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ValvesystemThevalvemechanismconsistsofvalves,rockerarms,push-rods,lifersandthecamshaft,Itmakesair,fuelmixtureandwastegasinandoutoftheengineatthepropertime.Tocoordinatethefour-strokecycle,agroupofpartscalledthevalvetrainopensandclosesthevalves(movesthemdownanduprespectively).Thesevalvemovementsmusttakeplaceatexactlytherightmoment.Theopeningofeachvalveiscontrolledbyacamshaft.1.OverheadCamshaft(OHC)ValveTrainThecamisanegg-shapedpieceofmetalonashaftthatrotatesincoordinationwiththecrank-shaft.Themetalshaft,calledthecamshaft,typicallyhasindividualcamsforeachvalveintheengine.Asthecamshaftrotates,thelobe,orhighspotofthecam,pushesagainstpartsconnectedtothestemofthevalve.Thisactionforcesthevalvetomovedownward.Thisactioncouldopenaninletvalveforanintakestroke,oropenanexhaustvalveforanexhauststroke.Asthecamshaftcontinuestorotate,thehighspotmovesawayfromthevalvemechanism.Asthisoccurs,valvespringspullthevalvetightlyclosedagainstitsopening,calledthevalveseat.Valvesinmoderncarenginesarelocatedinthecylinderheadatthetopoftheengine.Thisisknownasanoverheadvalve(OHV)configuration.Inaddition,whenthecamshaftislocatedocerthecylinderhead,thearrangementisknownasanoverheadcamshaft(OHC)design.Somehigherformanceengineshavetwoseparatecamshafts,oneforeachsetofinletandexhaustvalves.Theseenginesarekonwnasdualoverhead-camshaft(DHOC)engines.2.Push-rodValveTrainThecamshaftalsocanbelocatedinthelowerpartoftheengine,withintheengineblock.Totransferthemotionofthecamupwardtothevalve,additionalpartsareneeded.Inthisarrangement,thecamlobespushagainstroundmetalcylinderscalledcamfollower.Asthelobeofthecamcomesupunderthecamfollower,itpushesthecamfollowerupward(awayfromthecamshaft).Thecamfollowerridesagainstapushrod,whichpushesagainstarockerarm.Therockerarmpicotsonashaftthroughitscenter.Asonesideoftherockerarmmovesup,theothersidemovesdown,justlikeaseesaw.Thedownward-movingsideoftherockerarmpushesonthevalvestemtoopenthevalve.Becauseapush-rodvalvetrainhasadditionalparts,itismoredifficulttorunathighspeeds.Push-rodenginestypicallyrunatslowerspeedsand,consequently,producelesshorse-powerthanoverhead-camshaftdesignsofequalsize.3.CamDesignAndControlDynamicsThefunctionofthecamistoopenandclosethevalvesasfaraspossible,asfastaspossibleandassmoothlyaspossible.Theclosingforceforthevalvesisappliedbythevalvespringswhichalsomaintaincontactbetweenthecamandthevalves.Dynamicforcesimposelimitsoncamandvalvelift.Theentirevalve-trainassemblycanbeviewedasaspring/masssysteminwhichtheconversionfromstoredtofreeenergycausesforcedvibration.Valve-trainassemblieswithoverheadcamshaftscanberepresentedwithsufficientaccuracybya1-masssystem(consistingofthemovingmass,thevalve-trainassemblystiffnessandcorrespondingdamping).Forsystemswithbottom-mountedcamshaftsandpushrods,a2-masssystemisbeingincreasinglyused.Themaximumpermissiblecontactstress,usuallyregardedastheparameterwhichlimitscam-loberadiusandtherateofopeningontheflank,currentlyliesbetween600～750MPadependinguponthematerialpairingsused.4.CamshaftDriveMechanismEachcammustrevolveonceduringthefour--strokecycletoopenavalve.Acycle,remember,correspondswithtworevolutionsofthecrankshaft.Therefore,thecamshaftmustrevolveatexactlyhalfhastwicethenumberofteethasagearconnectedtothecrankshaft.Thegearsarelinkedinoneofthreeways.1)BeltDriveAcog-typebeltcanbeused.Suchbeltsaremadeofsyntheticrubberandreinforcedwithinternalsteelorfiberglassstrands.Thebeltshaveteeth,orslottedspacestoengageanddriveteehtongearwheel.Abelttypicallyisusedonengineswithoverhead-camvalvetrains.2)ChainDriveOnsomeengines,ametalchainisusedtoconnectthecrankshaftandcamshaftgears.Mostpush-rodenginesandsomeOHCengineshavechains.3)GearDriveThecamshaftandcrankshaftgearscanbeconnecteddirectly,ormeshed.Thistypeofoperatinglinkagecommonlyisusedonoldersix-cylinder,inlineengines.Acamshaftdrivenbyachainorbeltturnsinthesamedirectionasthecrankshaft.Butacam-shaftdrivendirectlybythecrankshaftgearturnsintheoppositedirection.Timingbeltsareusedbecausetheycostlessthanchainsandoperatemorequietly.Atypicaltimingbeltismadeofneoprene(syntheticrubber)reinforcedwithfiberglass.