Energy Policy 38 (2010) 6919–6925

Contents lists available at ScienceDirect

Energy Policy

0301-42

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/enpol

Effect of hybrid system battery performance on determining CO2 emissions ofhybrid electric vehicles in real-world conditions

Robert Alvarez n, Peter Schlienger, Martin Weilenmann

Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Internal Combustion Engines, Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland

a r t i c l e i n f o

Article history:

Received 31 March 2010

Accepted 7 July 2010Available online 3 August 2010

Keywords:

Hybrid electric vehicle

CO2 emissions

Real-world

15/$ - see front matter & 2010 Elsevier Ltd. A

016/j.enpol.2010.07.008

esponding author. Tel.: +41 44 823 48 69; fa

ail address: robert.alvarez@empa.ch (R. Alvar

a b s t r a c t

Hybrid electric vehicles (HEVs) can potentially reduce vehicle CO2 emissions by using recuperated

kinetic vehicle energy stored as electric energy in a hybrid system battery (HSB). HSB performance

affects the individual net HEV CO2 emissions for a given driving pattern, which is considered to be

equivalent to unchanged net energy content in the HSB. The present study investigates the influence of

HSB performance on the statutory correction procedure used to determine HEV CO2 emissions in Europe

based on chassis dynamometer measurements with three identical in-use examples of a full HEV model

featuring different mileages. Statutory and real-world driving cycles and full electric vehicle operation

modes have been considered. The main observation is that the selected HEVs can only use 67–80% of the

charge provided to the HSB, which distorts the outcomes of the statutory correction procedure that does

not consider such irreversibility. CO2 emissions corrected according to this procedure underestimate the

true net CO2 emissions of one HEV by approximately 13% in real-world urban driving. The correct CO2

emissions are only reproduced when considering the HSB performance in this driving pattern. The

statutory procedure for correcting HEV CO2 emissions should, therefore, be adapted.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Hybrid electric vehicles (HEVs) represent a promising ap-proach to reduce vehicle exhaust emissions of CO2. An additionalelectric powertrain including an energy storage device, typically arechargeable battery or supercapacitors, is combined with acombustion engine to provide the desired overall vehicle poweroutput. This configuration makes it possible to design and employthe internal combustion engine in its most efficient operatingconditions and to recuperate kinetic vehicle energy duringdeceleration for further use, which leads to reduced overall CO2

vehicle emissions (Sundstrom, 2009; Sundstrom et al., 2008,2010). HEVs are typically categorized according to their capabilityfor full electric driving (full HEV) or not (mild HEV). HEV sales areincreasing (AFCD, 2010) and expected to attain a considerablemarket share in the near future (IIEA-HEV, 2008; Christidis et al.,2005; Duleep et al., 2004) because of specific CO2 vehicle emissionreduction policies (An, 2007) and legislation (Regulation EC443/2009) (Fontaras and Samaras, 2010). Furthermore, HEVs areassumed to open up the way towards electricity-based power-train solutions such as plug-in hybrid electric, electric or fuel cellvehicles (Van Mierlo et al., 2006). Initial studies have already been

ll rights reserved.

x: +41 44 823 40 44.

ez).

carried out to determine the real-world pollutant emissionperformance of HEVs (Fontaras et al., 2008).

A crucial feature of HEVs is the ability of their electric storagedevice to provide previously stored electric energy to be used asdrive energy; as a result, this drive energy does not need to beprovided by combustion engine, which reduces vehicle CO2

emissions. When a hybrid system battery (HSB) is employed,HSB performance generally depends on the battery technologyused and its layout; moreover, HSB performance is also device-specific due to manufacturing deviations and in-use deterioration.The net CO2 emissions of a HEV in any driving pattern are affectedbecause they need to be described as equivalent to the unchangednet energy content or state of charge (SOC) of its HSB in contrastto other pollutant emissions (Fontaras et al., 2008). Therefore, anadequate procedure for correcting the recorded raw CO2 emis-sions of individual HEVs has to be applied in most cases to reflectthe true emission level of the HEV because the battery SOC cannotbe measured externally.

To study the effect of HSB performance on determining net CO2

emissions of HEVs under real-world conditions, an experimentalinvestigation with three identical in-use examples of a full HEVmodel featuring different mileages has been conducted on achassis dynamometer. Test runs with the statutory cycle forEurope NEDC and the real-world Common Artemis Driving Cycle(CADC), which includes urban, rural and motorway drivingpatterns, have been performed together with constant-speed fullelectric driving and vehicle traction mode. The test results are

R. Alvarez et al. / Energy Policy 38 (2010) 6919–69256920

discussed in detail to highlight the importance of including theindividual HSB performance in contrast to the statutory correctionprocedure when determining true net CO2 emissions of HEVs for agiven driving pattern. The implications of this finding are outlinedand possible alternatives are proposed.

2. Methodology

2.1. Vehicle sample

The main characteristics of the three identical in-use examplesof the HEV model selected for the test series with differentmileages are summarized in Table 1. This HEV is categorized as afull hybrid, i.e., the HSB is capable of full electric vehicle operationin certain driving situations in addition to assisting thecombustion engine and recuperating kinetic vehicle energyduring deceleration (Danisch and Goppelt, 2004). Therefore, acertain SOC range of the HSB is made available by the powertraincontrol system of this HEV; at its lower limit, the HSB is rechargedutilizing the combustion engine. Under normal operatingconditions, the HEV powertrain control system attempts tooperate around a defined SOC level of the HSB to prevent theSOC level from reaching its lower limit. Note that no particularservicing was carried out before the test runs except a generalvehicle function check.

2.2. Experimental program

Several driving cycles were employed in the test series toinvestigate the effect of HSB performance on determining CO2

emissions of the selected HEVs. The statutory cold-start drivingcycle for Europe NEDC (Council Directive 70/220/EEC) was included,as well as the real-world Common Artemis Driving Cycle (CADC).The warm-start CADC was derived from driving behavior studieswithin the ARTEMIS research program, and its sections representreal-world European urban, rural and motorway driving behaviorfor cars (Andre, 2004). Additionally, test runs with full electricdriving and vehicle traction mode, which simulates coastingconditions, were executed at a constant speed of 25 km h�1 todetermine the resulting net HSB charge flow when maximallydischarging and charging the HSB over its available SOC range.

The single cycle sections of the CADC were started withdifferent initial SOC levels of the HSB to investigate the sensitivityof the actual SOC of the HSB to each driving pattern with regard toCO2 emissions and to further apply a correction method todetermine the net CO2 emissions of the single HEVs as presented

Table 1Main characteristics of the considered vehicle sample. IC: internal combustion; El.: ele

Characteristic HEV A

Vehicle Make & model (–)

Inertia settinga (kg)

Gearbox (–)

Certification class (–)

1st certification (–) February 06

Mileage (km) 32 768

IC engine Displacement (cm3)

Rated power (kW)

El. motor Rated power (kW)

HSB Type (–)

Nominal voltage (V)

Number of cells (–)

a Empty mass plus 100 kg.

below. The maximum and minimum initial SOC of the consideredHSB are defined by having no additional charge leading to the HSBin vehicle traction mode or having the combustion engine startedby the HEV powertrain control system in full electric vehicledriving to ensure the minimum permitted SOC level, respectively.A medium initial SOC level of the HSB has also been included andis set identically for all tests using the information on theinstrument panel of the individual vehicles. However, this initialSOC condition was not strictly adjustable. The test runs with thestatutory cycle NEDC were all started with the maximum SOC ofthe HSB.

2.3. Experimental setup

Fig. 1 shows the overall experimental setup employed for thetest series. The exhaust was sampled with a constant volumesampling (CVS) system. Exhaust emissions of CO2 were measuredaccording to the statutory procedure specified in Council Directive70/220/EEC of storing a sample of diluted exhaust in a tedlar gassampling bag and analyzing its content offline after completion ofthe test run. Time-resolved measurements of raw exhaust CO2

emissions were also performed, correcting the resulting signaltraces with respect to time delay due to the length of the samplelines. In both cases, an adequate exhaust gas analyzer (HORIBAAIA-110S) as specified by Council Directive 70/220/EEC wasemployed. The time-resolved HSB wire current was measuredwith a clamp-on ammeter (LeCroy CP500) to meet the criteriaspecified in Regulation ECE R-101 of Council Directive70/220/EEC. In addition, the terminal voltage of the HSB wasmeasured using differential probe analyzers (LeCroy ADP305).Both measurements were recorded with a digital samplingoscilloscope (LeCroy WaveRunner 44Xi).

The chassis dynamometer and its settings were appliedaccording to the provisions of Council Directive 70/220/EEC. Thedriving resistance of the vehicle was simulated using the coast-down data provided by the manufacturer, and the inertia settingswere set at empty vehicle mass plus 100 kg payload (Table 1). Theambient conditions of the test cell were controlled to 23 1C and50% relative air humidity to prevent ambient conditions frominfluencing HSB operation and, therefore, CO2 emissions (Fontaraset al., 2008). All HEVs were operated with the same standard fuelwith low sulfur content.

2.4. Correction of raw CO2 emissions

Net CO2 emissions of a HEV in any driving pattern must beequivalent to unchanged net energy content or state of charge

ctric; HSB: hybrid system battery.

HEV B HEV C

Toyota Prius II

1425

CVT

Euro-4

August 06 June 05

60 761 104 266

1497

57

50

NiMH

201.6

168

DilutionTunnelBlower

SampleBags

CO2BagOnline

ebuTdetaeH

Dilution Air Inlet

ChassisDynamometer

(dry&cold)

Heated Conduit

HFM

Online sampling

CVS-System

I V

Fig. 1. Schematic diagram of the test setup. HFM: hot-film air-mass flow meter; CVS: constant volume sampling; V: measurement of HSB terminal voltage; I: measurement

of HSB wire current.

SOC (Q)

ηout

ηin Qin

Qout

Fig. 2. Schematic flow diagram of the charge flows of a HSB.

R. Alvarez et al. / Energy Policy 38 (2010) 6919–6925 6921

(SOC) of the HSB to avoid considerable under- or overestimation.However, a driving pattern with unchanged net SOC of the HSB isunlikely to be performed for most HEVs; thus, the resulting rawCO2 emissions need to be adjusted. Because the SOC of the HSBcannot be measured externally, an adequate correction procedurehas to be derived to achieve this aim.

The statutory procedure for Europe for correcting CO2 emis-sions of HEVs is described in Regulation ECE R-101 of CouncilDirective 70/220/EEC. This procedure is prescribed for every HEVwhose HSB provides electric energy of more than 1% compared tothe total fuel energy employed within a given driving cycle, i.e.,mild HEVs are typically exempted. The change in electric energyof the HSB DEbatt is defined to be equivalent to the nominalHSB energy content ETEbatt weighted by the change in SOC of theHSB and expressed by the product of the net charge flow Q

recorded on the HSB wire in a driving pattern and the HSBnominal voltage Vbatt:

DEbatt ¼DSOC½�� � ETEbatt ¼Q � Vbatt ð1Þ

The measured charge balance Q is therefore the only indicatorused to reflect changes in the SOC level of the HSB. Given this, n

measurements of a certain driving pattern are recorded for aparticular HEV to obtain a data set that includes raw CO2

emissions Mi and their respective Qi; at least one of the lattershould be negative to derive a correction factor for CO2 byapplying a linear regression on this data set:

KCO2¼

nP

Qi �Mi�P

Qi �P

Mi

nP

Q2i �

PQi

� �2ð2Þ

This correction factor is provided by the manufacturer forcertification purposes. The resulting net emissions of CO2 M0 fromthe raw emissions M of this particular HEV obtained for the givendriving pattern are then defined as

M0 ¼M�KCO2� Q ð3Þ

Two assumptions are made in this statutory procedure todescribe DEbatt of the HSB using DSOC and correct HEV CO2

emissions: first, it is assumed that the HSB terminal voltageremains constant and equal to its nominal voltage. Second, noirreversibility during storage and further usage of the chargeprovided to the HSB is implicitly assumed.

In real operation, however, neither of the two assumptions islikely to occur; in particular, the charge provided to the HSB Qin

and used from Qout are to be efficiency-delimited, as depicted in

Fig. 2. The single efficiencies Zin and Zout generally depend on theactual SOC, current, voltage, temperature and state of deteriorationof the HSB. The resulting charge balance equivalent to DSOC of theHSB in a driving pattern is then

Q ¼

ZZinðtÞ �

@Qin

@tdt�

Z1

ZoutðtÞ�@Qout

@tdt ð4Þ

Furthermore, when Zin and Zout are assumed to be constant, therelation between the charge flows provided to and used from theHSB per unit battery SOC can be expressed as

Qout ¼ Zin � Zout � Qin ð5Þ

Therefore, only the fraction Zin�Zout of the charge provided tothe HSB is used per unit battery SOC; thus, the HSB performance isstudied in terms of its individual ability to store and further usecharge provided to the HSB. This performance needs to beconsidered when using charge balance to describe the SOC ofthe HSB and subsequently to derive adequate correction factors todetermine the net CO2 emission of HEVs.

3. Results

3.1. Maximum hybrid system battery charging and discharging

Three repetitions of test runs were performed operating thesingle vehicles in full electric driving and vehicle traction mode at aconstant speed of 25 km h�1 and utilizing the whole permitted SOCrange of their HSBs. The resulting net average charge flows whenmaximally charging and discharging the same SOC range of thesingle HSBs are presented in Fig. 3 with their standard deviations. Itcan be seen that the maximum amount of charge available from thesingle HSBs decreases significantly with increasing vehicle mileage,

R. Alvarez et al. / Energy Policy 38 (2010) 6919–69256922

which indicates possible deterioration effect caused by in-useageing. The maximum amount of charge required by the singleHSBs is always substantially higher than the available charge. Thisobservation is independent of vehicle mileage; thus, the ratios ofcharge used from and provided to the HSB in this driving regimewere 78%, 67% and 72% for HEV A, B and C, respectively.

Because the demand on the HSB was almost constant duringfull electric constant-speed operation of the vehicles, these ratiosmay represent the characteristic Zin�Zout of the single HSBs andthus reflect the individual HSB performance with respect to usingstored charge provided to the HSB in this driving regime. Theselevels of performance are more likely to be affected by thenumber and depth of in-use HSB charge and discharge cycles thanby HSB ageing due to absolute vehicle mileage. The excess electricenergy provided to the HSB is assumed to dissipate as heatthrough the air cooling system of the HSBs.

3.2. Statutory emission performance

Fig. 4 shows the single HEV emission performance in thestatutory driving cycle NEDC of raw and corrected CO2 emissions

-2.0

-1.0

0.0

1.0

2.0

3.0

HEV A HEV B HEV CCha

rge

[Ah]

Maximum Charge Maximum Discharge

Fig. 3. Average and standard deviation of maximum net charge flows used from

and provided to the individual HSBs of the HEVs during full electric driving and

vehicle traction mode at 25 km h�1.

60

70

80

90

100

110

120

130

HEV A

UDC

CO

2 em

issi

ons

[g k

m-1

]

corrected emission (E

HEV B HEV C HEV A

Fig. 4. Raw and corrected CO2 emissions according to the statutory correction procedure

of the HSB.

according to Regulation ECE R-101, i.e., based on a single test runand employing the same CO2 correction factors for that cycleprovided by the HEV manufacturer. No particular trends for thesingle HEVs can be detected from the raw CO2 emissions, whichindicates that the low driving dynamics of the NEDC do not makegreat demands on their hybrid powertrains. All three vehiclesexhibit final CO2 emissions similar to the official value of104 g km�1 CO2 stated for this HEV model (Danisch and Goppelt,2004) even though the chosen inertia settings exceed the respectivecertification specification by 65 kg. Additionally, it is important tonote that a moderate measurement variability regarding vehicleCO2 emissions is implicitly allowed within the provisions ofRegulation ECE R-101 because of the single test run prescription.

Although these test runs were started with a maximallycharged HSB, a correction toward lower CO2 emissionsalready needs to be applied in the first cycle section of the NEDC,the Urban Driving Cycle (UDC). According to the correctionmethodology described above, this observation would imply anincrease in SOC of the HSB during the UDC, which cannot haveoccurred, also according to the information on the instrumentpanel of the vehicles in these test runs.

This observation can be explained by the findings summarizedin Fig. 3. More charge than can be used per unit SOC must beprovided to the HSBs of the single HEVs, i.e., whenever thesevehicles attempt to maintain a certain SOC level in regular drivingconditions, as in the UDC, more charge will have to be provided tothe HSB than is used. This circumstance distorts the outcomes ofthe statutory correction procedure, which does not consider anyirreversibility in HSB charge flow per unit battery SOC and resultsin the misleading indication that all of the excess charge providedto the HSB is available to further reduce vehicle CO2 emissions.Therefore, the HSB performance levels with regard to the ability tofurther use provided charge presented in Fig. 3 are also reflectedhere: the lower the stated ratio the greater the resultant absolutestatutory CO2 correction.

3.3. Real-world emission performance

The test results for raw CO2 emissions obtained for thedifferent cycle sections of the CADC representing real-worldurban, rural and motorway driving patterns and started with

NEDCEUDC

CE R-101) raw emission

HEV B HEV C HEV A HEV B HEV C

of the single HEVs in the statutory driving cycle NEDC started with maximum SOC

0

20

40

60

80

100

120

140

160

180

200

HEV A

CADC urban

CO

2 em

issi

ons

[g k

m-1

]

SOC max SOC med SOC min

HEV B HEV C HEV A HEV B HEV C HEV A HEV B HEV C

CADC rural CADC mway

Fig. 5. Raw CO2 emissions of the single HEVs in the real-world driving cycle CADC started with maximum, medium and minimum HSB SOC.

Table 2Corrected CO2 emissions M0 according to the statutory procedure and their

correction factors KCO2as well as the coefficient of determination R2 for the HEVs

in the real-world driving cycle CADC.

M0

(g km�1)

KCO2

(g km�1 Ah�1)

R2 (–)

CADC urban HEV A 123.2 40.233 0.9939

HEV B 106.8 40.259 0.9983

HEV C 129.8 37.958 0.9475

CADC rural HEV A 100.2 11.252 0.9838

HEV B 97.5 8.232 0.9812

HEV C 102.8 7.798 0.8587

CADC motorway HEV A 152.2 14.387 0.9996

HEV B 147.9 11.026 0.9878

HEV C 152.9 0.992 0.8179

90 100 110 120 130 140 150 16090

100

110

120

130

140

150

160

Correction based on net HSB charge flow

Cor

rect

ion

base

d on

net

HS

B w

ire e

lect

ric e

nerg

y Corrected CO2 emissions of single HEVs in CADC

CO2 [g km−1]

Fig. 6. Comparison of corrected CO2 emissions in the CADC derived from the

statutory correction procedure based on both net HSB charge and net HSB wire

electric energy.

R. Alvarez et al. / Energy Policy 38 (2010) 6919–6925 6923

minimum, medium and maximum SOC of the HSB are summar-ized in Fig. 5. The initial SOC of the HSB significantly influencesthe resulting raw CO2 emissions of the single HEVs for real-worldurban driving: the results vary by 30–40% of the average. Thiseffect is less pronounced for rural driving and almost non-existentfor motorway driving, which indicates that the hybridpowertrains of the considered HEVs influence their raw CO2

emissions most effectively in real-world urban driving patterns.In this driving pattern, raw CO2 emissions of the HEVs tend to

increase with vehicle mileage for the different initial SOCconditions of their HSBs, whereas the spread in raw CO2 emissionsis reduced. The absolute amount of charge that the single HSBscan facilitate decreases as the mileage increases (Fig. 3), whichleads to a more extensive demand on the combustion enginewhen performing this driving pattern.

Given the data set for the CADC, the statutory procedure forderiving CO2 correction factors according to Eq. (2) is applied tothe HEVs, and the outcomes are summarized in Table 2. This dataset complies with all requirements for applying the statutorycorrection procedure specified in Regulation ECE R-101 for eachHEV and cycle section of the CADC. However, a CO2 correctionprocedure for the motorway section of the CADC is not neededbecause of the limited energetic contribution of the HSB in thatdriving pattern, which reflects the moderate quality of the linearregression for HEV C.

Again, HEV B requires the largest correction in net CO2

emissions in all driving patterns presumably because of its lowHSB performance in terms of using charge provided to the HSB(Fig. 3). The individual correction factors derived for each HEV inthe single driving patterns also vary considerably, which reflectsthe effect of their particular HSB performances on the resultingnet HSB charge flows and the raw CO2 emissions in the differentdriving patterns (Fig. 5). Therefore, using a common correctionfactor to determine the CO2 emissions of individual HEVs in adefined driving pattern as considered in the statutory procedure isnot adequately accurate.

The corrected CO2 emissions summarized in Table 2 generallycomply with emission results obtained from another experimen-tal investigation carried out with comparable methodology andemploying the same real-world driving cycle and HEV model

R. Alvarez et al. / Energy Policy 38 (2010) 6919–69256924

(Fontaras et al., 2008). However, slightly lower CO2 emissionlevels are reported there for some driving patterns, which isattributed to the lower inertia settings that were set equal to thecertification specifications of the vehicle and to the presumablybetter overall HSB performance of that HEV because of its verylow mileage.

The assumption within the statutory correction procedure fordetermining CO2 correction factors of having constant HSBnominal voltage is also verified by the CADC test data. Therefore,the measured net HSB wire energy in the single driving patterns,which is calculated from the product of the measured HSB wirecurrent and its terminal voltage, forms the supporting points ofthe linear regression instead of the net HSB charge flow Q. Fig. 6compares the corrected CO2 emissions of each HEV in the singlecycle sections of the CADC that result from the two variants of theCO2 correction procedure. An average deviation of around 3% wasfound for the corrected CO2 emissions based on net HSB wireenergy. Therefore, the assumption of having constant terminalvoltage around the nominal voltage of the HSB as implied in thestatutory correction procedure is acceptable.

To determine the effect of HSB performance on the outcomesof the statutory procedure for correcting CO2 emissions of HEVs,subsequent repetitions of the CADC urban section starting withmaximum SOC of the HSB were carried out with HEV B until theresulting raw CO2 emissions reached stable values. Because theHSB of the present HEVs is self-sustaining and maintained at aspecific SOC level in normal vehicle operation, the stabilized rawCO2 emissions are assumed to reflect the true CO2 emission levelof this vehicle in the given driving pattern. Within theserepetitions of the CADC cycle section, it can also be assumed thatthe net SOC of the HSB remains essentially unchanged.

Fig. 7 summarizes the findings derived from this test series.After two out of six repetitions of the urban cycle section of theCADC, stabilized raw CO2 emissions are recorded for HEV B with astandard deviation of less than 2%. The corresponding CO2

emissions calculated according to the statutory correctionprocedure using the respective correction factor stated inTable 2 clearly underestimate these stabilized CO2 emissions byaround 13%. This observation indicates that more charge isrequired by the HSB than is used by the HEV to maintain theSOC of the HSB at a specific level. A stable average ratio of 79.5%between the net charge flows used from and provided to the HSBis detected in stabilized conditions (Fig. 7B), which also reinforcesthe assumption that the net SOC of the HSB in the latter four

60

70

80

90

100

110

120

130

140

1no. of repetitions of CADC urban

CO

2 em

issi

on [g

km

-1]

raw emission ECE R-101 adjusted procedure

Qout / Qin = 1KCO2 = 40.259

R2 = 0.9983

Qout / Qin = 0.795KCO2 = 44.535

R2 = 0.9944

2 3 4 5 6

Fig. 7. Raw emissions of CO2 of HEV B in subsequent repetitions of the urban cycle sect

correction procedure described in Regulation ECE R-101 and an adjusted procedure with

the HSB in the same cycle repetitions (B).

repetitions remains unchanged. This ratio indicates the particularHSB performance of HEV B in terms of using charge provided to itsHSB per unit battery SOC within this driving pattern, which differsfrom the ratio obtained from the test runs depicted in Fig. 3.Obviously, the driving pattern also affects the individual HSBperformance because it determines the theoretically possible HSBusage of an HEV.

When applying the mentioned ratio of 79.5% to the measuredcharge flows of the test results presented in Fig. 5, an adjustedcorrection factor according to Eq. (2) can be derived for HEV B inthe urban cycle section of the CADC (Fig. 7A). Considering both theratio of the charge flows and the adjusted correction factor,corrected CO2 emissions are obtained for HEV B that match theraw stabilized CO2 emissions for all repetitions of the urban cyclesection of the CADC, especially for the first repetition that featuresa considerable change in SOC of the HSB. This finding demon-strates that a correction procedure based on net HSB charge flowonly provides representative corrected CO2 emissions for anindividual HEV in a certain driving pattern if its particular HSBperformance with regard to charge flow per unit battery SOC ofthe HSB is taken into account.

4. Summary and conclusions

The present experimental investigation with three examples ofa full HEV offers insight into the effect of HSB performance ondetermining CO2 emissions of HEVs in real-world conditions. It isshown that the individual HSBs of the HEVs perform differentlyunder equivalent driving conditions. First, the total charge outputof the HSB is lower with increasing mileage, which is probablycaused by in-use deterioration. Second, it is observed that only afraction of the charge provided to the single HSBs can always beused per unit battery SOC, which also varies considerably for eachHSB. This phenomenon is presumably attributed to the numberand depth of in-use charge and discharge cycles performed byeach HSB.

These findings influence the raw CO2 emission performance ofthe considered HEVs in driving patterns in which the contributionof their hybrid powertrain is valuable. The outcomes of thestatutory procedure for Europe for correcting raw CO2 emissionsare particularly affected because this methodology reproduces thechange in SOC of the HSB in the measured net HSB charge flow butwithout considering irreversibility per unit battery SOC. It is

0.5

0.6

0.7

0.8

0.9

1.0

1no. of repetitions of CADC urban

Qou

t / Q

in [-

]

Qout / Qin

0.795

2 3 4 5 6

ion of the CADC together with corrected CO2 emissions based on both the statutory

weighted charge flows (A). The ratio of net charge flows used from and provided to

R. Alvarez et al. / Energy Policy 38 (2010) 6919–6925 6925

observed that the CO2 correction factors derived for the singleHEVs according to this statutory procedure differ from each otherwithin the same driving patterns, which reflects the individualeffect of HSB performance on the resulting net HSB charge flowand the raw HEV CO2 emissions. This circumstance is notaccounted for within the statutory correction procedure, whichprescribes a common correction factor for a HEV model in acertain driving pattern. In addition, the resulting corrected CO2

emissions considerably underestimate the true CO2 emission levelof a HEV in any driving cycle because they do not consider theirreversibility of HSB charge flow per unit battery SOC. Stabilizedraw CO2 emissions of a single HEV resulting from subsequentrepetitions of a real-world urban driving pattern are under-estimated by around 13% when the statutory correction proce-dure is applied. These stabilized raw CO2 emissions are onlyreproduced by an adjusted correction procedure that takes intoaccount the particular HSB performance of the HEV with regard tothe battery SOC in a given driving pattern when both deriving arepresentative CO2 correction factor and executing the respectivecorrection.

It can be concluded that the statutory procedure for correctingCO2 emissions of HEVs for certification purposes in Europesystematically underestimates their true CO2 emission levelsbecause it does not consider irreversibility in the measured netHSB charge flow per unit battery SOC when reproducing thechange in energy content of the HSB. This systematic under-estimation should be amended to avoid undermining theobjective of CO2 vehicle emission reduction legislation for Europe,which is based on certified vehicle CO2 emissions, and discrimi-nating against other powertrain technologies that are not affectedby similar underestimation. Consequently, the most appropriateapproach to determine true CO2 emissions of a HEV in a drivingpattern is to apply the appropriate correction procedure to theSOC information from the HSB energy management of the HEVpowertrain control system. This SOC information is most suitablebecause it accounts for irreversibility in HSB charge flow per unitbattery SOC. Alternatively, repeating a driving pattern untilstabilized CO2 emissions are reached is also feasible because theenergy content of the HSB of a HEV is self-sustaining.

Acknowledgements

The authors thank the Swiss Federal Office for the Environ-ment (FOEN) for principally funding the study and Toyota MotorEurope for providing valuable technical information.

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