A　vehicle　diesel　engine-organic　Rankine　cycle　(ORC)　combined　system　was　designed　based　on　the　variation　law　of　engine　exhaust　energy　under　various　operating　conditions.　The　zeotropic　mixture　R416A　was　used　as　the　working　fluid　for　ORC　waste　heat　recovery　system　to　absorb　the　engine　exhaust　energy　efficiently.　Meanwhile,　as　the　key　component　of　the　ORC　system,　a　screw　expander　was　used　to　produce　useful　work.　Via　experiments,　the　best　operating　point　of　the　screw　expander　was　determined　at　the　inlet　pressure　of　1.7　MPa,　the　expansion　ratio　of　8　and　the　isentropic　efficiency　of　0.65,　from　which　the　best　operating　parameters　for　ORC　waste　heat　recovery　system　were　set.　The　results　show　that,　using　the　combined　system　will　make　the　power　output　increase　by　30.6　kW,　and　the　thermal　efficiency　increase　by　10.99%,　while　the　waste　heat　recovery　efficiency　reach　its　maximum　value　of　10.61%,　and　the　brake　specific　fuel　consumption　(BSFC)　reaches　decrease　by　35　g/(kW·h)　at　most.　©　2016,　Chinese　Society　for　Internal　Combustion　Engines(CSICE).　All　right　reserved.