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Flowthroughvalves

1.Timing

Figure612showsthemaingeometricparametersofapoppetvalveheadandseat.

Figure612:

Figure613showstheproportionsoftypicalinletandexhaustvalvesandports,relativetothevalveinnerseatdiameterD.Theinletportisgenerallycircular,ornearlyso,andthecrosssectionalareaisnolargerthanisrequiredtoachievethedesiredpoweroutput.Fortheexhaustport,theimportanceofgoodvalveseatandguidecooling,withtheshortenlengthofexposedvalvestem,leadstoadifferentdesign.Althoughacircularcrosssectionisstilldesirable,arectangularorovalshapeisoftenessentialaroundtheguidebossarea.TypicalvalveheadsizesfordifferentshapedcombustionchambersintermsofcylinderboreBaregiveninTable6.1.Eachofthesechambershapesallowhighermaximumairflowsforagivencylinderdisplacement.

Figure613:

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Typicalvalvetiming,valveliftprofiles,andvalveopenareasforafourstrokecyclesparkignitionengineareshowninFig.614.Thereisnouniversallyacceptedcriterionfordefiningvalvetimingpoints.Somearebaseduponaspecificliftcriterion.Forexample,SAEdefinesvalvetimingeventsbasedonreferencevalveliftpoints:

1. Hydrauliclifters.Openingandclosingpositionsarethe0.15mm(0.006in)valveliftpoints.

2. Mechanicallifters.Valveopeningandclosingpositionsarethe0.15mm(0.006in)valveliftpointsplusthespecifiedlash.

Table61:

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Alternatively,valveeventscanbedefinedbasedonangularcriteriaalongtheliftcurve.Whatisimportantiswhensignificantgasflowthroughthevalveopenareaeitherstartsorceases.

Theinstantaneousvalveflowareadependsonvalveliftandthegeometricdetailsofthevalvehead,seat,andsteam.Therearethreeseparatestagestothedetailsoftheflowareadevelopmentasvalveliftincreases,asshowninFig.614b.

Forlowvalvelifts,theminimumflowareacorrespondstoafrustrumofarightcircularconewheretheconicalfacebetweenthevalveandtheseat,whichisperpendiculartotheseat,definestheflowarea.Forthisstage:

Formula67:

whereBisthevalveseatangle,Lvisthevcalvelift,Dvisthevalveheaddiameterandwistheseatwidth.

Forthesecondstage,theminimumareaisstilltheslantsurfaceofafrustrumofarightcircularcone,butthissurfaceisnolongerperpendiculartothevalveseat.Thebaseangleoftheconeincreasesfrom(90B)towardthatofacylinder,90.Forthisstage:

Formula68:

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whereDpistheportdiameter,DsisthevalvestemdiameterandDmisthemeanseatdiameter(Dvw).

Figure614:

(a)Typicalvalvetimingforhighspeed2.2dm3fourcylindersparkignitionengine.(b)Schematicshowingthreestagesofvalvelift.(c)Valveliftcurveandcorrespondingminimum intakeandexhaustvalveopenareasasa functionofcamshaftangle.Inletandexhaustvalvediametersare3.6and3.1cm,respectively.

Finally,whenthevalveliftissufficientlylarge,theminimumflowareaisnolongerbetweenthevalveheadandseatitistheportflowareaminusthesectionalareaofthevalvestem.

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Thusfor

Formula69

IntakeandexhaustvalveopenareascorrespondingtoatypicalvalveliftprofileareplottedversuscamshaftangleinFig.614c.Thesethreedifferentflowregimesareindicated.Themaximumvalveliftisnormallyabout12percentofthecylinderbore.

Inletvalveopening(IVO)typicallyoccurs10to25BTDC.Engineperformanceisrelativlyinsensitivetothistimingpoint.ItshouldoccursufficientlybeforeTDCsothatcylinderpressuredoesnotdipearlyintheintakestroke.

Inletvalveclosing(IVC)usuallyfallsintherange40to60afterBDC,toprovidemoretimeforcylinderfillingunderconditionswherecylinderpressureisbelowtheintakemanifolspressureatBTC.IVCisoneoftheprincipalfactorsthatdetermineshighspeedvolumetricefficiency(VE)italsoaffectslowspeedVEduetobackflowintotheintake.

ExhaustValveOpening(EVO)occurs50to60beforeBDC,wellbeforetheendoftheexpansionstroke,sothatblowdowncanassistinexpellingtheexhaustgases.ThegoalhereistoreducecylinderpressuretoclosetotheexhaustmanifoldpressureassoonaspossibleafterBDCoverthefullenginespeedrange.NotethatthetimingofEVOaffectsthecycleefficiencysinceitdeterminestheeffectiveexpansionratio.

Exhaustvalveclosing(EVC)endstheexhaustprocessanddeterminesthedurationofthevalveoverlapperiod.EVCtypicallyfallsintherange8to20afterTDC.Atidleandlightload,insparkignitionengines(whicharethrottled),itthereforeregulatesthequantityofexhaustgasesthatflowbackintothecombustionchamberthroughtheexhaustvalveundertheinfluenceofintakemanifoldvaccum.Athighenginespeedsandloads,itregulateshowmuchofthecylinderburnedgasesareexhausted.EVCtimingshouldoccursufficientlyfarafterTDCsothatthecylinderpressuredoesnotriseneartheendoftheexhauststroke.LateEVCfavorshighpowerattheexpenseoflowspeedtorqueandidlecombustionquality.Notefromthetimingdiagram(Fig.614a)thatthepointsofmaximumvalveliftandmaximumpistonvelocitydonotcoincide.

Theeffectofvalvegeometryandtimingonairflowcanbeillustratedconceptuallybydividingtherateofchangeofcylindervolumebytheinstantaneousminimumvalveflowareatoobtainapseudoflowvelocityforeachvalve:

Formula610:

whereVisthecylindervolume,Bisthecylinderbore,sisthedistancebetweenthewristpinandcrankaxis

andAmisthevalveareagivenbyEqs(6.7),(6.8)or(6.9).

Instantaneouspseudoflowvelocityprofilesfortheexhaustandintakestrokesofafourstrokefourcylinderengine

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areshowninFig.615.

Figure615:

Notetheappearanceoftwopeaksinthepseudoflowvelocityforboththeexhaustandtheintakestrokes.Thebroadpeaksoccuringatmaximumpistonvelocityreflectthefactthatvalveflowareaisconstantatthispoint.ThepeaksclosetoTDCresultfromtheexhaustvalveclosingandintakevalveopeningprofiles.Thepeakattheendoftheexhauststrokeisimportantsinceitindicatesagighpressuredropacrossthevalveatthispoint,whichwillresultinhighertrappedresidualmass.Themagnitudeofthisexhauststrokepseudovelocitypeakdependsstronglyonthetimingofexhaustvalveclosing.

Thepseudovelocitypeakatthestartoftheintakestrokeinmuchlessimportant.

Thatthepseudovelocitiesearlyintheexhauststrokeandlateintheintakestrokearelowindicatesthatphenomenaotherthanquasisteadyflowgovernetheflowrate.Thesearetheperiodswhenexhaustblowdownandramandtuningeffectsintheintakearemostimportant.