车辆工程专业英语原文Word文档格式.docx
- 文档编号:19779712
- 上传时间:2023-01-10
- 格式:DOCX
- 页数:16
- 大小:31.50KB
车辆工程专业英语原文Word文档格式.docx
《车辆工程专业英语原文Word文档格式.docx》由会员分享,可在线阅读,更多相关《车辆工程专业英语原文Word文档格式.docx(16页珍藏版)》请在冰豆网上搜索。
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.
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 车辆 工程 专业 英语 原文