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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
