汽車專業(yè)英語讀譯教程 第3版 教案 UNIT 2 ENGINE MECHANICAL

授課題目TEXTAOperation,MainComponentsandClassificationoftheEngine授課類型理論課首次授課時間學時2學時教學目標1、發(fā)動機主要部件的結構、材料、功能；2、發(fā)動機分類：燃料分類、氣缸布置分類、冷卻方式分類、工作循環(huán)分類等）；重點與難點重點：發(fā)動機主要部件的結構、材料、功能；難點：發(fā)動機分類：燃料分類、氣缸布置分類、冷卻方式分類、工作循環(huán)分類等）；教學手段與方法多媒體教學教學過程：（包括授課思路、過程設計、講解要點及各部分具體內容、時間分配等）1.授課思路在學習了前置課程之后，通過專業(yè)英語的學習，進行與汽車相關技術結構原理的中文英文對接，培養(yǎng)和提高英文專業(yè)資料（信息）的閱讀能力，從而為與專業(yè)英語密切相關的后續(xù)課程奠定了必要的基礎。2.過程設計：（1）課程內容簡介（5分鐘）（2）導入新課（15分鐘）。（3）新課內容（75分鐘）教師根據學生學習的情況，對課程內容的重點和難點進行講解。（4）課程小結（5分鐘）3.授課內容：Theautomotiveengineisessentiallyaheatengine.Theheatenginesusedinautomobilesareinternalcombustionengines.1.PrincipleofOperationThespark-ignitionengineisaninternal-combustionenginewithexternallysuppliedinignition.Thegasolineengineisakindofspark-ignitionengines.Thefour-stroke-cyclegasolineenginecycleisspreadoverfourpistonstrokes.TheoperationstrokesareThefirststrokeinyourengineiscalledtheintakestroke.Insteadofopeningtheintakevalveafteryouhavedrawnthepistondown,youwillfinditbettertoopentheintakevalveasthepistonstartsdown[1].Thisallowstheairtodrawfuelintheentiretimethepistonismovingdown.Remember,theintakestrokestartswiththepistonatthetopofthecylinder(intakevalveopenandexhaustvalveclosed)andstopswiththepistonatthebottomofitstravel[2].Thisrequiresone-halfturnofthecrankshaft.Asthecrankshaftcontinuestomove,thepistonisforcedupthroughthecylinder.Ifyoukeepbothvalvesclosed,thefuelmixturewillbesqueezed,orcompressed,asthepistonreachesthetop.Thisiscalledthecompressionstroke.It,too,requiresahalfturnofthecrankshaft.Thecompressionstrokeservestobreakupthefuelintoevensmallerparticles.Thishappensduetothesuddenswirlingandchurningofthemixtureasitiscompressed.Whentheair-fuelmixtureissubjectedtoasuddensharpcompressionforce,itstemperaturerises.ThisincreaseintemperaturemakesthemixtureeasiertoigniteandcausesittoexplodewithgreaterforceAsthepistonreachesthetopofitstravelonthecompressionstroke,ithasreturnedtotheproperpositiontobepushedbackdownbytheexplosion.Remember,thecompressionstrokestartswiththepistonatthebottomofthecylinder(bothvalvesclosed)andstopswiththepistonatthetopofthecylinder.Thisrequiresanadditionalhalfturnofthecrankshaft.Asthepistonreachesthetopofthecompressionstroke,themixtureisbrokenintotinyparticlesandheatedup.Whenignited,itwillexplodewithgreatforce.Thisistherighttimetoexplodethemixture.Asparkplugprovidesasparkinsidethecombustionchamber.Thesparkproducedattheplugisformedbytheignitionsystem.ThiswillbediscussedinUnit6.Whilethecrankshaftisturningaroundtwice,itisreceivingpoweronlyduringonehalfturn,oronefourthofthetime.Assoonasthepistonreachesthetopoftheexhauststroke,itstartsdownonanotherintake,compression,power,andexhaustcycle.Thiscycleisrepeatedoverandover.Eachcompletecycleconsistsoffourstrokesofthepiston,hencethenamefour-strokecycle.2.MainEngineComponentsEngineblockTheengineblockservesasarigidmetalfoundationforallpartsofanengine.Itcontainsthecylindersandsupportsthecrankshaftandcamshaft.Inolderengines,thevalveseats,ports,andguidesarebuiltintotheblock.Accessoryunitsandtheclutchhousingareboltedtoit.Blocksaremadeofeithercastironoraluminum.Thelightertheblock(providingithassufficientstrength),thebetter.Themodernthin-wallcastingprocesscontrolscoresizeandplacementmuchmoreaccuratelythantheoldercastingprocess.CylindersThecylinderisaroundholeformedintheblock.Itformsaguideforthepistonandactsasacontainerfortakingin,compressing,firing,andexhaustingtheair-fuelcharge.Cylindershavebeenmadeofbothsteelandcastiron.Castironisbyfarthemostpopular.Whensteelcylindersaredesiredinanaluminumblock,theyareinstalledintheformofcylindersleeves(round,pipe-likeliners),Thesesleevesmaybeeithercastorpressedintotheblock.Someenginesuseremovablecylindersleeves.Whenthecylinderbecomesworn,theoldsleevescanbepulledoutandnewsleevescanbepressedin.PistonsThepistonmustmovedownthroughthecylindertoproduceavacuumtodrawafuelchargeintothecylinder.Itthentravelsupinthecylinderandcompressesthemixture.Whenthemixtureisfired,thepressureoftheexpandinggasistransmittedtothetopofthepiston.Thisdrivesthepistonbackdownthroughthecylinderwithgreatforce,transmittingtheenergyoftheexpandinggastothecrankshaft.Thepistonthentravelsupthroughthecylinderandexhauststheburnedfuelcharge.ConnectingRodsAsthenameimplies,connectingrodsareusedtoconnectpistonstothecrankshaft.Theupperendoftherodoscillates(swingsbackandforth),whilethelower,orbigend,bearingrotates(turns).Becausethereisverylittlebearingmovementintheupperend,thebearingareacanbereasonablysmall.Thelowerendrotatesveryfast,andthecrankshaftbearingjournalturnsinsidetheconnectingrod.Thisrotationalspeedtendstoproduceheatandwear.Tomaketherodwearwell,alargerbearingareaisrequired.CrankshaftTheenginecrankshaftprovidesaconstantturningforcetothewheels.Ithasthrowstowhichconnectingrodsareattached,anditsfunctionistochangethereciprocatingmotionofthepistontoarotarymotiontodrivethewheels.Crankshaftsaremadeofalloysteelorcastiron.Thecrankshaftisheldinpositionbyaseriesofmainbearings.Themaximumnumberofmainbearingsforacrankshaftisonemorethanthenumberofcylinders.Itmayhavefewermainbearingsthancylinders.Mostenginesuseprecisioninsertbearingsthatareconstructedliketheconnectingrodbearings,butaresomewhatlarger.Inadditiontosupportingthecrankshaft,oneofthemainbearingsmustcontroltheforwardandbackwardmovement.FlywheelAheavyflywheelisattachedtotherearofthecrankshaftwithbolts.Thefunctionoftheflywheelistosmoothoutenginespeedandkeepthecrankshaftspinningbetweenpowerstrokes,insomeengines,theflywheelalsoservesasamountingsurfacefortheclutch.Theouterrimoftheflywheelhasalargeringattachedwithgearteethcutintoit.Theteethofthestartermotorengagetheseteethandspintheflywheeltocranktheengine.Whenanautomatictransmissionisused,thetorqueconverterassemblyworkswiththeflywheel.CamshaftThecamshaftisusedtoopenandclosethevalves.Thereisonecamonthecamshaftforeachvalveintheengine.Generallyonlyonecamshaftisusedinmostengines.NewerenginesareincreasinglyequippedwithtwoormorecamshaftsValvesEachenginecylinderordinarilyhastwovalves.However,modernenginesoftenusefourvalvespercylinder(twointakeandtwoexhaust).Afewenginesusedinsmallervehicleshavethreeorfivevalvespercylinder:twointakevalvesandoneexhaustvalveorthreeintakevalvesandtwoexhaustvalves.Becausetheheadofanexhaustvalveoperatesattemperaturesupto1300℉(704℃),valvesaremadeofheat-resistantmetal.Inordertopreventburning,thevalvemustgiveoffheattothevalveseatandtothevalveguide.Thevalvemustmakegoodcontactwiththeseatandmustrunwithminimumclearanceintheguide.ValveLiftersMechanicalvalveliftersareusuallymadeofcastiron.Thebottompartthatcontactsthecamshaftishardened.Someliftersarehollowtoreduceweight.Mostvalvetrainsthatcontainmechanicalliftershavesomeprovisionforadjustingclearance.Mechanicalvalvelifterswereusedinolderengines.Hydraulicvalveliftersperformthesamejobasmechanicallifters.However,hydraulicliftersareself-adjusting,operatewithnolifter-to-rockerarmclearance,andusesengineoilunderpressuretooperate.Hydraulicliftersarequietinoperation.3.EngineClassificationCycleClassificationEnginesareoftenclassifiedaccordingtocycle.Mostinternalcombustionpistonenginesuseatwo-orfour-strokecycle.Allmodernautomobileenginesusethefour-strokecycleengine.Thetwo-strokecycleengineperformstheintake,compression,firing,andexhaustsequenceinonerevolutionofthecrankshaft.CylinderClassificationTheinlineenginehasitscylindersarrangedoneaftertheotherinastraightline.Theyareinavertical,ornearverticalposition.Mostmoderninlineenginesarefourcylinders.AV-typeengineplacestwobanksorrowsofcylindersatanangletoeachother—generallyat60°or90°.TheV-typeenginehasseveraladvantages:shortlength,extrablockrigidity,ashort,heavycrankshaft,andlowprofilet