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英文文摘Four-stage-engineandTwo-stage-engineOperationTheactionstakingplaceintheenginecylindercanbedividedintofourstages,orstrokes.“Stroke”referstopistonmovement;astrokeoccurswhenthepistonmovesfromonelimitingpositiontotheother.TheupperlimitofpistonmovementiscalledTDC(topdeadcenter).ThelowerlimitofpistonmovementiscalledBDC(bottomdeadcenter).AstrokeispistonmovementfromTDCtoBDCorfromBDCtoTDC.Inotherwords,thepistoncompletesastrokeeachtimeitchangesitsdirectionofmotion.Wheretheentirecycleoreventsinthecylinderrequiresfourstrokes(ortwocrankshaftrevolutions),theengineiscalledafour-stroke-cycleengine,orafour-cycleengine.Thefourpistonstrokesareintake,compression,power,andexhaust.Intakestroke.Ontheintakestroke,theintakevalvehasopened,thepistonismovingdownward,andamixtureofairandvaporizedgasolineisenteringthecylinderthroughthevalveport.Themixtureofairandvaporizedgasolineisdeliveredtothecylinderbythefuelsystemandcarburetor.Compressionstroke.AfterthepistonreachesBDC,orthelowerlimitofitstravel,itbeginstomoveupward.Asthishappens,theintakevalvecloses.Theexhaustvalveisalsoclosed,sothatthecylinderissealed.Theexhaustvalveisalsoclosed,sothatthecylinderissealed.Asthepistonmovesupward(pushednowbytherevolvingcrankshaftandconnectingrod),theair-fuelmixtureiscompressed.BythetimethepistonreachesTDC,themixturehasbeencompressedtoaslittleasonetenthofitsoriginalvolume,orevenless.Thiscompressionoftheair-fuelmixtureincreasesthepressured,notonlydoesthepressureinthecylindergoup,butthetemperatureofthemixturealsoincreases.Powerstroke.AsthepistonreachesTDConthecompressionstroke,anelectricsparkisproducedatthesparkplug.Theignitionsystemdeliversahigh-voltagesurgeofelectricitytothesparkplugtoproducethespark.Thesparkignites,orsetsfireto,theair-fuelmixture.Itnowbeginstoburnveryrapidly,andthecylinderpressureincreasestoasmuchas3-5MPaorevenmore.Thisterrificpushagainstthepistonforcesitdownward,andapowerimpulseistransmittedthroughtheconnectingrodtothecrankpinonthecrankshaft.Thecrankshaftisrotatedasthepistonispusheddownbythepressureaboveit.Exhauststroke.AsthepistonreachesBDCagain,theexhaustvalveopens.Now,asthepistonmovesupontheexhauststroke,itforcestheburnedgasesortofthecylinderthroughtheexhaust-valveport.Then,whenthepistonreachesTDC,theexhaustvalveclosesandtheintakevalveopens.Now,afreshchargeofair-fuelmixturewillbedrawnintothecylinderasthepistonmovesdownagainto-wardBDC,Theabovefourstrokesarecontinuouslyrepeated.Inthefour-stroke-cycleengine,alreadydiscussedinLesson1、2,thecompletecycleofeventsrequiresfourpistonstrokes(intake,compression,power,andexhaust).Inthetwo-stroke-cycle,ortwo-cycle,engine,theintakeandcompressionstrokesandthepowerandexhauststrokesareinasensecombined.Thispermitstheenginetoproduceapowerstrokeeverytwopistonstrokes,oreverycrankshaftrotation.Inthetwo-cycleengine,thepistonactsasavalve,clearingvalveportsinthecylinderwallasitnearsBDC.Afreshair-fuelchargeentersthroughtheintakeport,andtheburnedgasesexitthroughtheexitthroughtheexhaustport.Thecompletecycleofoperationisasfollows:AsthepistonnearsTDC,ignitiontakesplace.Thehighcombustionpressuresdrivethepistondown,andthethrustthroughtheconnectingrodturnsthecrankshaft.AsthepistonnearsBDC,itpassestheintakeandexhaustportsinthecylinderwall.Burnedgases,stillundersomepressure,begintostreamortthroughtheexhaustport.Atthesametime,theintakeport,nowclearedbythepiston,beginstodeliverair-fuelmixture,underpressure,tothecylinder.Thetopofthepistonisshapedtogivetheincomingmixtureanupwardmovement.Thishelpstosweeptheburnedgasesaheadandoutthroughtheexhaustport.AfterthepistonhaspassedthroughBDCandstartsupagain,itpassesbothports,thussealingthemoff.Nowthefreshair-fuelchargeabovethepistoniscompressedandignited.Thesameseriesofeventstakesplaceagainandcontinueaslongastheengineruns.Wementionedthattheair-fuelmixtureisdeliveredtothecylinderunderpressure.Inmostengines,thispressureisputonthemixtureinthecrankcase.Thecrankcaseissealedexceptforaleaf,orreed,valveatthebottom.Thereedvalveisaflexible,flatmetalplatethatrestssnuglyagainstthefloorofthecrankcase.Thereareholesunderthereedvalvethatconnecttotheenginecarburetor.Whenthepistonismovingup,apartialvacuumisproducedinthesealedcrankcase.Atmosphericpressureliftsthereedvalveofftheholes,andair-fuelmixtureentersthecrankcase.AfterthepistonpassesTDCandstartsdownagain,pressurebeginstobuildupinthecrankcase.Thispressureclosesthereedvalvesothatfurtherdownwardmovementofthepistoncompressesthetrappedair-fuelmixtureinthecrankcase.Thepressurewhichisbuiltupontheair-fuelmixturethencausesittoflowupthroughtheintakeportintotheenginecylinderwhenthepistonmovesdownfarenoughtocleartheintakeport.Thetwo-strokeengineisnotonlyverysimplebutgivesnearlytwicethepowerofafourstrokeenginefromacylinderofgivensize,butitiswastefulofgasoline,assomemixtureinevitablyfindsitswayintotheexhaustsystemonthecombinesintake/exhauststroke,andtherearealwayssomecombustionproductsleftinthecylinderwhichreducetherapidburningofthefuel.Thiskindofengineisalwaysusedinmotorcycles.中文翻译四行程发动机和二行程发动机的工作过程发动机气缸内的工作过程,可以分为四个阶段,或行程。行程涉及塞的运动;活塞从某一限定位置到另一限定位置的动动称为一行程。活塞运动的上限称为TDC(上止点),下限称为BDC(下止点)。一个行程就是活塞从上止点到下止点,或下止点到上止点的运动。换句话说,活塞每完成一个行程,就改变一次其运动的方向。发动机气缸中的全部工作过程分为四个行程的(或嗜曲轴旋转两周的),叫做四行程循环发动机。发动机的四个活塞行程是进气、压缩、做功、排气。进气行程:在进气行程中,进气门打开,活塞向下移动,可燃混合气通过进气门进入气缸。适当浓度可燃混合气是由燃料系统和化油器提供的。压缩行程:在活塞到达下止点时,或者是活塞下限时,活塞开始向上运动。同时,进气门关闭,排气门也关闭,所以这时的气缸是封闭的。当活塞向上运动时(这时是由转动的曲轴和连杆推动活塞),可燃混合气被压缩。当活塞到达上止点时,可燃混合气被压缩到只有原体积的十分之一,甚至更少。当油气混合燃料被压缩时,不仅气缸里的压力上升,可燃混合气的温度也随之增加了。做功行程:当活塞到达压缩行程到上止点时,火花塞产生电火花。电火花是由点
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