Organic　Rankine　cycle　(ORC)　system　is　considered　as　a　promising　technology　for　energy　recovery　from　the　waste　heat　rejected　by　internal　combustion　(IC)　engines.　However,　such　waste　heat　is　normally　contained　in　both　coolant　and　exhaust　gases　at　quite　different　temperatures.　A　single　ORC　system　is　usually　unable　to　efficiently　recover　energy　from　both　of　these　waste　heat　sources.　A　dual　loop　ORC　system　which　essentially　has　two　cascaded　ORCs　to　recover　energy　from　the　engine＇s　exhaust　gases　and　coolant　separately　has　been　proposed　to　address　this　challenge.　In　this　way,　the　overall　efficiency　of　energy　recovery　can　be　substantially　improved.　This　paper　examines　a　regenerative　dual　loop　ORC　system　using　a　pair　of　environmentally　friendly　refrigerants,　R1233zd　and　R1234yf,　as　working　fluids,　to　recover　energy　from　the　waste　heat　of　a　compressed　natural　gas　(CNG)　engine.　Unlike　most　previous　studies　focusing　on　the　ORC　system　only,　the　present　research　analyses　the　ORC　system　and　CNG　engine　together　as　an　integrated　system.　As　such,　the　ORC　system　is　analysed　on　the　basis　of　real　data　of　waste　heat　sources　of　the　CNG　engine　under　various　operational　conditions.　A　numerical　model　is　employed　to　analyse　the　performances　of　the　proposed　dual　loop　cycle　with　four　pairs　of　working　fluids.　The　effects　of　a　regenerative　heat　exchanger　and　several　other　key　operating　parameters　are　also　analysed　and　discussed　in　detail.　The　performance　of　the　integrated　engine-ORC　system　is　then　analysed　under　actual　engine　operating　conditions　which　were　measured　beforehand.　The　performance　of　the　proposed　system　under　off　design　conditions　has　also　been　analysed.　The　obtained　results　show　that　the　proposed　dual　loop　ORC　system　could　achieve　better　performance　than　other　ORC　systems　for　similar　applications.　(C)　2017　The　Authors.　Published　by　Elsevier　Ltd.