车辆工程专业英语原文完整.docx
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车辆工程专业英语原文完整.docx
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车辆工程专业英语原文完整
2.5ValveSystem
Thevalvesystemismadeupofthosepartsneededtoopenandclosethevalveatjusttherighttime.
2。
5.1ValveSystem
Tocoordinatethefour—strokecycle,agroupofpartscalledthevalvetrainopensandclosesthevalves(movesthemdownandup,respectively)。
Thesevalvesmovementsmosttakeplaceatexactlytherightmovements。
Theopeningofeachvalveiscontrolledbyacamshaft。
1.OverheadCamshaft(OHC)ValveTrain
Thecamisanegg-shapedpieceofmetalonashaftthatrotatesincoordinationwiththecrankshaft.Themetalshaft,calledthecamshaft,typicallyhasindividualcamsforeachvalveintheengine.Asthecamshaftrotates,thelobe,orhighspotofthecam,pushesagainstpartsconnectedtothestemofthevalve。
Thisactionforcesthevalvetomovedownward.Thisactioncouldopenaninletvalveforanintakestroke,oropenanexhaustvalveforanexhauststroke.
Asthecamshaftcontinuestorotate,thehighspotmovesawayfromthevalvemechanism.Asthisoccurs,valvespringspullthevalvetightlyclosedagainstitsopening,calledthevalveseat。
Valveinmoderncarenginesarelocatedinthecylinderheadatthetopoftheengine。
Thisisknownasanoverheadvalve(OHV)configuration.Inaddition,whenthecamshaftislocatedoverthecylinderhead,thehigh-performanceengineshavetwoseparatecamshaft,oneforeachsetofinletandexhaustvalves。
Theseenginesareknownasdualoverhead-camshaft(DHOC)engines。
2.Push—rodValveTrain
Thecamshaftalsocanbelocatedinthelowerpartofengine,withintheengineblock。
Totransferthemotionofthecamupwardtothevalve,additionalpartsareneeded。
Inthisarrangement,thecamlobespushagainstroundmetalcylinderscalledcamfollower.Asthelobeofthecamcomesupunderthecamfollower,itpushesthecamfollowerupward(awayfromthecamshaft).Thecamfollowerridesagainstapushrod,whichpushesagainstarockerarm.Therockerarmpivotsonashaftthroughitscenter.Asonesideoftherockerarmmovesup,theothersidemovesdown,justlikeaseesaw。
Thedownward-movingsideoftherockerarmpushesonthevalvestemtoopenthevalve。
Becauseapush—rodvalvetrainhasadditionalparts,itismoredifficulttorunathighspeeds。
Push—roadenginestypicallyrunatslowerspeedsand,consequently,producelesshorse—powerthanoverhead—camshaftdesignsofequalsize.(Remember,poweristherateatwhichworkisdone.)
2.5.2ValveClearance
Whentheenginerunsincompressionstrokeandpowerstroke,thevalvemustclosetightlyontheirseatstoproduceagas-tightsealandthuspreventthegasesescapingfromthecombustionchamber.Ifthevalvedonotclosefullytheenginewillnotdevelopfullpower。
Alsothevalveheadswillbeliabletobeburntbythepassinghotgases,andthereisthelikelihoodofthepistoncrowntouchinganopenvalve,whichcanseriouslydamagetheengine。
Sothatthevalvescanclosefullysomeclearanceisneededintheopeningmechanism.Thismeansthattheoperatingmechanismmustbeabletomovesufficientlyfarenoughawayfromthevalvetoallowthevalvestobefullyclosedagainstitsseatbythevalvespring。
However,iftheclearanceissettoogreatthiswillcausealightmetallictapingnoise。
2。
5.3ValveTiming
Thetimeatwhichvalvesopenandclose(valvetiming)andthedurationofvalvesopeningisstatedindegreesofcrankshaftrotation.Forinstance,theintakevalvenormallybeginstoopenjustbeforethepistonhasreachedtopdeadcenter.ThevalveremainsopenasthepistontravelsdowntoBDCandevenpastBDC.Thisisintakevalveduration.Anexampleofthiscouldbestatedasfollows;IOat17°BTDC,ICat51°ABDC(or,intakeopens17°beforetopdeadcenter,intakecloses51°afterbottomdeadcenter)。
Intakevalvedurationinthiscaseis248°ofcrankshaftrotation.
Thisleaves129durationforthecompressionstrokesincecompressionendswhentheexhaustvalvebeginstoopenapproximatelyat51beforebottomdeadcenter.Thedurationofthepowerstrokeinthiscaseisalso129.
Sincetheexhaustvalveisopeningat51BBDC,thisbeginstheexhauststroke。
TheexhauststrokecontinuesasthepistonpassesBDCandmoveupwardtothepastTDC.Withtheexhaustvalveclosingat17ATDC,thedurationoftheexhauststrokeis248.
Itisapparentfromthisdescriptionthattheexhaustvalvestaysopenforashortperiodoftimeduringwhichtheintakevalveisalsoopen。
Inotherwords,theendoftheexhauststrokeandthebeginningoftheintakestrokeoverlapforashortperiodoftime。
Thisiscalledvalveoverlap.Valvetimingandvalveoverlapvaryondifferentengines。
OpeningtheintakevalvebeforeTDCandclosingitafterBDCincreasesthefillofair-fuelmixtureinthecylinder。
Openingtheintakevalveearlyhelpsovercomethestaticinertiaoftheair-fuelmixtureatthebeginningoftheintakestroke,whileleavingtheintakevalveopenafterBDCtakesadvantageofthekineticinertiaofthemovingair-fuelmixture。
Thisincreasesvolumetricefficiency.
Asthepistonmovesdownonthepowerstrokepastthe90ATDCposition,pressureinthecylinderhasdropped,andtheleveragetothecrankshafthasdecreasesduetoconnectingrodangleandcrankshaftposition.Thisendstheeffectivelengthofthepowerstoke,andtheexhaustvalvecannowbeopenedtobeginexpellingtheburnedgases。
TheexhaustvalveremainsopenuntilthepistonhasmoveduppasttheTDCposition.Thishelpstoremoveasmuchoftheburnedgasesasispossibleandincreasesvolumetricefficiency。
2。
5。
4CamDesignandControlDynamics
Thefunctionofthecamistoopenandclosethevalveasfaraspossible,asfastaspossibleandassmoothlyaspossible。
Theclosingforceforthevalveisappliedbythevalvespringwhichalsomaintaincontactbetweenthecamandthevalves。
Dynamicsforceimposelimitsoncamandvalveslift.
Theentirevalve-trainassemblycanbeviewedasaspringsysteminwhichtheconversionfromstoredtofreeenergycausesforcedvibration.Valve-trainassemblieswithoverheadcamshaftcanberepresentedwithsufficientaccuracybya1-masssystem。
Forsystemwithbottom—mountedcamshaftsandpushrods,a2-masssystemisbeingincreasinglyused。
Themaximumpermissiblecontactstress,usuallyregardedastheparameterwhichlimitscam-loberadiusandtherateofopeningontheflank,currentlyliesbetween600—750MPadependinguponthematerialpairingused.
2.5.5CamshaftDriveMechanism
Eachcammustrevolveonceduringthefour—strokecycletoopenavalve。
Acycle,remember,correspondswithtworevolutionofthecrankshaft.Therefore,thecamshaftmustrevolveatexactlyhalfthespeedofthecrankshaft。
Thisisaccomplishedwitha2:
1gearratio。
Agreatconnectedtothecamshafthastwicethenumberofteethasagearconnectedtothecrankshaft。
Thegearsarelinkedinoneofthreeways.
1.BeltDrive
Acog-typebeltcanbeused.Suchbeltsaremadeofsyntheticrubberandreinforcedwithinternalsteelorfiberglassstrands。
Thebeltshaveteeth,orslottedspacestoengageanddriveteethongearwheels。
Abelttypicallyisusedonengineswithoverhead-camvalvetrain。
2.ChainDrive
Onsomeengines,ametalchainisusedtoconnectthecrankshaftandcamshaftgears.Mostpush—rodenginesandsomeOHCengineshavechains。
3。
GearDrive
Thecamshaftandcamshaftgearscanbeconnecteddirectly,ormeshed。
Thistypeofoperatinglinkagecommonlyisusedonoldersix—cylinder,inlineengines。
Acamshaftdrivenbyachainsorbeltturnsinthesamedirectionasthecrankshaft.Butacamshaftdrivendirectlybythecrankshaftgearturnsintheoppositedirection.Timingbeltsareusedbecausetheycostlessthanchainsandoperatemorequietly。
Atypicaltimingbeltismadeofneoprenereinforcewithfiberglass.
2.5。
6ElectronicValveControlSystem
Anelectronicvalvecontrolsystemreplacethemechanismcamshaft,controllingeachvalvewithactuatorsforindependentvalvetiming。
TheEVCsystemcontrolstheopeningandclosingtimeandliftamountofeachintakeandexhaustvalveintoindependentcontrolledactuatorvalvesprovidesahugeamountofflexibilityinenginecontrolstrategy。
VehicleutilizingEVCcanrealizeseveralbenefitsincluding:
1)increasesenginepowerandfueleconomy。
2)allowscentralizedanddistributedEVCsystemstoperformattheirfullpotential。
3)adaptstoenginesofvariedcylindercounts。
Withalloftheimprovedefficienciesandconsumerbenefits,automanufacturersareeagertogettheirfirstEVCsystemontheirroad。
TheEVCsystemistargetedtooperatedintemperatureupto125,whiletheactuatoristargetedtorunupto6000r∕min.Theactuatorcanbecontrolledinacentralizedsystemwithahigh-speedmultiplexbusorinadistributedsystemwithanormalspeedbus。
EVCsystemmustbecompactinsize,specificallytheactuatorsthatmustbesmallenoughtofitintheenginespace.Avehiclethatusesa42VsystemisidealforEVCbecauseitrequireshighvoltagetocontrolthevalveactuators,andEVCistargetedforV8andV12engines.TheEVCsystemisalsohighlyflexible,allowingadaptabilityforanumberofcylinderengines.
2.6GasolineFuelSystem
2。
6.1Gasoline
Gasolineisdistilledfromcrudepetroleum。
Gasolineishighlyflammable,meaningitburnseasilyinthepresenceofair.
Gasolinemustvaporizeeasily.Thischaracteristic,calledvolatility,isimportant。
However,itmustnotvaporizetooeasily,oritwillturntovaporinsidethefueltankorfuellines.Insidethefuelline,fuelvapormayblocktheflowofliquidgasoline。
Thisiscalledvaporlock.Vaporlockiscommoninfuellineswheretheinletsideofthepumpisexposedtohightemperature.
Thefla
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