汽车专业英语PPT课件-section 1ppt课件Chapter 2 3-2 5

Chapter2InternalCombustionEngine2.3Engineblockandhead,NewWords,Chapter2InternalCombustionEngine2.3Engineblockandhead,2.3.1EngineblockTheengineblock(orcylinderblock)servesasarigidmetalfoundationforallpartsofanengine(Fig.2-4).Itcontainsthecylindersandsupportsthecrankshaftandcamshaft.Accessoryunitsandtheclutchhousingareboltedtoit.Blocksaremadeofeithercastironoraluminum.Thecylinderisaroundholeformedintheblock.Itformsaguideforthepistonandactsasacontainerfortakingin,compressing,firing,andexhaustingtheair-fuelcharge.Cylindershavebeenmadeofbothsteelandcastiron.Castironisbyfarthemostpopular.Whensteelcylindersaredesiredinanaluminumblock,theyareinstalledintheformofcylindersleeves(round,pipe-likeliners),Thesesleevesmaybeeithercastorpressedintotheblock.Someenginesuseremovablecylindersleeves.Whenthecylinderbecomesworn,theoldsleevescanbepulledoutandnewsleevescanbepressedin.,Chapter2InternalCombustionEngine2.3Engineblockandhead,Fig.2-4Cylinderblock,cylinderheadandcamshaft,Chapter2InternalCombustionEngine2.3Engineblockandhead,2.3.2CylinderheadThecylinderheadfastenstothetopoftheblock(Fig.2-4).Theundersideformsthecombustionchamberwiththetopofthepiston.In-lineenginesoflightvehicleshavejustonecylinderheadforallcylinders;largerin-lineenginescanhavetwoormore.Justaswithengineblocks,cylinderheadscanbemadeofcastironoraluminumalloy.,Chapter2InternalCombustionEngine2.4Pistons,ConnectingRodsandCrankshaft,NewWords,2.4.1PistonandconnectingrodThepistonmustmovedownthroughthecylindertoproduceavacuumtodrawafuelchargeintothecylinder.Itthentravelsupinthecylinderandcompressesthemixture.Whenthemixtureisfired,thepressureoftheexpandinggasistransmittedtothetopofthepiston.Thisdrivesthepistonbackdownthroughthecylinderwithgreatforce,transmittingtheenergyoftheexpandinggastothecrankshaft.Thepistonthentravelsupthroughthecylinderandexhauststheburnedfuelcharge.Pistonsareusuallymadeofaluminum.Often,aluminumpistonsaretin-platedtoallowproperbreak-inwhentheengineisstarted.Aluminumpistonscanbeforged,buttheyaremorecommonlycast.Castironisagoodmaterialforpistonsusedinaslow-speedengine.Ithasexcellentwearcharacteristicsandwillprovidegoodperformance.,Chapter2InternalCombustionEngine2.4Pistons,ConnectingRodsandCrankshaft,Fig.2-5Pistons,connectingrodsandcrankshaft,Asthenameimplies,connectingrodsareusedtoconnectpistonstothecrankshaft(Fig.2-5).Theupperendoftherodhasaholethroughitforthepistonpin.Thebottomofthelargeendoftheconnectingrod(Fig.2-6)mustberemovedsotherodcanbeinstalledonthecrankshaftjournal.Thesectionthatisremovediscalledtheconnectingrodcap.Connectingrodsarenormallymadeofalloysteel.Theyaredrop-forgedtoshapeandthenmachined.,Fig.2-6Connectingrod,Chapter2InternalCombustionEngine2.4Pistons,ConnectingRodsandCrankshaft,2.4.2CrankshaftTheenginecrankshaft(Fig.2-7)providesaconstantturningforcetothewheels.Ithasthrowstowhichconnectingrodsareattached,anditsfunctionistochangethereciprocatingmotionofthepistontoarotarymotiontodrivethewheels.Crankshaftsaremadeofalloysteelorcastiron.Thecrankshaftisheldinpositionbyaseriesofmainbearings.Themaximumnumberofmainbearingsforacrankshaftisonemorethanthenumberofcylinders.Itmayhavefewermainbearingsthancylinders.,Chapter2InternalCombustionEngine2.4Pistons,ConnectingRodsandCrankshaft,Mostenginesuseprecisioninsertbearingsthatareconstructedliketheconnectingrodbearings,butaresomewhatlarger.Inadditiontosupportingthecrankshaft,oneofthemainbearingsmustcontroltheforwardandbackwardmovement.,Fig.2-7Crankshaft,Chapter2InternalCombustionEngine2.5ValveSystem,NewWords,Thecamshaftmustturnatone-halfcrankshaftspeed.Thecamshaftislinkedwithcrankshaftbyoneofthreeways:beltdrive,chaindriveandgeardrive.Themaincomponentsofthevalvesystemare:Camshaft.Itisusedtoopenandclosethevalves.Generallyonlyonecamshaftisusedinmostengines.Newerenginesareincreasinglyequippedwithtwoormorecamshafts(Figure2-8).Valves.Eachenginecylinderordinarilyhastwovalves.However,modernenginesoftenusefourvalvespercylinder(twointakevalvesandtwoexhaustvalves).Inordertopreventburning,thevalvemustgiveoffheattothevalveseatandtothevalveguide.Thevalvemustmakegoodcontactwiththeseatandmustrunwithminimumclearanceintheguide.,Chapter2InternalCombustionEngine2.5ValveSystem,Valvelifters.Mechanicalvalvelifterswereusedinolderengines.Mostvalvetrainsthatcontainmechanicalliftershavesomeprovisionforadjustingclearance.Hydraulicvalveliftersperformthesamejobasmechanicallifters.However,hydr