A study of emissions from a Euro 4 light duty diesel vehicle with the European particulate measurement programme

Atmospheric Environment 44 (2010) 3469e3476

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A study of emissions from a Euro 4 light duty diesel vehicle with the European particulate measurement programme Harry Dwyer a, *, Alberto Ayala b, *, Sherry Zhang b, John Collins b, Tao Huai b, Jorn Herner b, Wilson Chau b a b

Uni. of California, Davis, Dept. of Mechanical and Aerospace Engineering, USA California Air Resources Board, Research Division, 1001 I Street, Sacramento, CA 95812, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 November 2009 Received in revised form 9 June 2010 Accepted 14 June 2010

The California Air Resources Board, CARB, has participated in a program to quantify particulate matter (PM) emissions with a European methodology, which is known as the Particulate Measurement Programme (PMP). The essence of the PMP methodology is that the diesel PM from a Euro 4 vehicle equipped with a Diesel Particulate Filter (DPF) consists primarily of solid particles with a size range greater than 23 nm. The PMP testing and the enhanced testing performed by CARB have enabled an increased understanding of both the progress that has been made in PM reduction, and the future remaining challenges for new and improved DPF-equipped diesel vehicles. A comparison of measured regulated emissions and solid particle number emissions with the results obtained by the PMP participating international laboratories was a success, and CARB’s measurements and standard deviations compared well with the other laboratories. Enhanced measurements of the influence of vehicle conditioning prior to testing on PM mass and solid particle number results were performed, and some significant influences were discovered. For example, the influence of vehicle preconditioning on particle number results was significant for both the European and USA test driving cycles. However, the trends for the cycles were opposite with one cycle showing an increase and the other cycle showing a decrease in particle number emissions. If solid particle size distribution and total particle numbers are to be used as proposed in PMP, then a greater understanding of the quality and errors associated with measurement technologies is advisable. In general, particle counting instruments gave results with similar trends, but cycle-to-cycle testing variation was observed. Continuous measurements of particle number concentrations during test cycles have given detailed insight into PM generation. At the present time there is significant variation in the capabilities of the particle counting instruments in terms of particle size and concentration. Current measurements show the existence of a large number of volatile and semi-volatile particles of yet-to-be-resolved chemical composition in diesel exhaust, especially during DPF regeneration, and these particles are not included in the PMP methodology because they are smaller than 20 nm. It will be very challenging to improve our understanding of this class of diesel particulate matter. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Traffic emission Automotive emissions Ultrafine particles Particle number Particle measurement

1. Introduction and background The California Air Resources Board (CARB) and the Joint Research Center of the European Commission (JRC) have collaborated on a number of research areas of mutual interest under a Memorandum of Understanding on Emissions from Transport signed on October 2005. One of those research topics is the Particulate Measurement Programme (PMP) launched under the auspices of the United Nation’s Economic Commission for Europe - Group of Experts on Pollution and Energy, (Andersson and Clark, 2004;

* Corresponding authors. E-mail addresses: [email protected] (H. Dwyer), [email protected] (A. Ayala). 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.06.028

Andersson et al., 2007). The California study conducted emission testing of the PMP’s Golden Vehicle (GV) at its Haagen-Smit Laboratory (HSL) in Los Angeles. The essence of the PMP methodology is that the diesel PM from a Euro 4 vehicle with a Diesel Particulate Filter, DPF, consist primarily of solid particles with a size range greater than 23 nm. The testing was an informal participation by CARB in the ligh-duty vehicle Inter-laboratory Correlation Exercise (ILCE_LD) that PMP conducted formally including 11 laboratories from Europe and Asia. The GV was a reference standard that was circulated around to all of the participating laboratories. The California testing was unique and expanded the ILCE_LD by including enhanced emissions testing which was not originally part of the PMP. The present paper is primarily concerned with results obtained with the use of the PMP, however results from some

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enhanced emission testing are also described in the paper. Also, the testing and reporting of the GV results (Ayala et al., 2009) was the culmination for a number of initial pilot investigations completed by CARB to assess the PMP protocol for heavy-duty vehicle classes (Ayala et al., 2007; Herner et al., 2007, 2009; Robertson et al., 2007; Zhang et al., 2008). A recent investigation of the PMP method for heavy-duty diesel engines was conducted in partnership with researchers from the University of California Riverside, the University of Minnesota, Matter Engineering, TTM, and Ricardo (Durbin et al., 2008). The investigation of the PMP methodology as a new measurement tool is the primary focus of the present paper, but a detailed study of tailpipe emissions from the GV is also described. The important research objectives were the following: (a) Compare CARB laboratory test results for criteria gases, particle mass, and particle number with results from PMP participating laboratories; (b) Evaluate the effects of vehicle soak-time conditioning on PM and particle number results; (c) Evaluate the effects of vehicle pre-conditioning cycles on PM and particle number results; (d) Compare various particle sampling instruments; (e) Determine some physical and chemical characteristics of the PM emissions; and (f) Study the emissions during regeneration of the DPF. CARB believes the areas of DPF regeneration and the determination of the physical and chemical characteristics of PM emissions need further study. 2. Test vehicle, the measurement systems, and test procedures The GV employed in the testing was a Peugeot 407 diesel passenger car equipped with a DPF, and it represents the most mature DPF technology present on the market and fully meets EURO4 emission standards (Blanchard et al., 2002; Giechaskiel et al., 2007, 2008; Quigley and Seguelong, 2002; Coroller and Plassat, 2003; Mohr et al., 2006). The DPF system consists of an oxidation catalyst upstream of an uncoated silicon carbide wallflow DPF and a cerium-based fuel-borne catalyst to reduce the DPF regeneration temperature. The cerium based fuel-born catalyst is housed separately from the fuel tank, and the method of doping the fuel with fuel-born catalyst is controlled by the vehicle’s on-board dosing system. The fuel used for the California testing was UltraLow Sulfur Diesel fuel, and no attempt was made to duplicate the diesel fuels employed at the other participating laboratories. At the present time the PMP methodology only attempts to measure solid particles with a diameter that is greater than 23 nm. PMP targets this size of particle for practical reasons and in an effort to reduce the variability in the measurement. It is recognized that diesel PM consists of large numbers of semi-volatile particles in the sub-23 nm size range. It is also well known that semi-volatile particles can contribute to PM measurements based on the gravimetric measurement. The role of sub-23 nm particles has to be studied more extensively for possible health effects. Therefore, there will likely be a considerable amount of further research as improved diesel emission reduction technology is introduced into vehicle fleets. The GV was shipped with instrumentation described as the Golden Particle Measurement System, GPMS, which was slightly modified and enhanced by CARB due to a failure of a GPMS instrument during the testing. Fig. 1 shows the major components of test set-up for particle measurements. It consisted of the following major parts: (a) A Constant Volume Sampling Tunnel; (b) A cyclone with a 2.5 mm size cut-point; and (c) Five independent particle counting systems with different features. Three of the counting systems contained an evaporation tube (ET) while two did not, and this setup allowed us to differentiate between solid and total (volatile, semivolatile, and solid) particles. The role of the ET is to remove volatile

Fig. 1. Schematic of typical sampling setup during ARB emission testing.

and semi-volatile particles before the particles in the exhaust reach the PMP measuring instrument. Volatile and semi-volatile particle formation is a strong function of sampling conditions and including them in an exhaust sample has potential to lead to high variability in the measurement, something the PMP was charged to improve upon. The particle counting systems also had different particle size resolutions, and this feature gave additional information concerning particle size. It should be mentioned that the California testing included particle instruments not used by the other participating laboratories, and this enhancement to the testing program allowed for a comparison of the other instruments to the PMP instruments. A list of the various instruments is given in Table 1. The test set-up also included measurement of regulated exhaust emissions (HC, NOX, CO, PM mass) and CO2. The sampling system, calculations, calibrations, and quality control conformed to the requirements of 40 CFR 86.1 Sampling and calculations for PM mass on TX40 and Teflon filters conformed to 40 CFR 1065.2 In addition to sampling for the regulated pollutants, CARB collected samples for analysis of organic and elemental carbon (OC/EC) and metals. Quartz fiber filters were collected at ambient temperature for determination of OC/EC (Ayala et al., 2008). Exhaust emission tests were conducted to study a number of the following issues:  The comparability of CARB laboratory results over the New European Driving Cycle (NEDC), with results from PMP participating laboratories.  The results and performance of the PMP protocols using the Federal Test Procedure cycle, FTP.  The effects of vehicle soak time on PM and particle number results.  The effects of vehicle pre-conditioning cycles on PM and particle number results.  The comparability of various particle sampling instruments with each other.

1 Code of Federal Regulations: PART 86-CONTROL OF EMISSIONS FROM NEW AND IN-USE HIGHWAY VEHICLES AND ENGINES 86.084-40, Automatic expiration of reporting and recordkeeping www.access.gpo.gov/nara/cfr/waisidx_04/40cfr86_04. html. 2 Code of Federal Regulations: PART 1065-CONTROL OF EMISSIONS FROM NEW AND IN-USE HIGHWAY VEHICLES AND ENGINES 1065.084-40, Automatic expiration of reporting and recordkeeping www.access.gpo.gov/nara/cfr/waisidx_04/ 40cfr1065_04.html.

H. Dwyer et al. / Atmospheric Environment 44 (2010) 3469e3476 Table 1 Summary of important instrumentation. Instrument

Function

CPC ET or VPR Ref CPC SPCS Grimm EEPS PND

Condensation particle counter Evaporation tube or volatile particle remover Matter Engineering Diluter and TSI 3010D CPC Horiba: Solid Particle Counting System using TSI 3010D CPC Grimm CPC Engine exhaust particle sizer and spectrometer e Spectrometer Particle number diluter

The study of pre-conditioning effects was a specific request by JRC as it had not carried out this type of study in the international program. In general, CARB’s exploration of the capabilities of the PMP method was driven by the need to advance understanding of improved approaches for measuring with regulatory rigor ultralow emissions from advanced, clean diesel technology. The study hypothesis is that PMP offers a real advantage for assessing PM emissions relative to existing methods. 3. Test results and discussion 3.1. Comparison between CARB and PMP participating laboratories To examine inter-laboratory variations, the regulated emissions and particle number emissions from five cold start NEDC cycles were conducted. The numbers associated with NEDC cycle refer to the order in which the tests were carried out, and both special preparation and events occurred during some of the individual cycles. For example, some tests were used for instrument comparison, regeneration, and other studies in the CARB investigation. The tests were run in the morning with an overnight soak, no pre-conditioning, and with the NEDC as the last cycle of the prior evening. This is important and the only means to empirically ascertain if and to what extent vehicle conditioning has an impact on test results, an area of research in need of increased attention as emissions decrease. The data from individual test runs are shown in Fig. 2, and the comparison of CARB results with the other PMP participating laboratories is shown in Fig. 3. The PMP averaged results were obtained from 11 tests in 9 countries and laboratories (Andersson et al., 2007). In general, regulated emissions measured at CARB’s HSL were within the range of other PMP results, except for CO, for which CARB’s average value is about three times higher. It should be noted that the average value of Cetane number for California Ultra-Low Sulfur Diesel fuel is slightly lower than the fuel used for PMP testing, and this could be a possible contributor to increased CO levels. 0.8

Regulated emissions

0.7 0.6 0.5

NEDC -19 NEDC -21 NEDC -22 NEDC -24 NEDC -26

0.4 0.3

0.2 0.1 0.0 THCx10 (g/km)

CO (g/km) CO2 (kg/km)NOx (g/km) PM (mg/km) PN x 10^11

Fig. 2. Emissions over five NEDC tests with overnight soak and no pre-conditioning. PN was measured with the Reference CPC.

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The Horiba SPCS employed in this study counted somewhat fewer solid particles than the results reported by other PMP laboratories, which is consistent with the comparisons of different instruments shown later in this paper. The particle number variability and standard deviation from the SPCS was similar to the results from the other participating laboratories. 3.2. Effects of vehicle soak time and pre-conditioning The NEDC test procedures specify a minimum soak time of 6 h, whereas the US FTP procedures specify a minimum soak time of 12 h. The impact of soak period and pre-conditioning on criteria emissions and particle number emissions was studied for both the NEDC and the FTP cycles. Of course these details are very important as regulatory authorities are expected to be very prescriptive in their descriptions of test procedure imposed upon car makers for emission compliance determination. The details of the preparation of the vehicle for the various tests can be found in reference (Ayala et al., 2008), and some results are presented in Figs. 4 for NEDC cycles and 5 for FTP cycles. The results clearly show that the soak period did not appear to have a substantial effect on emissions. This finding is relevant because it brings closer the emissions tests results obtained in Europe and in the US. The pre-conditioning of the GV vehicle does not seem to have substantially affected gas phase or PM mass emissions, however solid particle number emissions appear sensitive to pre-conditioning. The large variability in particle number emissions shown in Figs. 4 and 5 precludes any strong conclusions, but pre-conditioning appears to have increased particle number emissions over the NEDC cycle but decreased them over the FTP cycle. Since the trends for the NEDC and FTP cycles were opposite, there is a need for further research efforts in this area. The data contained in Figs. 4 and 5 is for a limited number of both NEDC and FTP tests cycles, and more tests are desirable to improve the statistics. It is noteworthy that PM emissions were not sensitive to vehicle pre-conditioning but solid particle numbers were. If solid particle numbers do not correlate with PM, it is implied that semi-volatile particles are playing a substantial role in PM gravimetric measurements. The semi-volatile particles, which are only included in the total particle counts, have to be understood and their physical and chemical characteristics measured. This result strongly points to the need for further continued study of the total number of particles from clean diesel engines. 3.3. Comparison of particle sampling and counting systems The sensitivity of the particle number measurements to the choice of sampling instrument was tested over two NEDC cycles. NEDC-25 followed a 6-h soak, and NEDC-26 followed an overnight soak of 12-h. Fig. 6 compares solid particle number emissions measured using the GPMS Condensation Particle Counters, CPCs, with solid particle number emissions measured using the Horiba SPCS. The Horiba SPCS was sampling downstream of the cyclone at the primary dilution ratio, the Reference CPCs sampled downstream of the first dilution stage, and the Gold CPC sampled downstream of the second dilution stage, i.e. downstream of the tertiary dilution stage. It is clearly seen from Fig. 6 that all three instruments gave similar particle number for NEDC 25 with pre-conditioning. Again it should be noted that these results are for two tests. Unfortunately after these two tests, the Gold CPC stopped functioning, and all remaining tests used a CARB CPC, the Grimm CPC in Fig. 1. All three working systems gave very similar results for the 6-hour soak test with pre-conditioning, however the Horiba SPCS gave substantially lower results than the GPMS CPC for the 12-hour soak test without pre-conditioning.

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Test Type

Test Date

Prior Cycle

Soak Time

THC

CO

CO2

NOx

PM

Filter Mass

Horiba SPCS

(g/km)

(g/km)

(g/km)

(g/km)

(mg/km)

(mg)

(#/km)

NEDC-19

(AM)

NEDC Overnight

0.009

0.141

175.82

0.291

0.715

0.0389

6.36E+10

NEDC-21

(AM)

NEDC Overnight

0.009

0.158

167.54

0.254

0.567

0.0308

2.79E+10

NEDC-22

(AM)

NEDC Overnight

0.013

0.206

164.87

0.257

0.513

0.0279

1.64E+10

NEDC-24

(AM)

NEDC Overnight

0.008

0.175

168.07

0.267

0.538

0.0294

3.41E+10

NEDC-26

(AM)

NEDC Overnight

0.012

0.201

167.09

0.267

0.533

0.0292

4.55E+10

Avg.

0.010

0.176

168.68

0.267

0.573

0.0312

3.75E+10

COV (%)

21.69

15.72

2.48

5.36

14.21

14.11

47.90

Range

0.0020.01

< 0.1

150170

0.190.27

0.2-0.6

0.0050.06

5E+101.3E+11

Avg.

0.056

161

COV (%)

5-50

<3

JRC Report

2-10

0.57

8.70E+10

10-65

12-72

Fig. 3. Comparison of Emission Results from Haagen Smit Laboratory Cell 7 and other PMP Participating Laboratories.

Another series of seven NEDC cycles were run near the time for DFP regeneration with all of the particle instruments running simultaneously. Fig. 7 shows the results from seven different instruments (the instruments shown in parentheses had some problems during the testing). The results are presented as number of particles per kilometer, however the seven NEDC cycles had different preparations such as soak times and pre-conditioning. At the beginning of the testing the Gold CPC was working and responding similar to the Reference CPC, but it gradually deteriorated during the course of the study. The TSI 3790 instrument displayed flow rate and concentration errors, while the EEPS was working correctly. The EEPS is shown in parentheses since the particle concentrations were sometimes near or below instrument noise levels, and these noise signals are included in the integrated results. The EEPS was the only instrument capable of measuring total particles less than 10 nm, and it was important to obtain some measure of total particles below this size, since we know that there are a considerable number of semi-volatile particles below 10 nm. Therefore, the EEPS results were included in the figure. All seven measurement systems showed similar trends from test to test in Fig. 7, even the systems with instrumentation problems.

The five correctly functioning systems (central five rows in the figure) are the TSI 3010, Grimm, Ref, Horiba, and EEPS. The TSI 3010 and EEPS measured total particles while the other three systems measured solid particles. All instruments exhibited a strong downward trend, and thus a large test to test variability, which is most likely due to being near the need for DPF regeneration. All five working systems show reasonable agreement with each other in magnitude as well as trend, especially when compared with test to test variability, Fig. 8. Another important result contained in Fig. 8 is that particle concentrations increase with a lower size cutoff limit and with total particles relative to solid particles. The Horiba and Ref. CPCs have the lowest concentrations, and they only measure solid particles with diameters above 23 nm. The Grimm CPC measures solid particles with diameters greater than 6 nm, while the TSI 3010 measures total particles with diameters greater than 10 nm. The most comprehensive instrument is the EEPS which measures total particles with a diameter greater than 6 nm. These results clearly show that sub 23 nm particles are numerous, and also that there are many more semi-volatile particles in the total particle count than solid particles.

1.4

Emissions

1.0

Soak Time

1.4 Soak Time

6-hr w/o precond. 12-hr w/o precond 6-hr w/ precond

1.2

Emissions (g/mile)

1.2

0.8 0.6 0.4 0.2

1.0

6 hr w/o precond 12-hr w/o precond 6-hr w/ precond 12-hr w/ precond

0.8 0.6

0.4 0.2

0.0

0.0

THC x 10 (g/km)

CO (g/km)

CO2 (kg/km)

NOx (g/km) PM (mg/km) PN x 10^11

Fig. 4. Emissions over NEDC cycles for various soak and pre-conditioning options.

THCx10 (g/mi)

CO (g/mi)

CO2 (kg/mi) Nox (g/mi)

PM/10 (mg/mi)

PN x 10^11

Fig. 5. Emissions over FTP for various soak and pre-conditioning options.

H. Dwyer et al. / Atmospheric Environment 44 (2010) 3469e3476

Solid particle number emissions (#/km)

1.2E+11 NEDC-26 (12-hr w/o precond) NEDC-25 ( 6-hr w/ precond.)

1.0E+11

8.0E+10

6.0E+10

4.0E+10

2.0E+10

0.0E+00 Horiba SPCS

Gold CPC

Ref. CPC

Fig. 6. NEDC particle number emissions using Horiba SPCS and GPMS CPCs.

If the results in Fig. 7 are compared to the five NEDC tests in Fig. 2, we do not see a downward trend in particle number emissions in Fig. 2. The five NEDC tests in Fig. 2 were not performed near the time that the DPF was scheduled for regeneration. If the particle number concentration measurement is very sensitive near the regeneration event, then this behavior should be addressed in future studies. 3.4. Real time particle emissions The CARB investigation studied particle emissions as a function of time during NEDC cycles. A comparison of real time emissions measured by four instruments is shown in Fig. 9. The ordinate in Fig. 9 is presented on a log scale, and the majority of the emissions occur during the first 300 s of the cycle during the 2nd and 3rd acceleration periods. After the first 300 s of the NEDC cycle in Fig. 9, particles measured from multiple particle counting systems are close to tunnel background. All CPCs overlap each other during parts of the NEDC cycle, but the TSI 3010 and Grimm CPCs have

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higher baseline levels. These baseline levels, although low, are not insignificant when included in the integrated total over the cycle for clean vehicles. However, it should be mentioned again that both the TSI 3010 and Grimm CPCs have smaller cut off ranges for particles, and they should give higher counts than the other particle instruments. The results in Fig. 9 clearly show that the majority of solid particles are generated during the cold start portion of the NEDC cycle, and that during cold starts the numbers of small total particles are not large. The four instruments shown have different size limits and particle type, but they are all measuring similar concentrations during the cold start part of the cycle. Therefore, during normal operation of a clean diesel vehicle the majority of the PM is generated at cold start, and that further reductions of PM will require cold starts to be mitigated and investigated. Another aspect of the particle number study was to investigate the repeatability of the real time results during repeat driving cycles. Fig. 10 shows five repeats of the NEDC cycle measured using the Horiba SPCS counter. The peak particle numbers during the cycles are in good qualitative agreement, however the logarithm scale of the plots hides some of the quantitative variability of the particle number counts. The average particle number per kilometer, PN/km, over the NEDC at CARB had a COV of 48% while the JRC participating laboratories had COVs ranging from 12% to 72%. The causes of the variability between repeat cycles cannot be determined at the present time, so there is a need for additional work to better understand particle counting systems and methodologies. Although the EEPS data was at noise levels during the parts of the tests, the EEPS collects sufficient particles to generate size distributions during the initial few hundred seconds of the NEDC test cycle. During this initial cold start period all particle instruments gave similar values for particle number, see Fig. 9. Therefore, the particles are primarily solid particles, and the size spectra is shown in Fig. 11. The data in Fig. 11 was taken at a time of peak concentration for test NEDC-32, and particle diameters are in the size range from 40 to 200 nm with a peak at 80 nm. It is implicit in the PMP methodology that the size distribution of the solid PM is

Fig. 7. NEDC particle number emissions using all of the particle instruments.

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90%

8.0E+10

80%

7.0E+10

70%

6.0E+10

60%

5.0E+10

50%

4.0E+10

40%

3.0E+10

30%

2.0E+10

20%

1.0E+10

10%

0.0E+00

Relative CoV

Particle Conc. (#/cm^3)

Particle Instruments Comparison 9.0E+10

0% Horiba SPCS

Ref. CPC

Grimm CPC

(EEPS)

TSI 3010

Solid Particle Solid Particle Solid Particle Total Particle Total Particle Fig. 8. Comparison of particle average concentrations by five different instruments.

similar for all makes of diesel engines and DPFs. For example, if an engine or DPF generated a much different size distribution, then the PM mass and particle concentration may not have a strong correlation with each other. Additional insight into the nature of the PM generated during the testing can be found from Fig. 12 where the organic carbon, OC, and elemental carbon, EC, content of the PM is presented for both the NEDC and FTP testing. The results have been further separated by the portion of the cycle during which the PM was generated, and this allows the cold start period to be studied. Phase 1, P1, of both the NEDC and FTP cycles has generated substantial amounts of both OC and EC during the cold start part of the cycle. During the latter phases of both cycles the amount of EC decreases substantially, while OC decreases by a lesser amount. The amount of OC generated in the latter phases of the NEDC and FTP cycles is comparable with OC generated in the cold start part of the cycles, and this implies that semi-volatile and small particles are probably responsible for the OC in the latter phases. This implication is supported by the very small number of solid particles measured by the Horiba SPCS and Reference CPC during the latter phases of the cycles. A chemical analysis was performed on the PM generated during the testing, but very few species were significantly above noise level for the NEDC tests. A larger amount of species were generated during long steady state tests, and the PM mass data was dominated by

PM10 that is not part of regulated data. It is interesting to note that cerium levels were always above noise levels, and this is most likely due to the presence of cerium in the fuel borne catalyst. However, the cerium levels were well below environmental standards. 3.5. A closer look at the golden vehicle Some extensions or enhancements of the GV investigation were carried out in order to gain knowledge of the processes occurring during DPF regeneration and also of the characteristics of the emitted particulate matter. The processes occurring during DPF regeneration are quite complex, and only a brief overview will be given in this paper (Andersson et al., 2007; Dwyer et al., 2010), in order to show the problems associated with the measurement of particle concentrations and PM during DPF regeneration. Values of particle concentration versus time are shown in Fig. 13 during a NEDC cycle in which a regeneration event occurred with the Horiba and EEPS instruments operating. The DPF regeneration event can be seen by the very large increase in concentrations measured by the EEPS instrument towards the end of the NEDC where the most aggressive driving occurs (approximately the last two hundred seconds). During the regeneration part of the cycle the Horiba instrument is showing a very modest increase in solid particle numbers compared

Fig. 9. Comparison of particle number concentrations from multiple particle counters during test NEDC-32.

H. Dwyer et al. / Atmospheric Environment 44 (2010) 3469e3476 1.00E+06

0.16 NEDC-19

NEDC-20

NEDC-21

NEDC-24

OC EC

NEDC-26

0.14 1.00E+05

Particle Carbon (mg/km)

Particle Number Conc. (#/cm3)

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1.00E+04

1.00E+03

1.00E+02

0.12 0.10 0.08 0.06

0.04 0.02

1.00E+01 0

200

400

600

0.00

800

NEDC P1

Time (s) Fig. 10. Particle number concentrations during five repeat NEDC cycles with the Horiba SPCS.

to cold start values, while the concentrations of the EEPS instrument are more than four orders of magnitude larger. If the solid particle numbers were correlated with PM mass then PM mass would not increase substantially during this NEDC cycle with regeneration. In reality, PM mass typically increased by a factor of 7e50 during NEDC cycles with regeneration, and this increase was not seen in solid particle concentrations. However, the PM mass increase is seen in the very large increase in semi-volatile and small particles measured by the EEPS instrument. Additional particle insight is shown in the contour plot in Fig. 14 where both particle size and concentration from the EEPS as a function of time during a regeneration event is presented (Note: A somewhat longer regeneration event than Fig. 13). The maximum concentrations occur for particles less than 10 nm, and these particles are semi-volatile particles that are not measured by an instrument for a 23 nm cutoff. The semi-volatile particles are so numerous that they are overwhelmingly responsible for the large increase in measured filter PM during the NEDC cycle. Therefore, it is clear that the DPF regeneration is a unique event for a diesel vehicle, and it will have to be addressed with different particle instruments or measurement methodologies. 4. Summary and conclusions

FTP P1

FTP P2

FTP P3

Fig. 12. Emission rates of Organic and Elemental Carbon during the NEDC and FTP cycles where the notation P1, P2, and P3 denotes the parts or phases of the NEDC and FTP cycles. For example, P1 e cold; P2 e transient and hot; and P3 is hot for FTP.

future challenges that will be faced by the introduction of new diesel vehicles. The major conclusions of the investigation are the following:  CARB measured regulated emissions and particle number emissions compared well with the JRC participating international laboratories in magnitude and test-to-test variability.  The influence of soak time on PM mass and particle number was not strong.  The influence of pre-conditioning on PM mass and particle number results was significant for both the NEDC and FTP cycles. However, the trends for the NEDC and FTP cycles were opposite.  Pre-conditioning of the NEDC cycle leads to a good comparison between CARB particle counters and GV particle counters, although limited tests were carried out.  In general all particle counters gave similar trends for integrated particle number emissions over the driving cycles, but test to test variation was observed. All of the particle counting instruments also gave similar trends for the real time data measurements  Real time measurements of particle concentrations during both NEDC and FTP cycles have given insight into PM generation during the cycles. The majority of the solid PM is generated during the initial cold phases of the cycles.

EEPS

SPCS

Speed

Particle Concentration #/cm3

The participation and testing by CARB of the PMP methodology has been a success, and it has increased the knowledge of both the European and California arms of the regulatory entities concerning PM emissions from diesel vehicles. Both the PMP testing and the enhanced testing performed by CARB have enabled us to understand the progress that has been made in PM reduction and the

NEDC P2

Time - seconds

Fig. 11. EEPS particle size distribution during cold start of NEDC32 cycle.

Fig. 13. Real time particle concentration emissions during the second DPF regeneration.

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The authors also wish to thank all of the CARB laboratory personnel, management, and executive staff for the support for this research and for the California-EC collaboration. Disclaimer: The statements and opinions expressed in this paper are solely the authors’ and do not represent the official position of the California Air Resources Board. The mention of trade names, products, and organizations does not constitute endorsement or recommendation for use. The Air Resources Board is a department of the California Environmental Protection Agency. CARB’s mission is to promote and protect public health, welfare, and ecological resources through effective reduction of air pollutants while recognizing and considering effects on the economy. CARB oversees all air pollution control efforts in California to attain and maintain health-based air quality standards. Fig. 14. EEPS particle concentrations and size distributions during a regeneration event.

 A substantial amount of OC is generated during the latter phases of the NEDC and FTP cycles. It is implied from the results that this OC is due to semi-volatile particles which are not measured by PMP CPCs.  There is significant variation in the capabilities of the particle counting instruments in terms of particle size and particle concentration magnitudes. It is very important for the measurement of volatile particles that particles of smaller size and increased concentrations be included in the instrument range.  Measurements of particle concentrations during DPF regeneration have yielded very high concentrations of total particles. The PMP methodology was not designed to measure these particles, and a different methodology would be needed to capture the regeneration events. Regeneration of DPFs is an area that should to be addressed in the future.

Acknowledgements CARB staff wishes to extent its appreciation to JRC staff for the opportunity to conduct this research. Of special note are the helpful interactions with Dr. Panagiota Dilara, Dr. Barouch Giechaskiel, and Mr. Giorgio Martini. Dr. Qiang Wei of Horiba, Inc. is specially acknowledged for providing in-kind an SPCS unit for this research. Dr. Wei’s technical input and participation was critical for the emission measurement comparisons in this project. We thank Mr. Jon Andersson of Ricardo and Mr. Chris Parking of the UK’s Department for Environment, Food and Rural Affairs for their expert advice during this work. We also thank Ms. Emma Sandbach from AEA Energy & Environment for calibration of the GPMS and TSI, Inc. for the loan of particle counters. The assistance with logistics for shipping the Golden Vehicle from Europe to California and back by Dr. Whitney Leeman and Dr. Pablo Cicero-Fernandez of CARB and Dr. Tom Durbin of the University of California, Riverside was greatly appreciated. The University of Wisconsin played a vital part in this study by their preparation and analysis of the filters for PM and metal determination.

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