《发动机原理》双语教案

Chapter2 EngineGasExchangeProcesses

Keypoints:Gasexchangeprocessoffour-strokeinternalcombustionengineandventilationlosses,theconceptofcombustionenginefillingcoefficient.

Difficultpoints:Themeasurestoimproveinternalcombustionenginefillingquantitycoefficient.

Inletandexhaustprocessofthefour-strokeengine

Thepurposeoftheexhaustandinletprocessesorofthescavengingprocessistoremovetheburnedgasesattheendofthepowerstrokeandadmitthefreshchargeforthenextcycle.

Inductingthemaximumairmassatwide-openthrottleorfullloadandretainingthatmasswithinthecylinderistheprimarygoalofthegasexchangeprocesses.

Enginegasexchangeprocessesarecharacterizedbyoverallparameterssuchasvolumetricefficiencyforfour-strokecycles.

Gasexchangeprocessoffour-strokeengine

Thegasexchangeprocessofanengineconsistsofthedurationfromopeningtheexhaustvalvetoclosingtheinletvalve,itextendsapproximately410~480ºCAandcanbedividedintofourphases—blowdown,displacement,inductionandscavenging.

Blowdown

Thedurationfromexhaustvalveopeningtocylinderpressureclosestopipepressureisreferredtoasblowdownphase.

Theburnedcylindergasesaredischargedduetothepressuredifferencebetweenthecylinderandtheexhaustsystem.IftheexhaustvalvebeginstoopenwhenthepistonreachedBDC,backpressureagainsttheupwardpistonmustbeextremelyhigh.Thustheexhaustprocessusuallybegins40~60ºCAbeforeBDC(exhaustleadcrankangle).

Blowdownphasesendswhenpressuredifferencebetweenthecylinderandtheexhaustsystemdisappears,about10~30ºCAafterBDC.Thoughthefreeexhaustphasecoversonlyabout1/10ofexhauststroke,itdischarges60%ofburnedgas.

Displacement

Theexhaustgasisscavengedbypiston’supwardmotionthatistheburntgasisforcedoutofthecylinder.Itisapositivedisplaceprocess.

Theexhaustvalvecloses15~30ºCAafterTDC(exhaustlagcrankangle)toimproveemptyingthecylindersandmakethebestuseoftheinertiaofthegasesintheexhaustsystems.

Inductionprocess

Fromtheinletvalvesopentoclose,thewholeprocessthatinternalcombustionengineinhalesfreshchargeiscalledintakeprocess.

Theusualpracticeistoextendthevalveopenphasesbeyondtheintakestrokestoimprovechargingofthecylindersandmakethebestuseoftheinertiaofthegasesintheintakesystems.

Theintakevalveopens10~20ºCAbeforeTDC(theinletleadcrankangle)andcloses40~70ºCAafterBDC(theinletlagcrankangle).

Valveoverlapandscavenging

Theexhaustvalvecloses15to30ºCAafterTDCandtheinletvalveopens10to20ºCAbeforeTDC.Thedurationthatbothvalvesareopenarecalledanoverlapperiod.

Withbothvalvesopening,theinertiaoffreshchargecanbeusedtosweeptheexhaustgasesoutofcylinderwithoutanylossiftheoverlapisproper,thatissocalledscavenging.

Theadvantageofvalveoverlapoccursathighenginespeedswhenthelongervalve-openperiodsimprovevolumetricefficiency.Ifthevalveoverlapistoolarge,backflowofexhaustedgasintothecylindergasesintotheintakewillusuallyoccur.

Valvetiming

Thevalvetimingaremodifiedtosetbetterchargingandexhaustingperformanceasthereisalwaysadifferencebetweentheoryandpractical.

Volumetricefficiency

Oneofthemostimportantprocessesthatgovernhowmuchpowerandperformancecanbeobtainedfromanengineisgettingthemaximumamountofairintothecylinderduringeachcycle.

Definitionofvolumetricefficiency

Volumetricefficiencyisameasureoftheeffectivenessoftheinductionandexhaustprocesses.

Intermsofquantitiesapplyingtoanactualengine,volumetricefficiencyis

definedasthemassoffreshmixturewhichpassesintothecylinderinonesuctionstroke,dividedbythemassofthismixturewhichwouldfillthepistondisplacementatinletdensity.

ηmaV1

(2.1)

m

V

v

s s

Wherema=massofairinhaledpercylinderpercycle;

ms=massofairtooccupysweptvolumepercylinderat“ambient”pressureandtemperature;

V1=volumeof“ambient”airinhaledpercylinderpercycle;Vs=cylindersweptvolume.

Theinfluencefactorsofvolumetricefficiency

Wheninletvalveclosed,theoverallvolumeofcylinderisVs’+Vc,themassof

trappedworkingfluidisma:

ma(VcVs')ρa

(2.2)

Massofresidualswhenexhaustvalveclosed:

mrVrρr

(2.3)

From(2.1),(2.2)and(2.3),freshchargesinhaledpercylinderpercycleis:

ηvVsρsVcVs'ρaVrρr

(2.4)

Consideringinfluenceofintakeandexhaustvalvelagangle,make

ξVcVs',

VcVs

v

a

r

φVr,then: η 1 (ξερφρ)

(2.5)

V

c (ε1)ρs

ByapplyingIdealGasEquation

ρp/(RT)

toEq.(2.5):

η 1 Ts(ξεpaφpr)

(2.6)

s

a

r

v ε1p T T

Thelevelofexhaustresidualstrappedinthecylinderhasasignificanteffectonthecycle-by-cyclevariationsincombustion,andtheemissionsofNOX.Theresidualcoefficientγisdefinedasmassofresidualtrappedincylinderattheendofintakestroke,dividedbymassoffreshchargeinhaledintheintakestroke.Itisusedto

evaluatetheresidualpercentageofmixtureincylinder.From(2.3)and(2.4):

γ mr

ηvVsρs

Vrρr

(VcVs')ρaVrρr

φVcρr ρr

ξVaρaφVcρr ξ

(2.7)

Byapplying(2.6)to(2.7):

ηξε

Tspa 1

ερaρr

φ

s

v ε1pT1γ

a

Inaqualitativeanalysis,volumetricefficiencyηvincreaseswith:

Increasingmixturepressureattheendofintakestrokepa;

DecreasingmixturetemperatureattheendofintakestrokeTa;

Reducingresidualcoefficientγ;

Increasingcompressionratioɛ;

Suitablevalveparametersξandφ.

Effectofoperatingconditionsanddesignonvolumetricefficiency

InletMachindex:

Forconveniencetheratioofthetypicalvelocitytotheinletsonicvelocity,u/α,iscalledtheinletMachindex.

Thegasvelocityischosenbythefollowingequation:

uApVp

CiAi

Whereu=gasvelocitythroughtheinletvalveatsmallestarea;Ap=pistonarea;Vp=meanpistonspeed;

Ai=nominalintakevalveopeningarea;Ci=inletvalveflowcoefficient.

And:

Zu

α

ApVp

AiCiα

(b)2

Di

Vp

Ciα

WhereZ=inletvalveMachindex;α=inletsonicvelocity;b=cylinderdiameter;Di=inletvalvediameter.

Fromagreatnumberofexperiments,itcouldbeseenthatthemaximumvolumetricefficiencyisobtainableforaninletMachnumberof0.55.Therefore,enginedesignersmusttakecareofthisfactortogetthemaximumvolumetricefficiencyfortheirengines.

Effectonintakesystemfriction

j

Duringtheintakestroke,duetofrictionineachpartoftheintakesystem,thepressureinthecylinderpcislessthantheatmosphericpressurepatmbyanamountdependentonthesquareofthespeed.

pp

atm

pc

pj

kρv2

Wherek=theresistancecoefficientforthatcomponentwhichdependsonits

geometricdetails;ρ=densityoffreshair; vj=thelocalvelocity.

Effectofinhaledchargeheating

Designswhichminimizethetemperaturesofinletmanifolds,inletports,inletvalvesandvalveseatsaredesirable.Improvementofheatconductivitybetweenthesepartsandthecoolantiseffectiveinreducingtemperatureoffreshcharge.

Effectofspeedandvalvetiming

Floweffectsonvolum