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GENERALCOMMENTSOFAUTOMOBILEENGINEENGINEISTHESOURCEOFAUTOMOTIVESOFAR,AUTOMOTIVEENGINESAREALLPOWEREDBYHEATEXCEPTFORAFEWOFAUTOMOTIVESDRIVEDBYELECTRICITYMODERNAUTOMOTIVEENGINESARECALLEDINTERNALCOMBUSTIONENGINESBECAUSEFUELBURNSINSIDETHEENGINETHEENGINECONVERTSTHEBURNINGFUELSTHERMALENERGYTOMECHANICALENERGYBYCOOLINGSYSTEMSLIQUIDCOOLEDENGINESANDAIRCOOLEDENGINESAREBEINGUSEDLIQUIDCOOLEDENGINESARETHEMOSTCOMMONINTHEDIESELINDUSTRYBYFUELSYSTEMGASOLINEDIESELANDPROPANEFUELSYSTEMSARECURRENTLYUSEDINAWIDEVARIETYOFENGINESBYIGNITIONMETHODGASENGINESUSETHESPARKELECTRICALIGNITIONDIESELENGINESUSETHEHEATFROBDCTOTDCITVARIESWITHCYLINDERBORESIZE,LENGTHOFPISTONSTROKE,ANDNUMBSYSTEMINJECTIONTHECALORYOFDIESELENGINECOMEFROMTHEFUELEMBLAZEDBYTHECOMPRESSEDAIRTHEDIESELENGINECOMPRESSIONRATIONISMUCHBIGGERTHANTHEGASENGINEITSSUFFICIENTCALORYISFROMTHEFUELBURNEDBYTHEPRESSEDAIRBYVALVEARRANGEMENTFOURTYPESOFVALVEARRANGEMENTSHAVEBEENUSEDINGASOLINEANDDIESELENGINESOFTHEFOURTYPESL,T,F,ANDIHEADS,THEIHEADISCOMMONLYUSEDONDIESELENGINESBYCYLINDERARRANGEMENTENGINEBLOCKCONFIGURATIONORCYLINDERARRANGEMENTDEPENDSONCYLINDERBLOCKDESIGNCYLINDERSMAYBEARRANGEDINASTRAIGHTLINEONEBEHINDTHEOTHERTHEMOSTCOMMONINLINEDESIGNSARETHEFOURANDSIXCYLINDERENGINESTHEVTYPEOFCYLINDERARRANGEMENTUSESTWOBANKSOFCYLINDERSARRANGEDINA60TO90VDESIGNTHEMOSTCOMMONEXAMPLESARETHOSEWITHTWOBANKSOFTHREETOEIGHTCYLINDERSEACHTHEOPPOSEDENGINEUSESTWOBANKSOFCYLINDERSOPPOSITEEACHOTHERWITHTHECRANKSHAFTINBETWEENENGINECLASSIFICATIONACCORDINGTOTHEDIFFERENCESOFTHEPISTONMOVEMENT,THEPISTONINTENALCOMBUSITIONENGINEWILLBECLASSIFIEDRECIPROCATINGINTENALCOMBUSITIONENGINEANDROTARYPISTONINTENALCOMBUSITIONENGINETHENWEWILLINTRODUCEWORKINGPRINCIPLEDIAGRAMOFRECIPROCATINGINTERNALCOMBUSTIONENGINEEXCEPTFORTHEWANKELROTARY,ENGINE,ALLPRODUCTIONAUTOMOTIVEENGINESARETHERECIPROCATING,ORPISTON,DESIGNRECIPROCATINGMEANS“UPANDDOWN“OR“BACKANDFORTH“ITISTHISUPANDDOWNACTIONOFAPISTONINACYLINDERTHATGIVESTHERECIPROCATINGENGINEITSNAMEALMOSTALLENGINESOFTHISTYPEAREBUILTUPONACYLINDERBLOCK,ORENGINEBLOCKTHEBLOCKISANIRONORALUMINUMCASTINGTHATCONTAINSTHEENGINECYLINDERSTHETOPOFTHEBLOCKISCOVEREDWITHTHECYLINDERHEAD,WHICHFORMSTHECOMBUSTIONCHAMBERSTHEBOTTOMOFTHEBLOCKISCOVEREDWITHANOILPAN,OROILSUMPAMAJOREXCEPTIONTOTHISTYPEOFENGINEONSTRUCTIONISTHEAIRCOOLEDVOLKWAGENENGINEITISREPRESENTATIVEOFTHEHORIZONTALLYOPPOSEDAIRCOOLEDENGINESUSEDBYPORSCHE,CHEVROLETCORVAIR,ANDSOMEOTHERAUTOMOBILEMANUFACTURERSINYEARSPASTPOWERISPRODUCEDBYTHEINLINEMOTIONOFAPISTONINACYLINDERHOWEVER,THISLINEARMOTIONMUSTBECHANGEDTOROTATINGMOTIONTOTURNTHEWHEELSOFACARORTRUCKTHEPISTONISATTACHEDTOTHETOPOFACONNECTINGRODBYAPIN,CALLEDAPISTONPINORWRISTPINTHECONNECTINGRODTRANSMITSTHEUPANDDOWNMOTIONOFTHEPISTONTOTHECRANKSHAFT,WHICHCHANGESITTOROTATINGMOTIONTHECONNECTINGRODISMOUNTEDONTHECRANKSHAFTWITHLARGEBEARINGSCALLEDRODBEARINGSSIMILARBEARINGS,CALLEDMAINBEARINGS,AREUSEDTOMOUNTTHECRANKSHAFTINTHEBLOCKTHECRANKSHAFTCHANGESTHERECIPROCATINGMOTIONOFTHEPISTONSTOROTATINGMOTIONTHECOMBUSTIBLEMIXTUREOFGASOLINEANDAIRENTERSTHECYLINDERSTHROUGHVALVESAUTOMOTIVEENGINESUSEPOPPETVALVESTHEVALVESCANBEINTHECYLINDERHEADORINTHEBLOCKTHEOPENINGANDCLOSINGOFTHEVALVESISCONTROLLEDBYACAMSHAFTLOBESONTHECAMSHAFTPUSHTHEVALVESOPENASTHECAMSHAFTROTATESASPRINGCLOSESEACHVALVEWHENTHELOBEISNOTHOLDINGITOPENTHEMOSTCOMMONARRANGEMENTSOFENGINECYLINDERSANDVALVESAREDISCUSSEDLATERTHEBASICSINGLECYLINDERENGINECONSISTSOFACYLINDERENGINEBLOCK,AMOVABLEPISTONINSIDETHISCYLINDER,ACONNECTINGRODATTACHEDATTHETOPENDTOTHEPISTONANDATTHEBOTTOMTOTHEOFFSETPORTIONOFACRANKSHAFT,ACAMSHAFTTOOPERATETHETWOVALVESINTAKEANDEXHAUST,ANDACYLINDERHEADAFLYWHEELISATTACHEDTOONEENDOFTHECRANKSHAFTTHEOTHERENDOFTHECRANKSHAFTHASAGEARTODRIVETHECAMSHAFTGEARTHECAMSHAFTGEARISTWICEASLARGEASTHECRANKSHAFTGEARTHISDRIVESTHECAMSHAFTATHALFTHESPEEDOFTHECRANKSHAFTONFOURSTROKECYCLEENGINES,THECRANKSHAFTANDCAMSHAFTRUNATTHESAMESPEEDENERGYCONVERSIONTHEINTERNALCOMBUSTIONDIESELENGINEISADEVICEUSEDTOCONVERTTHECHEMICALENERGYOFTHEFUELINTOHEATENERGYANDTHENCONVERTTHISHEATENERGYINTOUSABLEMECHANICALENERGYTHISISACHIEVEDBYCOMBININGTHEAPPROPRIATEAMOUNTSOFAIRANDFUELANDBURNINGTHEMINANENCLOSEDCYLINDERATACONTROLLEDRATEAMOVABLEPISTONINTHECYLINDERISFORCEDDOWNBYTHEEXPANDINGGASESOFCOMBUSTIONTHEMOVABLEPISTONINCYLINDERISCONNECTEDTOTHETOPOFACONNECTINGRODTHEBOTTOMOFTHECONNECTEDRODISATTACHEDTOTHEOFFSETPORTIONISTRANSFERREDTOTHECRANKSHAFT,ASTHEPISTONISFORCEDDOWN,THISOFFSETPORTIONOFACRANKSHAFT,TOROTATETHERECIPROCATINGBACKANDFORTHORUPANDDOWNMOVEMENTOFTHEPISTONISCONVERTEDTOROTARYTURNINGMOTIONOFTHECRANKSHAFT,WHICHSUPPLIESTHEPOWERTODRIVETHEVEHICLEINGENERALANAVERAGEAIRFUELRATIOFORGOODCOMBUSTIONISABOUT15PARTSOFAIRTO1PARTOFFUELBYWEIGHTHOWEVER,THEDIESELENGINEALWAYSTAKESINAFULLCHARGEOFAIRSINCETHEREISNOTHROTTLEPLATEINMOSTSYSTEMS,BUTONLYASMALLPARTOFTHISAIRISUSEDATLOWORIDLEENGINESPEEDSAIRCONSISTSOFABOUT20PERCENTOXYGENWHILETHEREMAINING80PERCENTISMOSTLYNITROGENTHISMEANSTHAT,FOREVERYGALLONOFFUELBURNED,THEOXYGENIN9,000TO10,000GALLONSOFAIRISREQUIREDFOURSTROKECYCLEGASOLINEBYITSELFWILLNOTBURN,ITMUSTBEMIXEDWITHOXYGENAIRTHISBURNINGISCALLEDCOMBUSTIONANDISAWAYOFRELEASINGTHEENERGYSTOREDINTHEAIRFUELMIXTURETODOANYUSEFULWORKINANENGINE,THEAIRFUELMIXTUREMUSTBECOMPRESSEDANDBURNEDINASEALEDCHAMBERHERETHECOMBUSTIONENERGYCANWORKONTHEMOVABLEPISTONTOPRODUCEMECHANICALENERGYTHECOMBUSTIONCHAMBERMUSTBESEALEDASTIGHTLYASPOSSIBLEFOREFFICIENTENGINEOPERATIONANYLEAKAGEFROMTHECOMBUSTIONCHAMBERALLOWSPARTOFTHECOMBUSTIONENERGYTODISSIPATEWITHOUTADDINGTOTHEMECHANICALENERGYDEVELOPEDBYTHEPISTONMOVEMENTTHE4STROKEENGINEISALSOCALLEDTHEOTTOCYCLEENGINE,INHONOROFTHEGERMANENGINEER,DRNIKOLAUSOTTO,WHOFIRSTAPPLIEDTHEPRINCIPLEIN1876INTHE4STROKEENGINE,FOURSTROKESOFTHEPISTONINTHECYLINDERAREREQUIREDTOCOMPLETEONEFULLOPERATINGCYCLETWOSTROKESUPANDTWOSTROKESDOWNEACHSTROKEISNAMEDAFTERTHEACTIONITPERFORMSINTAKE,COMPRESSION,POWER,ANDEXHAUST1、INTAKESTROKEASTHEPISTONMOVESDOWN,THEVAPORIZED,MIXTUREOFFUELANDAIRENTERSTHECYLINDERPASTTHEOPENINTAKEVALVE2、COMPRESSIONSTROKETHEPISTONRETURNSUP,THEINTAKEVALVECLOSES,THEMIXTUREISCOMPRESSEDWITHINTHECOMBUSTIONCHAMBER,ANDIGNITEDBYASPARK3、POWERSTROKETHEEXPANDINGGASESOFCOMBUSTIONFORCETHEPISTONDOWNINTHECYLINDERTHEEXHAUSTVALVEOPENSNEARTHEBOTTOMOFTHESTROKE4、EXHAUSTSTROKETHEPISTONMOVESBACKUPWITHTHEEXHAUSTVALVEOPEN,ANDTHEBURNEDGASESAREPUSHEDOUTTOPREPAREFORTHENEXTINTAKESTROKETHEINTAKEVALVEUSUALLYOPENSJUSTBEFORETHETOPOFTHEEXHAUSTSTROKETHIS4STROKECYCLEISCONTINUOUSLYREPEATEDINEVERYCYLINDERASLONGASTHEENGINEREMAINSRUNNINGTWOSTROKECYCLETHETWOSTROKECYCLEDIESELENGINECOMPLETESALLFOUREVENTSINTAKE,COMPRESSION,POWER,ANDEXHAUSTINONEREVOLUTIONOFTHECRANKSHAFTORTWOSTROKESOFTHEPISTONASERIESOFPORTSOROPENINGSISARRANGEDAROUNDTHECYLINDERINSUCHAPOSITIONTHATTHEPORTSAREOPENWHENTHEPISTONISATTHEBOTTOMOFITSSTROKEABLOWERFORCESAIRINTOTHECYLINDERTHROUGHTHEOPENPORTSEXPELLINGALLREMAININGEXHAUSTGASESPASTTHEOPENEXHAUSTVALVESANDFILLINGTHECYLINDERWITHAIRTHISISCALLEDSCAVENGINGASTHEPISTONMOVESUP,THEEXHAUSTVALVESCLOSEANDTHEPISTONCOVERSTHEPORTSTHEAIRTRAPPEDABOVETHEPISTONISCOMPRESSEDTONCOVERSTHEPORTSTHEAIRTRAPPEDABOVETHEPISTONISCOMPRESSEDSINCETHEEXHAUSTVALVEISCLOSEDJUSTBEFORETHEPISTONREACHESTOPDEADCENTER,THEREQUIREDAMOUNTOFFUELISINJECTEDINTOTHECYLINDERTHEHEATGENERATEDBYCOMPRESSINGTHEAIRIGNITESTHEFUELALMOSTIMMEDIATELYCOMBUSTIONCONTINUESUNTILTHEFUELINJECTEDHASBEENBURNEDTHEPRESSURERESULTINGFROMCOMBUSTIONFORCESTHEPISTONDOWNWARDONTHEPOWERSTROKEWHENTHEPISTONISAPPROXIMATELYFALFWAYDOWN,THEEXHAUSTVALVESAREOPENED,ALLOWINGTHEEXHAUSTGASESTOESCAPEFURTHERDOWNWARDMOVEMENTUNCOVERSTHEINLETPORTS,CAUSINGFRESHAIRTOENTERTHECYLINDERANDEXPELTHEEXHAUSTGASESTHEENTIREPROCEDUREISTHENREPEATED,ASTHEENGINECONTINUESTORUNTHEDIFFERENCESOFTHETWOINTENALCOMBUSTIONENGINEITCOULDBEASSUMEDTHATATWOCYCLEENGINEWITHTHESAMENUMBEROFCYLINDERS,THESAMEDISPLACEMENT,COMPRESSIONRATIO,ANDSPEEDASAFOURCYCLEENGINEWOULDHAVETWICETHEPOWERSINCEITHASTWICEASMANYPOWERHOWEVER,THISISNOTTHECASE,SINCEBOTHTHEPOWERANDCOMPRESSIONSTROKESARESHORTENEDTOALLOWSCAVENGINGTOTAKEPLACETHETWOCYCLEENGINEALSOREQUIRESABLOWER,WHICHTAKESENGINEPOWERTODRIVEABOUT160DEGREESOUTOFEACH360DEGREESOFCRANKSHAFTROTATIONAREREQUIREDFOREXHAUSTGASEXPULSIONANDFRESHAIRINTAKESCAVENGINGINATWOCYCLEENGINEABOUT415DEGREESOFEACH720DEGREESOFCRANKSHAFTROTATIONINAFOURCYCLEENGINEAREREQUIREDFORINTAKEANDEXHAUSTTHESEFIGURESINDICATETHATABOUT445OFCRANKROTATIONISUSEDFORTHEPOWERPRODUCINGEVENTSINTHETWOCYCLEENGINE,WHILEABOUT59OFCRANKROTATIONISUSEDFORTHESEPURPOSESINTHEFOURCYCLEENGINEFRICTIONLOSSESARECONSEQUENTLYGREATERINTHEFOURCYCLEENGINEHEATLOSSES,HOWEVER,AREGREATERINTHETWOCYCLEENGINETHOUGHBOTHTHEEXHAUSTANDTHECOOLINGSYSTEMSINSPITEOFTHESEDIFFERENCES,BOTHENGINETYPESENJOYPROMINENTUSEWORLDWIDEENGINECONSTRUCTIONCYLINDERBLOCKTHECYLINDERBLOCKISCASTINONEPIECEUSUALLY,THISISTHELARGESTANDTHEMOSTCOMPLICATEDSINGLEPIECEOFMETALINTHEAUTOMOBILETHECYLINDERBLOCKISACOMPLICATEDCASTINGMADEOFGRAYIRONCASTIRONORALUMINUMITCONTAINSTHECYLINDERSANDTHEWATERJACKETSTHATSURROUNDTHEMTOMAKETHECYLINDERBLOCK,ASANDFORMCALLEDAMOLDISMADETHENMOLTENMETALISPOUREDINTOTHEMOLDWHENTHEMETALHASCOOLEDTHESANDMOLDISBROKENUPANDREMOVEDTHISLEAVESTHETOUGHCYLINDERBLOCKCASTINGTHECASTINGTHECASTINGISTHENCLEANEDANDMACHINEDTOMAKETHEFINISHEDBLOCKCYLINDERBLOCKSFORDIESELENGINESAREVERYSIMILARTOTHOSEFORSPARKIGNITIONENGINESTHEBASICDIFFERENCEISTHATTHEDIESELENGINECYLINDERBLOCKISHEAVIERANDSTRONGERTHISISBECAUSEOFTHEHIGHERPRESSURESDEVELOPEDINTHEDIESELENGINECYLINDERSSEVERALENGINESHAVEALUMINUMCYLINDERBLOCKSALUMINUMISRELATIVELYLIGHTMETAL,WEIGHINGMUCHLESSTHANCASTIRONALSO,ALUMINUMCONDUCTSHEATMORERAPIDLYTHANCASTSOFTTOUSEASCYLINDERWALLMATERIALITWEARSTOORAPIDLYTHEREFORE,ALUMINUMCYLINDERBLOCKSMUSTHAVECASTIRONCYLINDERLINERSORBECASTFROMANALUMINUMALLOYTHATHASSILICONPARTICLESINITSOMEMANUFACTURESMAKEANALUMINUMCYLINDERBLOCKTHATDOESNOTHAVECYLINDERLINERS,ORSLEEVESINSTEAD,THEALUMINUMISLOADEDWITHSILICONPARTICLESSILICONISAVERYHARDMATERIALAFTERTHECYLINDERBLOCKISCAST,THECYLINDERSAREHONEDTHENTHEYARETREATEDWITHACHEMICALTHATETCHESEATSAWAY,THESURFACEALUMINUMTHISLEAVESONLYTHESILICONPARTICLESEXPOSEDTHEPISTONANDRINGSSLIDEONTHESILICONWITHMINIMUMWEARPISTONTHEPISTONCONVERTSTHEPOTENTIALENERGYOFTHEFUELINTOTHEKINETICENERGYTHATTURNSTHECRANKSHAFTTHEPISTONISACYLINDRICALSHAPED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OGREATERTHAN115TO1RATIOSTHISHIGH,HOWEVER,CREATEHIGHCOMBUSTIONCHAMBERTEMPERATURESTHISINTURNCREATESOXIDESOFNITROGENNOX,APRIMARYAIRPOLLUTANTINTHEEARLY1970S,COMPRESSIONRATIOSWERELOWEREDTOAROUND8TOPERMITTHEUSEOFLOWEROCTANELOWLEADORUNLEADEDFUEL,ANDTOREDUCENOXFORMATIONADVANCESINELECTRONICENGINECONTROLINTHE1980SHAVEALLOWEDENGINEERSTORAISECOMPRESSIONRATIOSTOTHE9AND10TO1RANGEFOROPTIMUMPERFORMANCEANDECONOMY发动机概述发动机是汽车的动力源。迄今为止除为数不多的电动汽车外，汽车发动机都是热能动力装置。现代汽车发动机因为燃料发动机内部燃烧而被称为内燃机。发动机将燃料燃烧产生的热能变为机械能。冷却系统水冷发动机和风冷发动机被应用，在柴油引擎制造业中，水冷发动机应用最广泛。燃料系统汽油，柴油，丙烷燃料系统在各式各样的发动机中广泛应用。点火方式汽油机采用电子点火系统；电子点火系统在压缩行程终了时通过气缸内的火花塞高压电板产生火花，点燃雾化的燃油空气混合物。柴油机的能量来源于当燃料被注入气缸时压缩空气点燃燃料。柴油机的压缩比远远大于汽油机的压缩比。在注入燃料时靠压缩空气点燃燃料能产生足够的热量。阀门装置已经有四种阀门配备被应用于汽油机和柴油机上。在这四种类型中（I、T、F和I头型），I型在柴油机中最常用。气缸排列方式发动机的外形结构或气缸排列方式取决于气缸的设计，气缸一个接一个地被排成直线，直线式最常用于四缸或六缸发动机；V型的气缸排列方式是两排气缸成60到90角度。最常用的机型每排气缸有三到八个。对置式发动机是指两列气缸水平相对曲轴位于二者之间发动机的分类按活塞运动方式的不同，活塞式内燃机可分为往复活塞式和旋转活塞式两种，前者在现代车上被广泛的运用，下面就以往复活塞式内燃机为例，介绍一下发动机的工作原理。除了旋转活塞式发动，所有生产的汽车发动机都是往复式的。往复的意思就是“上下运动”或者“前后运动”。正是由于活塞在气缸内做上下运动而被称为往复式发动机。几乎所有的这种类型的发动机都是坐在气缸上或发动机机体上的，机体是铸有发动机气缸的铁或铝的铸造物。机体的上端被气缸盖住，形成燃烧室。机体的低部被油底壳或油箱覆盖。对于这种类型的发动机在构造上有一个例外风冷的发动机，它是典型的水平对置式风冷发动机过去的几年里被其他一些汽车厂商广泛应用。活塞在气缸内的直线运动产生动力。然而，这种直线运动必须转变成驱动轿车或卡车车轮转动的旋转运动。活塞被一个销称为活塞销连接在连杆的顶端。连杆将活塞的上下运动传递给曲轴，曲轴将直线运动转变为旋转运动。连杆由大轴承安装在曲轴上。用类似的轴承称为主轴承，将曲轴安装在机体上。曲轴将活塞的往复运动转变为旋转运动。可燃的汽油和空气混合物通阀门进入气缸。汽车发动机使用POPPET阀门。这种阀门可以安装在机体或气缸头部。阀门的开启与关闭被凸轮轴控制。当凸轮轴旋转时凸轮上的凸起使阀门打开。当凸起不在维持阀门开启时，弹簧使阀门关闭。下面讨论气缸和阀门最常见的安排方式。基本的单气缸发动机包括一个气缸（发动机机体），气缸内可移动的活塞，连杆头端连接活塞，底端连接曲轴上的分支部分，操纵两个阀门（进气门，排气门）的凸轮轴以及汽缸盖。飞轮安装在曲轴的一端。曲轴的一端有一个齿轮来驱动凸轮轴上的齿轮。凸轮轴上的凸轮是曲轴上齿轮的两倍。当凸轮轴与曲轴以相同的转速转动时，在四冲程发动机中凸轮轴上吃捆的转速是曲轴上齿轮转速的一半。能量转化方面内燃机柴油机是将化学能转变为热能进而转化为可使用的机械能。这是由密闭的汽缸内燃烧适量的控制比例混合的空气与燃料的混合物获得的。汽缸内可移动的活塞被燃烧后膨胀的气体向下推动，汽缸内可运动的活塞与连杆的头端连接，连杆的底部与曲轴连接。当活塞向下运动时，曲轴开始旋转。活塞的往复式运动（前后或上下运动）转化为曲轴的旋转运动这为驱动汽车提供动力。通常情况下，空燃比为151时能获得叫好的燃烧。然而，进入柴油机的空气往往很多，但在发动机转速很慢时只有一部分被利用。空气大约由20的氧气而剩余的80几乎全为氮气。这就意味着，每一轮燃料的燃烧需要9000至10000加仑空气中含的氧气。四冲程发动机汽油单独不会燃烧，必须与氧气（空气）混合。这种燃烧即为氧化，是一种释放在燃油混合物中的能力方式。燃油混合气必须在密封小室内被压缩并燃烧，这样燃烧的能量使可移动的活塞工作产生机械能。为保证发动机的效率，燃烧室尽可能的牢固密封。燃烧室任何的泄露都会造成燃烧能量的耗散，而不会增加由活塞运动产生的机械能。为了纪念德国工程师尼克劳斯奥托博士，首位在1876年实行这个原则的博士，所以四冲程发动机又被称为奥托发动机，在四冲程发动机中需要气缸内活塞的四个行程来完成全额作业周期两个向上的行程，两个向下的行程，每个行程是以其发挥的作用来命名的进气，压缩，做功，排气。1、进气行程当活塞向下运动时，其气状的燃料与空气的混合物通过开启的进气阀进入汽缸。2、压缩行程活塞返回向上运动，进气门关闭，混合物在燃烧室内被压缩并且被火花点燃。3、作功行程燃烧的膨胀的气体使汽缸内地活塞向下运动。在做功行程终了时排气门打开。4、排气行程活塞随着排气门的开启，又返回向上运动，燃烧后的其他气体被排出，为下一个进气形成做准备。进气门往往在排气开始之前开启。只要发动机在运转，这四个冲程的循环就会在每个气缸内周而复始的运动。两冲程发动机两冲程柴油机完成这四个过程进气，压缩，做功，排气。曲轴的一次旋转或活塞的两个行程中。气缸周围排列着一系列的开口，当活塞在行程终了时，开口就会打开。一个鼓风机使空气通过开口进入气缸。驱使所有剩余的废气通过开启排气阀排除气体，并且使汽缸内充满空气。这叫做排除废气。当活塞向上运动时，排气门关闭，活塞覆盖开口。在排气关闭时留在活塞上部的空气被压缩。在活塞到达上止点之间，所需要的燃料被注入气缸。即刻因压缩空气而产生的热量将燃料点燃。燃烧一直持续到被注入的燃料燃烧完毕。在作功行程中，来自燃烧的压力使活塞向下运动。当活塞大约运动到一半的时候，排气门打开，废气排出，活塞继续向下运动进气门打开，新鲜空气进入气缸，排出废气。发动机继续运转时，则重复上述整个过程。二者比较假设，一台两冲程发动机与一台四冲程发动机有相同数量的气缸，相同的排气量，压缩比以及转速，那么，四冲程产生的动力将会是两冲程的两倍，因为四冲程做功的次数是两冲程的两倍。然而，这并不是事实，因为有排气废气的过程，做功行程及压缩行程均被缩短。两冲程发动机也需要一个鼓风机，使发动机动力驱动。两冲程发动机，在360的曲轴旋转中大约有160被用来排除废气和进入新鲜空气。在四冲程发动机中，每720的曲轴旋转中大约有415被用来进气和排气。这些数据表明，在两冲程发动机