A　regenerative　organic　Rankine　cycle　(RORC)　system　is　designed　to　recover　the　exhaust　heat　of　a　diesel　engine,　and　the　influence　of　the　intermediate　pressure　(the　pressures　at　which　the　steam　is　extracted　from　the　expander)　on　performance　parameters　such　as　net　power　output,　thermal　efficiency　and　mass　flow　rate　of　the　working　fluid　are　analyzed.　The　organic　working　fluids　under　investigation　are　R245fa　and　the　zeotropic　mixture　isopentane/R245fa　(in　a　0.7/0.3　mol　fraction).　Based　on　initial　calculations　of　RORC　system　performance,　the　intermediate　pressure　is　set　to　1.15　MPa　for　the　RORC　system　when　using　isopentane/R245fa　(in　a　0.7/0.3　mol　fraction)　as　the　working　fluid,　and　1.2　MPa　when　using　R245fa　as　the　working　fluid.　A　performance　analysis　of　the　RORC　system　using　the　two　different　working　fluids　is　then　conducted　over　the　whole　operating　range　of　a　diesel　engine.　The　results　show　that　the　zeotropic　mixture　isopentane/R245fa　(in　a　0.7/0.3　mol　fraction)　performs　better.　Finally,　a　combined　diesel　engine　and　RORC　system　is　defined　to　evaluate　the　performance　improvement　of　such　a　combined　system　over　the　whole　operating　range.　Results　show　that,　for　the　combined　system,　a　10.54%　improvement　in　power　output　and　a　9.55%　improvement　in　fuel　economy　can　be　achieved　at　the　engine＇s　rated　condition.　Crown　Copyright　(C)　2014　Published　by　Elsevier　Ltd.　All　rights　reserved.