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第1页共16页dieselEnginesInternalcombustiongasolineenginesrunonamixtureofgasolineandair.Theidealmixtureis14.7partsairtoonepartofgasoline（byweight.）Sincegasweightsmuchmorethanair,wearetalkingaboutawholelotofairandtinybitofoil.Onepartofoilthatiscompletelyvaporizedinto14.7partsofaircanproducetremendouspowerwhenignitedinsideanengine.Let'sseehowthemodernengineusesthatenergytomakethewheelsturn.Airenterstheenginethroughtheaircleanerandproceedstothethrottleplate.Youcontroltheamountofairthatpassesthroughthethrottleplateandintotheenginewiththegaspedal.Itisthendistributedthroughaseriesofpassagescalledtheintakemanifold,toeachcylinder.Atsomepointaftertheaircleaner,dependingontheengine,fuelisaddedtotheair-streambyeitherafuelinjectionsystemor,inoldervehicles,bythecarburetor.1.EnginetypeTheenginescanbeclassifiedinthefollowingseveralways:operatingcycle;pistonaction;pistonconnection;cylinderarrangement;methodoffuelinjection,andspeed.(1)operatingcycleDieselandgas-burningenginescanbedividedintotwogroupsbasedonthenumberofpistonstrokespercycle,eitherfourortwo.Anenginewhichneedsfourstrokestcompleteonecycleisfour-strokecycleengineor,forshort,afour-cycleengine.Ifitneedsonlytwostrokestocompleteacycle,itiscalledatwo-strokecycleengineor,forshort,atwo-cycleengine.Thus,atwo-cycleenginefirestwiceasoftenasafour-cycleengine.(2)pistonactionAnengine’spistonactionmaybeclassifiedassingle-action,第2页共16页double-action,oropposed-piston.Single-actingenginesuseonlyoneandendofthecylinderandonefaceofthepistontodeveloppower.Thisworkingspaceisattheendawayfromthecrankshaft,thatis,attheupperendofaverticalengine.Double-actingengineusebothendsofthecylinderandbothfacesofthepistontodeveloppowerontheupstrokeaswellasonthedownstroke.Theconstructioniscomplicated;therefore,double-actingenginesarebuiltonlyinlargeandcomparativelylow-speedunits,generallytopowermotorships.Anopposed-pistonenginehascylindersineachofwhichtwopistonstravelinoppositedirections.Thecombustionspaceisinthemiddleofthecylinderbetweenthepistons.Therearetwocrankshafts;theupperpistonsdriveone,thelowerpistonstheother.Notethateachpistonissingle-acting;thatis,itdevelopspowerwithonlyonefaceofthepiston.(3)pistonconnectionThepistonmaybeconnectedtotheupperendoftheconnectingrodeitherdirectly(trunkpistontype),orindirectly(crossheadtype).Intrunk-pistonengines,ahorizontalpinwithinthepistonisencircledbytheupperendoftheconnectingrod.Thisbyfarthemostcommonconstruction.Incrosshead-typeengines,thepistonfastenstoaverticalpistonrodwhoselowerendisattachedtoaslidingmembercalledacrosshead,whichslidesupanddowninguides.Thecrossheadcarriesacrossheadpinwhichisencircledbytheupperendoftheconnectingrod.Thismorecomplicatedconstructionisrequiredindouble-actingengines.Itisalsousedinsomelarge,slow-speed,single-actingengines.(4)cylinderarrangementThefourbasiccylinderarrangementsofadieselorgas-burningengineare:cylinder-in-line;V-arrangement;Flat,andradial.Acylinder-in-linearrangement.Thisisthesimplestandmostcommon第3页共16页arrangement,withallcylindersarrangedverticallyinline.Thisconstructionisusedforengineshavingupto12cylinders.Enginesarealsobuiltwithhorizontalcylinders,usuallyoneortwo,inafewcaseswiththreecylinders.Ifanenginehasmorethaneightcylinders,itbecomesdifficulttomakeasufficientlyrigidframeandcrankshaftwithanin-linearrangement.Also,theenginebecomesquitelongandtakesupconsiderablespace.TheV-arrangement,withtwoconnectingrodsattachedtoeachcrankpin,permitsreducingtheenginelengthbyalmostone-half,thusmakingitmuchmorerigid,withastiffcrankshaft.Italsocostslesstomanufactureandinstall.Thisisacommonarrangementforengineswitheight,twelve,andsixteencylinders.Cylinderslyinginonelinearecalleda“bank”,andtheanglebetweenthebanksmayvary,inmanufacturingpractice,from30゜to120゜,themostcommonanglesbeingbetween40゜and75゜.(Acompletecircleis360゜)AflatengineisaV-enginewiththeanglebetweenthebanksincreasedto180゜.Thisarrangementisusedwherethereislittleheadroom,asintrucks,buses,andrailcars.Flatenginesarealsocalled“opposed-cylinder”engines.Inaradialengineallthecylindersaresetinacircleandallpointtowardthecenterofthecircle.Theconnectingrodsofallthepistonsworkonasinglecrankpin,whichrotatesaroundthecenterofthecircle.Sucharadialengineoccupieslittlefloorspace.Byattachingtheconnectingrodstoamasterdisksurroundingthecrankpin,asmanyastwelvecylindershavebeenmadetoworkonasinglecrankpin.(5)methodoffuelinjectionDieselenginesaredividedintoair-injectionenginesandsolidormechanicalinjectionengines.Air-injectionenginesuseablastofhighlycompressedairtoblowthefuelintothecylinder.Airinjectionwascommonlyuseonearlydieselenginesbutwiththedevelopmentofsolid-injectionsystemstheair-injectionengineisrapidlydisappearing.第4页共16页(6)speedAlldieselandgas-burningenginesmaybedividedintothreeclassesaccordingtospeed;low-speed,medium-speed,andhigh-speedengines.Automotivedieselenginesoftenrunfasterthan1200rpm,butthegreatmajorityofotherenginesrunbetween350and1200rpmandaretermedmedium-speedengines.2.HowanEngineWorksSincethesameprocessoccursineachcylinder,wewilltakealookatonecylindertoseehowthefourstrokeprocessw