汽车构造英文Chapter 9 Engine measurement and performanceWord格式.docx

汽车构造英文Chapter 9 Engine measurement and performanceWord格式.docx

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Technicalterms

Reviewquestions

Manufacturersprovidespecificationoftheirenginesandalsoperformancedatawhichincludespoweroutputandtorque.Thischapterprovidesdefinitionsofsomeofthesebasictermsaswellasotherinformationthatrelatestoengines.

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.

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,soanenginewith2012cm³

displacementhasacapacityof2.012litres.Thiswouldberoundedoffandtheenginewouldbeclassedasa2litreengine.

LitresareusedforenginecapacityintheInternationalSystemofUnits（SI）becauseitistheunitforfluids.Fluidsarebothliquidsandgasesanditisthegascapacityoftheenginecylindersthatisbeingmeasured.

■Olderenginesthathavetheirboreandstrokemeasuredinincheshavetheircapacitystatedincubicinches.

Compressionratio

Thecompressionratioofanengineisameasurementofhowmuchtheair-fuelchargeiscompressedinthecylinder,Itiscalculatedbydividingthevolumeofthecylinder（anditscombustionchamber）withthepistonatBDC,bythevolumewiththepistonatTDC.ThisisshownforasimplecylinderinFigure9.4.

Figure9.4ThecompressionratioisthevolumeinacylinderwiththepistonatBDCdividedbyitsvolumewiththepistonatTDC

■ThevolumewiththepistonatTDCiscalledtheclearancevolume,sinceitistheclearancethatremainsabovethepistonwhenitisatTDC.

Example

Acylinderhasatotalvolumeof600cm³

andaclearancevolumeof75cm³

.Thecompressionratiois600dividedby75cm³

-thatis8:

1.Duringthecompressionstroke,theair-fuelmixtureiscompressedfromavolumeof600cm³

to75cm³

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.

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.