At　present,　the　most　prominent　technical　problems　of　small　and　medium　capacity　expanders　are　low　internal　efficiency　and　small　expansion　ratio.　In　order　to　promote　the　practical　application　of　small　and　medium　capacity　Organic　Rankine　Cycle　(ORC)　system,　improving　the　thermodynamic　performance　of　the　expander　was　a　remarkable　issue,　and　the　systematic　research　on　the　irreversibility　of　expander＇s　working　process　is　the　fundamental　work　to　achieve　this　goal.　Summarizing　the　related　research　on　various　small　capacity　volumetric　expanders,　the　research　on　flow　problem　is　lacked.　For　an　atypical　volumetric　expander,　the　flow　mechanism　of　single　screw　expanders　(SSEs)　is　more　complex.　In　this　paper,　we　discussed　the　effect　of　the　pressure　loss　in　the　inlet　and　exhaust　passages　on　the　performance　of　SSEs.　We　selected　two　SSE　prototypes　developed　by　our　laboratory　to　carry　out　the　experiment　under　variable　working　conditions.　The　internal　volume　ratios　are　2.95　and　3.98　respectively.　The　pressure　differences　of　inlet　and　exhaust　passages　were　measured.　In　order　to　evaluate　the　effect　of　the　flow　losses　on　the　performance,　we　defined　two　indexes:　pressure　loss　rate　(PLR)　and　pressure　energy　loss　influence　rate　of　shaft　efficiency　(PELRe).　From　the　results,　the　pressure　losses　of　inlet　and　exhaust　passages　for　each　SSE　increase　gradually,　PLRin　increases　slightly　and　PLRout　increases　gradually.　At　the　maximum　point　of　shaft　efficiency,　the　losses　of　the　SSE　with　2.95　are　28.06　kPa　and　46.09　kPa,　and　the　values　of　the　SSE　with　3.98　are　37.58　kPa　and　33.79　kPa,　respectively.　It　shows　that　the　pressure　losses　are　relatively　high.　The　maximum　values　of　internal　and　external　expansion　ratio　for　the　SSE　with　2.95　are　4.8　and　6　respectively,　and　the　values　of　the　SSE　with　3.98　are　5.7　and　7　respectively.　For　ideal　volume　expansion,　the　internal　expansion　ratio　of　two　prototypes　should　be　4.55　and　6.9,　respectively.　Summarizing　the　above　results,　how　to　enhance　the　pressure　drop　effect　of　leakage　and　reduce　the　flow　resistance　loss　of　inlet　and　exhaust　passages　simultaneously　is　an　important　technical　premise　to　improve　the　expansion　ratio　by　structural　optimization.　Meanwhile,　PELRe　of　two　prototypes　are　16.01%　and　13.59%　respectively　at　the　point.　It　shows　that　the　flow　losses　of　inlet　and　exhaust　passages　could　significantly　influence　the　performance　of　SSEs.　(C)　2021　Elsevier　Ltd.　All　rights　reserved.