The　fuel　injectors　in　a　heavy-duty　diesel　engine　with　a　selective　catalytic　reduction　(SCR)　system　operate　in　harsh　conditions　that　cause　them　to　deteriorate,　resulting　in　changes　in　the　fuel　injection　quantity.　In　this　study,　a　method　for　determining　the　injection　quantity　of　fuel　injected　using　the　output　of　a　NOx　sensor　in　a　heavy-duty　diesel　engine　equipped　with　a　SCR　system　is　developed.　A　mathematical　model　was　derived　to　calculate　the　fuel　injection　quantity　based　on　the　oxygen　concentration　in　the　exhaust　gases.　Bench　tests　were　conducted　at　various　engine　operating　conditions.　The　results　indicate　that　at　high　engine　loads,　the　absolute　error　between　the　oxygen　concentration　measured　by　the　NOx　sensor　and　that　measured　by　a　gas　analyser　was　less　than　2%,　which　is　acceptable,　and　the　reactions　in　the　SCR　reactor　consumed　no　more　than　1%　of　the　oxygen　in　the　exhaust　gases,　which　is　negligible.　An　analysis　of　the　model　demonstrates　that　the　calculated　value　of　the　injection　quantity　based　on　the　oxygen　concentration　in　the　exhaust　is　more　accurate　for　lower　oxygen　concentrations　(i.e.,　at　greater　engine　loads).　The　test　data　reveal　that　the　error　in　the　fuel　injection　quantity　obtained　from　the　model　was　less　than　2%　at　high　engine　loads.　This　research　demonstrates　that　the　on-board　calculation　of　the　fuel　injection　quantity　based　on　the　oxygen　concentration　signals　from　the　NOx　sensor　is　possible　at　high　engine　loads.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.