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外文翻译---四行程发动机和二行程发动机的工作过程.doc

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外文翻译---四行程发动机和二行程发动机的工作过程.doc

英文文摘FourstageengineandTwostageengineOperationTheactionstakingplaceintheenginecylindercanbedividedintofourstages,orstrokes.Strokereferstopistonmovementastrokeoccurswhenthepistonmovesfromonelimitingpositiontotheother.TheupperlimitofpistonmovementiscalledTDC(topdeadcenter).ThelowerlimitofpistonmovementiscalledBDCbottomdeadcenter.AstrokeispistonmovementfromTDCtoBDCorfromBDCtoTDC.Inotherwords,thepistoncompletesastrokeeachtimeitchangesitsdirectionofmotion.Wheretheentirecycleoreventsinthecylinderrequiresfourstrokesortwocrankshaftrevolutions,theengineiscalledafourstrokecycleengine,orafourcycleengine.Thefourpistonstrokesareintake,compression,power,andexhaust.Intakestroke.Ontheintakestroke,theintakevalvehasopened,thepistonismovingdownward,andamixtureofairandvaporizedgasolineisenteringthecylinderthroughthevalveport.Themixtureofairandvaporizedgasolineisdeliveredtothecylinderbythefuelsystemandcarburetor.Compressionstroke.AfterthepistonreachesBDC,orthelowerlimitofitstravel,itbeginstomoveupward.Asthishappens,theintakevalvecloses.Theexhaustvalveisalsoclosed,sothatthecylinderissealed.Theexhaustvalveisalsoclosed,sothatthecylinderissealed.Asthepistonmovesupwardpushednowbytherevolvingcrankshaftandconnectingrod,theairfuelmixtureiscompressed.BythetimethepistonreachesTDC,themixturehasbeencompressedtoaslittleasone–tenthofitsoriginalvolume,orevenless.Thiscompressionoftheairfuelmixtureincreasesthepressured,notonlydoesthepressureinthecylindergoup,butthetemperatureofthemixturealsoincreases.Powerstroke.AsthepistonreachesTDConthecompressionstroke,anelectricsparkisproducedatthesparkplug.Theignitionsystemdeliversahighvoltagesurgeofelectricitytothesparkplugtoproducethespark.Thesparkignites,orsetsfireto,theairfuelmixture.Itnowbeginstoburnveryrapidly,andthecylinderpressureincreasestoasmuchas35MPaorevenmore.Thisterrificpushagainstthepistonforcesitdownward,andapowerimpulseistransmittedthroughtheconnectingrodtothecrankpinonthecrankshaft.Thecrankshaftisrotatedasthepistonispusheddownbythepressureaboveit.Exhauststroke.AsthepistonreachesBDCagain,theexhaustvalveopens.Now,asthepistonmovesupontheexhauststroke,itforcestheburnedgasesortofthecylinderthroughtheexhaustvalveport.Then,whenthepistonreachesTDC,theexhaustvalveclosesandtheintakevalveopens.Now,afreshchargeofairfuelmixturewillbedrawnintothecylinderasthepistonmovesdownagaintowardBDC,Theabovefourstrokesarecontinuouslyrepeated.Inthefourstrokecycleengine,alreadydiscussedinLesson1、2,thecompletecycleofeventsrequiresfourpistonstrokesintake,compression,power,andexhaust.Inthetwostrokecycle,ortwocycle,engine,theintakeandcompressionstrokesandthepowerandexhauststrokesareinasensecombined.Thispermitstheenginetoproduceapowerstrokeeverytwopistonstrokes,oreverycrankshaftrotation.Inthetwocycleengine,thepistonactsasavalve,clearingvalveportsinthecylinderwallasitnearsBDC.Afreshairfuelchargeentersthroughtheintakeport,andtheburnedgasesexitthroughtheexitthroughtheexhaustport.ThecompletecycleofoperationisasfollowsAsthepistonnearsTDC,ignitiontakesplace.Thehighcombustionpressuresdrivethepistondown,andthethrustthroughtheconnectingrodturnsthecrankshaft.AsthepistonnearsBDC,itpassestheintakeandexhaustportsinthecylinderwall.Burnedgases,stillundersomepressure,begintostreamortthroughtheexhaustport.Atthesametime,theintakeport,nowclearedbythepiston,beginstodeliverairfuelmixture,underpressure,tothecylinder.Thetopofthepistonisshapedtogivetheincomingmixtureanupwardmovement.Thishelpstosweeptheburnedgasesaheadandoutthroughtheexhaustport.AfterthepistonhaspassedthroughBDCandstartsupagain,itpassesbothports,thussealingthemoff.Nowthefreshairfuelchargeabovethepistoniscompressedandignited.Thesameseriesofeventstakesplaceagainandcontinueaslongastheengineruns.Wementionedthattheairfuelmixtureisdeliveredtothecylinderunderpressure.Inmostengines,thispressureisputonthemixtureinthecrankcase.Thecrankcaseissealedexceptforaleaf,orreed,valveatthebottom.Thereedvalveisaflexible,flatmetalplatethatrestssnuglyagainstthefloorofthecrankcase.Thereareholesunderthereedvalvethatconnecttotheenginecarburetor.Whenthepistonismovingup,apartialvacuumisproducedinthesealedcrankcase.Atmosphericpressureliftsthereedvalveofftheholes,andairfuelmixtureentersthecrankcase.AfterthepistonpassesTDCandstartsdownagain,pressurebeginstobuildupinthecrankcase.Thispressureclosesthereedvalvesothatfurtherdownwardmovementofthepistoncompressesthetrappedairfuelmixtureinthecrankcase.Thepressurewhichisbuiltupontheairfuelmixturethencausesittoflowupthroughtheintakeportintotheenginecylinderwhenthepistonmovesdownfarenoughtocleartheintakeport.Thetwostrokeengineisnotonlyverysimplebutgivesnearlytwicethepowerofafourstrokeenginefromacylinderofgivensize,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。电火花是由点火系统向火花塞提供高压电脉冲而产生的。电火花点燃可燃混合气。可燃混合气开始发生剧烈燃烧,气缸内压力达到35兆帕,甚至更高。作用于活塞上强大的推动力推动活塞下运动,并将这一推力通过连杆传到曲轴上的连杆轴颈上。因此,当活塞受压向下动动时,推动曲轴转动。排气行程当活塞受压向下运动时,推动曲轴转动。排气行程当活塞再一次到达下止点时,排气门打开。同时,活塞向上移动,把废气经排气门排出气缸。随后活塞达到上止点,排气门关闭,进气门打开。当活塞又一次向下移动到达下止点时,新鲜可燃混合气被吸入气缸,上述的四个行程又继续重复。四行程发动机整个工作循环需要活塞完成进气、压缩、作功、排气四个行程。在二行程发动机中,进气和压缩行以及作功和排气行分别合并在一起的,这使得二行程发动机每两个行程,即曲轴旋转一周,就完成一次工作循环。在二行程发动机中,活塞起到阀门的作用,当活塞接近下止点时,使气缸壁上的进排气口打开。新的可燃混合气通过进气口进入,燃烧过的废气从排气口排出。整个工作过程如下当活塞接近上止点时,发动机点火。燃烧产生的高压驱动活塞向下运动,通过连杆,推动曲轴转动。当活塞接近下止点时,以过气缸壁上的进排气口,燃烧过的气体,仍有剩余压力,开始从排气口喷出。同时,活塞使进气口露出,受压缩的可燃混合气开始进气缸。活塞顶部的形状使进入气缸的可燃混合气向上运动,这样有助于扫气,引导燃烧过的废气向上运动并从排气口排出。

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