on–road measurement of gaseous emissions and fuel consumption for two hybrid electric vehicles in Macao



AtmosphericPollutionResearch6(2015)858Ͳ866

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On–road measurement of gaseous emissions and fuel consumption for two hybrid electric vehicles in Macao XiaomengWu1,ShaojunZhang1,YeWu1,2,ZhenhuaLi 1,WenweiKe 1,LixinFu 1, 2,JimingHao1,2 1 2

SchoolofEnvironment,andStateKeyJointLaboratoryofEnvironmentSimulationandPollutionControl,TsinghuaUniversity,Beijing100084,China StateEnvironmentalProtectionKeyLaboratoryofSourcesandControlofAirPollutionComplex,Beijing100084,China

ABSTRACT



Hybridelectricvehicles(HEVs)arepromotedinChinatoeaseincreasingpressuresofurbanairpollutionandoilsecurity.In thispaper,wemeasuredtwoToyotaPriusHEVsbyusingaportableemissionmeasurementsystem(PEMS)toevaluatetheir real–world performance with regard to gaseous emission factors and fuel consumption. Our results indicated that their averageexhaustemissionfactorsofCO,THC,NOXandCO2were0.25±0.08gkm–1,0.015±0.002gkm–1,0.009±0.005gkm–1 and136±21gkm–1(i.e.,5.81±0.90L100km–1forfuelconsumption)respectively,whiledrivingtheaveragedon–roadtraffic pattern. Compared to conventional gasoline and diesel vehicles, the tested HEVs demonstrated significant advantages in simultaneously mitigating major air pollutants (e.g., NOX), greenhouse gas emissions (CO2) and fuel consumption. For example,averageCO2emissionfactorsarereducedbyapproximately35%and15%relativetoconventionalgasolineand diesel cars in Macao. Unlike conventional gasoline and diesel cars, relative CO2 emission factors of HEVs were much less sensitive to speed change, while their relative NOX emission factors were reduced as average speed became lower. This indicates significant environmental and energy benefits from HEVs under congested driving conditions. Our assessment suggeststhatHEVsareacompetitivetechnologyoptionfor thetaxifleet inMacao withstrongadvantages insaving fuel costfortaxidriversandmitigatingNOXemissions.  Keywords:Hybridelectricvehicle,vehicleemissions,CO2,fuelconsumption,trafficspeed

CorrespondingAuthor:

Ye Wu

:+86Ͳ10Ͳ62796947 :+86Ͳ10Ͳ62773597

:[email protected] 

ArticleHistory: Received:20November2014 Revised:09February2015 Accepted:09March2015

doi:10.5094/APR.2015.095 

1.Introduction 

Thesurgeofvehiclepopulationoverthepasttwodecadeshas raised great challenges to national oil security, urban air quality and public health in many emerging countries (e.g., China, India). (Haoetal.,2000;Duetal.,2012;WangandHao,2012;Jaffeetal., 2014;Sharmaetal.,2014;SindhwaniandGoyal,2014).Manycities ofdevelopingcountriesarefacingsubstantialpressurestomitigate vehicle emissions to meet the stringent limits needed to comply with the National Ambient Air Quality Standard (NAAQs) of their own countries (MoEF, 2009; MEP China, 2012; Wu et al., 2012a). China has implemented a series of vehicle emission control measuressincethelate1990s(Zhouetal.,2010;Wuetal.,2011; Wuetal.,2012a;Zhangetal.,2013;Zhangetal.,2014a),including implementation of increasingly stringent standards for vehicle emissions and fuel quality, scrappage of older vehicles, enhanced inspection and maintenance (I/M) programs and city–level traffic control and management actions. Recently, alternative fuel and advanced powertrain technologies are promoted by the Chinese government, including hybrid electric vehicles (HEVs), plug–in hybrid electric vehicles (PHEVs) and battery electric vehicles (BEVs), due to their significant advantages in reducing oil consumption and on–road emissions (Samaras and Meisterling, 2008;Jietal.,2012;Wuetal.,2012b;Zhangetal.,2014b).  Macao, one of the two Special Administrative Regions in China,liesonthewesternsideofthePearlRiverDelta.Itisoneof the most populated cities in Asia (a19 000 people km–2) together with an extremely high traffic density. On–road vehicles are considered to be the dominant local source of air pollution, since

Macaoisnotdirectlyinfluencedbylocalindustrialemissions(Huet al., 2012;Zhou etal., 2014).Thevertical,horizontal andchemical profiles offineparticle matter(PM2.5)nearmajorroadshave also confirmed the significance of road traffic to the local air quality (Wuetal.,2002;Wuetal.,2003;TangandWang,2006;Shengand Tang, 2013; Song et al., 2014). The “street canyon” effect would extend the residence time of pollutants, by amplifying the accumulationofprimaryairpollutants(e.g.,CO,NOXandPM2.5)in areas of high building density (Hu et al., 2012; Sheng and Tang, 2013).Forexample,theannualconcentrationofNO2inthetraffic– populated Shui Keng Wei (the Rua do Campo Road) site is 64.5ʅgm–3 (DMSG, 2012; Song et al., 2014), higher by 61% than theWorldHealthOrganization(WHO)guidelineannualmeanvalue (i.e.,40ʅgm–3)(WHO,2005).  TrafficcongestionoccursveryfrequentlyinMacao.According totheTransportationBureauofMacao,averagespeedduringrush hoursontheMacaoPeninsulawasonly15kmh–1.Thelow–speed trafficconditionswouldsubstantiallyincreaseNOXemissionfactors of conventional vehicles (Ntziachristos and Samaras, 2000; Wang etal.,2014;Zhangetal.,2014c).Greateffortsshouldbemadeto mitigateNOXemissionsfromon–roadvehicles,especiallyfordiesel vehiclesoperatinginthetraffic–congestedMacaoPeninsula(Huet al., 2012). Technically, NOX is formed as atmospheric nitrogen competes with fuel molecules to couple with oxygen in the extremely hot exhaust gases behind the proceeding flame front (Zeldovichtype)andasreactionsoccurdirectlyinthecombustion flamezone("prompt"type)intheautomobileengine(Studzinskiet al., 1993). Several control technologies regarding both internal combustion (e.g. exhaust gas recirculation, EGR) and after–

©Author(s)2015.ThisworkisdistributedundertheCreativeCommonsAttribution3.0License.

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 companies may have distinctive characteristics of on–road emissions and energyconsumption (Adlyetal., 2006;Fontaras et al., 2008). Due to very limited HEVs in Macao compared with conventional cars, so far we are unable to recruit other HEV modelsfromHEVowners.  Theon–roadmeasurementwasconductedduringMarch2011 inMacao.Macaohasahumidsubtropicalclimate,withanaverage humidity of approximately 80% and an average temperature around 20 °C in March. We employed one Sensor Inc. SEMTECH– DSPEMStocollectsecond–by–secondemissionprofilesofgaseous pollutants.ItshouldbenotedthattheoperatingprincipleofPEMS is different from remote sensing or plume chasing technologies, which depend on background levels of pollutants and CO2 and estimateemissionfactorsbasedonacarbonbalancemethod.For example, the SEMTECH–DS employs a heated flame ionization detector (HFID) to measure total hydrocarbons (THC), a non– dispersive infrared analyzer (NDIR) to measure CO and CO2 and a non–dispersiveultravioletanalyzer(NDUV)tomeasureNOX(Wuet al., 2012a; Zhang et al., 2014c; Zhang et al., 2014d). The flow measurementwasconductedwithaSEMTECHexhaustflowmeter (SEMTECH–EFM),whichcannormalizetheinstantaneousflowrate according to the real–time exhaust temperature. In addition, vehicle speed and location data (e.g., latitude, longitude and altitude information) were continuously recorded by a GPS receiver during the tests. The measurement accuracy of the SEMTECH–DS PEMS has passed validation in the U.S. for in–use compliancemeasurements(U.S.EPA,2008).Wecalibratedallthe analyzersandtheexhaustflowmeterbeforeandafterthetesting toensuretheanalyticaluncertaintywasminor(within±5%).  The test route in Macao includes typical urban freeways, arterialroadsandresidentialroads,withatotaldistanceof36km, whichwasconsistentwiththeonesusedinourprevioustestsfor gasoline and diesel cars in Macao (Hu et al., 2012; Wang et al., 2014). The average speeds of the on–road testes were 26 km h–1 and 29kmh–1, respectively.Due tothecity speed limit in Macao (i.e.,80kmh–1),thedatawithinstantaneousspeedover80kmh–1 was very rare (i.e., less than 0.6%). Although we were unable to measure instantaneous performance of the HEV electric motors, unlikeconventionalpowertrains,wekepttheirbatteries’stateof charge(SOC)attheendoftestsalmostthesameasthebeginning ofthetests.  2.2.Dataprocessing

treatment (e.g. selective catalytic reduction for diesel vehicles, SCR; three–way catalyst for gasoline vehicles, TWC) have been appliedtoreducevehicleemissionsofNOX(Hebbar,2014;Murata etal.,2015).  Among a variety group of alternative fuel and advanced technologies,theofficialstatisticsdataindicategasoline–powered HEVs are the dominant technology in Macao. Several tests regarding fuel economy and pollutant emissions for HEVs under various test cycles or driving conditions have been performed in the past years and results showed that the HEVs could have substantial advantages in mitigating of fuel consumption and emissions of air pollutants (Lave and MacLean 2002; Adly et al., 2006;Granovskiietal.,2006;KarnerandFrancfort2007;Fontaras et al., 2008; Frey et al., 2009; Zhai et al., 2011; Holmen et al., 2014). In this study, we have a special interest in those emerging HEVtechnologyinMacaoandcomparewithconventionalvehicles, since we have completed on–road emission measurement for typical gasoline and diesel cars in Macao in our previous studies (Hu et al., 2012; Wang et al., 2014; Zhang et al., 2014d). Consequently,werecruitedtwoHEVsandmeasuredtheiron–road emission profiles by using a portable emission measurement system.Tobetterevaluatethebenefitsofreducingemissionsand fuel consumption, we compared their emission factors and fuel consumption with gasoline and diesel cars tested in a previous study.Furthermore, we exploredtheimpacts oftrafficconditions indicated by average speed on their real–world emission factors and fuel consumption. Finally, we made a comprehensive assessmentforthepenetrationofHEVsinthetaxifleetinMacao withtheirenergy,environmentalandeconomicbenefitstakeninto account. Our study can improve the understanding of real–world performance regarding emissions of air pollutants and fuel consumptionforHEVs.  2.ExperimentsandDataAnalysis



2.1.Experimentalsection



According to the official statistics data provided by the Transport Bureau of Macao, Toyota HEVs are responsible for a majorpart(nearly50%andmostlythePriusmodels)ofallexisting HEVsinMacao.Asaresult,twoToyotaPriusHEVswererecruited in this study (see Table 1). They were manufactured in 2007 and 2010, one Generation II model and one Generation III model respectively. Both vehicleshavethe similar levelsof hybridization (i.e., full hybrid) although the Prius III model has a larger engine displacementandgrossvehicleweight.Itshouldbenotedthatas one of the two Special Administrative Regions in China, China’s national emission and fuel economy standards are not implemented for vehicles in Macao (Hu et al., 2012; Wang et al., 2014). Those two HEVs comply with the 2005 emission standards fornewvehiclesmanufacturedinJapan,whichcanbeconsidered as stringent as the Euro 5 emission standards. It should be noted that different HEV models manufactured by various automotive 



In this study, we applied an operating mode binning method torelateinstantaneousemissionsofgaseouspollutants(e.g.,CO2, CO, THC and NOX) to real–world driving conditions indicated by vehiclespecificpowerandinstantaneousspeed.VSPisdefinedas instantaneous power per unit mass of the vehicle (Jimenez– Palacios,1999;Freyetal.,2008).Weusedthesimplealgorithmfor calculatingVSPoflightdutyvehiclesprovidedbyJimenez–Palacios (1999)inthisstudy[seeEquation(1)](Zhangetal.,2014d).

Table1.VehicleinformationoftwotestedHEVs TestedVehicleModel YearofManufacture

PriusII

PriusIII

2007

2010

Mileage(km)

29542

495

GrossVehicleWeight(kg)

1720

1805

EngineDisplacement(L)

1.5

1.8

ElectricMotorMaximumPower(kW)

50

60

Fullhybrida

Fullhybrid

Nickel–metalhydride(NiMH)battery

Nickel–metalhydride(NiMH)battery

HybridizationLevel Battery a

FullhybridincludesthefollowingHEVcharacteristics:engineshut–off,regenerativebraking,smallerinternalcombustion enginecomparedtoconventionalandelectricdrive(Fontarasetal.,2008)





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 ܸܵܲ ൌ ‫ݒ‬ሺͳǤͳܽ ൅ ͻǤͺͳ •‹ሺ݃‫݁݀ܽݎ‬ሻ ൅ ͲǤͳ͵ʹሻ ൅ ͵ǤͲʹ (1) ൈ ͳͲିସ ‫ ݒ‬ଷ   where, VSP is the calculated vehicle specific power, kW t–1; v is instantaneous vehicle speed, m s–1; a is instantaneous vehicle acceleration, m s–2; and grade is angle of inclination of the road. Wefurtherestablished22operatingmodebinsdefinedbyVSPand instantaneousvehiclespeed(v),includingadecelerationorbraking bin, an idling bin, and 20 bins to represent cruise or acceleration drivingmodes(seeTable2)  We calculated instantaneous fuel consumption rates for the two HEVs in this study based on real–time emission rates of carbonaceous pollutants (i.e., CO2, CO and THC), illustrated as Equation(2)(AQSIQ,2003).  ͳ ‫ ܴܥܨ‬ൌ ൈ ሺͲǤͺ͸͸ ൈ ‫்ܴܧ‬ு஼ ൅ ͲǤͶʹͻ ൈ ‫ܴܧ‬஼ை ൅ ͲǤʹ͹ ܹ஼ (2) ൈ ‫ܴܧ‬஼ைమ 

 where,FCRisthereal–worldfuelrateconsumption,gs–1;WCisthe ratio of carbon mass to total fuel mass, 0.866 for gasoline. We calculated average emission and fuel consumption rates of all gaseous pollutants by operating mode for those two HEVs respectively.Inordertoeliminatethedifferenceintheactualon– road measurement, we normalized distance–specific emission factors (g km–1)andfuel consumption (L100 km–1)toanaverage on–road driving pattern for comparison. In this study, we applied the average real–world driving pattern (i.e., baseline driving pattern) based on previous on–road tests, with an average speed of31kmh–1(Zhangetal.,2014d).  Furthermore, weapplied a micro–trip method toexplorethe impacts of driving conditions indicated by average speed on emissions and fuel consumption of tested HEVs. We divided the entire test trips of the Prius II and Prius III HEVs into 16 and 13micro–trips, respectively, which lasted 308±48 s on average. Relativeemissionfactorsandrelativefuelconsumptionaredefined as the ratio of emission factors of each micro–trip to those normalizedtothe baselinetrafficpattern.Theyare dimensionless variables and can be regarded as the speed correction factors in vehicle emission models. In this study, we employed the average on–road traffic pattern (i.e., average speed of 31 km h–1) as the baselinetrafficpattern.

 3.ResultsandDiscussion



3.1.On–roademissionsandfuelconsumptionforHEVs  Figure1presentstheaverageemissionandfuelconsumption rates by operating mode for two HEVs. In general, emission and fuelconsumptionratesofeachoperatingmode increasewiththe VSP,whichare similar tothetrendsofconventionalgasolinecars (Freyetal.,2008).ThePriusIIIHEVhasahigherfuelconsumption 

andCO2emissionratescomparedtothePriusIIHEV,primarilydue to its larger engine size and higher vehicle weight. On the other hand, average NOX emission rates of the Prius III HEV were substantiallylowerthanthoseofthePriusIIHEVwhileCOandTHC emissionratesofPriusIIIHEVwerehigher.Forgaseouspollutants, much more uncertain impacts than those on CO2 emission rates could result in on–road discrepancy of emissions, such as vehicle mileage. However, due to limited samples tested, we could not findsolidcausesforthedifferentemissionrates,sincebothofthe two HEVs’ gaseous emissions were far lower than the Euro 5 standard,whichwillbeaddressedinthefollowingtext.  We calculated distance–specific emission factors of gaseous pollutantsandfuelconsumptionundertheaverageon–roadtraffic pattern,presentedinTable3.Furthermore,wecollectedemission factors of HEVs and conventional gasoline cars from previous studies for comparative analysis. Compared to nine gasoline cars measuredinMacaowiththeirmodelyearslaterthan2000(seethe footnote of Table 3), the HEVs were comparable in CO emission factorsbutcouldsubstantiallyreduceexhaust emissionfactors of THC, NOX and CO2 and fuel consumption. For example, on–road NOX emission factors of the two HEVs were both significantly reduced by over 90% relative to the conventional gasoline counterparts in Macao, accompanied by an energy benefit of reduced fuel consumption of 27%a40%. If compared to eleven gasolinetaxistestedinBeijingwhichcanmeettheEuro4standard, thetestedHEVscansignificantlymitigategaseousemissionfactors by70%a90%forCO,THCandNOXandsaveabout40%real–world fuel consumption. The HEVs were further identified to be more competitive in controlling vehicle emissions than the new Euro 5 gasolinecars. Compared toon–roademissionfactorsfor the new Euro5gasolinecarswiththeEMBEVmodel(Zhangetal.,2014a), those two HEVs were able to reduce emission factors of CO, THC andNOXby47%,75%and53%,respectively.Wecomparedthose emissionmitigationresultsagainstconventionalgasolinecarswith previous findings from other countries and our on–road tests foundaclosereductioninfuelconsumptionandhigherreductions in NOX and THC, important precursors of ozone and secondary aerosol(Laveetal.,2002;Zhaietal.,2011;Holmenetal.,2014).  Itshouldbenotedthatsomepreviousstudieshaveindicated vehiclespecifications(e.g.,modelyear,vehicleweight,andengine displacement) can influence real–world fuel consumption and emissions(Cappielloetal.,2002;Li,2006;Freyetal.,2007;Zhang et al., 2014d). Taking CO2 emissions for example, we applied the correction function suggested by the COPERT4 model and estimatedapossibleimpactwithin3%asaresultoflargervehicle sizeforHEVs,whichisaveryminorpartrelativetothegapofCO2 emissions and energy consumption between those conventional gasoline cars and HEVs emission factors of gaseous pollutants for HEVs might rise as they accumulate vehicle kilometers, while the performance of their after–treatment devices diminish with distance travelled (Zhou et al., 2014). Further on–road measurements should be conducted to determine the impact of emissionfactordeteriorationunderreal–worldconditions.

Table2.Definitionofoperatingmodebinsaccordingtovehiclespecificpower(VSP)andinstantaneousvehiclespeed VehicleSpeed(kmhͲ1)

VSP(kWtͲ1) VSP<Ͳ4

–4чVSP<–2 –2чVSP<0

Bin0 Decelerationorbraking

v<1.6

1.6чv<40

40чv<80

v>80

Bin1 Idle

Bin11

Bin21

Bin35

Bin12

Bin22

Bin13

Bin23 Bin24

0чVSP<2

Bin14

2чVSP<4

Bin15

Bin25

4чVSP<6

Bin16

Bin26

Bin36

6чVSP<8

Bin17

Bin27

Bin37

VSPш8

Bin18

Bin28

Bin38

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 Table3.ExhaustemissionfactorsofgaseouspollutantsandfuelconsumptionforHEVsandconventionalgasolineanddieselcars CO

VehicleCategory HEVs GasolineCars

DieselCars

THC

NOX

CO2

Ͳ1

FuelConsumptiona (L100kmͲ1)

(gkm ) PriusII,thisstudy

0.19

0.013

0.012

121

PriusIII,thisstudy

0.30

0.016

0.005

151

5.18 6.45

9gasolinecarsinMacaob

0.26±0.33

0.021±0.017

0.14±0.16

205±28

8.86±1.22

11Euro4taxisinBeijingb

0.86±0.33

0.086±0.064

0.091±0.060

230±16

10.00±0.67

Euro4,theEMBEVmodelc

0.67(0.17a1.69)

0.077(0.034a0.16)

0.031(0.004a0.094)





Euro5,theEMBEVmodelc

0.46(0.12a1.15)

0.058(0.026a0.13)

0.018(0.003a0.052)





11ToyotadieseltaxisinMacaod

0.39±0.13

0.042±0.016

0.59±0.16

161±16

5.96±0.59

a

Thedensityvaluesofgasolineanddieselusedinthisstudyare730gL–1and850gL–1,respectively 9gasolinecarsinMacaoand11Euro4gasolinetaxisinBeijingweretestedon–roadbyusingaSEMTECH–DSPEMS.ThesamplesinMacaowereall manufacturedinJapanwiththeirmodelyearsfrom2002to2006.ThesamplesinBeijingwereallmanufacturedin2009tocomplywiththeEuro4emission standards(Zhangetal.,2014d).Averageemissionfactorsandfuelconsumption(±standarddeviations)areallnormalizedtotheaverageon–roadtraffic pattern c EmissionfactorsofCO,THCandNOXforEuro4andEuro5gasolinecarsaresimulatedbyusingtheEMBEVmodelata95%confidencelevel,whichare correctedtotheaveragespeedof31kmh–1(Zhangetal.,2014a) d 11dieseltaxisinMacaoweretestedon–roadbyusingaSEMTECH–DSPEMS.TheywereallToyotaCorollamodels,manufacturedinJapanduring2002– 2009(Huetal.,2012).Averageemissionfactorsandfuelconsumption(±standarddeviations)inthistableareallnormalizedtotheaverageon–roadtraffic pattern b



 Figure1.Averageemissionratesof(a)CO,(b)THC,(c)NOXand(d)CO2aswellas(e)averagefuelconsumptionratesbyoperating modefortwotestedHEVs.

 

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  The in–use taxis in Macao are powered by diesel fuel due to the better energy fuel economy relative to conventional gasoline engines.Ouron–roadmeasurementindicatesthatHEVscouldstill reduce real–world fuel consumption (unit in MJkmͲ1) and CO2 emission factorsby approximately15% over theaverageon–road traffic pattern. Furthermore, the average NOX emission factor of the two HEVs was lowered by almost two orders of magnitude comparedtothedieseltaxisinMacao(Huetal.,2012).Itisworth notingthatseveralemissionmeasurementprojectshaveidentified that real–world NOX emission factors of diesel cars in Europe are not significantly improved as their emission standards become increasinglystringentfromtheEuro1totheEuro5(Carslawetal., 2011; Chen and Borken–Kleefeld, 2014). Recent measurement results indicate real–world NOX emission factors of Euro 6 diesel carsstillexceedtheemissionstandardbyoneorderofmagnitude (Weiss et al., 2012). Furthermore, advanced after–treatment devices (e.g., diesel oxidation catalyst, DOC; diesel particle filter, DPF) applied to diesel cars significantly increase the ratio of directly–emitted NO2 (i.e., primary NO2) emissions to total NOX emissions by over 50%, increasing the atmospheric oxidation environment at the road curb (Roustan et al., 2011; Weiss et al., 2012). Therefore, HEVs can play an important role in mitigating fuelconsumptionandNOXemissionssimultaneouslyforpassenger cars,inparticularwhere severeairpollutionproblemsdue tothe NO2exceedanceofambientairqualitystandardfrequentlyoccurs owing to the operation of diesel cars. We will discuss this issue comprehensively through a case study of Macao’s taxi fleet in a latersection.  3.2. Correlations between relative emission factors and average speed  Figure2 presents correlations between relative emission factors of CO, NOX and CO2 for tested HEVs during the average speed range from 5 to 70 km h–1. In addition, as the micro–trip method can effectively eliminate the impacts of individual distinctions in emission factors, we also present the correlations for9gasolinecarsand11dieselcarsmeasuredinMacao(note:the same conventional vehicle samples applied in the comparative analysis of Table 3). The detailed vehicle information of each gasoline and diesel sample has been provided by Zhang et al. (2014d).ForHEVs,itisworthnotingthatthecorrelationcoefficient R2 between relative CO2 emission factors and average speeds of micro–trips is merely 0.04, significantly lower than those of gasoline and diesel cars (i.e., a0.9). This is because HEVs would largelyrelyontheelectricmotortolaunchbyusingtheelectricity generated by the regenerative braking system (Fontaras et al., 2008). Besides, if the SOC of the on–board battery is low, its gasoline engine would provide increased power to charge the batteryunderhigh–speedcruisedrivingconditions.Therefore,the assistance of electric motor makes the real–world CO2 emission factorsofHEVsalmostinsensitivetodrivingconditions.Inaddition, unlikegasolineanddieselcars,therelativeNOXemissionfactorsof HEVsbecomelowerastheaveragespeeddecreases.Forexample, under congested traffic conditions when average speed is lower than 20 km h–1, real–world NOX emission factors are reduced by a50%comparedtothoseunderdrivinganaverageon–roadtraffic pattern(i.e.,thebaselineaveragespeedat31kmh–1).Aprevious laboratorymeasurementstudybyusingachassisdynamometerto simulateavarietyofdrivingcyclesalsoindicatedthatNOXemission factorsofHEVswereincreasedwiththeaveragespeedofthetest cycles (Fontaras et al., 2008). THC emission factors of HEVs decreasedastheaveragespeedincreased,similartothetrendfor conventional gasoline and diesel cars. However, the correlation coefficients (R2) of the HEVs and gasoline cars are below 0.5, significantly lower than that of diesel cars (0.83). This is probably because the TWC installed on HEVs and gasoline cars have a significantimpactonexhaustemissionsofCO,THCandNOX,asthe correlations between air pollutants (e.g., CO, THC, NOX) and CO2 for TWC–equipped gasoline cars and HEVs are much lower than

those for diesel cars. The performance of this after–treatment technologycouldbegreatlyaffectedbyotheroperatingconditions (e.g.,coldstart)otherthandrivingspeed.  We acknowledge that there are other influencing factors leading to the uncertainty in those speed–dependent functions, such as road slope, different proportion of acceleration, weather conditions and cold start in real–world operation. Taking CO2 emissionsforexample,weusearesidualerroranalysistoquantify theuncertaintyfromotheroperatingconditions.Forexample,the relative uncertainty in our constructed speed–dependent CO2 curvesare–29%to+34%forHEVs,–13%to+21%forgasolinecars and–12%to+19%fordieselcars,allata90%confidencelevel(i.e., P5–P95).GreateruncertaintyforHEVsthanconventionalgasoline anddieselvehiclesalsorepresentshighercomplexitywithelectric motorfunctioningontheroad.  A sensitivity analysis for HEVs, gasoline and diesel cars was performed in order to observe the extent that relative emission factorsandfuelconsumptionwillbeaffectedbyagivenchangeof average speed. The changes in relative fuel consumption rate correspondingtoadecreasingaveragespeedrateof5kmh–1from 35 to 15 km h–1 are provided in Table 4; this range is an average speed with a high probability of occurring in urban areas. Not surprisingly, CO2 emission factors (i.e., fuel consumption) of HEVs areleastsensitivetospeedchangeamongthethreevehicletypes. In particular, under extremely congested traffic conditions when average speed is only 15 km h–1, average CO2 emission factors of HEVswillbeloweredby61%and42%respectively,relativetothe gasoline and diesel counterparts in Macao. The benefits of mitigatingNOXemissionfactorscomparedtoconventionalgasoline and diesel cars become more significant under increasingly congested traffic conditions. Therefore, our evaluation clearly indicates that HEVs have greater environmental and energy benefits under the congested operating conditions compared to conventional vehicle technologies. However, it should be noted that due to the speed limit in Macao, we were unable to collect test data with speed higher than 80 km h–1. The image of high– speeddrivingconditionsforbothgasolineandHEVsisgoingtobe updated with more test data in other regions. Based on previous researches, CO2 emission factors of gasoline cars would be significantlyelevatedwithintheaveragespeedrangeof80–140km h–1(NtziachristosandSamaras,2000).TheenergybenefitofHEVs relative to their conventional gasoline counterparts would be narrowedunderthehigh–speeddrivingconditions(Fontarasetal., 2008). Similarly, for NOX emissions, previous studies also have identified an elevation of speed correction factors under high– speed conditions (Ntziachristos and Samaras, 2000; Zhou et al., 2010;Kousoulidouetal.,2013).Therefore,theemissionreduction benefits of HEVs compared to conventional gasoline cars under free driving conditions (e.g., inter–city expressways) should be carefully assessed in the future with more high–speed measureͲ mentdataavailable.  3.3. Energy, environmental and economic assessment of the penetrationofHEVsintothetaxifleetinMacao



InmanyofChina’scities,taxishavesignificantlyhigherannual vehiclekilometerstravelled(VKT)comparedtoprivatecars,usually over100000kmperyear.Thisisbecausetaxidriversmakeprofits proportionately to distance traveled, and they often need to extendtheirservicehourstomakeadditionalprofitsaftercovering fixed contract and management fees. As a result, taxi drivers are verysensitivetothetotalfuelcost,whichisgreatlyinfluencedby the on–road energy economy of taxis. Furthermore, taxis are alwaysthe subjectofattentionbylocalenvironmentalprotection bureaus in China, since the deterioration of emission factors for taxisismoresignificantthanprivatecarsduetotheextremelyhigh vehicle–useintensity. 

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  (a)TwoPriusHybridVehicles

AverageSpeed(kmhͲ1)

AverageSpeed(kmhͲ1)

AverageSpeed(kmhͲ1)

(b)NineGasolineCarsManufacturedDuring2002–2007

AverageSpeed(kmhͲ1)

AverageSpeed(kmhͲ1)

AverageSpeed(kmhͲ1)

(c)ElevenDieselCarsManufacturedDuring2002–2009

AverageSpeed(kmhͲ1)

AverageSpeed(kmhͲ1)

AverageSpeed(kmhͲ1)

Figure2.Correlationsbetweenrelativeemissionfactorsof(a)CO2(b)NOXand(c)THCandaveragespeedofmicro–tripsforHEVs, gasolinecarsanddieselcars(Thecorrelationsbetweenrelativefuelconsumptionandaveragespeedhighlyresemblethosefor relativeCO2emissionfactorsforthreevehiclecategoriesrespectively.ThecorrelationsbetweenrelativeCOemissionfactorsand averagespeedaresimilartothepatternsofrelativeTHCemissionfactors).

 Table4.ResultsofsensitivityanalysisofrelativeemissionfactorsforHEVs,gasolinecarsanddieselcarstoaveragespeedchanges ChancesofRelativeEmissionFactors a

VehicleCategory

HEVs

GasolineCars

DieselCars

a

Pollutant

 ଷ଴   ଷହ

CO THC NOX CO2 CO THC NOX CO2 CO THC NOX CO2

1.22 1.20 0.91 1.01 1.20 1.24 1.31 1.12 1.12 1.10 1.10 1.09

 ଶହ   ଷ଴

 ଶ଴   ଶହ

 ଵହ   ଶ଴

 ଵହ   ଷହ

1.24 1.21 0.92 1.01 1.26 1.31 1.33 1.15 1.14 1.12 1.12 1.10

1.24 1.21 0.93 1.00 1.28 1.33 1.31 1.18 1.18 1.15 1.15 1.13

1.23 1.20 0.94 1.00 1.28 1.31 1.29 1.24 1.24 1.20 1.20 1.17

2.28 2.13 0.74 1.02 2.48 2.82 2.94 1.88 1.87 1.72 1.73 1.57

Thesubscriptnumberindicatetheaveragespeed(kmh–1)forthatrelativeemissionfactor(REF)

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  In Macao, there were a total 1 163 registered taxis in May 2013.Theirfleet–averageannualVKTisapproximately130000km per year based on our investigation in Macao. It is also very commoninMacaothattwotaxidriversworkshiftsofasingletaxi, whichmeanstheycanoperatethattaxialmost24–hoursadayand sharethecostoffees.ThetaxisinMacaoareallpoweredbydiesel duetobetterenergyeconomythangasolinecars.The2.0LToyota Corolla diesel cars with model years starting from 2004 are the dominant taxi model and compose nearly 85% of the total taxi fleet in Macao. We assess the energy, environmental and economicbenefits of the penetration of HEVs against the current diesel taxis together with a gasoline taxi scenario developed for comparison.AsTable5illustrates,althoughthevehiclesalesprice of HEVs is higher compared to the diesel counterparts, their significant advantage in fuel economy could provide a breakeven point of total fuel cost plus vehicle cost in less than 3 years (see Figure3). This could stimulate taxi drivers in Macao to embrace HEVs in the future, if the government and taxi companies notice the substantial benefits and allow HEVs into the taxi market in Macao. Furthermore, HEVs could significantly mitigate NOX emissions compared to diesel taxis under the congested driving conditionsinMacao.Ifthetotaltaxifleetiscompletelycomposed ofHEVs,nearly85tonsofNOXemissionscouldbeabated,whichis equivalent to 5% of total vehicular NOX emissions in Macao. Recently, advanced after–treatment devices (e.g., DPF) are being installed in new diesel cars in order to meet stringent emission standards. Specific deNOX devices (e.g., lean NOX trap, LNT; selectivecatalyticreduction,SCR)willbenecessaryfornewdiesel carsinEuropecomplyingwiththeEuro6emissionstandard,which will increase the manufacture cost of diesel cars (Boulter et al., 2013; Wallington et al., 2013). Therefore, the future payback periodmayprobablybeshortenedwhenreplacingadieseltaxiby anHEV.  4.SummaryandConclusions



In this study, we measured on–road emissions of gaseous pollutants from two HEVs (Toyota Prius II and Prius III models) in Macao by using a SEMTECH–DS PEMS. Our results show that average emission rates of all pollutant categories for each operatingmodegenerallyincreasewithincreasingVSP.Drivingthe averageon–roadtrafficpattern,averageexhaust emissionfactors of CO, THC, NOX and CO2 were 0.25±0.08 g km–1, 0.015±0.002g 

km–1, 0.009±0.005 g km–1 and 136±21 g km–1 respectively. Based onthecarbonbalancemethod,averagefuelconsumptionofthose two HEVs was 5.81±0.90 L 100 km–1. Compared to previous on– road measurement results of conventional gasoline and diesel vehicles, those two HEVs presented significant advantages in simultaneously mitigating major air pollutants (e.g., NOX), greenhouse gas emissions (CO2), and fuel consumption especially under the low–speed traffic conditions. Average CO2 emission factors are reduced by approximately 35% and 15% relative to conventionalgasolineanddieselcarsinMacao.Meanwhile,those two HEVs can also substantially reduce real–world NOX emission factors by over 90% compared to their gasoline and diesel counterparts. We also observed correlations between relative emission factors and average speed for tested HEVs based on a micro–trip method. Unlike conventional gasoline and diesel cars, relative CO2 emission factors of HEVs were identified as being almost insensitive to speed change. Their relative NOX emission factors even decreased as average speed became lower, representing more congested traffic conditions. Therefore, the benefits of mitigating CO2 and NOX emissions for HEVs compared to their gasoline and diesel counterparts would be increasingly significant under extremely congested traffic conditions. For example, the HEVs can reduce CO2 emission factors by over 60% comparedtogasolinecarswhenaveragespeedismerely15kmh– 1. Considering different vehicle technologies adopted by various vehicle models and intrinsic intra–vehicle discrepancy, our results do not necessarily represent on–road emission characteristics for allHEVs.Futureupdatesarerequiredwithon–roadmeasurement profilesforavarietygroupofHEVmodels.  Wefinallyperformedanenergy,environmentalandeconomic assessmentofthepenetrationofHEVsintothetaxifleetinMacao. Our results showed that although HEVs have higher vehicle cost, their better energy economy can save annual fuel costs up to 25 thousandRMBpertaxirelativetodieselcars.Thiscouldprovidea paybackperiodofonly2a3years,muchshorterthanthetypicalin– service duration of diesel taxis in Macao (8 years). On the other hand,ifthetotaltaxifleetiscomposedofHEVs,nearly85tonsof NOX emissions could be mitigated, which is equivalent to 5% of totalvehicularNOXemissionsinMacao.Therefore,ourassessment indicates that HEVs are a competitive technology option for taxi fleetswithstrongadvantagestosavingfuelcostandmitigatingair pollutants.

Table5.Energy,environmentalandeconomicassessmentofthepenetrationofHEVsinthetaxifleetinMacao 

DieselTaxi

HEV

GasolineTaxi

ToyotaCorolla,2.0L

PriusIIandPriusIIIa

ToyotaCorolla,1.8L

VehicleSalePrice(1000RMB)b

190

250

150

AverageSpeed(kmhͲ1)

24

24

24

130000

130000

130000 9.79

ReferenceVehicleModel

AnnualVKT(km) FuelPrice(RMBLͲ1)c

a

10.56

9.79

DistanceͲSpecificFuelConsumption(L100kmͲ1)

7.1

5.7

11.1

AnnualFuelConsumptionperVehicle(m3)

9.28

7.44

14.45 141

AnnualFuelCostperVehicle(1000RMB)

98

73

AnnualNOXEmissionsperVehicle(kg)d

77

1.1

18

LifeͲtimeFuelCost(1000RMB)e

785

583

1132

WeusetheaverageddataforthePriusIIandPriusIIIHEVsinordertoreducetheuncertaintyinmeasuredfueleconomyand emissionfactors b DuetothelackofpriceinformationinMacao,wereferthevehiclesalepriceinMainlandChina c WeconverttheaveragefuelpriceofgasolineanddieselduringQuarter1inMacaobasedonthecurrencyrate(0.79)between MacaoandMainlandChina. d Weusetheon–roademissionfactorsfrom9testedgasolinecarsinMacaotocalculateannualNOXemissionsfromagasolinetaxi e ThetypicallifetimeoftaxisinMacaois8years



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  Fontaras, G., Pistikopoulos, P., Samaras, Z., 2008. Experimental evaluation of hybrid vehicle fuel economy and pollutant emissions over real– world simulation driving cycles. Atmospheric Environment 42, 4023– 4035. Frey, H.C., Zhai, H.B., Rouphail, N.M., 2009. Regional on–road vehicle running emissions modeling and evaluation for conventional and alternative vehicle technologies. Environmental Science & Technology 43,8449–8455. Frey, H.C., Zhang, K.S., Rouphail, N.M., 2008. Fuel use and emissions comparisons for alternative routes, time of day, road grade, and vehicles based on in–use measurements. Environmental Science & Technology42,2483–2489. Frey, H.C., Rouphail, N.M., Zhai, H.B., Farias, T.L., Goncalves, G.A., 2007. Comparing real–world fuel consumption for diesel– and hydrogen– fueled transit buses and implication for emissions. Transportation ResearchPartD–TransportandEnvironment12,281–291. Figure3.Thetotalcostofvehiclepurchaseandfuelconsumptionfor dieselcars,HEVsandgasolinecarsinthetaxifleetofMacao(The potentialdifferenceinmaintenancecostamongthreevehicle technologiesisnotconsideredinthisstudy).



Acknowledgments



This work was supported by the National Natural Science Foundation of China (No. 51322804 and No. 51378285), the National Science & Technology Pillar Program of China (2013BAC13B03), the Environmental Public Welfare Project (201309071), and the Program for New Century Excellent Talents in University (NCET–13–0332). The authors thank Mr. Charles N. Freed(formerlyofU.S.EPA)forhishelpfulinimprovingthispaper. The contents of this paper are solely the responsibility of the authors and do not necessarily represent official views of the sponsors. 

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