机械类文献翻译发动机的分类和总体结构1.docx

机械类文献翻译发动机的分类和总体结构1.docx

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机械类文献翻译发动机的分类和总体结构1

外文翻译

1EngineClassificationandOverallMechanics

Theautomobileenginescanbeclassifiedaccordingto:

1.numberofcylinders;2.arrangementofcylinders;3.arrangementofvalves;4.typeofcooling;5.numberofcycles（twoorfour）;6.typeoffuelburned;7.typeofignition.

Theengineisthesourceofpowerthatmakesthewheelsgoaroundandthecarmove.theautomobileengineisaninternal-combustionenginebecausethefuel（gasoline）isburnedinsideit.theburningofgasolineinsidetheengineproduceshighpressureintheenginecombustionchamber.thishighpressureintheenginecombustionchamber.thishighpressureforcespistontomove,themovementiscarriedbyconnectingrodstotheenginecrankshaft.thecrankshaftisthusmadetorotate;therotarymotioniscarriedthroughthepowertraintothecarwheelssothattheyrotateandthecarmoves.

Theenginerequiresafuelsystemtosupplyitwithamixtureofairandfuel.thefuelsystemdoesthisbypumpingliquidgasolinefromtankintothecarburetoramixingdevicethatmixesthegasolinewithair.themixtureisdeliveredtoenginewhereitisburned.

Theenginealsoneedsacoolingsystem,thecombustionoftheair-fuelmixtureintheenginecreatesaveryhightemperature（ashighas2000to2700C）.thecoolingsystemtakeheatawayfromtheenginebycirculatingaliquidcoolant（watermixedwithantifreeze）betweentheengineandaradiator.thecoolantgetshotasitgoesthroughtheengine.itcoolsoffasitgoesthroughtheradiator.Thus,thecoolantcontinuallytakesheatawayfromtheengine,whereitcoulddodamage,anddeliversittotheradiator.Airpassingthoughtheradiatortakesheatawayfromtheradiator.

Theenginealsoincludesalubricatingsystem.Thepurposeofthelubricatingsystemistosupplyallmovingpartsinsidetheenginewithlubricatingoil;theoilkeepsmovingpartsfromwearingexcessively.

Theenginerequiresafourthsystem,theignitionsystem.Theignitionsystemprovideshigh-voltageelectricsparksthatignite,orsetfireto,thechargesofair-fuelmixtureintheenginecombustionchambers.

Thefifthisstartingsystemanditspurposeistochangetheelectricalcurrentintothemechanicalenergytopushthecrank-shaftaround.Bymeansofthis,theenginecanbestarted.

2Four–stage–engineOperation

Theactiontakingplaceintheenginecylindercanbedividedintofourstages,orstrokes.Storereferstopistonmovement;astrokeoccurswhenthepistonmovesfromonelimitingpositiontotheother.TheupperlimitofpistonmovementiscalledTDC（topdeadcenter）.ThelowerlimitofpistonmovementiscalledBDC（bottomdeadcenter）.AstrokeispistonmovementfromTDCtoBDCorfromBDCtoTDC.Inotherwords,thepistoncompletesastrokeeachtimeitchangesitsdirectionofmotion.

Wheretheentireofeventsinthecylinderrequiresfourstrokes（ortwocrankshaftrevolution）,theengineiscalledafour-stroke-cycleengine,orafour-cycleengine.Thefourpistonstrokesareintake,compressionpower,andexhaust.

Intakestroke.Ontheintakestroke,theintakevalvehasopened,thepistonismovingdownward,andamixtureofairandvaporizedgasolineisenteringthecylinderthroughthevalveport.Themixtureofairandvaporizedgasolineisdeliveredtothecylinderbythefuelsystemandcarburetor.

Compressionstroke.AfterthepistonreachesBDC,orthelowerlimitofitstravel,itbeginstomoveupward.Asthishappens,theintakevalvecloses.Theexhaustvalveisalsoclosed,sothatthecylinderissealed.asthepistonmovesupward（pushednowbytherevolvingcrankshaftandconnectingrod）,theair-fuelmixtureiscompressed.BythetimethepistonreachesTDC,themixturehasbeencompressedtoaslittleasone-tenthofitsoriginalvolume,orevenless.Thiscompressionoftheair-fuelmixtureincreasesthepressureinthecylinder.Whentheair-fuelmixtureiscompressed,notonlydoesthepressureinthecylindergoup,butthetemperatureofthemixturealsoincreases.

Powerstroke.AsthepistonreachesTDConthecompressionstroke,anelectricsparkisproducedatthesparkplug.Theignitionsystemdeliversahigh-voltagesurgeofelectricitytothesparkplugtoproducethespark..Thesparkignites,orair-fuelmixture.Itnowbeginstoburnveryrapidly,andthecylinderpressureincreasestoasmuchas3-5MPaorevenmore.Thisterrificpushagainstthepistonforcesitdownward,andapowerimpulseistransmittedthroughtheconnectingrodtothecrankpinonthecrankshaft.thecrankshaftisrotatedasthepistonispusheddownbythepressureaboveit.

Exhauststroke.AsthepistonreachesBDCagain,theexhaustvalveopens.Now,asthepistonmovesupontheexhauststroke,itforcestheburnedgasesoutofthecylinderthroughtheexhaust-valveport.Then,whenthepistonreachesTDC,theexhaustvalveclosesandtheintakevalveopens.Nowafreshchargeofair-fuelmixturewillbedrawnintothecylinderasthepistonmovesdownagaintowardBDC.Theabovefourstrokesarecontinuouslyrepeated.

3Two–stage-engineOperation

Inthefour-stroke-cycleengine,alreadydiscussedinLesson1,2,thecompletecycleofeventsrequiresfourpistonstrokes（intake,compression,powerandexhaust）.Inthetwo-stroke-cycle,ortwo-cycle,engine,theintakeandcompressionstrokesandthepowerandexhauststrokesareinasensecombined.Thispermitstheenginetoproduceapowerstrokeeverytwopistonstrokes,ofeverycrankshaftrotation.

Inthetwo-cycleengine,thepistonactsasavalve,clearingvalveportsinthecylinderwallasitnearsBDC.

Afreshair-fuelchargeentersthroughtheintakeport,andtheburnedgasesexitthroughtheexhaustport.Thecompletecycleofoperationisasfollows:

asthepistonnearsTDC,ignitiontakesplace.Thehighcombustionpressuresdrivethepistondown,andthetrustthroughtheconnectingrodturnsthecrankshaft.AsthepistonnearsBDC,itpassestheintakeandexhaustportsinthecylinderwall.Burnedgases,stillundersomepressure,begintostreamoutthroughtheexhaustport.Atthesametime,theintakeport,nowclearedbythepiston,beginstodeliverair-fuelmixture,underpressuretothecylinder.Thetopofthepistonisshapedtogivetheincomingmixtureanupwardmovement.Thishelpstosweeptheburnedgasesaheadandoutthroughtheexhaustport.

AfterthepistonhaspassedthroughBDCandstartsupagain,itpassesbothports,thussealingthemoff.Nowthefreshair-fuelchargeabovethepistoniscompressedandignited.Thesameseriesofeventstakesplaceagainandcontinueaslongastheengineruns.

Wementionedthattheair-fuelmixtureisdeliveredtothecylinderunderpressure.Inmostengines,thispressureisputonthemixtureinthecrankcase.Thecrankcaseissealedexceptforaleaf,orreed,valveatthebottom.Thereedvalveisaflexibleflatmetalplatethatrestssnuglyagainstthefloorofthecrankcase.Thereareholesunderthereedvalvethatconnecttotheenginecarburetor.Whenthepistonismovingup,apartialvacuumisproducedinthesealedcrankcase.Atmosphericpressureliftsthereedvalveofftheholes,andair-fuelmixtureentersthecrankcase.AfterthepistonpassesTDCandstartsdownagain,pressurebeginstobuildupinthecrankcase.Thispressureclosesthereedvalvesothatfurtherdownwardmovementofthepistoncompressesthetrappedair-fuelmixtureinthecrankcase.Thepressurewhichisbuiltupontheair-fuelmixturethencausesittoflowupthroughtheintakeportintotheenginecylinderwhenthepistonmovesdownfarenoughtocleartheintakeport.

Thetwo-strokeengineisnotonlyverysimplebutgivesnearlytwicethepowerofafourstrokeenginefromacylinderofgivensize,butitiswastefulofgasoline,assomemixtureinevitablyfindsitswayintotheexhaustsystemonthecombinesintake/exhauststroke,andtherearealwayssomecombustionproductsleftinthecylinderwhichreducetherapidburningofthefuel.Thiskindofengineisalwaysusedinmotorcycles.

4EngineCylinderBlockCrankcase

wehaveseenhowthemixtureofairandfuelisdeliveredbythefuelsystemtotheenginecylinder,whereitiscompressed,ignited,andburned.Wehavenotedthatthiscombustionproducesahighpressurethatpushesthepistondownsothatthecrankshaftisrotated.Nowletusexaminethevariouspartsoftheengineindetail.

Enginecylinderblock.

Thecylinderblockofliquid-cooledenginesformsthebasicframeworkoftheengine.Otherpartsareattachedtothecylinderblockorareassembledinit.Theblockiscastinonepiecefromgrayironorironalloyedwithothermetals,suchasnickelorchromium.Someblocksarecastfromaluminum.Theblockcontainsnotonlythecylindersbutalsothewaterjacketsthatsurroundthem.Inaluminumblocks,cast-ironofsteelcylindersleeves（alsocalledboreliners）areused.Theremetalshavebetterwearingqualitiesthanaluminumandcanbetterwithstandthewearingeffectofthepistonsandringsmovingupanddowninthecylinders.Formostengines,castironhasbeenfoundtobeasatisfactorycylinder-wallmaterial.However,insomesmallengines,thecylinderwallsareplatedwithchromium,averyhardmetal,toreducewallwearandlengthentheirlife.

CylinderHead.

Thecylinderheadisusuallycastinonepiecefromiron,fromironalloyedwithothermetals,orfromaluminumalloy.Aluminumhastheadvantageofcombiningheadtendstoruncooler,otherfactorsbeingequal.Therearetwotypesofhead,LheadandIhead.Cylinderheadcontainswaterjacketsforcooling;intheassembledengine,cylinder-blockwaterjackets.Spark-plugholesareprovided,alongwithpocketsintowhichth