汽车专业外文翻译5Word文档下载推荐.docx
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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.
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