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.中文翻譯四行程發(fā)動機(jī)和二行程發(fā)動機(jī)的工作過程發(fā)動機(jī)氣缸內(nèi)的工作過程，可以分為四個階段，或行程。行程涉及塞的運(yùn)動；活塞從某一限定位置到另一限定位置的動動稱為一行程?；钊\(yùn)動的上限稱為TDC（上止點(diǎn)），下限稱為BDC（下止點(diǎn)）。一個行程就是活塞從上止點(diǎn)到下止點(diǎn)，或下止點(diǎn)到上止點(diǎn)的運(yùn)動。換句話說，活塞每完成一個行程，就改變一次其運(yùn)動的方向。發(fā)動機(jī)氣缸中的全部工作過程分為四個行程的（或嗜曲軸旋轉(zhuǎn)兩周的），叫做四行程循環(huán)發(fā)動機(jī)。發(fā)動機(jī)的四個活塞行程是進(jìn)氣、壓縮、做功、排氣。進(jìn)氣行程：在進(jìn)氣行程中，進(jìn)氣門打開，活塞向下移動，可燃混合氣通過進(jìn)氣門進(jìn)入氣缸。適當(dāng)濃度可燃混合氣是由燃料系統(tǒng)和化油器提供的。壓縮行程：在活塞到達(dá)下止點(diǎn)時，或者是活塞下限時，活塞開始向上運(yùn)動。同時，進(jìn)氣門關(guān)閉，排氣門也關(guān)閉，所以這時的氣缸是封閉的。當(dāng)活塞向上運(yùn)動時（這時是由轉(zhuǎn)動的曲軸和連桿推動活塞），可燃混合氣被壓縮。當(dāng)活塞到達(dá)上止點(diǎn)時，可燃混合氣被壓縮到只有原體積的十分之一，甚至更少。當(dāng)油氣混合燃料被壓縮時，不僅氣缸里的壓力上升，可燃混合氣的溫度也隨之增加了。做功行程：當(dāng)活塞到達(dá)壓縮行程到上止點(diǎn)時，火花塞產(chǎn)生電火花。電火花是由點(diǎn)