汽车构造英文Chapter 9 Engine measurement and performance.docx
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汽车构造英文Chapter9Enginemeasurementandperformance
Chapter9
Basictermsanddefinitions
Enginetermsanddefinitions
Enginepower
Enginetorqueandpower
Engineefficiency
Performanceratings
Technicalterms
Reviewquestions
Manufacturersprovidespecificationoftheirenginesandalsoperformancedatawhichincludespoweroutputandtorque.Thischapterprovidesdefinitionsofsomeofthesebasictermsaswellasotherinformationthatrelatestoengines.
Basictermsanddefinitions
Thereareanumberofdefinitionsthatrelatetobasicmechanics.Eachofthesehasabearingontheothers.Ifforceisconsideredfirst,thenitsmeaningcanbeusedtounderstandwork.Workcanbeusedtounderstandpower,andsoon.
Force
Aforceexistswhenevermovementoccursortendstooccur.Forcesarepresenteverywhere.Theystartmovement,stopmovementandcausechangesindirection.Theyarealsopresentevenwhennomovementoccurs.Thecommontermspushandpullarereallypushingforceandpullingforce.
Abriefdefinitionofforceis“thatwhichchangesortendstochangethestateofmotionofabody”.Forceisstatedinnewtons(N).
InFigure9.1,pressurefromcombustionproducesaforceontheheadofthepistonandthisistransmittedthroughtheconnectingrodtothecrankshaft.
Figure9.1Pressurefromcombustionproducesaforceontheheadofthepiston
Work
Workisdonewhenevertheforceappliedtoanobjectcausesittomove.Workisequaltotheforceappliedmultipliedbythedistancemoved.Thatis,thenewtons(force)multipliedbymetres(distance).Theresultisnewtonmetres(Nm).Onenewtonmetreisgiventhenameofjoule,andsoworkisstatedinjoules(J).
■Forworktobedone,movementmustoccur.
Relatingthistoapistoninacylinderofanengine,theworkthatisdoneasthepistonmovesdownonitspowerstrokeisequaltotheaverageforceonthepistonmultipliedbythepistonstroke.
Thepressureonthepistonthatproducestheforceisreferredtoasthemeaneffectivepressure.Mean,inthisinstance,hasthemeaningaverage,someaneffectivepressureistheaveragecombustionpressureonthepistonduringthepowerstroke.
Power
Poweristherate(orspeed}atwhichworkisdone.Thisintroducestheunitoftimeinadditiontotheunitofwork(J).Powerisstatedinwatts(W).
Onewattisonejoule(ofwork)performedinonesecond.Ifthisisrelatedbacktothebasicunits,thenonewattisequaltotheforceofonenewtonbeingmovedthroughadistanceofonemetreinonesecond.
Alargerunitofpoweristhekilowatt(kW)(1000W).Enginepowerisstatedinkilowatts.
Horsepower
Anotherunitofpower,whichwasusedtoindicatethepowerofengines,ishorsepower.Onehorsepowerisequalto746watts(unitsofpower).Itsnamecomesfromthepowerofahorse,andoriginatedwhencomparisonswerebeingmadebetweentherateatwhichworkcouldbedonebythenewlydevelopedmachinesoftheday,andbyhorses,whichwerethenthesourceofpower.Horsepowerhasbeensupersededbykilowatts.
Energy
Thisisthecapacityorabilitytodowork.Energyisstatedinthesameunitsaswork,thatis,joules,asitcanbeclassedasstoredwork.Forexample,energyisstoredinarotatingflywheel.Theflywheelabsorbsenergyduringthepowerstrokesandgivesupenergyduringtheotherstrokes.
Torque
Torqueisatwistingorturningforce,whichisequaltotheforcemultipliedbytheperpendiculardistancetotheturningpoint.Torqueisstatedinnewtonmetres(Nm).
Inthecylinderofanengine,torqueisproducedbytheforcefromtheconnectingrodapplied1othecrankofthecrankshaft(Figure9.2).Thediagramshowsthecrankat90°totheconnectingrod.Thiswouldbethepositionofgreatesttorqueiftheforcefromtheconnectingrodwasconstant,butthisisnotso.
Figure9.2EnginetorqueisequaltotheforceFmultipliedbytheperpendiculardistanced
Thetorquechangecontinuouslyasthecrankshaftrotatesbecausetheforceonthepistonvariesthroughoutitspowerstroke,andalsobecausetheanglebetweentheconnectingrodandthecrankxvaries.
Enginetermsanddefinitions
Thefollowingaretermsanddefinitionsthatareusedinrelationtoengines.
Boreandstroke
Thesizeofanenginecylinderisreferredtointermsofitsboreandstroke(Figure9.3).Theboreisthediameterofthecylinder.ThestrokeisthedistancethepistontravelsfromTDC(topdead-centre)toBDC(bottomdeadcentre).
Theboreisusuallymentionedfirst.Forexample,thecylindercouldhaveadiameter,orbore.of80mmandastrokeof100mm.Thesemeasurementsareusedtodeterminethepistondisplacement.
Enginescanhavesmallboresandlongstrokes,orlargeboresandshortstrokes.Ifanenginehasaborethesamedimensionasitsstroke,itwouldbereferredtoasasquareengine.Ifthediameteroftheboreislargerthanthestroke,thenitisreferredtoasanoversquareengine.
Pistondisplacement
PistondisplacementisthevolumethatthepistondisplacesasitmovesfromBDCtoTDC.PistondisplacementofthecylinderinFigure9.3(asanexample)wouldbethevolumeofacylinderthathasa4cmradiusandis10cmhigh.
Thevolumeis:
Iftheenginehasfourcylinders,thetotaldisplacementwouldbe2012
.
Enginecapacity
Thetotalpistondisplacementisalsoreferredtoasthecapacityoftheengine.Thecapacityisstatedinlitres,soanenginewith2012cm³displacementhasacapacityof2.012litres.Thiswouldberoundedoffandtheenginewouldbeclassedasa2litreengine.
LitresareusedforenginecapacityintheInternationalSystemofUnits(SI)becauseitistheunitforfluids.Fluidsarebothliquidsandgasesanditisthegascapacityoftheenginecylindersthatisbeingmeasured.
■Olderenginesthathavetheirboreandstrokemeasuredinincheshavetheircapacitystatedincubicinches.
Compressionratio
Thecompressionratioofanengineisameasurementofhowmuchtheair-fuelchargeiscompressedinthecylinder,Itiscalculatedbydividingthevolumeofthecylinder(anditscombustionchamber)withthepistonatBDC,bythevolumewiththepistonatTDC.ThisisshownforasimplecylinderinFigure9.4.
Figure9.4ThecompressionratioisthevolumeinacylinderwiththepistonatBDCdividedbyitsvolumewiththepistonatTDC
■ThevolumewiththepistonatTDCiscalledtheclearancevolume,sinceitistheclearancethatremainsabovethepistonwhenitisatTDC.
Example
Acylinderhasatotalvolumeof600cm³andaclearancevolumeof75cm³.Thecompressionratiois600dividedby75cm³-thatis8:
1.Duringthecompressionstroke,theair-fuelmixtureiscompressedfromavolumeof600cm³to75cm³,ortoone-eighthofitsoriginalvolume.
Thevolumeoftheactualpistondisplacementisoftenreferredtoasthesweptvolume,sincethisisthevolumeofairthatissweptintothecylinderbytheactionofthepistonasitmovesdownitsstroke.Thetotalcylindervolumeisthenequaltothesweptvolumeplustheclearancevolume.Thecompressionratiocanthenbestatedas:
(sweptvolume+clearancevolume)/(clearancevolume)
Thisisthesameasthetotalvolumedividedbytheclearancevolume.
Acompressionratioofaround8:
1wouldbeusedforapetrolengine.Highcompressionratiosprovidehighpressureoftheair-fuelmixture.Asaresult,highercombustionpressuresareobtainedandmoreforceisexertedonthepistonduringthepowerstroke.
However,thereisalimittothecompressionratioofpetrolengines.Iftheair-fuelmixtureiscompressedtoomuch,detonationandroughrunningwilloccur.
Dieselengineshavecompressionratiosfromabout15:
1to21:
1.Highcompressionratiossuchastheseareneededtoobtainthehighantemperaturesforcompressionignition.
Enginepower
Thepoweroutputofanengineismeasuredinkilowatts(kW).Ifanengineisratedat100kW,thismeansthattheenginecanproducethispowerataparticularenginerpmatfullthrottle.Manufacturersrateenginesatthemaximumkilowattsthatcanbeproduced.
Tomeasurepower,theengineiscoupledtoadynamometerinatestinglaboratory(Figure9.5).Thisisabrakingdevice,whichcanbeintheformofawaterpumporanelectricdynamo.Thedynamometerappliesaloadandalsoenablestheloadtobevaried.Instrumentsarearrangedtomeasuretheengine'srpmandtheloadplacedontheengine.ThisenablesthepoweroutputinkWtobedetermined.
Figure9.5AnengineonatestbedwithanenginedynamometerbeingusedtocheckengineoutputTOYOTA
Thedynamometerusedforserviceasworkcheckstheengineinthevehicle.Thistypeofunitiscalledachassisdynamometer(Figure9.6).Thedrivingwheelsofthecarareplacedonroller;theenginedrivesthewheelsandthewheelsdrivetherollers.Therollerscanbeloadedandtheloadvariedsothatoutputcanbemeasured.
Figure9.6Front-wheeldrivevehicleontherollersofachassisdynamometerMAZDA
Thepowerdeliveredbythewheelstotherollersisshownonagaugeasroadpower.Thisisapracticaltermbecausethisisthepowerthatisavailableatthewheelstomovethevehicle.
■Duetofrictionlossesinthedriveline,roadpowerwillbelessthanthepoweroutputoftheengine.
Indicatedpower(powerinput)
Indicatedpowerisbasedonthepowerdevelopedinsidetheenginecy1indersbythecombustionprocesses.Thisisthepowerinputoftheengine.
Tomeasurethepowerinput(indicatedpower)aspecialindicatingdevice(anoscilloscope)isrequired.Thisdevice,whichisusedinalaboratory,measuresthepressureinthecylinderthroughoutthefourpistonstrokes(intake,compression,power,exhaust).
AgraphofthecylinderpressuresforapetrolengineisshowninFigure9.7.