f23-5dbc-7ee5-acdb-e7086399d397_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_GranteeStatusAxis_019b0f23-5dbc-7ee5-acdb-e7086399d397" xlink:to="loc_us-gaap_GranteeStatusDomain_019b0f23-5dbc-7ee5-acdb-e7086399d397_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GranteeStatusDomain_019b0f23-5dbc-7376-bd79-aaf136c31216" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GranteeStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_GranteeStatusAxis_019b0f23-5dbc-7ee5-acdb-e7086399d397" xlink:to="loc_us-gaap_GranteeStatusDomain_019b0f23-5dbc-7376-bd79-aaf136c31216" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedPaymentArrangementEmployeeMember_019b0f23-5dbc-75b9-a59e-9dfb3fccfbc9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedPaymentArrangementEmployeeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GranteeStatusDomain_019b0f23-5dbc-7376-bd79-aaf136c31216" xlink:to="loc_us-gaap_ShareBasedPaymentArrangementEmployeeMember_019b0f23-5dbc-75b9-a59e-9dfb3fccfbc9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019b0f23-5dbc-79e9-b176-934ee3d5703d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-744a-9270-36013e59f150" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019b0f23-5dbc-79e9-b176-934ee3d5703d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_019b0f23-5dbc-71b6-b377-8a0953bfa097" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-744a-9270-36013e59f150" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_019b0f23-5dbc-71b6-b377-8a0953bfa097" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019b0f23-5dbc-74d8-8348-b38f482379d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-744a-9270-36013e59f150" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019b0f23-5dbc-74d8-8348-b38f482379d3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ShareBasedPaymentArrangementNumberOfSharesReceivedShareOfCommonStock_019b0f23-5dbc-7843-9c6d-c6ff4f063df8" xlink:href="psix-20251231.xsd#psix_ShareBasedPaymentArrangementNumberOfSharesReceivedShareOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-744a-9270-36013e59f150" xlink:to="loc_psix_ShareBasedPaymentArrangementNumberOfSharesReceivedShareOfCommonStock_019b0f23-5dbc-7843-9c6d-c6ff4f063df8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019b0f23-5dbc-7605-8bcf-8eb0580d80e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-744a-9270-36013e59f150" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019b0f23-5dbc-7605-8bcf-8eb0580d80e0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019b0f23-5dbc-7f14-a589-729c077dfe43" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-744a-9270-36013e59f150" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019b0f23-5dbc-7f14-a589-729c077dfe43" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/StockBasedCompensationSARAwardsNarrativeDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#StockBasedCompensationSARAwardsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/StockBasedCompensationSARAwardsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7e13-9851-17fda3099554" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7a79-be78-29398470b40a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7e13-9851-17fda3099554" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7a79-be78-29398470b40a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-726e-9bdb-3210b87c9444" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7a79-be78-29398470b40a" xlink:to="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-726e-9bdb-3210b87c9444" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-726e-9bdb-3210b87c9444_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-726e-9bdb-3210b87c9444" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-726e-9bdb-3210b87c9444_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7314-8096-99f26380e267" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-726e-9bdb-3210b87c9444" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7314-8096-99f26380e267" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockAppreciationRightsSARSMember_019b0f23-5dbc-7fa3-9372-ecb68aee808e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockAppreciationRightsSARSMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7314-8096-99f26380e267" xlink:to="loc_us-gaap_StockAppreciationRightsSARSMember_019b0f23-5dbc-7fa3-9372-ecb68aee808e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted_019b0f23-5dbc-794c-bd31-c15636a8d651" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7e13-9851-17fda3099554" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted_019b0f23-5dbc-794c-bd31-c15636a8d651" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019b0f23-5dbc-7ce7-b6bd-d31d0f487f4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7e13-9851-17fda3099554" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019b0f23-5dbc-7ce7-b6bd-d31d0f487f4a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested_019b0f23-5dbc-76aa-b46b-0f1d38a8c88e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7e13-9851-17fda3099554" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested_019b0f23-5dbc-76aa-b46b-0f1d38a8c88e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019b0f23-5dbc-74c7-971e-2ee2cda3ee22" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7e13-9851-17fda3099554" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019b0f23-5dbc-74c7-971e-2ee2cda3ee22" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019b0f23-5dbc-7baf-954e-f930c3a001a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7e13-9851-17fda3099554" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019b0f23-5dbc-7baf-954e-f930c3a001a9" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/StockBasedCompensationScheduleofAssumptionsUsedforDeterminingFairValueofSARsDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#StockBasedCompensationScheduleofAssumptionsUsedforDeterminingFairValueofSARsDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/StockBasedCompensationScheduleofAssumptionsUsedforDeterminingFairValueofSARsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7932-bbf5-e6019959f7d8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7284-b1d0-f32a168d1b1e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7932-bbf5-e6019959f7d8" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7284-b1d0-f32a168d1b1e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7283-bb14-3dba2064f599" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7284-b1d0-f32a168d1b1e" xlink:to="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7283-bb14-3dba2064f599" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7283-bb14-3dba2064f599_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7283-bb14-3dba2064f599" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7283-bb14-3dba2064f599_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-705c-8003-3eee58c7d490" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7283-bb14-3dba2064f599" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-705c-8003-3eee58c7d490" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockAppreciationRightsSARSMember_019b0f23-5dbc-7a81-a1e2-885a48c3bf4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockAppreciationRightsSARSMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-705c-8003-3eee58c7d490" xlink:to="loc_us-gaap_StockAppreciationRightsSARSMember_019b0f23-5dbc-7a81-a1e2-885a48c3bf4a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019c2c03-2501-7015-8604-8ae8f2132373" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7284-b1d0-f32a168d1b1e" xlink:to="loc_srt_RangeAxis_019c2c03-2501-7015-8604-8ae8f2132373" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c2c03-2501-7015-8604-8ae8f2132373_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019c2c03-2501-7015-8604-8ae8f2132373" xlink:to="loc_srt_RangeMember_019c2c03-2501-7015-8604-8ae8f2132373_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c2c03-2501-74cc-bbc7-57e747ce8e07" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019c2c03-2501-7015-8604-8ae8f2132373" xlink:to="loc_srt_RangeMember_019c2c03-2501-74cc-bbc7-57e747ce8e07" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019c2c03-8153-77df-ade6-00cab0adc45e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c2c03-2501-74cc-bbc7-57e747ce8e07" xlink:to="loc_srt_MinimumMember_019c2c03-8153-77df-ade6-00cab0adc45e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019c2c03-aa8f-7760-83c2-b8343b44c627" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c2c03-2501-74cc-bbc7-57e747ce8e07" xlink:to="loc_srt_MaximumMember_019c2c03-aa8f-7760-83c2-b8343b44c627" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharePrice_019b0f23-5dbc-7d7d-8b8c-0c8cc07f21a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7932-bbf5-e6019959f7d8" xlink:to="loc_us-gaap_SharePrice_019b0f23-5dbc-7d7d-8b8c-0c8cc07f21a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice_019b0f23-5dbc-71fe-a3b7-c792fabe9a64" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7932-bbf5-e6019959f7d8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice_019b0f23-5dbc-71fe-a3b7-c792fabe9a64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019b0f23-5dbc-7f31-98a2-4340d13ac8b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7932-bbf5-e6019959f7d8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019b0f23-5dbc-7f31-98a2-4340d13ac8b9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate_019b0f23-5dbc-75de-bd97-c24342a3e7f2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7932-bbf5-e6019959f7d8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate_019b0f23-5dbc-75de-bd97-c24342a3e7f2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum_019b0f23-5dbc-7934-b31b-375876c3b538" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7932-bbf5-e6019959f7d8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum_019b0f23-5dbc-7934-b31b-375876c3b538" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum_019b0f23-5dbc-7c94-ae9f-b8f934c72605" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7932-bbf5-e6019959f7d8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum_019b0f23-5dbc-7c94-ae9f-b8f934c72605" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019b0f23-5dbc-7702-a443-24a174b19114" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7932-bbf5-e6019959f7d8" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019b0f23-5dbc-7702-a443-24a174b19114" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019b0f23-5dbc-7ab0-a7d3-2005a4758b76" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7932-bbf5-e6019959f7d8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019b0f23-5dbc-7ab0-a7d3-2005a4758b76" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019b0f23-5dbc-70bd-b506-94ba6d1e1eee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7932-bbf5-e6019959f7d8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019b0f23-5dbc-70bd-b506-94ba6d1e1eee" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/StockBasedCompensationScheduleofSARActivityDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#StockBasedCompensationScheduleofSARActivityDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/StockBasedCompensationScheduleofSARActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7256-85e5-2f5ecd5331fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7c12-a592-b6d52463008b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7256-85e5-2f5ecd5331fe" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7c12-a592-b6d52463008b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7185-8a8d-6c5a7b637b89" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7c12-a592-b6d52463008b" xlink:to="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7185-8a8d-6c5a7b637b89" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7185-8a8d-6c5a7b637b89_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7185-8a8d-6c5a7b637b89" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7185-8a8d-6c5a7b637b89_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7a6b-9034-8e517f58b3ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7185-8a8d-6c5a7b637b89" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7a6b-9034-8e517f58b3ab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockAppreciationRightsSARSMember_019b0f23-5dbc-7f2e-becc-448b2ffb3925" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockAppreciationRightsSARSMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7a6b-9034-8e517f58b3ab" xlink:to="loc_us-gaap_StockAppreciationRightsSARSMember_019b0f23-5dbc-7f2e-becc-448b2ffb3925" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019b0f23-5dbc-7734-92db-36e654a39129" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7256-85e5-2f5ecd5331fe" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019b0f23-5dbc-7734-92db-36e654a39129" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_019b0f23-5dbc-7b0e-88d2-88fd87a9d3a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019b0f23-5dbc-7734-92db-36e654a39129" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_019b0f23-5dbc-7b0e-88d2-88fd87a9d3a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted_019b0f23-5dbc-7200-80c5-aec62506b44a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019b0f23-5dbc-7734-92db-36e654a39129" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted_019b0f23-5dbc-7200-80c5-aec62506b44a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOther_019c7b04-c435-7522-a2a3-398eb96d1e22" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019b0f23-5dbc-7734-92db-36e654a39129" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOther_019c7b04-c435-7522-a2a3-398eb96d1e22" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised_019b0f23-5dbc-7d33-b077-5bca190bb744" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019b0f23-5dbc-7734-92db-36e654a39129" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised_019b0f23-5dbc-7d33-b077-5bca190bb744" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsForfeitures_019b0f23-5dbc-77bd-93b0-caa3247dda08" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsForfeitures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019b0f23-5dbc-7734-92db-36e654a39129" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsForfeitures_019b0f23-5dbc-77bd-93b0-caa3247dda08" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExpirations_019b0f23-5dbc-74bc-8af1-bbcdb8d1a0ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExpirations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019b0f23-5dbc-7734-92db-36e654a39129" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExpirations_019b0f23-5dbc-74bc-8af1-bbcdb8d1a0ec" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_019b0f23-5dbc-760f-b869-851055746094" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableNumber_019b0f23-5dbc-7a10-86ee-b1aa66056db0" xlink:href="psix-20251231.xsd#psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7256-85e5-2f5ecd5331fe" xlink:to="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableNumber_019b0f23-5dbc-7a10-86ee-b1aa66056db0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019b0f23-5dbc-7a77-a3cd-db7a4858d8fd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7256-85e5-2f5ecd5331fe" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019b0f23-5dbc-7a77-a3cd-db7a4858d8fd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice_019b0f23-5dbc-7750-a26b-d746f71994fb" xlink:href="psix-20251231.xsd#psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019b0f23-5dbc-7a77-a3cd-db7a4858d8fd" xlink:to="loc_psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice_019b0f23-5dbc-7750-a26b-d746f71994fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsGrantsInPeriodWeightedAverageExercisePrice_019b0f23-5dbc-7f08-b0f4-9f6c19e5dc33" xlink:href="psix-20251231.xsd#psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019b0f23-5dbc-7a77-a3cd-db7a4858d8fd" xlink:to="loc_psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsGrantsInPeriodWeightedAverageExercisePrice_019b0f23-5dbc-7f08-b0f4-9f6c19e5dc33" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOtherWeightedAverageExercisePrice_019c9567-f4bf-7aa7-b508-2c14e8aab34c" xlink:href="psix-20251231.xsd#psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOtherWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019b0f23-5dbc-7a77-a3cd-db7a4858d8fd" xlink:to="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOtherWeightedAverageExercisePrice_019c9567-f4bf-7aa7-b508-2c14e8aab34c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsExercisesInPeriodWeightedAverageExercisePrice_019b0f23-5dbc-7bf5-93e0-52c0ed8c8c92" xlink:href="psix-20251231.xsd#psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019b0f23-5dbc-7a77-a3cd-db7a4858d8fd" xlink:to="loc_psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsExercisesInPeriodWeightedAverageExercisePrice_019b0f23-5dbc-7bf5-93e0-52c0ed8c8c92" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsForfeituresInPeriodWeightedAverageExercisePrice_019b0f23-5dbc-7b03-8260-b3e4662686ee" xlink:href="psix-20251231.xsd#psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsForfeituresInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019b0f23-5dbc-7a77-a3cd-db7a4858d8fd" xlink:to="loc_psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsForfeituresInPeriodWeightedAverageExercisePrice_019b0f23-5dbc-7b03-8260-b3e4662686ee" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsExpirationsInPeriodWeightedAverageExercisePrice_019b0f23-5dbc-7bed-a194-ebfdd4ee1798" xlink:href="psix-20251231.xsd#psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsExpirationsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_019b0f23-5dbc-7a77-a3cd-db7a4858d8fd" xlink:to="loc_psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsExpirationsInPeriodWeightedAverageExercisePrice_019b0f23-5dbc-7bed-a194-ebfdd4ee1798" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice_019b0f23-5dbc-7af6-aff3-e51b5156303f" xlink:href="psix-20251231.xsd#psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsExercisableWeightedAverageExercisePrice_019b0f23-5dbc-7877-80ba-2d83a4c6dd6f" xlink:href="psix-20251231.xsd#psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7256-85e5-2f5ecd5331fe" xlink:to="loc_psix_SharebasedCompensationArrangementBySharebasedPaymentAwardNonOptionEquityInstrumentsExercisableWeightedAverageExercisePrice_019b0f23-5dbc-7877-80ba-2d83a4c6dd6f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualTermAbstract_019b0f23-5dbc-7122-bdb6-c210a6062729" xlink:href="psix-20251231.xsd#psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualTermAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7256-85e5-2f5ecd5331fe" xlink:to="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualTermAbstract_019b0f23-5dbc-7122-bdb6-c210a6062729" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_019b0f23-5dbc-7599-858f-5ed691b9587c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualTermAbstract_019b0f23-5dbc-7122-bdb6-c210a6062729" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_019b0f23-5dbc-7599-858f-5ed691b9587c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableWeightedAverageRemainingContractualTerm_019b0f23-5dbc-798d-a573-db647f09fcb9" xlink:href="psix-20251231.xsd#psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableWeightedAverageRemainingContractualTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualTermAbstract_019b0f23-5dbc-7122-bdb6-c210a6062729" xlink:to="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableWeightedAverageRemainingContractualTerm_019b0f23-5dbc-798d-a573-db647f09fcb9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_019b0f23-5dbc-736c-8e6d-e422b101973e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7256-85e5-2f5ecd5331fe" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_019b0f23-5dbc-736c-8e6d-e422b101973e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_019b0f23-5dbc-7e42-985b-573475838d0f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_019b0f23-5dbc-736c-8e6d-e422b101973e" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_019b0f23-5dbc-7e42-985b-573475838d0f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueExercised_019c7b0d-1def-71fa-859e-f4fa47921873" xlink:href="psix-20251231.xsd#psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_019b0f23-5dbc-736c-8e6d-e422b101973e" xlink:to="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueExercised_019c7b0d-1def-71fa-859e-f4fa47921873" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableAggregateIntrinsicValue_019b0f23-5dbc-7dbb-8402-7145745fe174" xlink:href="psix-20251231.xsd#psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableAggregateIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueAbstract_019b0f23-5dbc-736c-8e6d-e422b101973e" xlink:to="loc_psix_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableAggregateIntrinsicValue_019b0f23-5dbc-7dbb-8402-7145745fe174" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/StockBasedCompensationScheduleofRestrictedStockActivityDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#StockBasedCompensationScheduleofRestrictedStockActivityDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/StockBasedCompensationScheduleofRestrictedStockActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7d51-a625-a35469ff9f99" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7a86-8393-6d473ce27988" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7d51-a625-a35469ff9f99" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7a86-8393-6d473ce27988" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-704c-b28c-6a77638e8234" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7a86-8393-6d473ce27988" xlink:to="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-704c-b28c-6a77638e8234" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-704c-b28c-6a77638e8234_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-704c-b28c-6a77638e8234" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-704c-b28c-6a77638e8234_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7d4b-8f7f-1ec46601c252" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-704c-b28c-6a77638e8234" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7d4b-8f7f-1ec46601c252" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019b0f23-5dbc-775a-b659-491cb709af21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7d4b-8f7f-1ec46601c252" xlink:to="loc_us-gaap_RestrictedStockMember_019b0f23-5dbc-775a-b659-491cb709af21" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019b0f23-5dbc-721a-a6e4-813a6e3f9eec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7d51-a625-a35469ff9f99" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019b0f23-5dbc-721a-a6e4-813a6e3f9eec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019b0f23-5dbc-72ea-bb8f-339e46b7baa6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019b0f23-5dbc-721a-a6e4-813a6e3f9eec" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019b0f23-5dbc-72ea-bb8f-339e46b7baa6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019b0f23-5dbc-73c7-ba37-a7a7026c0abb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019b0f23-5dbc-721a-a6e4-813a6e3f9eec" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019b0f23-5dbc-73c7-ba37-a7a7026c0abb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019b0f23-5dbc-70c6-8f63-e3edc2518b64" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019b0f23-5dbc-721a-a6e4-813a6e3f9eec" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019b0f23-5dbc-70c6-8f63-e3edc2518b64" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019b0f23-5dbc-7011-84b6-f28f27c2de01" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019b0f23-5dbc-721a-a6e4-813a6e3f9eec" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019b0f23-5dbc-7011-84b6-f28f27c2de01" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019b0f23-5dbc-78b3-bbf4-95fafb0a6552" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019b0f23-5dbc-7b26-b00d-ab213e44b170" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7d51-a625-a35469ff9f99" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019b0f23-5dbc-7b26-b00d-ab213e44b170" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019b0f23-5dbc-7580-a78c-83a451546671" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019b0f23-5dbc-7b26-b00d-ab213e44b170" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019b0f23-5dbc-7580-a78c-83a451546671" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019b0f23-5dbc-7a0e-bdad-99b439b59b5d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019b0f23-5dbc-7b26-b00d-ab213e44b170" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019b0f23-5dbc-7a0e-bdad-99b439b59b5d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019b0f23-5dbc-7a27-9edc-ab7203223db3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019b0f23-5dbc-7b26-b00d-ab213e44b170" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019b0f23-5dbc-7a27-9edc-ab7203223db3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019b0f23-5dbc-7f93-b3ff-cb6e9d6ca36b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019b0f23-5dbc-7b26-b00d-ab213e44b170" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019b0f23-5dbc-7f93-b3ff-cb6e9d6ca36b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019b0f23-5dbc-791d-b8dd-674d775eb6a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/StockBasedCompensationRestrictedStockAwardsNarrativeDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#StockBasedCompensationRestrictedStockAwardsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/StockBasedCompensationRestrictedStockAwardsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7ea4-9534-fc155762abd1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7cca-950d-a282253670e1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7ea4-9534-fc155762abd1" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7cca-950d-a282253670e1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7287-8509-247aa7881cd0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019b0f23-5dbc-7cca-950d-a282253670e1" xlink:to="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7287-8509-247aa7881cd0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7287-8509-247aa7881cd0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7287-8509-247aa7881cd0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7287-8509-247aa7881cd0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7668-b748-99e9a632b7a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019b0f23-5dbc-7287-8509-247aa7881cd0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7668-b748-99e9a632b7a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019b0f23-5dbc-72cd-bd93-32e550251c4c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019b0f23-5dbc-7668-b748-99e9a632b7a1" xlink:to="loc_us-gaap_RestrictedStockMember_019b0f23-5dbc-72cd-bd93-32e550251c4c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019b0f23-5dbc-7a0b-a21c-344744efee61" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7ea4-9534-fc155762abd1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_019b0f23-5dbc-7a0b-a21c-344744efee61" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested_019b0f23-5dbc-729c-bd8e-60854a6e0e30" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7ea4-9534-fc155762abd1" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested_019b0f23-5dbc-729c-bd8e-60854a6e0e30" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019b0f23-5dbc-7d08-8c83-5328313c508e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019b0f23-5dbc-7ea4-9534-fc155762abd1" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019b0f23-5dbc-7d08-8c83-5328313c508e" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/RelatedPartyTransactionsNarrativeDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#RelatedPartyTransactionsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/RelatedPartyTransactionsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_019ba163-157a-7187-b9ef-29b2c8ffc470" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ba163-157a-7c44-9b6c-1cd3ca7ad706" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ba163-157a-7187-b9ef-29b2c8ffc470" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ba163-157a-7c44-9b6c-1cd3ca7ad706" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019ba163-157a-7ec0-846c-1105a3070c18" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ba163-157a-7c44-9b6c-1cd3ca7ad706" xlink:to="loc_srt_CounterpartyNameAxis_019ba163-157a-7ec0-846c-1105a3070c18" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ba163-157a-7ec0-846c-1105a3070c18_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019ba163-157a-7ec0-846c-1105a3070c18" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ba163-157a-7ec0-846c-1105a3070c18_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ba163-157a-71bb-9469-fd816a4dfe0d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019ba163-157a-7ec0-846c-1105a3070c18" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ba163-157a-71bb-9469-fd816a4dfe0d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_SWIECMember_019ba163-157a-7ea1-b1b8-01cb00882cf9" xlink:href="psix-20251231.xsd#psix_SWIECMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ba163-157a-71bb-9469-fd816a4dfe0d" xlink:to="loc_psix_SWIECMember_019ba163-157a-7ea1-b1b8-01cb00882cf9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_WeichaiAmericaCorpMember_019ba163-157a-72a7-9127-3ceb8a44f1b4" xlink:href="psix-20251231.xsd#psix_WeichaiAmericaCorpMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ba163-157a-71bb-9469-fd816a4dfe0d" xlink:to="loc_psix_WeichaiAmericaCorpMember_019ba163-157a-72a7-9127-3ceb8a44f1b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_KNAMember_019c6580-8b77-7477-90d0-2b921e0c865f" xlink:href="psix-20251231.xsd#psix_KNAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ba163-157a-71bb-9469-fd816a4dfe0d" xlink:to="loc_psix_KNAMember_019c6580-8b77-7477-90d0-2b921e0c865f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019ba163-157a-7985-97c0-b2ab4e98006c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ba163-157a-7c44-9b6c-1cd3ca7ad706" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019ba163-157a-7985-97c0-b2ab4e98006c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019ba163-157a-7985-97c0-b2ab4e98006c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019ba163-157a-7985-97c0-b2ab4e98006c" xlink:to="loc_us-gaap_RelatedPartyDomain_019ba163-157a-7985-97c0-b2ab4e98006c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019ba163-157a-7811-adec-a79c7ce8124b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019ba163-157a-7985-97c0-b2ab4e98006c" xlink:to="loc_us-gaap_RelatedPartyDomain_019ba163-157a-7811-adec-a79c7ce8124b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019ba163-157a-7e3a-a32c-9a695058e56e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019ba163-157a-7811-adec-a79c7ce8124b" xlink:to="loc_us-gaap_RelatedPartyMember_019ba163-157a-7e3a-a32c-9a695058e56e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionAxis_019c0e29-1997-7583-9498-affdd2289c3e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ba163-157a-7c44-9b6c-1cd3ca7ad706" xlink:to="loc_us-gaap_RelatedPartyTransactionAxis_019c0e29-1997-7583-9498-affdd2289c3e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_019c0e29-1997-7583-9498-affdd2289c3e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_019c0e29-1997-7583-9498-affdd2289c3e" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_019c0e29-1997-7583-9498-affdd2289c3e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_019c0e29-1997-7575-b78a-808ce433bb15" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_019c0e29-1997-7583-9498-affdd2289c3e" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_019c0e29-1997-7575-b78a-808ce433bb15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_MORAgreementFeeMember_019c0e29-1997-7fa6-b8b8-a0048cbb0e5f" xlink:href="psix-20251231.xsd#psix_MORAgreementFeeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019c0e29-1997-7575-b78a-808ce433bb15" xlink:to="loc_psix_MORAgreementFeeMember_019c0e29-1997-7fa6-b8b8-a0048cbb0e5f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_DevelopmentCostsReimbursementMember_019c7ae1-216d-7a81-9f2b-1409981b62bc" xlink:href="psix-20251231.xsd#psix_DevelopmentCostsReimbursementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019c0e29-1997-7575-b78a-808ce433bb15" xlink:to="loc_psix_DevelopmentCostsReimbursementMember_019c7ae1-216d-7a81-9f2b-1409981b62bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermPurchaseCommitmentPeriod_019ba163-157a-7601-9b20-d02ce4bc6c20" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermPurchaseCommitmentPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ba163-157a-7187-b9ef-29b2c8ffc470" xlink:to="loc_us-gaap_LongtermPurchaseCommitmentPeriod_019ba163-157a-7601-9b20-d02ce4bc6c20" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_LongTermPurchaseCommitmentMinimumTargetPeriodForRenewal_019ba163-157a-724d-b67b-9ce32f41b091" xlink:href="psix-20251231.xsd#psix_LongTermPurchaseCommitmentMinimumTargetPeriodForRenewal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ba163-157a-7187-b9ef-29b2c8ffc470" xlink:to="loc_psix_LongTermPurchaseCommitmentMinimumTargetPeriodForRenewal_019ba163-157a-724d-b67b-9ce32f41b091" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_OtherCommitmentFeeAsAPercentageOfGrossRevenue_019ba163-157a-7c80-b893-f2c226863811" xlink:href="psix-20251231.xsd#psix_OtherCommitmentFeeAsAPercentageOfGrossRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ba163-157a-7187-b9ef-29b2c8ffc470" xlink:to="loc_psix_OtherCommitmentFeeAsAPercentageOfGrossRevenue_019ba163-157a-7c80-b893-f2c226863811" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionAmountsOfTransaction_019c0e29-1997-7fcd-a77e-a632b32adaf6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionAmountsOfTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ba163-157a-7187-b9ef-29b2c8ffc470" xlink:to="loc_us-gaap_RelatedPartyTransactionAmountsOfTransaction_019c0e29-1997-7fcd-a77e-a632b32adaf6" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/RelatedPartyTransactionsScheduleofAnnualMinimumRequirementsDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#RelatedPartyTransactionsScheduleofAnnualMinimumRequirementsDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/RelatedPartyTransactionsScheduleofAnnualMinimumRequirementsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_019ba162-f3c1-7447-bddb-406193e178a4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ba162-f3c1-7103-af98-8df7cda6a78c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ba162-f3c1-7447-bddb-406193e178a4" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ba162-f3c1-7103-af98-8df7cda6a78c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019ba162-f3c1-7828-9a02-35d43a939373" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ba162-f3c1-7103-af98-8df7cda6a78c" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019ba162-f3c1-7828-9a02-35d43a939373" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019ba162-f3c1-7828-9a02-35d43a939373_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019ba162-f3c1-7828-9a02-35d43a939373" xlink:to="loc_us-gaap_RelatedPartyDomain_019ba162-f3c1-7828-9a02-35d43a939373_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019ba162-f3c1-7bcc-883e-f98f200c629a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019ba162-f3c1-7828-9a02-35d43a939373" xlink:to="loc_us-gaap_RelatedPartyDomain_019ba162-f3c1-7bcc-883e-f98f200c629a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019ba162-f3c1-7e3f-a726-a7569986d6b8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019ba162-f3c1-7bcc-883e-f98f200c629a" xlink:to="loc_us-gaap_RelatedPartyMember_019ba162-f3c1-7e3f-a726-a7569986d6b8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PurchaseObligationDueInNextTwelveMonths_019ba162-f3c1-7af7-9890-6aabcef499ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PurchaseObligationDueInNextTwelveMonths"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ba162-f3c1-7447-bddb-406193e178a4" xlink:to="loc_us-gaap_PurchaseObligationDueInNextTwelveMonths_019ba162-f3c1-7af7-9890-6aabcef499ee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PurchaseObligationDueInSecondYear_019ba162-f3c1-7167-a8c4-b35d225b30d8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PurchaseObligationDueInSecondYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ba162-f3c1-7447-bddb-406193e178a4" xlink:to="loc_us-gaap_PurchaseObligationDueInSecondYear_019ba162-f3c1-7167-a8c4-b35d225b30d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PurchaseObligationDueInThirdYear_019ba162-f3c1-799e-aa09-6e8e7b52a74c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PurchaseObligationDueInThirdYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ba162-f3c1-7447-bddb-406193e178a4" xlink:to="loc_us-gaap_PurchaseObligationDueInThirdYear_019ba162-f3c1-799e-aa09-6e8e7b52a74c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PurchaseObligationDueInFourthYear_019ba2c4-7972-74d7-9e41-8ce145874cc1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PurchaseObligationDueInFourthYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ba162-f3c1-7447-bddb-406193e178a4" xlink:to="loc_us-gaap_PurchaseObligationDueInFourthYear_019ba2c4-7972-74d7-9e41-8ce145874cc1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PurchaseObligation_019ba162-f3c1-7687-9918-f1b20afd19c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PurchaseObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ba162-f3c1-7447-bddb-406193e178a4" xlink:to="loc_us-gaap_PurchaseObligation_019ba162-f3c1-7687-9918-f1b20afd19c1" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://powersint.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="psix-20251231.xsd#SubsequentEventsDetails"/>
  <link:definitionLink xlink:role="http://powersint.com/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_019b0f23-5dbc-7fde-85d4-1f50944d8d4b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_019b0f23-5dbc-7fed-bf2c-c3d4962172bb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_019b0f23-5dbc-7fde-85d4-1f50944d8d4b" xlink:to="loc_us-gaap_SubsequentEventTable_019b0f23-5dbc-7fed-bf2c-c3d4962172bb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019c0437-af48-73a1-9097-19ef123b3d29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019b0f23-5dbc-7fed-bf2c-c3d4962172bb" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019c0437-af48-73a1-9097-19ef123b3d29" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c0437-af48-73a1-9097-19ef123b3d29_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019c0437-af48-73a1-9097-19ef123b3d29" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c0437-af48-73a1-9097-19ef123b3d29_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c0437-af48-7a9e-814c-7ed05e1568e2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019c0437-af48-73a1-9097-19ef123b3d29" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c0437-af48-7a9e-814c-7ed05e1568e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_psix_MTLManufacturingEquipmentInc.Member_019c0437-af48-7d18-870a-846497e4e6c2" xlink:href="psix-20251231.xsd#psix_MTLManufacturingEquipmentInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c0437-af48-7a9e-814c-7ed05e1568e2" xlink:to="loc_psix_MTLManufacturingEquipmentInc.Member_019c0437-af48-7d18-870a-846497e4e6c2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019c0437-af48-7de7-b3ef-501e699aaec3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019b0f23-5dbc-7fed-bf2c-c3d4962172bb" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019c0437-af48-7de7-b3ef-501e699aaec3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019c0437-af48-7de7-b3ef-501e699aaec3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019c0437-af48-7de7-b3ef-501e699aaec3" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019c0437-af48-7de7-b3ef-501e699aaec3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019c0437-af48-751c-8401-937c07d12700" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019c0437-af48-7de7-b3ef-501e699aaec3" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019c0437-af48-751c-8401-937c07d12700" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019c0437-af48-7b85-82ee-d9d6c60b303d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019c0437-af48-751c-8401-937c07d12700" xlink:to="loc_us-gaap_SubsequentEventMember_019c0437-af48-7b85-82ee-d9d6c60b303d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019c0437-af48-77bc-bb99-a2b82c460b21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019b0f23-5dbc-7fde-85d4-1f50944d8d4b" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019c0437-af48-77bc-bb99-a2b82c460b21" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-roles-2025.xsd#eedm"/>
  <link:definitionLink xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization" xlink:type="arc" order="101"/>
  </link:definitionLink>
</link:linkbase>
