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英文.docx
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英文
Fueleconomyinautomobiles
Thefueleconomyofanautomobileisthefuelefficiencyrelationshipbetweenthedistancetraveledandtheamountoffuelconsumedbythevehicle.Consumptioncanbeexpressedintermsofvolumeoffueltotraveladistance,orthedistancetravelledperunitvolumeoffuelconsumed.Sincefuelconsumptionofvehiclesisasignificantfactorinairpollution,andsinceimportationofmotorfuelcanbealargepartofanation'sforeigntrade,manycountriesimposerequirementsforfueleconomy.Differentmeasurementcyclesareusedtoapproximatetheactualperformanceofthevehicle.Theenergyinfuelisrequiredtoovercomevariouslosses(windresistance,tiredrag,andothers)inpropellingthevehicle,andinprovidingpowertovehiclesystemssuchasignitionorairconditioning.Variousmeasurescanbetakentoreducelossesateachoftheconversionsbetweenchemicalenergyinfuelandkineticenergyofthevehicle.Driverbehaviorcanaffectfueleconomy;suddenaccelerationandheavybrakingwastesenergy.
Unitsofmeasure
Fueleconomyistherelationshipbetweenthedistancetraveledandfuelconsumed.
Fueleconomycanbeexpressedintwoways:
Unitsoffuelperfixeddistance
Generallyexpressedaslitersper100kilometers(L/100km),usedinsomeEuropeancountries,China,SouthAfrica,AustraliaandNewZealand.Canadianlawallowsforuseofeitherlitersper100kilometresormilesperimperialgallon.Recently,thewindowstickeronnewUScarshasstarteddisplayingthevehicle'sfuelconsumptioninUSgallonsper100miles.
Unitsofdistanceperfixedfuelunit
Milespergallon(mpg)iscommonlyusedintheUnitedStates,theUnitedKingdom,andCanada(alongsideL/100km).Kilometresperlitre(km/L)ismorecommonlyusedelsewhereintheAmericas,NorthernEurope,Asia,partsofAfricaandOceania.Whenthempgunitisused,itisnecessarytoidentifythetypeofgallonused,astheimperialgallonis4.5litersandtheUSgallonis3.785liters.
Fueleconomystatistics
Whilethethermalefficiency(mechanicaloutputtochemicalenergyinfuel)ofpetroleumengineshasincreasedsincethebeginningoftheautomotiveera,thisisnottheonlyfactorinfueleconomy.Thedesignofautomobileasawholeandusagepatternaffectsthefueleconomy.Publishedfueleconomyissubjecttovariationbetweenjurisdictionduetovariationsintestingprotocols.
Theaveragefueleconomyin2008fornewcars,lighttrucksandSUVsintheUnitedStateswas26.4mpg(US)(8.9L/100km).2008modelyearcarsclassifiedas"midsize"bytheUSEPArangedfrom11to46mpg(US)(21to5L/100km)However,duetoenvironmentalconcernscausedbyCO2emissions,newEUregulationsarebeingintroducedtoreducetheaverageemissionsofcarssoldbeginningin2012,to130g/kmofCO2,equivalentto4.5L/100kmforadiesel-fueledcar,and5.0L/100kmforagasoline(petrol)-fueledcar.
Theaverageconsumptionacrossthefleetisnotimmediatelyaffectedbythenewvehiclefueleconomy,forexampleAustralia'scarfleetaveragein2004was11.5L/100kmcomparedwiththeaveragenewcarconsumptioninthesameyearof9.3L/100km
Speedandfueleconomystudies
Fueleconomyatsteadyspeedswithselectedvehicleswasstudiedin2010.Themostrecentstudyindicatesgreaterfuelefficiencyathigherspeedsthanearlierstudies;forexample,somevehiclesachievebetterfueleconomyat100km/hratherthanat70km/h,althoughnottheirbesteconomy,suchasthe1994OldsmobileCutlass,whichhasitsbesteconomyat55milesperhour,andgets2mpgbettereconomyat65mphthanat45mph.Theproportionofdrivingonhighspeedroadwaysvariesfrom4%inIrelandto41%inNetherlands.
WhentheUSNationalMaximumSpeedLaw's55mphspeedlimitwasmandated,therewerecomplaintsthatfueleconomycoulddecreaseinsteadofincrease.The1997ToyotaCelicagot1mpgbetterfuel-efficiencyat65mphthanitdidat45mph,althoughalmost5mpgbetterat60mphthanat65mph,anditsbesteconomyatonly25mph.Othervehiclestestedhadfrom1.4to20.2%betterfuel-efficiencyat55mphvs65mph.Theirbesteconomywasreachedatspeedsof25to55mph.
Officialshopedthatthe55mphlimit,combinedwithabanonornamentallighting,nogasolinesalesonSunday,anda15%cutingasolineproduction,wouldreducetotalgasconsumptionby200,000barrelsaday,representinga2.2%dropfromannualized1973gasolineconsumptionlevels.Thiswaspartlybasedonabeliefthatcarsachievemaximumefficiencybetween40and50mphandthattrucksandbusesweremostefficientat55mph.
However,theUnitedStatesDepartmentofTransportation'sOfficeofDriverResearchfoundtotalfuelsavingsofthe55mphlimittobe1%,andindependentstudiesfounda0.5%savings.
Energyconsiderations
Sincethetotalforceopposingthevehicle'smotion(atconstantspeed)multipliedbythedistancethroughwhichthevehicletravelsrepresentstheworkthatthevehicle'senginemustperform,thestudyoffueleconomy(theamountofenergyconsumedperunitofdistancetravelled)requiresadetailedanalysisoftheforcesthatopposeavehicle'smotion.Intermsofphysics:
Foravehiclewhosesourceofpowerisaheatengine(anenginethatusesheattoperformusefulwork),theamountoffuelenergythatavehicleconsumesperunitofdistance(levelroad)dependsupon:
1Thethermodynamicefficiencyoftheheatengine;
2Theforcesoffrictionwithinthemechanicalsystemthatdeliversengineoutputtothewheels;
3Theforcesoffrictioninthewheelsandbetweentheroadandthewheels(rollingfriction);
4Otherinternalforcesthattheengineworksagainst(electricalgenerator,airconditioneretc.,waterpump,enginefanetc.);
5Externalforcesthatresistmotion(e.g.,wind,rain);
6Non-regenerativebrakingforce(brakesthatturnmotionenergyintoheatratherthanstoringitinausefulformelectricalenergyinhybridvehicles).
Ideally,acartravelingataconstantvelocityonlevelgroundinavacuumwithfrictionlesswheelscouldtravelatanyspeedwithoutconsuminganyenergybeyondwhatisneededtogetthecaruptospeed.Lessideally,anyvehiclemustexpendenergyonovercomingroadloadforces,whichconsistofaerodynamicdrag,tirerollingresistance,andinertialenergythatislostwhenthevehicleisdeceleratedbyfrictionbrakes.Withidealregenerativebraking,theinertialenergycouldbecompletelyrecovered,buttherearefewoptionsforreducingaerodynamicdragorrollingresistanceotherthanoptimizingthevehicle'sshapeandthetiredesign.Roadloadenergy,ortheenergydemandedatthewheels,canbecalculatedbyevaluatingthevehicleequationofmotionoveraspecificdrivingcycle.Thevehiclepowertrainmustthenprovidethisminimumenergyinordertomovethevehicle,andwilllosealargeamountofadditionalenergyintheprocessofconvertingfuelenergyintoworkandtransmittingittothewheels.Overall,thesourcesofenergylossinmovingavehiclemaybesummarizedasfollows:
∙Engineefficiency,whichvarieswithenginetype,themassoftheautomobileanditsload,andenginespeed.
∙Aerodynamicdragforce,whichincreasesroughlybythesquareofthecar'sspeed,butnotethatdragpowergoesbythecubeofthecar'sspeed.
∙Rollingfriction.
∙Braking,althoughregenerativebrakingcapturessomeoftheenergythatwouldotherwisebelost.
∙Lossesinthetransmission.Manualtransmissionscanbeupto94%efficientwhereasolderautomatictransmissionsmaybeaslowas70%efficient.Automaticallycontrolledshiftingofgearboxesthathavethesameinternalsasmanualboxeswillgivethesameefficiencyasapuremanualgearboxplusthebonusofaddedintelligenceselectingoptimalshiftingpoints
∙Airconditioning.Thepowerrequiredfortheenginetoturnthecompressordecreasesthefuel-efficiency,thoughonlywheninuse.Thismaybeoffsetbythereduceddragofthevehiclecomparedwithdrivingwiththewindowsdown.TheefficiencyofACsystemsgraduallydecreaseduetodirtyfiltersetc;regularmaintenancepreventsthis.Theextramassoftheairconditioningsystemwillcauseaslightincreaseinfuelconsumption.
∙Powersteering.Olderhydraulicpowersteeringsystemsarepoweredbyahydraulicpumpconstantlyengagedtotheengine.Powerassistancerequiredforsteeringisinverselyproportionaltothevehiclespeedsotheconstantloadontheenginefromahydraulicpumpreducesfuelefficiency.Moremoderndesignsimprovefuelefficiencybyonlyactivatingthepowerassistancewhenneeded;thisisdonebyusingeitherdirectelectricalpowersteeringassistanceoranelectricallypoweredhydraulicpump.
∙Cooling.Oldercoolingsystemsusedaconstantlyengagedmechanicalfantodrawairthroughtheradiatorataratedirectlyrelatedtotheenginespeed.Thisconstantloadreducesefficiency.Moremodernsystemsuseelectricalfanstodrawadditionalairthroughtheradiatorwhenextracoolingisrequired.
∙Electricalsystems.Headlights,batterycharging,activesuspension,circulatingfans,defrosters,mediasystems,speakers,andotherelectronicscanalsosignificantlyincreasefuelconsumption,astheenergytopowerthesedevicescausesincreasedloadonthealternator.Sincealternatorsarecommonlyonly40–60%efficient,theaddedloadfromelectronicsontheenginecanbeashighas3horsepower(2.2kW)atanyspeedincludingidle.IntheFTP75cycletest,a200wattloadonthealternatorreducesfuelefficiencyby1.7MPG.Headlights,forexample,consume110wattsonlowandupto240wattsonhigh.TheseelectricalloadscancausemuchofthediscrepancybetweenrealworldandEPAtests,whichonlyincludetheelectricalloadsrequiredtoruntheenginean
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