Environ. Sci. Technol. 2009, 43, 5928–5933
Effect of Advanced Aftertreatment for PM and NOx Control on Heavy-Duty Diesel Truck Emissions J O R N D I N H H E R N E R , * ,† S H A O H U A H U , † WILLIAM H. ROBERTSON,† TAO HUAI,† JOHN F. COLLINS,† HARRY DWYER,† AND A L B E R T O A Y A L A †,‡ California Air Resources Board, 1001 “I” Street, P.O. Box 2815, Sacramento, California 95812, and Mechanical and Aerospace Engineering, West Virginia University, P.O. Box 6201, Morgantown, West Virginia
Received March 18, 2009. Revised manuscript received June 11, 2009. Accepted June 18, 2009.
Emissions from four heavy-duty and medium-duty diesel vehicles were tested in six different aftertreatment configurations using a chassis dynamometer. The aftertreatment included four different diesel particle filters (DPF) and two prototype selective catalytic reduction (SCR) devices for NOx control. The goal of the project was to fully characterize emissions from various in-use vehicles meeting the 2007 particulate matter (PM) standard for the United States and California and to provide a snapshot of emissions from 2010 compliant vehicles. The aftertreatment devices all worked as designed, realizing significant reductions of PM and NOx. The DPF realized >95% PM reductions irrespective of cycle and the SCRs >75% NOx reductions during cruise and transient modes, but no NOx reductions during idle. Because of the large test matrix of vehicles and aftertreatment devices, we were able to characterize effects on additional emission species (CO, organics, and nucleation mode particles) from these devices as a function of their individual characteristics. The two predicting parameters were found to be exhaust temperature and available catalytic surface in the aftertreatment, which combine to create varying degrees of oxidizing conditions. The aftertreatments were not found to incur a fuel penalty.
Introduction Emissions from diesel exhaust continue to receive attention because of their adverse health effects (1-4) and environmental impacts (5). For example, diesel particulate matter was identified as a toxic air contaminant by the state of California in 1998 (6). In addition to particulate matter (PM), diesel exhaust contains various gaseous pollutants of concern such as NOx, CO, and volatile and semivolatile organic compounds (7) that have direct health implications and act as precursors to photochemical ozone formation (8). Diesel emissions constitute approximately 40-60% of the on-road PM and NOx inventories in California and United States (9, 10). In response to these concerns, the California Air Resources Board (CARB) and the United States Environmental Protection Agency (EPA) started regulating heavy* Corresponding author phone: (916) 324-9299; fax: (916) 3224357/(916) 323-1045; e-mail:
[email protected]. † California Air Resources Board. ‡ West Virginia University. 5928
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duty diesel (HDD) emissions in the late 1980s and have since tightened the emissions standards periodically as new diesel technology has made it possible to lower emissions while retaining the high thermal efficiency characteristics of diesel engines. The most sweeping reduction in the emissions standards from heavy-duty diesel engines is a 90% reduction of PM and NOx, effective as of 2007 and 2010, respectively. In the past, engine manufacturers have realized reductions in PM and NOx emissions for HDD engines with engine design modifications and combustion process improvements. While the new PM and NOx standards are technology neutral, it is widely expected that sophisticated aftertreatment technologies will be required to meet the limits, namely, diesel particulate filters (DPF) for PM and selective catalytic reduction (SCR) and exhaust gas recirculation (EGR) for NOx. DPFs and SCRs, in addition to filtering soot and converting NOx, change the character of the diesel engine emissions qualitatively by acting as chemical reactors that create varying degrees of either oxidizing or reducing conditions with associated effects on other exhaust gas species (11-13). The extent to which this happens depends on the individual characteristics of the DPF or SCR. While epidemiological evidence exists linking PM2.5 mass with adverse health effects (14-18) and, therefore, a reduction is beneficial, it still remains to be shown directly that the toxicity of the treated exhaust is also reduced (19, 20). The primary goal of this study is to investigate the effectiveness and effects of advanced emission control technologies. Effectiveness is defined as reductions of the targeted pollutants PM mass and NOx. Effects include those on other criteria pollutants, SO2(g), particle number, volatility, toxicity, mutagenicity, toxic gas-phase compounds, ozone precursors, and greenhouse gases. While previous studies on the effects of DPF or SCR technologies have been limited to studying only one or two devices, the current study looks at four different vehicles and six different aftertreatment configurations and a baseline (no aftertreatment). This allows for a comprehensive and holistic analysis of effects of each device on emissions, not as a generic entity but on the basis of the individual characteristics of the device. The present paper deals only with criteria pollutants and associated fuel economy effects from aftertreatment in HDD vehicles, while companion papers present physical (21), chemical (22-24), and toxicological (25) effects of these aftertreatments on HDD vehicles.
Methods Vehicle emissions testing took place at the CARB heavy-duty diesel emissions testing laboratory in Los Angeles, CA. The facility has been described in detail previously (19) and is only briefly discussed here. The laboratory is equipped with a heavy-duty, Schenck-Pegasus chassis dynamometer driven by a direct current 675 hp motor that can absorb up to 660 hp. Vehicles operated on the chassis dynamometer have their exhaust connected to a full-exhaust dilution tunnel in the form of a Horiba critical flow venturi constant volume sampler (CVS) operated at either 1500 or 2500 standard cubic feet per minute (scfm) during the current study. The tunnel flow rate selections depends on vehicle size and cycle. Filtered and carbon-scrubbed ambient air is used for dilution. The CVS and sampling lines, which have been in use testing uncontrolled diesel vehicles since 1990, were disassembled and cleaned to prevent any contamination of samples for the current ultralow emitting vehicles. Criteria pollutants were collected and measured in accordance with reference 10.1021/es9008294 CCC: $40.75
2009 American Chemical Society
Published on Web 07/13/2009
TABLE 1. List of Vehicles Tested, and Engine and Aftertreatment Specifics vehicle vehicle number/ID
make
model
year
miles
curb weight (lb)
curb GVWR (lb)
tested weight (lb)
Veh1-baseline Veh1-DPF1 Veh1-DPF1+SCR1 Veh1-DPF1+SCR2 Veh2-DPF2 Veh3-DPF3 Veh4-DPF4
Kenworth Kenworth Kenworth Kenworth International Thompson Gillig (35 ft)
T800B T800B T800B T800B 4900 SafetyLiner Alison Hybrid Drive
1998 1998 1998 1998 1999 1988 2007
374000 374000 360000 360000 40000 325000 1000
26640 26640 26640 26640 15030 22200 27500
80000 80000 80000 80000 27500 36200 39600
53320 53320 53320 53320 20920 26720 30200
vehicle vehicle number/ID
aftertreatment (AT)a
engine model
year
size (L)
engine miles
make and type
Veh1-baseline Veh1-DPF1 Veh1-DPF1+SCR1c
b
Cummins M11 Cummins M11b Cummins M11b
1998 1998 1998
11 11 11
374000 374000 360000
none JM CRT JM SCRT
Veh1-DPF1+SCR2c
Cummins M11b
1998
11
360000
JM SCRT
Veh2-DPF2 Veh3-DPF3
International DT466E Cummins
1999 2003
7.6 5.9
40000 50000
Veh4-DPF4
Cummins
2006
5.9
1000
Engelhard DPX Cleaire Horizon + EGR JM CCRT
description
miles on AT
none DOC + uncatalyzed DPF CRT + vanadium SCR + small oxidation catalyst CRT + zeolite SCR + small oxidation catalyst catalyzed DPF uncatalyzed DPF
n.a. 64000 50000
DOC + catalyzed DPF
1000
SCR, 0; CRT, 50000 30000 31000
a
DOC, diesel oxidation catalyst; EGR, exhaust gas recirculation; JM, Johnson Matthey; SCR, selective catalytic reduction. b Reflashed. c Prototype systems, not commercial units. Both terminate with a small oxidation catalyst for ammonia slip control.
methods for emission certification described in the Code of Federal Regulations (CFR parts 86 and 1065). Figure S1 of the Supporting Information shows the sampling setup for the project in its entirety. Filter samples and cartridge-based samples were collected for chemical, toxicological, and morphology analysis. Numerous real-time instruments were employed to measure particle number, size distribution, particle surface area, and particle bound PAHs. The details and results of sampling for noncriteria pollutants are discussed in companion publications (21-24). Table 1 shows details of the test fleet, simulated test weights, engines, and aftertreatment devices tested. All but one of the aftertreatments tested were retrofits, which provide a snapshot of 2007 and 2010 compliant vehicles, but are not as highly optimized as will be the original equipment manufacturer (OEM)-installed devices. Vehicle 1 is powered by a 1998 Cummins 11 L diesel engine and was tested in four configurations: (1) as a baseline without aftertreatment (Veh1baseline), (2) with a Johnson Matthey CRT, which consists of a diesel oxidation catalyst (DOC) followed by an uncatalyzed DPF (Veh1-DPF1), (3) with a vanadium-based SCRT (Veh1-DPF1+SCR1), and (4) with a iron-based zeolite SCRT (Veh1-DPF1+SCR2). SCRT is Johnson Matthey’s trade name for a CRT followed by SCR for NOx control. The CRT in the two SCRT systems was the same as the one used in configuration 2. Although the systems are commercially available, both SCRs tested were prototype systems that used liquid urea injection and a small oxidation catalyst to prevent ammonia slip. Vehicle 2 is powered by a 1999 7.6 L International diesel engine and had been retrofitted with an Engelhard DPX. The DPX is a DPF containing a catalytic coating (Veh2-DPF2). Vehicle 3 is powered by a 2003 5.9 L Cummins diesel engine and had been retrofitted with a Cleaire Horizon DPF, which consists of an uncatalyzed silica carbide filter and requires electrical regeneration (Veh3DPF3). The last vehicle, Vehicle 4, was a hybrid electric bus powered by the Allison Hybrid drive and a 2006 Cummins 5.9 L diesel engine. This was a 2007 model year vehicle with a Johnson Matthey CCRT installed by the OEM (Veh4-DPF4). The CCRT consists of a DOC followed by a catalyzed DPF. All of the testing was done using a single batch of com-
mercially available ultra-low-sulfur diesel (ULSD) provided by British Petroleum (BP) with a measured sulfur content of 6 ppm. When first installed on the dynamometer, a new vehicle was operated at highway cruise speeds for 4 to 8 h to condition the sampling system. Because the tested engine sizes and model years are very different and therefore comply with different PM and NOx engine standards, the reader is advised to exercise caution when using these results to compare retrofit devices. For example, it would be incorrect to conclude that the CCRT is better at reducing PM than the DPX on the basis of the current data set, which shows lower PM emissions from Veh4 with the CCRT than from Veh2 with the DPX because the former has a newer and smaller engine than the latter. Intercomparisons of aftertreatment devices can be made between the four configurations tested on Veh1. The vehicles were all tested using three driving modes: idle, cruise at 50 mph, and transient cycle. The transient cycle selected was the EPA urban dynamometer driving schedule (UDDS) for heavy-duty chassis dynamometer testing (Figure S2 of the Supporting Information). The UDDS cycle duration is 1060 s, has an average speed of 18.86 miles per hour, a total length of 5.55 miles, and a maximum speed of 58 miles per hour. One of the main goals of the project was to collect sufficient PM mass with standard filter media for subsequent physicochemical analysis. The idle and cruise cycles were therefore long, lasting 45 min, while the UDDS was driven in duplicate, back-to-back for a single sample lasting approximately 35 min. All samples were collected in triplicate at minimum, with some samples from very clean vehicles repeated more than 30 times to collect sufficient mass for subsequent analysis. This process is arduous and resource consuming but necessary and an aspect of the study that makes it unique. The only exceptions to this sampling schedule were Veh1-DPF1 from which no idle sample was collected and Veh4-DPF4 from which neither an idle or cruise at 50 mph sample was collected.
Results and Discussion Table 2 shows the average PM mass, NOx, total hydrocarbons (THC), and CO emissions ( one standard deviation (these VOL. 43, NO. 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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90.5 ( 2.9 77.2 ( 2.5 43.0 ( 2.5 56.5 ( 1.3
a All vehicles are equipped with some type of DPF except the baseline. Veh3 has EGR for NOx control. Veh1-DPF1+SCR1 and Veh1-DPF1+SCR2 have SCR for NOx control. DPF1 consists of a DOC followed by an uncatalyzed filter. DPF2 consists of a catalyzed filter. DPF3 consists of an uncatalyzed filter. DPF4 consists of a DOC followed by a catalyzed filter. Both SCR systems terminate with a small oxidation catalyst for ammonium slip control. These data need to be analyzed cognizant of the amount of catalyst in each aftertreatment system. b Blank entries represent cycles not measured. Concentrations are expressed in g/mi or g/h instead of g/bhp-hr because the testing was done with a chassis dynamometer rather than an engine dynamometer. This precludes the ability to compare the results directly with the heavy-duty diesel engine standards. c ND is nondetect.
31.9 ( 1.4 34.8 ( 1.8 22.2 ( 1.5 9.1 ( 2.0
30.3 ( 3.1
4.04 ( 0.34 62.1 ( 3.1 1.55 ( 0.05 7.2 ( 0.3 0.33 ( 0.12 0.10 ( 0.001 0.70 ( 0.12 0.06 ( 0.05 31.7 ( 5.9 0.04 ( 0.01 0.24 ( 0.09 0.01 ( 0.01 14.4 ( 13.2 0.014 ( 0.01 0.10 ( 0.09 0.05 ( 0.02 4.8 ( 0.3 0.006 ( 0.002 0.42 ( 0.06 0.13 ( 0.03 6.9 ( 3.8 0.70 ( 0.03 2.2 ( 0.2 0.01 ( 0.004 0.025 ( 0.01 0.73 ( 0.03 0.01 ( 0.001 0.002 ( 0.003 0.001 ( 0.003 0.001 ( 0.004 0.07 ( 0.01 80.9 ( 0.9
17.3 ( 0.12 21.1 ( 0.19 16.3 ( 0.05 18.8 ( 0.04 0.024 ( 0.021 2.7 ( 0.5 5.3 ( 1.9 0.054 ( 0.040 3.6 ( 1.2 5.3 ( 1.9 0.021 ( 0.04 7.9 ( 0.2 10.6 ( 0.3 0.022 ( 0.012 3.5 ( 0.2 5.3 ( 0.6 8.0 ( 0.3 0.67 ( 0.04 0.014 ( 0.001 0.013 ( 0.010 0.008 ( 0.007 0.006 ( 0.003 0.001 ( 0.002 0.003 ( 0.006 Veh1-baseline Veh1-DPF1 Veh1-DPF1+SCR1 Veh1-DPF1+SCR2 Veh2-DPF2 Veh3-DPF3 Veh4-DPF4
0.13 ( 0.06 0.007 ( 0.001 0.017 ( 0.006 0.014 ( 0.005 0.005 ( 0.001 NDc
5.1 ( 1.4
idle (g/h) UDDS (g/mi) UDDS (g/mi) cruise (g/mi)
idle (g/h)
cruise)g/mi)
UDDS (g/mi)
idle (g/h)
cruise (g/mi)
UDDS (g/mi)
idle (g/h)
cruise (g/mi)
CO THC NOx PM
TABLE 2. Criteria Pollutant Resultsa,b
results are graphed in Figures S3-S6 of the Supporting Information). The results in Table 2 are best interpreted cognizant of the effects the aftertreatment is expected to have on each species, specifically the extent to which it affects redox chemistry. A discussion on redox chemistry in diesel aftertreatment is provided below for context prior to the discussion of the results in Table 2. Many DPFs contain some level of catalyst in the form of a DOC or some level of catalytic washcoat on the DPF itself (i.e., a “catalyzed” filter), which lowers THC, CO, and PM (through a reduction of soluble organic fraction) emissions and helps regenerate the filter (remove the collected soot) (26-29). The amount of catalyst in the aftertreatment determines its overall oxidation potential. DPF1, DPF2, and DPF4 in the current study utilize various levels of catalyzation to achieve soot removal. In general, less aggressive duty cycles result in low average exhaust temperatures and the need for more heavily catalyzed DPFs. DPF3 is an exception to this rule and achieves soot removal through an onboard electric heating element and blower to regenerate the DPF when the vehicle is parked and the device is plugged into offboard power. SCR also contributes to the overall catalytic loading of the aftertreatment. By definition, they are catalyzed, but as mentioned earlier many SCR systems, including SCR1 and SCR2, also terminate with a small oxidation catalyst to control for ammonia slip. Figure 1 conceptualizes graphically the expected effect of the aftertreatment device on THC, CO, the NO2/NOx ratio, and SO2(g) as a function of how heavily it is catalyzed and the operating temperature. These two parameters are deterministic in the physics, chemistry, and toxicity of the emissions for a given combination of engine, fuel, lubricant, and aftertreatment. The top right corner of Figure 1 represents the hottest operation of the engine (high-speed, high-load highway cruise) with a large catalytic surface/volume available in the aftertreatment where resultant oxidation will be strong. Under such conditions THC and CO emissions are expected to be very low and the NO2/NOx ratio very high. Also, SO2(g) is oxidized to SO3(g), which combines readily with water and ammonia leading to nucleation of sulfuric acid or ammonium sulfate particles. Alternatively, at hot temperatures with completely uncatalyzed aftertreatment, represented by the top left corner of Figure 1, the engine out NO2 will be reduced by the collected soot to NO, leading to very near-zero NO2/NOx ratios, while the effect on THC, CO, and SO2 will be small to negligible. These effects are dampened toward the lower part of Figure 1 where temperatures are low and less redox chemistry occurs in the aftertreatment irrespective of available catalytic surface/ volume. This concept can be illustrated with the NO2/NOx ratios measured in the current study. Using a ratio allows for comparisons between the four engines without regard for the difference in nominal NOx emissions Also, diesel engine out NO2/NOx ratios are relatively consistent in diesel exhaust, typically 5-15% (30). Figure 2 shows the NO2/NOx ratios measured during the current study. The seven configurations have been listed in order of increasing catalytic surface in the aftertreatment along the x-axis starting with Veh3-DPF3, which is completely uncatalyzed, and ending with Veh4DPF4, which contains a DOC and catalyzed filter. On the z-axis, the cycles tested are listed in order of increasing engine out temperature starting with idle and ending with cruise. This arrangement of configurations and cycles corresponds to the conceptual model presented in Figure 1. Several factors combine to make the comparison with the conceptual model in Figure 1 less than perfect such as the different exhaust temperatures between same cycles of different vehicles, the density and light off temperature of the different catalytic coatings, and state of aftertreatment soot loading. Also, it has been reported that SCRs can store NO2 during cold cycles
FIGURE 1. Two major factors in DPF chemistry ar catalytic loading and temperature. Shown here is their effect on the NO2:NOx ratio, THC, CO. and SO2(g) when compared to engine out exhaust. The more heavily catalyzed and hotter the exhaust temperature, the more strongly the aftertreatment will oxidize the exhaust. Uncatalyzed DPFs however, will not affect THC and CO but will reduce NO2 to NO as the collected soot acts as a reductant.
FIGURE 2. NO2:NOx ratio as a function of aftertreatment device and driving cycle. Aftertreatment devices are listed in order from least catalyzed to most catalyzed (left to right), while the driving cycles are listed in order of heat they produce (idle through cruise). Data not available for the idle cycle of Veh1-DPF1 and for idle and cruise of Veh4-DPF4. (31), which could explain the low NO2/NOx ratio during idle for the Veh1-DPF1+SCR1 and Veh1-DPF1-SCR2 configurations. Despite these interferences inherent in any comparison between many vehicles, the overall trend in the NO2/NOx ratio is clear, with the expected highest NO2/NOx ratio in the top right corner of Figure 2, the lowest in the top left corner, and the minimal effect due to the catalysts during the cold idle cycle. We now return to the PM results in Table 2 and Figure S3 of the Supporting Information, which show all the DPFs working effectively and achieving very low PM emission rates, in most cases less than 0.01 g/mile. Direct comparison of PM emissions from Veh1-baseline and Veh1-DPF1 suggests the PM removal efficiencies of the DPF is 95% and 98% for the cruise and UDDS cycles, respectively. Veh1-DPF1+SCR1 and Veh1-DPF1+SCR2 had slightly lower efficiencies during the cruise cycle where the oxidation of SO2(g) increased nucleation and the associated sulfate particle emissions (22, 32). The effect of aftertreatment on when and how nucleation occurs is of great importance and will be further discussed
FIGURE 3. Real-time NOx measured in a CVS tunnel during cruise with Veh1-DPF1+SCR2. The time urea injection initiates is clearly visible. This requirement for a high operating temperature for the SCR affects NOx reduction during low speed cycles. in a companion publication (32). The same publication will also discuss ammonia slip from the SCRs. Comparison of Veh1-baseline and Veh1-DPF1+SCR1 and Veh1-DPF1+SCR2 idle cycles shows the efficiency of the DPF is not temperature dependent with removal rates during this “cold” cycle of >99%. In the idle mode for all vehicles with DPFs and in all driving modes for the two smaller and newer engines, Veh3DPF3 and Veh4-DPF4, the measured emission rate is very low. The average PM emission for Veh3-DPF3 during cruise, for example, is negative, and is therefore listed as not detectable. The accuracy of the measured mass is expected to decrease with decreasing mass emissions, and the standard deviation is greater than the average PM emission rate for many of these measurements, reminding us of the remaining challenges for the gravimetric method above and beyond the new CFR 1065 recommendations partially adhered to during this study (33, 34). DPFs have been shown to have only a small effect on total NOx emissions (19, 26, 35), even if they significantly alter the NO2/NOx ratio as discussed previously. Comparing Veh1baseline and Veh1-DPF1 in Table 2 or Figure S4 of the Supporting Information, we show that during cruise and UDDS cycles, NOx concentrations only differ by 6% and 10%, respectively. The SCRs in the Veh1-DPF1+SCR1 and Veh-2 configurations function by injecting urea, which is converted to ammonia and reduces NOx to N2 and O2 (26). The NOx reductions of the SCR systems tested in the current study ranged from 84-79% for the cruise cycle and 75% for the UDDS cycle to 0% for the idle. These reductions and specifically the lower efficiency during the transient UDDS cycle is consistent with the NOx conversion being highly dependent on exhaust temperature and the UDDS cycle resulting in less heat in the exhaust than a cruise cycle (26, 27). The NOx reduction is nil during idle and other vehicle operation that leads to low exhaust temperatures. This is relevant for many reasons and highlights the need for specific assessment of exposures to elevated NOx in the urban environment where stop-and-go vehicle operation would be common. Figure 3 shows the real-time NOx mixing ratio measured in the CVS during a Veh1-DPF1+SCR2 cruise cycle. Figure 3 also shows the engine out and SCR out temperatures. While the exact temperature requirements for a SCR system to operate differ, it is obvious from Figure 3 that for this example the needed temperature is reached after 8 min of cruise operation when NOx mixing ratios in the dilution tunnel are suddenly and drastically reduced. Even so, it should be mentioned that the UDDS cycle is an aggressive and therefore “hot” transient cycle, with maximum speeds of almost 60 mph, which combines with the thermal mass of VOL. 43, NO. 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. There is no apparent fuel penalty associated with the aftertreatment tested in Veh1. Veh4 achieves similar fuel economy to Veh2 and Veh3. The hybrid system in Veh4 is expected to have an even greater fuel economy advantage on cycles with more stop-and-go activity. the aftertreatment to maintain operating temperatures for the SCR systems tested. Less aggressive transient cycles may not achieve the required temperatures and, therefore, the NOx reductions reported here for the UDDS (26). Similarly, EGR is often scaled in proportion to engine load and completely disabled during idle. Total hydrocarbons (THC) and CO are both constituents of diesel exhaust that are easily oxidized to the final products of complete combustion, H2O and CO2 (36). As such they are effectively controlled as an additional benefit by the added catalysts employed by DPFs to affect regeneration. The emission rates measured during the current study are shown in Table 2 and Figures S5 and S6 of the Supporting Information. Like the NO2/NOx ratios discussed earlier, the emissions of THC and CO can be predicted by exhaust temperature and the amount of catalyst in the aftertreatment. Emissions are very low to undetectable when the aftertreatment device is hot and has a large catalytic surface/volume such as during the cruise cycles for all the catalyzed aftertreatment devices (i.e., all except DPF3). DPF1 employed in Veh1 reduces THC and CO by 99% and 94%, respectively, during the cruise cycle, while the additional catalyst introduced in the Veh1-DPF1+SCR1 configuration removes 80% and 60% of the remainder, respectively, for those species. During the cruise cycle, the emission rates for THC and CO from the uncatalyzed Veh3-DPF3 are low compared with Veh1-baseline because the vehicle has a newer and smaller engine, but emissions are 1-2 orders of magnitude greater than configurations with catalyzed aftertreatment. The reductions and differences described for the cruise cycle are less pronounced during the UDDS cycle and would be even less so for a less aggressive (and therefore less heat producing) transient cycle. At low exhaust temperatures, the catalyzed aftertreatment still realized some THC reductions but little to no CO reductions. For example, during idle, when exhaust temperatures for the vehicles tested ranged from 90-120 °C, Veh1-DPF1+SCR1 achieves only a 50% reduction in THC but no reduction in CO emissions over Veh1-baseline. Figure 4 shows the fuel economy achieved by the configurations tested and shows emissions reductions were achieved without a fuel penalty in Veh1. The hybrid vehicle, Veh4, achieved similar fuel economy to Veh2 and Veh3, even though it was tested at a considerably higher simulated weight (Table 1). This is in line with previous studies showing an overall fuel benefit for this system of approximately 10% (37). 5932
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The current study shows that DPFs and SCRs can achieve their designed purpose of significantly reducing PM mass and NOx emissions achieving reductions of >95% and >75%, respectively. Considerations, however, have to be made in regard to the overall effect of aftertreatment on heavy-duty diesel emissions and especially the chemistry that takes place within them. As discussed, NOx reduction technologies tested in the current study, SCR and EGR, work best during heavy engine loads that yield higher exhaust temperatures, which in turn affects where NOx emissions will occur (city stopand-go versus highway). Also, it is well-established that catalyzed aftertreatment can increase NO2/NOx ratios, even as emissions of both are reduced because of new emissions standards. The strategy for DPF regeneration and SCR ammonia slip control can create strong oxidizing conditions or even reducing conditions as seen with the configurations tested here. Heavily catalyzed and, hence, oxidizing DPFs, will increase NO2/NOx ratios and promote nucleation but reduce emissions of CO and THC, including toxics such as PAHs, which have been linked to the formation of reactive oxygen species that may be an important indicator of the adverse health effects of diesel particulate matter (25, 38, 39). The noncatalyzed systems, which decrease NO2, are not likely to enhance nucleation but are not effective in eliminating THCs and CO. It is also noted that offboard regeneration emissions are not considered for Veh3-DPF3 (i.e., when plugged in and regenerating). These and the resultant pollutant mix expected in cities and near roadways are important factors to consider when designing diesel control programs relying on postcombustion aftertreatment devices. Additional strategies for regeneration exist but were not explored as part of the current study. These include fuel borne catalysts and active regeneration via fuel injection in the DPF to burn off soot. These strategies will most likely be used in OEM systems, and the effects on the emissions, including redox potential in the aftertreatment device, will have to be characterized.
Acknowledgments This project was funded by the California Air Resources Board, South Coast Air Quality Management District, and California Energy Commission. BP provided the diesel fuel for the study, and the California Department of Transportation, San Joaquin Regional Transit District, Elk Grove Unified School District, and Sanitation Districts of Los Angeles County provided test vehicles. The authors thank Dr. Subhasis Biswas, Dr. Harish Phuleria, Dr. Michael Geller, Dr. Constantinos Sioutas, Ning Zhi, Payam Pakbin, Mohammad Arhami, Ralph Rodas, George Gatt, and Keshav Sahay for their valuable support during the experimental phase. We also acknowledge the retrofit device makers, whose technologies made this investigation possible. 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 (CARB). 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.
Note Added after ASAP Publication Identification of one of the aftertreatment device/driving cycle configurations was incorrect in the Results and Discussion section of the version published ASAP July 13, 2009; the corrected version published ASAP July 30, 2009.
Supporting Information Available Figure S1 illustrates the sampling setup and Figure S2 the UDDS driving cycle. Figures S3-S6 graphically show the emission factors for PM, NOx, THC, and CO, respectively.
This material is available free of charge via the Internet at http://pubs.acs.org.
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