Particle Emissions from Diesel Passenger Cars Equipped with a

Publication Date (Web): March 3, 2006 ... Tail pipe particle emissions of passenger cars, with different engine and aftertreatment technologies, were ...
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Environ. Sci. Technol. 2006, 40, 2375-2383

Particle Emissions from Diesel Passenger Cars Equipped with a Particle Trap in Comparison to Other Technologies MARTIN MOHR,* ANNA-MARIA FORSS, AND URS LEHMANN Laboratory for Internal Combustion Engines, Empa (Swiss Federal Laboratories for Materials Testing and Research), CH-8600 Du ¨ bendorf, Switzerland

Tail pipe particle emissions of passenger cars, with different engine and aftertreatment technologies, were determined with special focus on diesel engines equipped with a particle filter. The particle number measurements were performed, during transient tests, using a condensation particle counter. The measurement procedure complied with the draft Swiss ordinance, which is based on the findings of the UN/ECE particulate measurement program. In addition, particle mass emissions were measured by the legislated and a modified filter method. The results demonstrate the high efficiency of diesel particle filters (DPFs) in curtailing nonvolatile particle emissions over the entire size range. Higher emissions were observed during short periods of DPF regeneration and immediately afterward, when a soot cake has not yet formed on the filter surface. The gasoline vehicles exhibited higher emissions than the DPF equipped diesel vehicles but with a large variation depending on the technology and driving conditions. Although particle measurements were carried out during DPF regeneration, it was impossible to quantify their contribution to the overall emissions, due to the wide variation in intensity and frequency of regeneration. The numbers counting method demonstrated its clear superiority in sensitivity to the mass measurement. The results strongly suggest the application of the particle number counting to quantify future low tailpipe emissions.

Today we have a more complex situation. A rapidly increasing fraction of the growing fleet of diesel passenger cars in Europe has very efficient diesel particle filters (DPF) as original equipment ex-factory (9). For instance, a major European automobile manufacturer is selling DPF fitted cars since 2000, and today more than 1 million such cars are on the road. Meanwhile, every European manufacturer offers at least some models of diesel passenger cars equipped with DPF. And the German car manufacturers announced DPF as standard equipment in all diesel models starting in 2008. The trends for the heavy-duty vehicles are less clear. Most manufacturers intend to meet the Euro 5 limit values (in force starting in 2007) without DPF. But, more and more city buses are retrofitted with DPFs. At large Swiss construction sites, all diesel engines rated above 18 kW must be retrofitted with DPF (10). Concurrently, there are advances in gasoline fueled vehicles. Fuel direct injection into the cylinder for spark ignition engines has been developed and introduced in the market, with the objectives of better fuel economy and higher power output. This combustion concept facilitates lean combustion but mimics a more diesel-like combustion principle that also results in higher particle emissions (9, 11). These trends will substantially change the contribution of traffic-related emissions to ambient fine particle concentrations. The new situation deserves a more careful examination of the specific emissions of the different vehicle technologies. Consequently, studies on source apportionment should be updated. This paper reports the results of a measurement program to better quantify the specific particle emissions from a wide variety of passenger car technologies. It focuses on diesel passenger cars equipped with different DPF concepts. Particle number counts and filter mass measurements characterized the particle emissions. A key aspect, for the comparison of low emitting sources, is the suitability of the measurement techniques applied. Hence, the particle measurement was implemented according to the initial findings of the experts group of ECE-GRPE Particle Measurement Program (PMP) (12). This program, under the auspices of the UNECE WP29/ GRPE Group, is evaluating a new measurement method for type approval by coordinating several national studies, e.g. ref 13.

Experimental Section Introduction Tail pipe particle emissions from road traffic are considered to be an important emission source with regard to impact on human health (1, 2). They are typically a small fraction of the PM10 mass in ambient air (3, 4), but the particle number concentration predominates in densely populated urban areas in the vicinity of heavy traffic streets (5-7). Formerly, a simplistic image represented the impact of the vehicle specific sources of particle emissions. Diesel vehicles were considered responsible for almost all tail pipe emissions, whereas the emissions of gasoline vehicles were neglected. This view was acceptable for older technologies and should be restricted to solid particles that consist mainly of soot and, to a much lesser extent, of ashes. However, volatile particles are also formed, mainly by sulfates and hydrocarbons, and create a separate mode (nucleation mode) in number size distributions (8). * Corresponding author e-mail: [email protected]. 10.1021/es051440z CCC: $33.50 Published on Web 03/03/2006

 2006 American Chemical Society

The measurements were done on a chassis dynamometer at the Empa laboratories. This facility has dual full-flow dilution tunnels: one tunnel for testing diesel vehicles, the other for gasoline vehicles. This arrangement excludes interference due to different exhaust gas compositions and exhaust gas temperatures. Transient measurements of gaseous components as well as temperatures and pressures at several points along the exhaust gas and sampling line, routinely accompanied the particle measurements. The exhaust flowed from the tailpipe to the tunnel through a heated/insulated (T > 353 K) corrugated, stainless steel tube of length about 5.5 m and diameter about 8 cm. The flow rate in the CVS (constant volume sampling) tunnel was set to a constant value of 12 m3/min for all vehicles. The probes for the particle number and mass sampling were installed more than 4 m, i.e., 10 tunnel diameters, downstream of the mixing point, to ensure complete mixing of dilution air and the exhaust gas. Figure 1 shows the schematic of the experimental setup. Particle Mass Measurement. Two separate setups, for the particle mass measurements, were simultaneously inVOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Schematic of the experimental setup for the measurement of the particle number and mass emissions: C, precyclone; D, dilution unit; ET, evaporation tube; FH, filter holder. stalled on the dilution tunnel. One gravimetric measurement procedure was done according to the European specifications (14) with two successive Teflon-coated glass-fiber filters (Pallflex, T60A20, retention for DOP 0.3 µm: 96.4%). The temperature of the filter holder was thermally uncontrolled, but the aerosol sample was held below 52 °C by the dilution. At the second setup, all parts were controlled to a temperature of 320 ( 5 K, compliant to draft PMP and EPA US2007 specifications. A cyclone, with a 50% cutoff at 2.5 µm diameter, was installed upstream of the filter holder. The particles were collected on single Teflon-coated microglass fiber filters (Pallflex, TX40HI20WW, retention for DOP 0.3 µm: 99.99%) with a higher collection efficiency than the T60 filter. Separate samples were taken for each phase of the test cycle. Before and after the sampling, the filters were conditioned at 298 ( 1 K and 50% relative humidity, for at least 2 h to achieve equilibrium. A microbalance (Mettler Toledo, UMX 2) with a resolution of 0.1 µg was used for the weighing. Particle Number Measurement. The particle number measurement setup complied with the Swiss draft ordinance on the determination of particle emissions from diesel passenger cars, which is based on the initial findings of the PMP expert group. The sample flow was extracted from the dilution tunnel and was diluted by a one- or two-stage ejector pump diluter (Palas GmbH; Dekati Ltd.) at a dilution ratio of 10 or 90, respectively (15). Dilution should ensure that the total number concentration of nonvolatile particles does not exceed 104 cm-3 in the particle counter. Subsequently, the aerosol flowed through a metal tube (length 50 cm, diameter 20 mm), heated to a temperature of 573 K, to evaporate volatile particles formed by nucleation in the exhaust and sampling line. A condensation particle counter (TSI, CPC3022A) then counted the particles. This instrument has single particle detection and a 50% counting efficiency for 7 nm diameter particles. More details about the CPC principles are published (16). The total transit time, between the probe in the tunnel and the particle counter, was 8.6 s. In the draft ordinance a maximum transit time of 10 s specified to minimize the diffusion losses. Upstream of the evaporation tube, another CPC (TSI, CPC3022A) was connected to the sampling line to measure the number of particles including the possible volatile fraction. An important issue is the specification of the ET. This unit should have the capability to evaporate a sufficient quantity of volatile material, yet the losses of solid particles by thermophoresis and diffusion should be minimal (17). The PMP findings and the Swiss draft ordinance specify that the evaporation unit should fulfill these two criteria: (a) a penetration of 90% or more for solid particles of 30 nm, 50 nm, and 100 nm diameter and (b) an evaporation efficiency of 99% or more of 2376

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tetracontan particles (C40H82) with a diameter of 30 nm and a concentration of at least 1000 cm-3. More details of the measurement setup and evaluation are published elsewhere (18). A correction of the systematic errors mentioned above would require a detailed knowledge of the number size distribution throughout the test cycle, which is usually not available. For this reason the PMP expert group agreed not to correct the losses but to define the measurement procedure in a way to minimize the losses as much as possible. Rough estimates result in total losses of less than 20% for a typical diesel exhaust aerosol. Vehicles and Fuels. The fleet of vehicles, investigated in detail, comprised three diesel passenger cars and one gasoline passenger car of latest technology. The diesel vehicles were equipped with diesel particle filters (DPF) as original equipment ex-factory, i.e., they were not retrofitted. The Volkswagen Passat had a SiC wall flow filter and used a fuel borne catalyst (FBC-DPF) to promote periodic filter regeneration every several hundreds of kilometers. The Opel Vectra was equipped with a catalytic coated SiC wall flow filter (CSF), which also is regenerated periodically. The third car, Toyota Avensis, had a combined particle filter and lean NOx-trap (D-cat) to simultaneously reduce particle and NOx emissions (19). The surface of this corderite wall flow filter is catalyst coated with catalyzing noble-metal for filter regeneration and with barium oxide for NO2 trapping during lean combustion phases. The gasoline vehicle, with direct injection and temporarily lean combustion strategy, also uses a lean NOx-trap, however, with an open cell structure, designed for NOx reduction only. Supplementing the four vehicles described above, single tests were performed on another diesel vehicle with CSF (BMW 530 d), a diesel vehicle without DPF (VW Touran TDI), and a conventional gasoline vehicle with multi point injection (MPI) (Audi A3). Specifications of the vehicles are listed in Table 1. The vehicles ran with commercial fuel from the same batch with a sulfur content of less than 10 ppm. The results of the fuel analysis are presented in Figure SI 1, Supporting Information. The brand-new vehicles Avensis, Vectra, and Passat were run-in for about 3000 km with the test fuel. The already used vehicle Touran was conditioned for about 500 km with the test fuel. Measurement Program. The reported results are mainly based on measurements during the New European Driving Cycle (NEDC), which is used for type approval purposes. The test cycle consists of an urban part, ECE (duration: 780 s, maximum speed: 50 km/h), and an extra-urban part, EUDC (duration 400 s, maximum speed: 120 km/h). Besides the NEDC, several other driving cycles were run but are not reported here.

Results Comparison of Vehicles. Figure 2 illustrates the particle numbers measured for the individual vehicles, obtained during the NEDC driving cycle. The selection of vehicles represents a rather complete set of the different automobile technologies prevalent in European traffic. The various technologies cover an emissions span of more than 3 orders of magnitude. The lowest emission was observed for the DPF equipped vehicles, the highest emissionsas expecteds emanates from the diesel vehicle without DPF. The absolute emissions, measured in this study, are in good agreement to measurements reported in ref 9 performed with a similar measurement system. Similar relative emissions of the different vehicle technologies, but at a higher absolute level, were reported in another study (20). They used an ELPI (21) for the particle number measurement, whose principle is not based on single particle detection. Due to the lack of data the instrument could not be calibrated for the actual particle densities.

TABLE 1. Specification of Vehicles Investigated in This Study manufacturer model fuel injection displacement no of cylinders power aftertreatment system

Toyota

Avensis 2.0 D-cat diesel direct 1995 cm3 4 85 combined NOx adsorber and DPF (D-cat), oxidation catalyst material Corderite certification Euro 4 odometer 3100 km no of NEDC-tests 7

Opel

VW

VW

BMW

VW

Audi

Vectra 1.9CDTI 16V diesel direct 1910 cm3 4 110 oxidation catalyst, catalyzed DPF (CSF)

Passat 2.0 TDI diesel direct 1968 cm3 4 100 oxidation catalyst, fuel borne catalyst DPF (FBC-DPF) Si-SiC Euro 4 3100 km 15

Touran 1.6 FSI gasoline direct 1598 cm3 4 85 NOx adsorber

530d diesel direct 2993 cm3 6 160 oxidation catalyst, catalyzed, DPF (CSF)

Touran 1.9 TDI diesel direct 1896 cm3 4 77 oxidation catalyst

A3 gasoline MPI 1595 cm3 4 75 three-way catalyst (TWC)

Euro 4 8100 km 7

Euro 4 3200 km 1

Euro 3 14900 km 1

Euro 4 40000 km 1

Si-SiC Euro 4 3100 km 6

FIGURE 2. Mean values of total particle number concentration for the NEDC as measured by a CPC with and w/o evaporation tube upstream of it. The error bars represent (1 standard deviation. There are several interesting points in Figure 2: (1) The diesel vehicle with combined particle and NOx aftertreatment shows a factor 100 higher particle emissions than the other diesel vehicles with DPF, indicating a distinctly lower efficiency of this system. (2) For the vehicles with the very efficient traps, significantly higher emissions of a factor 10 and more were observed in tests that were done after trap regeneration. No regeneration was observed during a NEDC cycle. Hence, regeneration was initiated by running the vehicles at high speed and increased load. After the regeneration, the vehicle was conditioned in the same way (15 min at 80 km/h and soak time at least 6 h) before the next test, as for the other test runs. In contrast, the Toyota did not show such an effect. An additional test series of five runs, without additional intermediate tests, was done on the Passat, because this vehicle showed the largest test-to-test variation, due to the history of prior test runs. The vehicle was only conditioned by 3 EUDC between the test cycles. Column 4 in Figure 2 shows the clear curtailment in the total emissions and in the variation of this measure. (3) The emissions of the gasoline vehicles clearly exceeded the emissions of the diesel vehicles with the very efficient DPF. For the gasoline vehicle with direct injection, two emission levels can be distinguished, depending on the combustion strategy. In four of the seven tests, the engine management switched, during short phases of the driving cycle, from the stoichiometric mode to the lean combustion

mode. Although these lean periods did not exceed 240 s in total of the whole cycle of 1180 s, the lean stratified combustion caused significantly higher particle emissions. This observation is consistent with previous results (9, 11). (4) The small difference between the emissions with and without evaporation tube indicates the absence of a nucleation mode formed by volatile particles, for all vehicles. A more detailed investigation on the composition of the detected particles was beyond the scope of this investigation and would be very difficult, due to the very low mass loadings. Influence of Preconditioning on the Particle Number Emissions. The influence of the trap burden on the particle emissions is corroborated by the correlation between number emissions and mean pressure drop over the particle trap. For the Passat and the Vectra, there is a clear tendency that the particle number decreased when the pressure drop increased (Figure 3). This observation suggests that the DPF efficiency is significantly reduced when the filter is empty and no soot cake has accumulated on the filter surface. However, some data points from the Passat measurements show that the penetration through the DPF is not fully characterized by its pressure drop. The spatial distribution of soot (and also of lube oil ash) in the filtersand as a consequence the retention efficiency of the soot cakescan vary with the driving conditions (22). In contrast, the mean pressure drop of the Avensis filter system was significantly higher, and there was no indication that the surface filtration plays a role in particle trapping as even a slight trend to VOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Correlation between particle number emission and mean pressure drop over the particle trap for the NEDC: squares, diesel w/FBC-DPF (Passat); circles, diesel w/CSF (Vectra). higher emission at higher pressure drop was observed. See Figure SI 2, Supporting Information. Background Concentrations. The background concentration can have a significant impact on the vehicle results for low emission measurements. Hence, particle concentration in the CVS tunnel was measured before each vehicle test by starting the measurement system several minutes before the engine start. The concentrations were converted into emissions per driven distance assuming a mean speed of 33 km/h, which corresponds to the mean speed of the test cycle NEDC. A mean concentration of 3.3 × 109 km-1 (rel. standard deviation: 84%) was determined for the total 39 test runs, which is in the range of the lowest measured exhaust emissions. However, the measured background concentration should not be interpreted such that it can be simply subtracted from the exhaust measurement, as flow distribution and temperature of the aerosol are different to vehicle tests. Time-Resolved Particle Number Emissions. Figure 4 illustrates that not only the absolute concentration but also the pattern of the time-resolved emission of the particle number differs for the various vehicles during the test cycle. The diesel vehicles with the very efficient traps show the highest emissions peaks at the beginning of the tests. Only in the first half of the cycle are the emissions related to the velocity profile and consequently to the exhaust gas flow rate. The independence of emissions and driving conditions indicates very high filter efficiency. For freshly regenerated filters the particle emissions exhibit a clear dependence on the driving conditions throughout the test, indicating a higher penetration of the trap, because a soot cake has not yet formed on the filter surface. For the diesel vehicle with the D-cat, higher emissions and a clear correlation to the velocity throughout the test cycle was observed in all tests. This result corroborates the significantly lower efficiency of the D-cat (Figure 2). A much stronger variation in emissions, as function of the test cycle, is observed for the conventional gasoline vehicle. The emissions cover a range of more than 3 orders of magnitude. Higher emissions occur during the cold start phase, during acceleration, and during the high-speed part of the cycle, which is in agreement to (11, 23). The emissions profile of the gasoline vehicle with direct injection (Touran FSI) is similar in shape but less pronounced. During phases, when the engine operates in the lean combustion mode, the emission looks very similar to the profile of the conventional diesel vehicle, Touran TDI, indicating a more diesel-like combustion process. 2378

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Number Size Distribution for Constant Speed Conditions. The SMPS (24) requires several tens of seconds to scan a number size distribution. This lag limited the measurement to constant speed driving conditions. Figure 5 shows the number size distributions of the particles at a speed of 80 km/h in the 4th gear. The samples were taken downstream of the evaporation tube. In contrast to the transient tests (Figure 2), the diesel vehicle with the D-cat shows significantly higher emissions than the gasoline vehicle with direct injection does. As described above, the gasoline vehicle had higher emission predominately during acceleration and in the lean combustion mode (Figure 4). In this stoichiometric mode, as verified by the CO2 concentration, emissions are relatively low. Particle Mass Emissions. The condensation particle counter delivers real-time data on vehicular emitted particle numbers. This provides insight into particle emissions with high sensitivity and time resolution. However, regulations for vehicle certification and emission inventories are based on mass measurement. It is therefore valuable to quantify the emissions of the test vehicles also on a mass basis. Filter mass measurements were performed according to the European specifications and/or in a modified version with expected improvements in sensitivity and repeatability. Figure 6 shows the mean values of the mass measurements for the individual test vehicles. Excluding the diesel vehicle without DPF, the particle mass differs within less than factor 3 between the lowest and highest emissions. Within the incomplete set of measurements, using the modified method, the span is within a factor of about 2, which indicates no improvement. In comparison, the emissions number counts span a factor of about 500 for the same group of vehicles. This significant divergence in sensitivity between number and mass measurement is in agreement to other published studies, e.g. ref 13. The small differences in mass are mainly explained by the low absolute emissions. At low particle burden, adsorption and desorption processes of gaseous compounds on the filter material become important and can cause positive and negative artifacts (25, 26). A U.S. research program (27) is currently investigating the influence of the filter material on this effect. In our case, the typical filter loading was around 20 µg on a filter, and we assume that the majority of the collected mass originated from adsorption of gaseous components of the diluted exhaust gas in the CVS tunnel. Obviously, this process is to a large extent independent of the vehicle technology. The higher average emissions for the modified method, compared to

FIGURE 4. Time-resolved particle number emissions during the NEDC driving cycle for the vehicles. the specified method, indicates that the higher adsorption efficiency of the TX40 filter dominates over the opposed effect of higher volatility caused by the higher mean filter temperature. Figure 7 plots the correlation of the two filter mass methods, for the mass emissions of Passat and Vectra,

classified in three phases of the test cycle. Significantly higher emissions for the modified filter method are observed only for the ECE part with cold start. A plausible hypothesis is that during the cold start phase initially a larger amount of semivolatile compounds is present in the exhaust gas. The VOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Number size distribution of the particle tailpipe emissions for a constant speed of 80 km/h in the 4th gear.

FIGURE 6. Mean values of total particulate mass concentrations for the NEDC measured with the standard and the modified method. The error bars represent (1 standard deviation. causes are incomplete combustion due to the cold engine walls, then the fact that the exhaust gas system has not yet reached the soot light-off temperature, and also due to the cold transfer line and CVS-tunnel. By the way, the higher speed phase does not produce higher emission per driven kilometer. This demonstrates that the filter traps function properly also at high exhaust volume rates. Emissions during Regeneration. To avoid DPF clogging and excessive back-pressure, the trapped soot must be periodically combusted. Soot combustion starts at a temperature of about 920 K, which is rarely reached by the exhaust gas. To lower the combustion temperature, the DPF surface is catalytic coated, or catalyst is added to the fuel (28). Alternatively or in addition, NO2 can lower the soot light-off temperature to about 520 K (29). The NO2 is produced from oxidation of NO in the exhaust gas, using an oxidation catalytic converter in the exhaust gas line upstream of the DPF, or by catalytic coating the DPF itself. Another 2380

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strategy, to promote the soot combustion process, is the periodic increase of the exhaust gas temperature by delayed injection in the cylinder or even a postinjection in the manifold downstream of the outlet valve. Figure 8 shows data recorded during a DPF regeneration process for the three diesel vehicles. These examples show the principle course of the processes rather than quantify the emissions, as these varied drastically from event to event. In the Vectra and the Passat, the regeneration was initiated by running the vehicles at a constant speed of 80 km/h and at maximum weight load and additional engine load. The Passat regeneration started about 1 min after the constant speed was reached, initiated by late fuel injection, as indicated by the sudden increase in the exhaust gas temperature downstream of the turbocharger (TC). Consequently, pressure drop across the DPF decreases and the T.HC signal increases due to incomplete combustion products. The subsequent increase in pressure drop is explained by the temperature increase of

FIGURE 7. Correlation between mean mass emissions measured by the regulated and a modified method, which includes a temperature-controlled sampling system, a precyclone with a 50% cut point diameter of 2.5 µm, and the use of a filter substrate of higher retention efficiency. The phases correspond to different parts of the test cycle: I, ECE; II, EUDC; III, German highway cycle (mean/ maximum speed: 117/160 km/h).

the exhaust gas. During the regeneration process, the particle number concentration measured without evaporation tube increases by more than 1 order of magnitude, whereas the concentration downstream of the evaporation tube does not change significantly. This difference clearly indicates that these higher particle emissions are caused by condensed hydrocarbon compounds, due to incomplete combustion, and not by penetration of solid soot or ash particles. Drastically higher emissions during the regeneration process are observed from the Opel Vectra. Here, the particle number concentration also downstream of the evaporation tube increases by more than 3 orders of magnitude. However, note that the vehicle was operated at full engine load to instigate the regeneration. The temperature before the Vectra’s DPF reached 820 K, which was significantly higher than for the Passat. Due to the extreme conditions, other effects could be involved, e.g. desorption from walls of the exhaust gas lines. Moreover, we cannot exclude that the amount of volatile material was too large to be evaporated in the evaporation tube or that recondensation took place after the ET due to saturation. The decrease in pressure drop and the higher particle concentration clearly indicate the regeneration of the DPF. Interestingly, no increase of the T.HC signal is observed during the regeneration, which could be explained by the complete combustion at the higher exhaust gas temperature.

FIGURE 8. Time course of particle emissions (left scale) and other relevant parameters (right scale) during a filter regeneration process. An example for the Passat (upper), Vectra (middle), and Avensis (lower graph) is shown. Graph in the middle: CPC w/o ET was for awhile in saturation. VOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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On the vehicle with combined particle and NOx-trap, the intervals between two regeneration processes were much shorter. The process is initiated by an exhaust port injection (EPI) that increases the exhaust gas temperature to about 870 K by combustion of the fuel in the D-cat. The catalytic coating and the high-temperature forces the soot oxidation. During the regeneration phases, particle emissions, including the nonvolatiles, increase by about a factor 10. EPI is used to increase the exhaust gas temperature for both the soot oxidation and the NOx curtailment. However, the processes are generally controlled separately, because richer combustion conditions are needed for NOx curtailment.

Discussion Nonvolatile particle number emissions of different passenger cars’ combustion and aftertreatment technologies varied up to 3 orders of magnitudes, whereas the difference in mass emissions was found to be within a factor of 3 only. The mass measurement sensitivity is not improved at all, when a modified mass method with a temperature controlled sampling and a more efficient filter material is applied. Another method to increase the sensitivity is the better particle filtering of the dilution air, which was not investigated. Although the background concentration can have an important impact on low emission measurements, the particle counting is superior to the mass measurement, as also previously reported (13). DPF equipped diesel cars have the lowest particle emissions, in all operation conditions. However, the filtration efficiency depends on the DPF soot burden. Not all DPFs show similar good performance, even if the actual mass limit value for particle mass was always well attained. The lower performance of the D-cat system is caused by a relatively large mean pore size of 25 µm and a high filter face velocity indicated by the significantly higher backpressure of the filter compared to the other investigated DPFs. The time-resolved particle measurements revealed that a conventional gasoline vehicle emits particles mainly during short durations, when acceleration or high load occurred. However, averaged over the driving cycle the total number emissions of gasoline vehicles exceeded that of diesel vehicles equipped with very efficient DPFs. The inferiority is even more pronounced for the spark ignition technology with direct injection that favors higher fuel efficiency. The more diesel-like combustion results in significantly higher emission in all operation conditions. In conclusion, emission models used to assess emission inventories must account for the higher complexity of the traffic emissions by the higher variety of engine and aftertreatment technologies. Uncertainty persists in the determination of the particle emissions during the DPF regeneration process. Although we believe that these do not drastically effect the overall emissions they vary considerably in concentration and frequency and were therefore beyond the resources of this study. To complete the picture, more research is needed on the regeneration process under real world conditions.

Acknowledgments We appreciate the excellent work of our colleagues Peter Stettler, Jan Stilli, and Philippe Novak in preparing the vehicles, running the chassis dynamometer tests, and compiling the data. The Swiss Agency for the Environment, Forests and Landscape (SAEFL) mainly funded this study.

Supporting Information Available Fuel specification, gaseous emissions, and correlation and another graph. This material is available free of charge via the Internet at http://pubs.acs.org. 2382

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Received for review July 22, 2005. Revised manuscript received December 21, 2005. Accepted January 31, 2006. ES051440Z

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