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Pollutant emissions from vehicles with regeneratingafter-treatment systems in regulatory andreal-world driving cycles

Robert Alvarez⁎, Martin Weilenmann, Philippe NovakaEmpa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Internal Combustion Engines, Ueberlandstrasse 129,CH-8600 Dübendorf, Switzerland

A R T I C L E I N F O

⁎ Tel.: +41 44 823 69 48; fax: +41 44 823 40 4E-mail address: robert.alvarez@empa.ch (R

0048-9697/$ – see front matter © 2008 Elsevidoi:10.1016/j.scitotenv.2008.02.022

A B S T R A C T

Article history:Received 23 October 2007Received in revised form31 January 2008Accepted 8 February 2008Available Online 16 April 2008

Regenerating exhaust after-treatment systems are increasingly employed in passenger carsin order to comply with regulatory emission standards. These systems include pollutantstorage units that occasionally have to be regenerated. The regeneration strategy applied,the resultant emission levels and their share of the emission level during normal operationmode are key issues in determining realistic overall emission factors for these cars. In orderto investigate these topics, test series with four cars featuring different types of such after-treatment systemswere carried out. The emission performance in legislative and real-worldcycles wasmonitored as well as at constant speeds. The extra emissions determined duringregeneration stages are presented together with the methodology applied to calculate theirimpact on overall emissions. It can be concluded that exhaust after-treatment systems withstorage units cause substantial overall extra emissions during regeneration mode and canappreciably affect the emission factors of cars equipped with such systems, depending onthe frequency of regenerations. Considering that the fleet appearance of vehicles equippedwith such after-treatment systems will increase due to the evolution of statutory pollutantemission levels, extra emissions originating from regenerations of pollutant storage unitsconsequently need to be taken into account for fleet emission inventories. Accuratelyquantifying these extra emissions is achieved by either conducting sufficient repetitions ofemission measurements with an individual car or by considerably increasing the size of thesample of cars with comparable after-treatment systems.

© 2008 Elsevier B.V. All rights reserved.

Keywords:VehicleParticle filterRegenerationEmissionReal-world

1. Introduction

In recent decades, legislation on the homologation of newpassenger car models regarding pollutant emission levelshas been successively tightened, with the objective of low-ering the environmental impact caused by pollutant emis-sions originating from internal combustion engines. Meetingthese requirements has led to the development and applica-tion of a series of different technical solutions aimed at pre-venting the formation of pollutants during the combustion

4.. Alvarez).

er B.V. All rights reserved

process in an engine, and further reducing emissions withexhaust after-treatment. The integration of exhaust gas re-cuperation and turbocharging, for instance, represent majoradvances in process design. Substantial improvements infuel-mixture generation have also been achieved with in-creased turbulence generation in the combustion chamber,and fuel-injection systems that aim at direct injection atthe highest possible fuel pressures. In addition, reduction ofpollutant emissions by exhaust after-treatment is attainedwith the use of oxidation catalytic converters and three-way

.

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catalytic converters. The latter requires combustion at stoi-chiometric conditions and its implementation therefore en-tails the application of closed-loop engine control systemsin parallel (Shelef and McCabe, 2000). This field has cometo predominate in current vehicle development efforts, dueto the wide range of optimization opportunities offered byincreasingly accurate physical models, together with itsprecise and partly adaptive mapping. Similar control systemsare nowadays also employed for other drive train and chassiscomponents, pointing the way to integrated vehicle controlsystems.

In this regard, vehicles with regenerating exhaust after-treatment systems have recently also come onto themarket tomeet the present emission limits for the homologation of newpassenger cars (Twigg, 2007). These after-treatment systemshave in common that they work in a non-continuous manner,i.e. they store a certain pollutant in a trap during normaloperating mode of the engine and decompose it chemically ina special procedure known as regeneration. The behavior ofsome of these systems employed as retrofits is known(Ntziachristos et al., 2005), but the operation and performanceof original equipment manufacturer (OEM) systems in real-world situations are not known yet. The scope of the presentstudy is to investigate these topics in order to be able to derivemethods for determining the emission level of pollutantscaused by cars with such after-treatment systems. A testserieswas therefore carried outwith four passenger cars of theEuro 4 certification class that feature different designs of theseafter-treatment systems. The emission levels of the cars weredetermined in single test series on a chassis dynamometertest bench including several repetitions of legislative cyclesand real-world cycles. Measurements at different constantspeeds were also considered to examine the possible effect ofregenerations on the emission level of the car in staticoperation mode.

2. Experimental section

2.1. Vehicle sample

One naturally-aspirated gasoline and three turbochargeddiesel passenger cars of the Euro 4 legislative period wereselected for the experimental campaign, see Table 1. Thesecars were selected because of the representative design and

Table 1 –Main characteristics of the passenger cars employed i

Designation G1 D1

Manufacturer Volkswagen VolkswagenModel Touran 1.6 FSI Passat Variant 2.0 TDFuel Gasoline DieselEmpty weight [kg] 1585 1649Displacement [cm3] 1598 1968Power [kW] 85 100Gearbox Manual 6 Manual 6Mileage [km] 7966 3057After-treatment NSCC FBC-DPFCertification class Euro 4 Euro 4

implementation of their after-treatment systems at that time.The gasoline car G1 featured direct fuel injection and wasable to run in lean combustion mode. A NOx storage catalyticconverter (NSCC) was therefore employed to prevent high NOx

emissions (Glück et al., 2000). One diesel car, namely D3, wasequipped with a so-called DPNR system that consists of a NOx

storage catalytic converter in combinationwith a particle filterdesigned as one unit (Tsuzuki et al., 2003) and had capabilityfor fuel injection in the exhaust outlet port. The other dieselcars were equipped with a diesel particle filter (DPF), whilethey differed in the procedure for filter regeneration: D2provides the heat required for particle burn-out inside itscatalytic coated particle filter (CSF-DPF) by late fuel injection inthe combustion chamber, and D1 employs an iron-based fuel-borne catalyst (FBC-DPF), as its unit-injector fuel-injectionsystem does not offer wide flexibility in fuel injection withregard to crank angle position. Except for this particular car,common rail fuel-injection systems are employed for thediesel cars. All four cars feature cooled exhaust gas recupera-tion and an oxidation catalytic converter used as a precatalyticconverter except for car D3, where it is located after the mainafter-treatment unit. Note that the cars feature rather lowmileage, i.e. they represent fairly new cars.

2.2. Experimental program

For each car, a test series was conducted on a chassisdynamometer test bench, by carrying out several repetitionsof the real-world cycle CACD, which includes as cycle sectionsrepresentative urban, rural and highway cycles derived fromdriving behavior studies within the ARTEMIS research project(André, 2004). The legislative cycle NEDC, the German real-world motorway cycle BAB (Alvarez et al., 2006) and therepetitive real-world cycle IUFC15 (André et al., 1999), suitablefor cold-start tests, were also considered. In addition, test runsat different constant speeds of 50, 80 and 120km/h using the4th, 5th and 6th gear (if available), respectively, were done inorder to investigate the influence of regenerations of the after-treatment system on the emission level, excluding distortioncaused by changing operating mode of the car.

The settings of the test bench, its ambient conditions, thetest procedure and the exhaust gas sampling and analyzingprocedure were applied according to European CouncilDirective 70/220/EEC for passenger cars, insofar as included.Time-resolved pollutant emissions were recorded at the

n the experimental campaign

D2 D3

Opel ToyotaI DPF Vectra Caravan 1.9 16V CDTI Avensis D-Cat

Diesel Diesel1605 15301910 1995110 85Manual 6 Manual 53093 3091CSF-DPF DPNREuro 4 Euro 4

Fig. 1 –Schematic of the experimental setup including measuring devices and sampling points. HFM: hot-film air-mass flowmeter; CVS: constant volume sampling; ET: evaporation tube; PM: gravimetric particle measurement; Δp: measurement of thebackpressure of the pollutant storage unit; λ: air–fuel ratio measurement.

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tailpipe and before the main after-treatment unit. Additionalsignals such as the pressure drop over the pollutant storageunit, the exhaust gas temperature and concentrations ofsingle gas components at selected duct points were alsodetected in order to obtain more information on the behaviorof the after-treatment systems. Furthermore, the number ofemitted particles PMN was measured with a condensationparticle counter (CPC) preceded by a unit for thermaldesorption of volatile compounds on the particle's surface(Mohr et al., 2006). An overview of the experimental setup isgiven in Fig. 1. The fuel used in the experimental campaigncorresponds to standard diesel and gasoline with an octanenumber of 98, both having a sulfur content of less than 10ppm.

2.3. Evaluation methodology

The method applied to identify the regenerations of the carsbasically consists in detecting any anomalous pattern of therelevant signal traces (e.g. the backpressure of the filter) anddetermining in each case if regeneration takes place or not,either controlled or spontaneously. Then, in order to estimatethe resulting effect on pollutant emissions, the overall relativeextra emissions eecp,tot,pol of a pollutant pol in a certain cy-cle section cp during the test series of the single car can becomputed. It is therefore necessary to calculate the differencebetween the emission level Ecp,i,pol,reg of the pollutant pol inthe particular cycle section cp of a cycle test run i affected by aregeneration and the average of its normal pollutant emissionlevel in that section, namely Ecp,i,pol,norm, which is determinedfrom the other cycle test runs that feature usual pollutantemission levels in their cycle section cp:

EEcp;i;pol ¼ Ecp;i;pol;reg �PEcp;i;pol;norm : ð1Þ

Relating the resulting extra emissionsEEcp,i,pol to its averagednormal emission level, as mentioned, gives the relative extraemissions eecp,i,pol:

eecp;i;pol ¼ EEcp;i;pol=PEcp;i;pol;norm : ð2Þ

This procedure can be applied to every regeneration thatoccur in the same cycle section of different cycle repetitionsconducted within the test series of a single car. Weightingthe resulting averaged relative extra emissions of a pollutantpol in a cycle section cp with the relative incidence of extraemissions in that particular cycle section xcp,ee gives the over-all relative extra emissions:

eecp;tot;pol ¼ xcp;ee �Peecp;i;pol : ð3Þ

The relative incidence of extra emissions in a particularcycle section xcp,ee is obtained by relating the total distance ofthe same cycle sections affected by regenerations to the totaldistance of the cycle repetitions conducted with a single car,which include cp as a cycle section:

xcp;ee ¼X

distcp;i;reg=X

distcycle cpð Þ: ð4Þ

3. Results

3.1. General emission performance

An overview of the emission performance of the cars can beobtained by comparing the emission levels in the legislativecycle NEDC to their respective limit values, see Table 2. It canbe seen that the gasoline car clearly fulfils the limits given,

Fig. 2 –NO- and NO2-ratio (mass) at different constant speedsfor cars D1 and D2. OCC: oxidation catalytic converter.

Table 2 – Pollutant emission levels in the cycle NEDCrelative to its limit values

Substance Unit G1 stoich. G1 lean D1 D2 D3

CO [%] 34.2 59.1 7.3 43.0 42.3HC [%] 40.2 52.4 – – –NOx [%] 15.9 17.4 96.1 113.9 61.3HC+NOx [%] – – 86.9 103.9 60.4PMm [%] – – 6.5 4.8 5.3

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although the good performance slightly deteriorates when thecar operates partly in lean combustionmode, especially for HCand CO emissions. This indicates possible incomplete com-bustion in combination with insufficient fuel-mixture gener-ation in this operating mode. Nor were notable NOx peaksduring after-cycle purge after finishing the NECD cycle mea-surements detected as in other experimental campaigns withsimilarly equipped cars (Mittermaier et al., 2003). But in mostruns where the engine operates in lean combustion mode, ahigher discharge of particle number is detected. The latter isassumed to result from somewhat disturbed combustion ofthe directly injected fuel (Mohr et al., 2003) rather than from aregeneration effect of the NOx storage catalytic converter. Notethat no definable criteria for switching the combustion modebetween stoichiometric and lean combustion of the enginewere found out of the data of the measured cycles.

All three diesel cars also show fairly good performance inCO emissions, whereas car D1 shows emissionswell below thelimit value. The same can also be observed for the three dieselcars with respect to the gravimetrically determined emissionsof particulatematter PMm, as themeasured emission level fallsfar below the respective limit value. However, the cars differconsiderably with regard to NOx emissions: car D3 features aquite low emission level for nitrogen oxides, and it can thus beassumed that its NOx storage catalytic converter makes anappreciable contribution to lowering emissions of this pollu-tant. In contrast, D1 remains below the limit value only by asmall margin and D2 even exceeds the emission level, byalmost 14%. As these two cars inhibit particle emission with aDPF, their discharge of nitrogen oxides could consequentlybe lowered by adjusting the engine management system inorder to minimize its formation during the combustion pro-cess. However, this strategy was apparently not entirely im-plemented in the present case, at least not for car D2. The HCemissions of the diesel cars do not represent a critical problem,as the emission levels improve when the resulting values ofNOx are comparedwith the respective figures for combined HCand NOx emissions.

3.2. NO/NO2 emissions

NO2 in the exhaust contributes to the oxidation of particlesstored in DPF units (Kandylas et al., 2002). Its appearancebefore a DPF is therefore desirable, but may result in tailpipeemissions if the particle oxidation mentioned does not occur(Gense et al., 2006). The latter is to be avoided, as high NO2

shares in NOx stimulate ozone formation in street canyonssignificantly faster than a comparable amount of NOx con-sisting mainly of NO. Moreover, NO2 is more noxious than NO

and it is not clear yet if the ambient NO2 limit concentrationsfor the European Union in force from 2010 on can be achievedin street canyons due to the increasing share of such vehiclesin the car fleet (Carslaw and Beevers, 2004). Hence, for somecars during single measurements at constant speed, the NO2

converter of the chemiluminescence analyzer (CLD) was by-passed in order to be able to measure NO as an alternativeto NOx. From these measured concentration levels, the massflow rates of NO and NO2 were derived in order to quantifytheir respective ratios, cf. Fig. 2. Car D1 features ratios of 50–60% NO tailpipe emissions of the total NOx at all measuredspeeds, which is mainly thought to be intentionally causedby the catalytic coating of the oxidation catalytic converter(Kandylas et al., 2002, Gense et al., 2006). Between engine-outand tailpipe emissions, a significant shift from NO to NO2 at120km/h can also be detected. Car D2 shows an increasingshare of NO2 for higher speeds, whereas at 120km/h it pre-dominates with a ratio of about 80%. Here the measurementsjust before the DPF show a shift from NO to NO2 in the DPFunit as well at all speeds. This observation indicates thatfurther oxidation from NO to NO2 may also occur in catalyticcoated particle filters, which complieswith the results of otherexperimental campaigns (Czerwinski et al., 2006).

3.3. Effect of regenerations

For car G1, measurements at constant speed are most suit-able to investigate the effect on pollutant emissions while theNOx storage catalytic converter regenerates. In this drivingregime, the engine usually runs in lean combustion modewith periodic short phases of air–fuel mixture enrichment toreduce the nitrogen oxides stored. The time intervals of thesephases are 80s, 60s and 110s for 30km/h, 50km/h and 80km/h,respectively, and last around 10s. At 120km/h the car runs

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exclusively in stoichiometric combustion mode and conse-quently no regenerations occur. The very short time inter-vals between the even shorter regeneration stages of this carallow the effect of regenerations on the pollutant emissionlevel to be included with the usual procedure for determiningits emission factors.

A more detailed analysis of emission behavior during theregeneration can be obtained from a single test run at 50km/h,see Fig. 3. The course of the air–fuel ratio signal clearly showsthe two different operating states. In normal operation, a highdischarge level of NOx before the converter can be detected,which is then stored in the converter unit as the tailpipeemissions are much lower. The latter rises slowly in the timeinterval between two regenerations, i.e. the storage ability ofthe converter decreases. Interestingly, this rise in NOx tailpipeemissions starts earlier in the intervals between successiveregenerations, and storage efficiency thus appears to decreaseprogressively. During regeneration, the air–fuel mixture isgreatly enriched, which causes peak emissions of CO that arenot completely compensated by the after-treatment system.

Fig. 3 –Course of the relevant signal traces during a re

Engine-out NOx emissions fall sharply during the regenerationstages, but high NOx peaks are observed at the tailpipe at thesame time as the converter releases some of the stored NOx

instead of reducing it. The resulting discharge of pollutantsper unit mass flow are less pronounced than their emissionson a concentration basis due to the fact that the mixtureenrichment mentioned is basically achieved by throttling theintake air and thus reducing the total gas flow, see Fig. 3.

Note that this effect also acts on CO2 concentrations incycle test runs, as shown in Fig. 4, in which the course of CO2

emissions originating from stoichiometric and lean combus-tion mode is compared during a cycle section of the NEDCcycle. Besides fuel cut-off in overrun conditions, G1 showslean combustion mode at idle and some constant speeds,but the CO2 discharge per unit mass flow shows no particularimprovement. Again the possible benefit in emissions that canbe achieved by dethrottling is compensated by an increasedintake air-mass flow to the engine.

In the test series involving car D1 not a single controlledregenerationwas observed. But the analysis of themeasurement

generation of car G1 at constant speed of 50 km/h.

Fig. 4 –Course of CO2 concentration and mass flow in a cycle section of the cycle NEDC for car G1.

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data, among other things the variation in backpressure ofthe DPF over the whole test series, cf. Fig. 5, indicates that theiron-based fuel-borne catalyst employed to regenerate the DPFallows particle burn-out at the usual exhaust gas temperaturesof car D1 and thus regenerates consecutively. However, singletime-resolved CPC measurements at constant speeds reflectpeak particle emissions at the beginning of the test that declinebefore the exhaust gas reaches normal operating temperature(Alvarez et al., 2006). An additional initial partial blow-out ofstored particles thus cannot be excluded, including in test runswith warm engine start, because particle number emission inthe first section of the cycle is significantly higher than in thefollowing sections (Alvarez et al., 2006).

Car D2 features similar behavior regarding the incidence ofregenerations. Within its test series of 1300min duration, onlya single controlled regeneration was detected with increasedexhaust gas temperature during post-cycle conditioning aftera measurement with the cycle NEDC, cf. Table 3. Here, CO andNOx emissions do not vary particularly from the respective

Fig. 5 –Course of the backpressure over the DPF and the exhaustmeasurements.

common values, but HC and CO2 emissions rise substantially.Unfortunately, no informationonparticlenumber emissions isavailable for this regeneration stage, as the measuring devicestops data logging at the very end of the cycle. Interestingly, ahighernumber of particleswas recordedduring theNEDCcycletest run mentioned preceding the regeneration, see Table 3.Here, however, the exhaust gas temperature does not exceedtypical values.

The after-treatment system of car D3 with its combined NOx

storagecatalytic converter andparticle filter exhibits interestingregenerative ability. Again measurements at constant speedgivemost insight, see Fig. 6. Innormaloperatingmode, i.e.whenthe exhaust gas temperature reaches common values, a similarstrategy as in car G1 is applied to reduce the stored nitrogenoxideswith short air–fuelmixture enrichment phases. The timeintervals between these enrichment phases (e.g. 25 s at 80km/h)and duration (e.g. 5 s at 80 km/h) are shorter at each speedcompared to the respective operation states of car G1. A two-stage regeneration strategy is visible in regeneration mode of

temperature before the DPF for car D1 in three subsequent

Table 3 – Overall relative extra emissions occurring in cycle sections due to regenerations weighted with their relativeoccurrence in test series

eecp,tot,pol

Car Cycle section cp xcp,ee [−] CO [−] HC [−] NOx [−] CO2 [−] PMm [−] PMN [−]

D2 L2 ECE 0.050 0.008 0.004 −0.001 0.000 −0.007 0.190L2 EUDC 0.084 0.028 0.070 0.000 0.004 −0.026 0.311Post-cycle NEDC 0.030 0.004 0.032 0.016 0.031 – –

D3 L2 BAB 0.172 0.036 −0.019 0.009 0.011 0.087 0.757CADC urban 0.017 −0.004 −0.005 0.040 0.004 0.015 0.027CADC rural 0.063 −0.018 −0.025 0.004 −0.001 0.005 0.069CADC highway 0.094 0.004 −0.030 0.028 0.009 0.039 0.367Constant 80 km/h 0.084 7.982 2.797 0.053 0.031 – 0.823Constant 120 km/h 0.286 6.738 −0.114 0.180 0.049 – 2.174

Values highlighted in gray indicate an increase in overall emissions of more than 10%.

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the particle filter. The first stage is used to recover the loadedparticle filter and the second to desulfurize the NOx storagecatalytic converter. The time intervals of these regenerationstages are 3.6 h and 0.8h for 80km/hand 120 km/h, respectively,with each stage lasting around 20 min.

Fig. 6 –Course of the relevant signal traces during particle

In the first stage of the particle regeneration, the highestexhaust gas temperature appears together with a sharp de-crease in the backpressure of the converter. The temperaturerise is achieved by injecting fuel into the exhaust outlet portas indicated by the increase in CO2 tailpipe emissions with

regeneration of car D3 at constant speed of 120 km/h.

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respect to the discharge level before the converter. The rateof exhaust gas recuperation is also assumed to be reduced, asit aids to reach the desired high exhaust temperatures. Theheat generated allows particle burn-out in its filter, but alsogives rise to considerable extra NOx emissions, especially atthe beginning of the regeneration phase. Pronounced CO andHC levels also appear before the converter, but they are al-most completely reduced in the oxidation catalytic converterlocated after the main converter unit. In the second stage ofthe regeneration, periodic mixture enrichment phases occurat high exhaust gas temperature to regenerate and possiblydesulfurize the NOx storage unit. These enrichment phasesare also caused by fuel injection in the exhaust outlet port, asthe intake air pressure remains constant during this period,but CO2 tailpipe emissions feature peak amplitudes. The re-sulting COpeaks are not completely compensated by the after-treatment system and peak NOx emissions at the end of asingle enrichment phase can also be detected. HC peak emis-sions are greatly reduced by the oxidation catalytic converter,although the resulting extra emissions are still considerable asthe normal HC discharge is even lower. The sudden changein the air–fuel mixture in the exhaust gas evidently preventsproper functioning of the converter.

The overall extra emissions of car D3 caused by regenerationsin the test runs at constant speedare calculated for the combinedregeneration phases mentioned above, cf. Table 3. The resultingextra emissions of NOx, PMm, and CO2 are notable, but theadditional discharge of CO, HC and PMN is striking. Another kindof regeneration observed in the first section of a transient CADCcycle only shows considerable extra emissions for nitrogenoxides. In addition, a notable increase in particle numberemissions was identified in some sections of different drivingcycles. These correspond to the gravimetrically determinedparticle discharges. In these cycle sections, the temperature ofthe exhaust gas after the converter reaches the level at whichparticle burn-out in the filter is assumed to occur according to theregeneration procedure reported above, but no significantdecrease in the already rather low backpressure of the lattercan be observed. It thus cannot be assumed that regenerationtakes place in the sense of filter recovery. However, thisrepresents a process conducted by the control unit of the after-treatmentsystem,because the temperature risementioned isnotobserved in the same cycle sections of other cycle repetitions.Interestingly, suchoperationphasesof carD3allhave incommonthat they appear at rather high engine load, as the rated power ofthe engine is moderate and the gearbox only features 5 gears. Itappears that the occurrence of such operating modes is some-times utilized to easily reach particle burn-out temperatures, sothat the particle filter may be partially recovered, even if itsparticle load does not strictly require a regeneration stage.

4. Discussion and conclusions

The present experimental campaign carried out with fourpassenger cars of the Euro 4 certification class featuring dif-ferent regenerating exhaust after-treatment systems providesvaried insight into its functioning and emission behavior.Measurements at constant speed on chassis dynamometertest benches are most suitable to obtain an understanding of

the particular regeneration procedure and its effect on thedischarge of pollutants. Series of driving cycles, especially real-world cycles, allow their frequency of occurrence in normaldriving conditions and the resulting change in the emissionlevel to be estimated.

Within the experimental campaign, the two NOx storagecatalytic converters installed in different cars exhibit very shortregeneration intervals, so that the resulting extra emissionsare included in the emission levels determined with the usualmeasurement effort. Overall NOx emissions are successfullylowered. But the strategy employed for regenerating theseunits,consisting of air–fuel mixture enrichment phases, occasionallycombined with rather high exhaust gas temperature to removepossibly deposited sulfur compounds, causes additional HC,CO and also NOx peak emissions that vary depending on itsquality of implementation. In fact, improving both the catalystproperties employed, together with the regeneration strate-gies applied, represents the main development task to be un-dertaken to further optimize NOx storage catalytic converters(Klingstedt et al., 2006).

The regeneration strategy of the particle filters applied inthreedifferent diesel cars results in various perceptions. The caremploying a fuel-borne catalyst featured not a single regenera-tion conducted by the respective control unit, while another carfeatured only one. Although regeneration stages only repre-sent a small part of the total operating mode of this car, theyaffect its resulting overall pollutant emission levels, because thepollutant discharge during the regeneration is considerable.Besides, the discharge of NO2 within their total NOx emissionsis pronounced with shares of 40% of 80%. The car with thecombined particle filter and NOx storage catalytic converterexhibits interesting behavior: the overall extra emissions dueto regenerations carried out in driving cycles are notable. Atdifferent constanthigh speeds, however, the rise inoverall extraemissions is substantial and can be attributed to both the extraemissions of a single regeneration and the high frequency ofregenerations in that driving mode.

The implemented regeneration strategy and the occurrenceof regeneration phases in the different driving situations ofa single car thus appreciably influence its emission behavior.In fact, both the extra emissions caused by regenerations andthe time interval between two regenerations greatly depend onthe driving pattern employed. Therefore, the European Unionelaborated a regulation for including these extra emissions,cf. Council Regulation ECE-R 83 Annex 13, whose applicationis indeed not yet mandatory. But reliable emission factors areanyway not obtained with this procedure as the only drivingpattern considered there consists of the statutory test cycle,which does not reflect real-world driving behavior. Consequent-ly, with regard to determining real-world emission factors of asample including cars with such regenerating after-treatmentsystems, either the number of measured driving cycles or thenumber of cars to bemeasuredhas to be adequately increased inorder to take into account the variations mentioned.

Acknowledgment

The authors thank the Swiss Federal Office for the Environ-ment (FOEN) for mainly funding the study.

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