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GasDynamicsofExhaustValves
MarcusWinroth
Duetotougherlegisla5onsonexhaustemissionsfromcombus5on-enginevehicles,thedevelopmentofmoreefficientengineswithloweremissionsisanimportanttopicwithinthevehicleindustry. Theexhaustgasesarehotandathighpressure andare thus rich in energy. Someof this energymaybe recoveredusing a turbocharger. Tomaximize theenergyrecovery,theenginesystemneedsop5misa5on.Whenop5mizinganenginesystemit iscommontouseone-dimensional,semi-empiricalmodels.Insuchmodels,complexflows(suchastheflowpasttheexhaustvalves)mayberepresentedbyastraightpipe-flowwithacorrespondingdischargecoefficient(CD).
Introduc9onandMo9va9on:In order to decrease computa5onal 5mewhen designing and op5mizing enginesystems, it is common to use 1D/0Dsimula5ons. In these simula5ons the flowpast the exhaust valve is simplified usingmapped coefficients to describe the flow.These maps are obtained experimentally,where the experiments are typicallyperformed with constant valve liOs at lowpressurera5os.Thisprojecthastestedtheassump5ons(quasi-steadinessandindependenceofpressurera5o)madewhendeterminingthedischargecoefficient(CD)oftheexhaustvalve.Thiswasdonebyperformingexhaus5onExperimentsatdifferentpressurera5os,withbothsta5cdynamicvalves(withequivalentenginespeedsranging800-1350rpm).
Setup:The photo shows the setup for thedynamic valve experiments. The valve isoperated using a linear motor, whichallows for a controllable valve liO profile.The rig has a straight outlet pipe (whichexhauststoatmosphere)connectedtotheexhaust port approximately 1 port-diameter downstream of the valve seat.The pressure ismeasured in the cylinder,in the valve seat and at two posi5ons intheoutletpipe.The ini5al temperature inthe cylinder is also monitored. Bymeasuring the pressure in the cylinder asfunc5onof5meanddeterminingthemassthat have leO the cylinder using theisentropicrela5onship(1)andthegas law(2),adynamicevalua5onofthemassflowcanbemade (3).This is illustratedby thetwofiguresbelowthephotoofthesetup.
Results:TheleOfigureshowsCDasafunc5onofvalveliOfordifferent ini5alcylinderpressures, foran equivalent engine speed of 900 rpm. ItcanfromthisplotbeseenthatanincreaseinpressurecausesadecreaseinCD.Intherightfigure CD for different equivalent enginespeeds is ploYed. The value of CD shows alargedependencyonenginespeed,whereafasteropeningspeedleadstoahighervalueof CD. In both figures CD obtained with asta5c valve can be seen to generallyoveres5matethevalueofCD.
SummaryandConclusion:Resultsshowalargedependenceonbothvalveopeningspeedandpressurera5o.Italsoshowsthatmeasurementsusingasta5cvalveoveres5mates the value of CD, compared to the dynamic valve.Indica5ngthatanyaYempttorepresenttheexhaustflows,fromarealengine,withasta5callyobtainedCDisboundtobeflawed.ReferenceM.Winroth(2017)“OnGasDynamicsofExhaustValves”.Licen5atethesis,KTHMechanics,March2017.
Supervisors:Prof.HenrikAlfredsson,Dr.RamisÖrlü
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