发动机原理双语教案

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1、Chapter2EngineGasExchangeProcessesKeypoints:Gasexchangeprocessoffour-strokeinternalcombustionengineandventilationlosses,theconceptofcombustionenginefillingcoefficient.Difficultpoints:Themeasurestoimproveinternalcombustionenginefillingquantitycoefficient.2.1Inletandexhaustproces

2、softhefour-strokeengineThepurposeoftheexhaustandinletprocessesorofthescavengingprocessistoremovetheburnedgasesattheendofthepowerstrokeandadmitthefreshchargeforthenextcycle.Inductingthemaximumairmassatwide-openthrottleorfullloadandretainingthatmasswithinthecylinderistheprimarygo

3、alofthegasexchangeprocesses.Enginegasexchangeprocessesarecharacterizedbyoverallparameterssuchasvolumetricefficiencyforfour-strokecycles.2.1.1Gasexchangeprocessoffour-strokeengineThegasexchangeprocessofanengineconsistsofthedurationfromopeningtheexhaustvalvetoclosingtheinletvalve

4、,itextendsapproximately410~480ºCAandcanbedividedintofourphases—blowdown,displacement,inductionandscavenging.1.BlowdownThedurationfromexhaustvalveopeningtocylinderpressureclosestopipepressureisreferredtoasblowdownphase.Theburnedcylindergasesaredischargedduetothepressuredifferen

5、cebetweenthecylinderandtheexhaustsystem.IftheexhaustvalvebeginstoopenwhenthepistonreachedBDC,backpressureagainsttheupwardpistonmustbeextremelyhigh.Thustheexhaustprocessusuallybegins40~60ºCAbeforeBDC(exhaustleadcrankangle).Blowdownphasesendswhenpressuredifferencebetweenthecylind

6、erandtheexhaustsystemdisappears,about10~30ºCAafterBDC.Thoughthefreeexhaustphasecoversonlyabout1/10ofexhauststroke,itdischarges60%ofburnedgas.2.DisplacementTheexhaustgasisscavengedbypiston’supwardmotionthatistheburntgasisforcedoutofthecylinder.Itisapositivedisplaceprocess.Theexh

7、austvalvecloses15~30ºCAafterTDC(exhaustlagcrankangle)toimproveemptyingthecylindersandmakethebestuseoftheinertiaofthegasesintheexhaustsystems.3.InductionprocessFromtheinletvalvesopentoclose,thewholeprocessthatinternalcombustionengineinhalesfreshchargeiscalledintakeprocess.Theusu

8、alpracticeistoextendthevalveopenphasesbeyondtheintakes