教学材料《汽车构造英语》-Unit4

UnitFourValveGearCamshaft’sfunctionistoopentheenginevalvespositivelyandtimely，inadefinitesequence，andtocontroltheirclosingagainstthereturnactionofthevalvesprings.Theshaftismadeintegralwithitscamsandbearingjournals.Eachcamcontrolsasinglevalve，eitherintakeorexhaust.Insomeautomobileengine，thecamshaftismadeintegralwithfuelpumpeccentricwheelandoilpumpdrivegear.Thecamshaftbearingsarelubricatedwithoilsuppliedunderpressurefromthemaingalleryinthecylinderblock.上一页返回下一页UnitFourValveGearThetappetstransmittheforcefromthecamshafttothepushrods.Thetappetsaresmallcylindricalboresreceivingthepushrods.Withtheenginerunning，thetappetscontinuouslyrotateabouttheiraxesforuniformwear.Therotationisensuredbyaconvexsurfaceoftheirbottomsandaslantedsurfaceofthecams.Thepushrodstransmittheforcefromthetappetstotherockerandaremadeassteelstemswithhardenedtipsorduralumintubeswithsphericalsteeltipspress-fittedatbothends.上一页返回下一页UnitFourValveGearThepushrodtipsbearagainstthetappethollowatoneendagainstthesphericalsurfaceoftherockeradjustingscrewattheother.Inmanyengineswiththecamshaftmountedintheblock，thepushrodsaremadefromhollowsteel.Eachendofthepushrodhasaroundtip.Thelowerpushrodtipfitsintothetappetcup，andtheupperpushrodtipislocatedinarockerarmrecess.上一页返回下一页UnitFourValveGearRockerarmsareusuallymanufacturedfromstampedsteelorhardenedironalloy.Therockerarmstransmittheforcefromthepushrodtothevalve.Therockerarmsfreelyoscillateabouttherockerarmshaftsupportedbyaseriesofpedestalswhichareboltedtothetopdeckofthecylinderhead.Anenginevalveisadevicedesignedtoopenorcloseapassage.Eachcylinderofafour-stroke-cycledieselorgasolineengineiscommonlyequippedwithanintakevalveandanexhaustvalve.上一页返回下一页UnitFourValveGearThepurposeoftheintakevalveistoallowtheair-fuelmixturetoenterthecylinder.Afterthecombustionprocess，theburnedgasesarepermittedtoescapefromthecylinderthroughtheexhaustvalve.Toobtainsufficientvalvearea，someautomobileshavetwointakevalvesandtwoexhaustvalves.2

Avalveconsistsofaheadandstem.Thevalveheadfacemustbeseatedevenlyagainstthevalveseatinthecylinderheadtopreventleaksfromthecylinderduringthecompressionandpowerstrokes.上一页返回下一页UnitFourValveGearManyvalvefacesandseatshaveanangleof45°.Theheadsoftheinletandexhaustvalvesareofdifferentdiameters.Forbetterenginebreathing，theinletvalvehasalargerdiameterthantheexhaustone.Exhaustvalvesmustwithstandextremelyhightemperaturesupto815(815℃degreesCelsius)withoutdamagingthevalve.Asthevalvesarenotequallyheatedintherunningengine，theyaremadefromdifferentmaterials.Theinletvalvesaremadefromchromiumsteel，theexhaustvalvesareofheat-resistantsteel.上一页返回下一页UnitFourValveGearSpringsreturnthevalvestotheirclosedpositionafterrockerarmspressdownonthevalves.Thesespringshavetobeverystrongbecauseathighenginespeeds，thevalvesarepusheddownveryquickly.Ifthespringswerenotstrongenough，thevalvesmightcomeawayfromtherockerarmsandsnapback.上一页返回下一页UnitFourValveGearNotes1.Forthispurpose，thevalvesatdefinitemomentsopenandclosetheintakeandexhaustportsinthecylinderhead，throughwhichthecylinderscommunicatewiththeintakeandexhaustmanifold.译文：为此，气门在确定时刻开启和关闭汽缸盖上的进、排气道，汽缸通过进、排气歧管与进、排气道相通。上一页返回下一页UnitFourValveGear2.Toobtainsufficientvalvearea，someautomobileshavetwointakevalvesandtwoexhaustvalves.译文：为了获得足够的气门通道面积，有些汽车每汽缸各有两个进、排气门。上一页返回下一页UnitFourValveGearExtraReadingConnectingRodsandCrankshaft

Theconnectingrodslinkthepistonswiththecrankshaftandtransmitforceandmotionfromthepistontothecrankpinonthecrankshaft.Inoperation,theconnectingrodissubjectedtobothgaspressureandinertialoads,andtherefore,itmustbeadequatelystrongandrigid.Atthesametime,therodisineccentricmotion.Tominimizevibrationandbearingloads,therodmustbelightinweight.上一页返回下一页UnitFourValveGearSo,manyconnectingrodsaresteel-forged,taperedI-beamdesign.Connectingrodshavering-shapedheadsatitsends,theheadsbeingknownastheconnectingrodbigendandsmallend.Theconnectingrodbigendfitsoverthecrankshaftjournal,andtheconnectingrodsmallendfitsoverthepistonpin(seeFig.4-2).上一页返回下一页UnitFourValveGearTheconnectingrodsarecomposedofaconnectingrodsmallend,aconnectingrodshank,aconnectingrodbigend,aconnectingrodcapandconnectingrodbearinghalfshells.Theconnectingrodbigendandtheconnectingrodcaparejoinedbymeansofspecialhigh-strengthboltsandnuts.Thenutsontheconnectingrodboltsaretightenedwithatorqueindicatingwrenchandthenpottered.Thebigendoftheconnectingrodhousesaslidingcontactbearingandfeaturesaholethroughwhichoilissquirtedontothecylinderwalls.1上一页返回下一页UnitFourValveGearThepartinglinebetweentheconnectingrodanditscapisgenerallyarrangedatrightangletotheaxisoftheconnectingrodshank,butinsomeengines,thepartinglineisnecessarilyarrangeddiagonally,becausetheproportionsofthebigendoftheconnectingrodaresuchthatthelowerpartoftherodcouldnototherwisebepassedthroughthecylinderforassemblypurpose.2Withsuchanangledbigend,thecapissecuredtotheconnectingrodbysetscrewsinsteadofboltsandnuts.上一页返回下一页UnitFourValveGearToresistthegreatertendencyfortheinertiaforcestodisplacethecapsidewaysrelativetotheconnectingrod,eitheraserratedorasteppedjointisgenerallypreferredfortheirabuttingfaces.Hence,theretainingsetscrewsintheirclearanceholesarecompletelyrelievedofshearloads.Tabwashersareusedundertheheadsofsetscrewsinordertopreventthelatterfromworkingloose.上一页返回下一页UnitFourValveGearThecrankshaft(seeFig4-3)takesthedownwardthrustsofthepistonsandconnectingrodswhentheair-fuelmixtureisburnedinthecylindersandchangesthesethrustsintotorquewhichistransferredtothedrivelineoftheautomobile.Thecrankshaftisusuallymadefromcarbonsteelwhichisalloyedwithasmallproportionofnickel.Themainbearingjournalsfitintothecylinderblockandthebigendjournalsalignwiththeconnectingrods.上一页返回下一页UnitFourValveGearThecrankshaftalsodrivesvariousenginemechanismsandcomponents.Attherearendofthecrankshaftisattachedtheflywheel,andatthefrontendarethedrivingwheelsforthetiminggears,fan,coolingwaterandalternator.上一页返回下一页UnitFourValveGearAllthemainbearingandconnectingrodbearingjournalsaremachinedtoahighlypolishedfinish.Bearinginsertsarelocatedbetweenthemainbearingboresandthemainbearingjournalsonthecrankshaft.Themainbearingjournalsmustbeperfectlyalignedwitheachother.ThecrankshaftjournalsmustbeproperlyspacedsothatthepistonsreachT.D.Cinthe