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1、2.5ValveSystemThevalvesystemismadeupofthosepartsneededtoopenandclosethevalveatjusttherighttime.2.5.1ValveSystemTocoordinatethefour-strokecycle,agroupofpartscalledthevalvetrainopensandclosesthevalves(movesthemdownandup,respectively).Thesevalvesmovementsmosttakeplaceatexactly
2、therightmovements.Theopeningofeachvalveiscontrolledbyacamshaft.1.OverheadCamshaft(OHC)ValveTrainThecamisanegg-shapedpieceofmetalonashaftthatrotatesincoordinationwiththecrankshaft.Themetalshaft,calledthecamshaft,typicallyhasindividualcamsforeachvalveintheengine.Asthecamshaft
3、rotates,thelobe,orhighspotofthecam,pushesagainstpartsconnectedtothestemofthevalve.Thisactionforcesthevalvetomovedownward.Thisactioncouldopenaninletvalveforanintakestroke,oropenanexhaustvalveforanexhauststroke.Asthecamshaftcontinuestorotate,thehighspotmovesawayfromthevalveme
4、chanism.Asthisoccurs,valvespringspullthevalvetightlyclosedagainstitsopening,calledthevalveseat.Valveinmoderncarenginesarelocatedinthecylinderheadatthetopoftheengine.Thisisknownasanoverheadvalve(OHV)configuration.Inaddition,whenthecamshaftislocatedoverthecylinderhead,thehigh
5、-performanceengineshavetwoseparatecamshaft,oneforeachsetofinletandexhaustvalves.Theseenginesareknownasdualoverhead-camshaft(DHOC)engines.2.Push-rodValveTrainThecamshaftalsocanbelocatedinthelowerpartofengine,withintheengineblock.Totransferthemotionofthecamupwardtothevalve,ad
6、ditionalpartsareneeded.Inthisarrangement,thecamlobespushagainstroundmetalcylinderscalledcamfollower.Asthelobeofthecamcomesupunderthecamfollower,itpushesthecamfollowerupward(awayfromthecamshaft).Thecamfollowerridesagainstapushrod,whichpushesagainstarockerarm.Therockerarmpivo
7、tsonashaftthroughitscenter.Asonesideoftherockerarmmovesup,theothersidemovesdown,justlikeaseesaw.Thedownward-movingsideoftherockerarmpushesonthevalvestemtoopenthevalve.Becauseapush-rodvalvetrainhasadditionalparts,itismoredifficulttorunathighspeeds.Push-roadenginestypicallyru
8、natslowerspeedsand,consequently,producelesshorse-powerthanoverhead-camshaftdesigns
