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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