汽车专业外文翻译5Word文档下载推荐.docx

汽车专业外文翻译5Word文档下载推荐.docx

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,ofafuelandconvertsthisheatintousefulworkfordrivingthecar.

Thefuelusedinthevastmajorityofcarenginesispetrol,whichisoneofthemanyproductsobtainedfromcrudeoilfoundintheearth.Petrol,whenmixedwiththerightamountofair,willburnwhenaflameorsparkisappliedtoit.

Inthecarengine,airmixedwithpetrolistakenintoaconfinedspaceandcompressed.Themixtureisthenignitedanditburns.Inburningitheatstheair,whichexpands,andtheforceofexpansionisthenconvertedintoarotarymovementtodrivethewheelsofthecar.

Tobeabletousethisenergyeffectivelywehavetocontroltheburningorcombustionprocessandtheforceofexpansion.Firstly,weneedatube,or'

cylinder'

closedatoneend,inwhichtocompressandburnthepetrolandairmixture.Thenweneedapistonwhichcanslidefreelyinthecylinder,andwhichcanbedrivenoutwardsbythe

forceofexpansion.Toconverttheoutwardmovementofthepistonintoarotarymovementwemustjoinitbyaconnectingrodstoacrankshaft.Weneedonepassagefortheentryofthemixtureintothecylinderandanothertoletouttheusedgases.Tocontroltheentryofthemixtureandtheexhaustofthegasesweneedvalves,andthesearecalledtheinletandexhaustvalves.Finally,weneedsomemeansofignitingthemixtureinthetopofthecylinder,thepartcalledthecombustionchamber;

andforthisweuseasparkingplug.

Bytimingtheopeningandclosingofthevalvesandbytimingthearrivalofthesparkwecancontrolthewholesequenceofeventsandmakethepistonmoveinandoutoverandoveragain.

THECYLINDERS

Motor-carenginesmayhavefour,six,oreightcylinders.Lookatfigure1.Thesecylindersareusuallymountedinacylinderblockontopoftheengine.Beneaththecylinderblockisthecrankcase,whichcontainstwoshafts,thecrankshaftandthecamshaft.Asyouhaveread,thecrankshaftisrevolvedbytheoutwardmovementofthepistonsinthecylinders.Thisrotarymovementofthecrankshafttransmitsthepowerdevelopedbytheenginethroughthegearboxtothedrivingwheelsandsetsthecarinmotion.

Whenthecrankshaftrotatesitalsocausestherotationofthecamshaft,whichliesalongsideitinthecrankcase.Asthecamshaftrotates,itpushesuprodsalongsideeachcylindertoopenandshutthevalvesatthetopofthecylinder.

Therearetwovalvestoeachcylinder.Theinletvalveletsairandpetrolintothecombustionchamberofthecylinderwhenitisopened.Whentheexhaustvalveisopenedthegasesformedafterthecombustioninthechamberareallowedtoescape.Thesegasesareledawayfromthecarthroughanexhaustpipe.

Letusexaminetheactionofoneofthecylindersinmoredetail.Lookatfigure2.In（a）thepistonisnearthetopofthecylinderwiththeinletvalveopenandtheexhaustvalveclosed,Ifthecrankshaftisturned,eitherbyhandorbythestartermotor,thepistonisdrawndownbytheconnectingrodandachargeofpetrol-airmixturerushesin.Whenthepistonreachesthebottomofitsstroketheinletvalveisclosedbytheactionofaspring.Thisstrokeiscalledtheinductionstroke.

In（b）bothvalvesareclosedandthecrankshaftforcesthepistonuptocompressthemixtureinthetopofthecylinder.This,then,iscalledthe'

compression'

stroke.Towardstheendofthecompressionstrokeasparkfromthesparkingplugcausesthemixturetoignite.

In（c）weseethattheheatofcombustionhascausedarapidriseinpressureinthecombustionchamberandthishasforcedthepistondown.Throughtheconnectingrodthepistoncausesthecrankshafttocontinuetorotate.Thisstrokeiscalledthe'

expansion'

or'

power'

stroke,andwecansaynowthattheenginehas'

fired'

.Attheendof

thisstroke,asthecrankshaftrotates,causingthecamshaftalongsideittocontinuetorotate,oneofthecamsonthecamshaftpushesuptherod,whichcausestheexhaustvalvetoopen,allowingtheexhaustgasestoescape.

In（d）weseethefourthandfinalstroke,the'

exhaust'

stroke.Theexhaustvalvehasbeenforcedopenbytherotationofthecamshaft,andthecrankshaft,continuingtorotate,drivesthepistonbackupthecylinder,forcingouttheexhaustgases.Attheendofthisstroketheexhaustvalveisclosedbytheactionofaspring,andthecamshaft,continuingtorotate,pushesupasecondrodtoforcetheinletvalveopen.Nowthecylinderwillreceiveanotherchargeofpetrol-airmixture,andthesequenceoffourstrokes,'

induction'

'

and'

willstartagain.

Becausethereisafour-strokesequence,or'

cycle'

inthistypeofinternalcombustionengineitiscalledafour-strokeengine.Therearealsotwo一strokeenginesused,forexample,formotorscooters,andforsomemotorcycles.

Letusthinkofacarwithfourcylinders.Rememberthatitisonlyontheexpansionstrokethatpoweristransmittedtomakethecrankshaftrotate.Letusnumberthecylinders,1,2,3,4.Theymaytransmitpowerinthisorder.1,2,4,3.Thismeansthatwhennumber1cylinderisontheexpansionstroke,number2isonthecompressionstroke,number4isontheinductionstroke,andnumber3cylinderisontheexhauststroke.Thefourpistons,movingupanddowninsidetheircylindersinthisorder,pushdownrodsconnectedtothecrankshaftatdifferenttimesandatdifferentpointsalongitsshaft.Thiskeepsthecrankshaftrevolvingandthecrankshaft,whenthegearsareengaged,keepsthecarmoving.

2.TheFuelandIgnitionSystems

Thefuelsystemincludesafueltank,afuelpump,andacarburetor.

Thefuelpumpmaybeoperatedeithermechanicallyfromtheenginecamshaftorelectrically.Itsfunctionistopumppetrolfromthepetroltankanddeliverittothecarburetor.Itcontainsafinefillertoexcludeparticlesofdustordirtwhichmayhavefoundtheirwayintothetank.

Thecarburetorismountedontheinletpipe,orinlet'

manifold'

whichleadstotheinletvalvesofeachcylinder.Itsfunctionisto'

carburet'

ormix,therequiredamountsofpetrolandairforcombustioninthecylinders.Itregulatesautomaticallytheproportionsofpetrolandairandalsoallowsthedrivertocontroltherateofdeliveryofthemixture,andsovarythespeedoftheengine.

Seefigure3.Thisisasimpletypeofcarburetor.Itmakesuseofafuelreservoiror"

floatchamber"

towhichpetrolispumpedbythepetrolpump.Thelevelofthepetrolin.thechamberiskeptconstantbytheactionofafloatwhich,asitriseswiththepetrol,closesaneedlevalvewhenthecorrectlevelisreached.Thepetrolfromthefloatchamberflowstoasmalljetsituatedinanarrowtubecalledthechoketube.Whentheengineisrunning,i.e.whenthepistonsaremovingupanddowninthecylinders,airisdrawninandpassesthroughthechoketube.Hereitmixeswiththepetrolandpassesoutintotheinletmanifoldoftheengine.Theamountofmixtureallowedtopassiscontrolledbyabutterflyvalve,or’throttle'

situatedinthecarburetoroutlet.Thisvalveisoperatedbytheacceleratorpedal.Anaircleanermaybeconnectedtotheairinlettoremovedustfromtheairandpreventitgettingintothecylindersanddownintotheenginelubricatingoil,whereitcancauseincreasedenginewear.

Theproportionofairandpetrolrequiredatvaryingenginespeedsiscontrolledautomatically.Whenstartingacoldengineamuchrichermixture,onewithahigherproportionofpetroltoair,isrequired.Thisisobtainedbyrestrictingtheamountofairenteringthecarburetor.Inmostcarsthedriverdoesthisbypullingoutaknobcalledthechokeandthispartlyclosesthetubecarryingairtothecylinders.Thedrivermustremembertopushthechokeinagainassoonaspossibleafterstartingtoavoiddamagetotheengineandwasteoffuel.

THEIGNITIONSYSTEM

Thefunctionoftheignitionsystemistoprovidethesparkinthecombustionchambertoignitethemixtureofpetrolandairattherightinstant.Thesystemnormanycomprisesacoil,adistributor,andsparkingplugs.

Thecoilproducesthehigh-voltageimpulserequiredtomakethesparkatthesparkingplugs.Itreallyconsistsoftwocoilsofinsulatedwirewoundonearoundtheothersothatthenumberofturnsintheinner,'

secondary'

coilismuchgreaterthanthatintheouter,'

primary'

coil.

Alowvoltageissuppliedtotheprimarycoilandsuddenlyinterrupted.Atthatmomentanimpulseatverymuchhighervoltageisinducedinthesecondarycoil.Voltagesof6or12voltssuppliedtotheprimary,low一tension,coilresultinvoltagesof10,000voltsormorebeingobtainedfromthesecondary,hightension,coil.Whenthecarisstartedthelow-tensionsupplyisprovidedbythecar'

sbattery.

Rememberthatthelow-tensionsupplymustbebrokensuddenlytoproducethehigh-tensionsupplyfromthesecondarycoil.Thisisdoneinaninstrumentcalledthedistributor.Thedistributorisplacedbetweenthecoilandthesparkingplugs.

Insidethedistributorarecontactpoints.Thecontactbetweenthemisbrokenbyarevolvingcam,whichforcesthemapart.Thentheyareforcedtogetheragainbyaspringpressingagainstthearmholdingonecontact.Itiswhenthecontactpointsareforcedapartthatthelow一tensionsupply,goingthroughaleadtothecoil,issuddenlyinterrupted.

Asecondlead,goingfromthecoilbacktothedistributor,conveysthehigh-tension（H.T.）supplytothedistributorhead.FromthereotherleadsdistributetheH.T.supplytoeachofthesparkingplugsinturn.