Organic　Rankine　Cycle　(ORC)　could　be　used　to　recover　low-grade　waste　heat.　When　a　vehicle　is　running,　the　engine　exhaust　gas　states　have　a　wide　range　of　variance.　Defining　the　operational　conditions　of　the　ORC　that　achieve　the　maximum　utilization　of　waste　heat　is　important.　In　this　paper　the　performance　of　different　working　fluids　operating　in　specific　regions　was　analyzed　using　a　thermodynamic　model　built　in　Matlab　together　with　REFPROP.　Nine　different　pure　organic　working　fluids　were　selected　according　to　their　physical　and　chemical　properties.　The　results　were　compared　in　the　regions　when　net　power　outputs　were　fixed　at　10　kW.　Safety　levels　and　environmental　impacts　were　also　evaluated.　The　outcomes　indicate　that　R11,　R141b,　R113　and　R123　manifest　slightly　higher　thermodynamic　performances　than　the　others;　however,　R245fa　and　R245ca　are　the　most　environment-friendly　working　fluids　for　engine　waste　heat-recovery　applications.　The　optimal　control　principle　of　ORC　under　the　transient　process　is　discussed　based　on　the　analytical　results.　(C)　2011　Elsevier　Ltd.　All　rights　reserved.