Influence of Real-World Engine Load Conditions on Nanoparticle

Article pubs.acs.org/est

Influence of Real-World Engine Load Conditions on Nanoparticle Emissions from a DPF and SCR Equipped Heavy-Duty Diesel Engine Arvind Thiruvengadam,*,† Marc C. Besch,† Daniel K Carder,† Adewale Oshinuga,‡ and Mridul Gautam† †

Mechanical and Aerospace Department, West Virginia University, Morgantown, West Virginia 26505, United States of America South Coast Air Quality Management District, Diamond Bar, California 91765, United States of America

‡

S Supporting Information *

ABSTRACT: The experiments aimed at investigating the effect of real-world engine load conditions on nanoparticle emissions from a Diesel Particulate Filter and Selective Catalytic Reduction after-treatment system (DPF-SCR) equipped heavy-duty diesel engine. The results showed the emission of nucleation mode particles in the size range of 6− 15 nm at conditions with high exhaust temperatures. A direct result of higher exhaust temperatures (over 380 °C) contributing to higher concentration of nucleation mode nanoparticles is presented in this study. The action of an SCR catalyst with urea injection was found to increase the particle number count by over an order of magnitude in comparison to DPF out particle concentrations. Engine operations resulting in exhaust temperatures below 380 °C did not contribute to significant nucleation mode nanoparticle concentrations. The study further suggests the fact that SCR-equipped engines operating within the Not-To-Exceed (NTE) zone over a critical exhaust temperature and under favorable ambient dilution conditions could contribute to high nanoparticle concentrations to the environment. Also, some of the high temperature modes resulted in DPF out accumulation mode (between 50 and 200 nm) particle concentrations an order of magnitude greater than typical background PM concentrations. This leads to the conclusion that sustained NTE operation could trigger high temperature passive regeneration which in turn would result in lower filtration efficiencies of the DPF that further contributes to the increased solid fraction of the PM number count.

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INTRODUCTION United States Environmental Protection Agency’s (USEPA) emission standards for heavy-duty diesel engines have gradually evolved toward stringent emissions control policy. The agency’s primary focus has been the reduction of mass based emission rates of oxides of nitrogen (NOx) and particulate matter (PM). The current 2010 USEPA emissions standards for NOx and PM have been set at 0.20 g/bhp-hr and 0.01 g/bhp-hr, respectively. In response to the legislation heavy-duty diesel engine manufacturers have adopted different technologies and engine control strategies for compliance. Of the different methods, combustion based reduction of NOx and PM through advanced engine control strategies face limitations due to the trade-off between these two criteria pollutants, whereby the reduction of one contributes to the increase of the other.1 Adding to the complexity is the fuel economy penalty that is incurred with strategies that employ in cylinder techniques such as high EGR rates to mitigate NOx emissions. Diesel particulate filters (DPF), the only proven after-treatment systems capable of meeting the USEPA PM standards and urea based selective catalytic reduction (SCR) exhaust after-treatment system have emerged as a feasible strategies to meet the 2010 USEPA NOx regulation. DPF systems are catalyzed to reduce soot-light off temperatures and enable passive regeneration when suitable exhaust temperatures are attained and they generally exceed 95% filtration efficiencies for solid PM.2−4 Although DPFs are © 2011 American Chemical Society

very effective in filtering the elemental carbon fraction of total PM, number counts of ultrafine PM have been shown to increase by several orders of magnitude under certain exhaust conditions.4 Furthermore, soot free exhaust downstream of a DPF is conducive for homogeneous nucleation of semivolatile and volatile organic fractions in a continuously diluting and cooling diesel engine exhaust.5 The formation of nucleation mode (particle diameters less than 50 nm) nanoparticles downstream of a catalyzed DPF is highly dependent upon engine operating conditions and exhaust dilution conditions. As a result the formation of nucleation mode particles does not occur over the entire operating regime of the engine. Studies have documented the temperature dependent nanoparticle emissions downstream of a catalyzed DPF.6,7 Results from Kittelson et al. show that sulfur content from lubrication oil and ultra low sulfur diesel results in high particle number concentration downstream of a Continuously Regenerating Trap (CRT).6 Urea based SCR systems have shown to be largely efficient through most part of the engines operating load conditions. Studies have shown NOx reduction efficiency to be greater than Received: Revised: Accepted: Published: 1907

September 2, 2011 December 5, 2011 December 22, 2011 December 22, 2011 dx.doi.org/10.1021/es203079n | Environ. Sci. Technol. 2012, 46, 1907−1913

Environmental Science & Technology

Article

study; for which the procedures are documented elsewhere.8 As part of the referenced work, the engine was calibrated to achieve the USEPA 2010 NOx standard, by involving strategies such as high EGR rates and optimization of the urea dosage in the SCR after-treatment system.8 Test engine specifications are listed in Table 1. The engine was equipped with a catalyzed

75% over various transient and steady-state operating regimes of the engine with use of SCR after-treatment system.4,8 An added advantage of SCR systems is that engine calibrations could be developed for greater fuel economy with the SCR system mitigating the higher engine out NOx. However, the optimum operation of the SCR systems depends on critical exhaust temperature thresholds. Below these thresholds, incomplete decomposition of urea within the after treatment system could produce secondary emissions which in turn can act as precursors for ultrafine PM formation.9 The contribution of a urea based SCR system toward ultrafine particle emissions is not clearly understood. Further, the operating efficiencies of the SCR after-treatment system, in certain applications such as a refuse truck application could be affected by the low exhaust temperatures. It is important to understand the possible formation of ultrafine PM at such operating parameters. This work presents the findings of PM nanoparticle emissions downstream of a DPF-SCR after-treatment system, while operating at low and high exhaust temperature load conditions. This study specifically focuses on the nanoparticle formation during real-world operating load conditions of a modern heavy-duty on highway diesel engine. All diesel engines manufactured to be used in the US are subject to the Federal Test Procedure (FTP), which mandates exercising the engine over the FTP engine dynamometer cycle. Although engines are certified on the FTP cycle, significant differences in emissions rates have been observed in real world operation.10 Therefore, the USEPA has defined a Not-ToExceed (NTE) control zone bound by certain speed and load conditions under the engines lug curve which encompasses a broad range of real world load conditions that are not covered in the FTP engine dynamometer cycle. The current 2010 emissions certification procedures now include these additional NTE test procedures in evaluating the engines for compliance purposes.11 Apart from emissions rates, exhaust temperature is believed to be a critical parameter in initiating the oxidation of sulfur, an important precursor to particle formation.6 Since, it is known that there are large differences in exhaust temperature between the FTP and NTE zone operation, it is imperative we investigate the particle size and number concentration emissions from a heavy-duty engine during predominant NTE zone operation. To that end, we have created an NTE engine dynamometer cycle from data gathered from Engine Control Unit (ECU) as part of the extensive in-use testing conducted on class 8 heavy-duty vehicles. Transient particle size and concentration measurements were performed over this NTE cycle and over select steady state points within this region to investigate the effects of high exhaust temperatures. Also, an engine dynamometer cycle representative for an in-use refuse truck application was created to explore the interactions of possible particle formation with a urea-SCR after-treatment system operating under conditions with low exhaust temperatures.

Table 1. Test Engine Specifications model

Volvo MD11, MY07

configuration displacement compression ratio aspiration fuel injectors peak torque rated power

6 cylinders, inline 10.8 L 16:1 variable geometry turbocharger (VGT) with intercooler dual solenoid electronic unit injectors 1300 ft-lbs @ 1300 rpm 339 bhp @ 1800 rpm

DPF system, with the capability to actively regenerate. Figure S1 of the Supporting Information, SI, shows the schematic of the DOC-DPF-SCR arrangement downstream of the engine. However, due to lack of control over the active regeneration fuel injector, active regeneration was disabled. The DPF was manufactured by Fleetguard and included a Diesel Oxidation Catalyst (DOC) upstream of the particulate filter. Urea-SCR system equipped with an ammonia slip catalyst was manufactured by Johnson Matthey and was designed to be compliant with Euro IV regulations. A urea dosing system, with an independent controller was used to integrate the engine speed and load with the optimized urea dosage map. The development of the urea dosage strategies are described in the work of Ardanese et al.8 The study was conducted in two exhaust configurations, by sampling exhaust downstream of only the DPF followed by sampling downstream of the complete DPF and SCR after-treatment package. Figure S1 of the SI shows the after-treatment systems arrangement during the different test phases. The fuel used in the study was Ultra Low Sulfur Diesel (ULSD) with sulfur concentration not exceeding 10 ppm. Test Cycles and Steady State Set Points. Particle size measurements were performed on two transient engine dynamometer cycles and 7 steady state points within the NTE zone. The two transient cycles used were the refuse truck cycle and the PA3 cycle. The refuse truck cycle is originally a chassis dynamometer cycle developed by West Virginia University that provided the basis for an engine dynamometer version. The speed and torque traces for the refuse truck engine dynamometer cycle is shown in Figure S2 of the SI. This cycle is characterized by low exhaust temperatures usually around the lower bounds of the optimum SCR catalyst operation. Another transient cycle consisting predominantly of NTE operation was created from ECU data logged during in-use operation. The PA3 route (Pittsburgh, PA to Morgantown, WV) contained a majority of the engine loads within the NTE zone. The speed and torque trace of the PA3 cycle is shown in Figure S3 of the SI. Figure 1 shows the PA3 cycle set points plotted within the test engine lug curve. The figure also illustrates the characteristic feature of the cycle with majority of the test points contained within the bounds of the NTE zone. The NTE zone in Figure 1 is represented as the region within the bounds of the red lug curve line and the blue speed and torque lines. The PA3 cycle is a complete highway based operation characterized

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EXPERIMENTAL PROCEDURE Engine, Fuel, and After-Treatment System. The engine used in this study was model year 2007 Volvo MD11 compliant with USEPA 2007 emissions standards. The engine was supplied by Volvo power train with complete access to the ECU. The calibration of the engine to achieve compliance to 2010 NOx and PM emissions standards was part of a larger 1908

dx.doi.org/10.1021/es203079n | Environ. Sci. Technol. 2012, 46, 1907−1913


Article

Figure 1. Test engine lug curve with PA3 set points, NTE zone and steady state test points.

Figure 2. Schematic of the particle size measurement setup.

air was HEPA filtered and relative humidity controlled at 40 ± 2% RH with dilution air temperature maintained at 30 ± 5 °C. The residence time within the CVS tunnel was approximately 1.3 s. The second stage dilution occurred with an ejector dilutor. The inlet of the ejector dilutor was fitted with a critical flow orifice to maintain a constant secondary dilution ratio of 24:1. A variable residence time mini dilution tunnel was setup for partial flow dilution of sample extracted from the CVS. HEPA filtered, dry air at 24 ± 3 °C was used for second stage dilution. The sampling probe was fixed at the maximum residence time sampling port of 2 s. A SMPS Model 3080 was used to characterize the size distribution and measure number concentration at steady state and a Cambustion DMS was used to measure real time size distribution and concentrations of aerosols for transient tests. The SMPS was operated with a Long Differential Mobility Analyzer (LDMA TSI Model 3081) and an Ultrafine Condensation Particle Counter (UCPC TSI Model 3025A).

by high engine loads and exhaust temperatures. Also, for the steady state set points 7 modes were selected within the NTE zone, based on the PA3 cycle set points for particle size measurements using the Scanning Mobility Particle Sizer (SMPS). The details of the 7 modes are given in Table S4 of the Supporting Information and also presented on the lug curve in Figure 1. Particle Size Measurements. The PM size distribution measurement used a two stage dilution tunnel. Figure 2 shows the setup for the particle size measurements. The primary dilution of the engine exhaust is within the Constant Volume Sampler (CVS) dilution tunnel whereby the dilution ratio is dependent on the flow rate of the exhaust. Kittelson has documented that dilution ratios in the range of 5−50 can create conditions for super saturation and hence initiate nucleation of vapor phase species.8 The dilution ratios within the CVS dilution tunnel are usually suitable to induce super saturation of volatile compounds in the engines exhaust. The CVS dilution 1909



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Table 2. Exhaust temperatures, Total Particle Concentrations and Urea Dosage mode mode mode mode mode mode mode mode

1 2 3 4 5 6 7

engine torque (ft-lb)

engine speed (rpm)

exhaust temperatures (°C)

1310 1650 1850 1650 1650 1650 1410

1200 1050 650 800 625 400 1100

449 410 365 397 367 326 425

SCR out total particle no. (#/cm3)

total dilution ratio

urea dosage (kg/h)

× × × × × × ×

111 87 102 104 121 158 105

1.44 2.16 1.08 1.08 0.72 0.36 2.16

8.02 1.41 6.26 8.72 2.55 1.30 3.64

109 108 105 107 105 106 109

The CPC was operated at high flow mode with a sample flow rate of 1.5 lpm. The Cambustion DMS operates at a flow rate of 7 lpm and is capable of measuring particle size and concentration at 10 Hz sampling frequency. Five SMPS scans of each 135 s were performed on each steady-state mode. Table 2 shows the test sequence of the different steady state modes.

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RESULTS AND DISCUSSION Table 2 summarizes the engine out exhaust temperature, total particle concentrations downstream of the SCR and the ufrea dosage corresponding to each steady-state mode. The urea dosage was optimized for optimum conversion efficiency with ammonia slip less than 10 ppm. The data show the relationship between engine exhaust temperature and total particle number concentration downstream of the SCR after-treatment system. Results show that below a threshold temperature of about 380 °C, the contribution of the catalyst toward precursors for nanoparticle formation is minimal. The resulting particle concentrations at these temperatures are close to typical background concentrations. However, higher exhaust temperatures resulted in total particle number concentration that were orders of magnitude higher suggesting a strong effects of the after-treatment system on production of precursors for nanoparticle formation. Particle concentrations showed in this section are corrected for their respective dilution ratios shown in the table. The particle size distributions plotted in the figures are measurements averaged over 5 SMPS scans, each 135 s in duration. Figures 3 and 4 show the steady-state particle size distribution

Figure 4. SMPS particle size distributions for Mode 3.

diameters between 6 and 15 nm. The error bars shown in figures indicate the maximum and minimum particle number concentration deviations of the 5 consecutive SMPS scans. During the steady state testing, it was observed that particle concentrations in the nucleation mode continued to increase until exhaust temperatures stabilized. Hence, the instantaneous particle size distribution measurements from the DMS were used to verify the stability of the distribution and the concentration before commencing data collection with the SMPS. The SCR out particle number concentration were an order of magnitude higher than the DPF out concentration, both in the nucleation region and the accumulation region of the distribution. The order of magnitude increase in accumulation mode particle concentration could be due to differences in loading state of the DPF during the different phases of the after-treatment testing configuration. A possible passive regeneration event prior to the Mode 1 testing of DPF-SCR out configuration could have resulted in lower filtration efficiencies, thereby contributing to higher accumulation mode particle number count in comparison to the results obtained while testing DPF out configuration of the same mode. To substantiate the particle size measurement results, filter weights were analyzed from Mode 1 testing of DPF out and DPF-SCR out configurations. The total PM mass collected during Mode 1 testing of the DPF out configuration was 0.087 mg and PM mass collected with DPF-SCR out configuration was 0.6962 mg. An order of magnitude increase in PM mass collected between the two exhaust configurations further substantiate the particle sizing results. Particle concentrations measured during operation in Mode 3 were close to the detection levels of the instrument, hence a particle size distribution with high variability in measured number counts were observed.


downstream of the DPF and SCR for Mode 1 (exhaust temperature of 449 °C) and Mode 3 (exhaust temperature of 365 °C), respectively. Mode 1, DPF out and DPF-SCR out particle size distribution exhibited peaks for particles with 1910



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Figure 5 shows the particle size distribution during steady state load condition of Mode 7. This mode also resulted in high

modes. However, SCR out distribution of Mode 2 (exhaust temperature of 410 °C) and Mode 4 (exhaust temperature of 397 °C) exhibited a single mode distribution peaking between 8 and 10 nm, with particle number concentrations 2 to 3 orders of magnitude greater than DPF out concentrations. Modes 5 and 6 resulted in exhaust temperatures below 380 °C and hence were characterized by low measured particle concentrations and high variability in measured particle size distribution. The steady-state results show a direct relationship between exhaust nanoparticle formations and exhaust temperatures in a DPF-SCR equipped diesel engine. The particles were primarily distributed in nucleation mode. Table 3 shows the relationship between exhaust temperature and the particle count median diameter (CMD) observed during the different steady state modes. The results show a repeatable particle CMD between 6 and 15 nm for modes with temperatures over 380 °C. For modes with temperatures below this value resulted in a particle size distribution not characterized by a nucleation mode, and with high variability in measured particle count. SMPS measurements of Modes 5 and 6 did not result in a normally distributed size distribution due to the low measured particle concentrations. As a result, CMD values for these modes could not be populated in Table 3. The results also show that the presence of the SCR after-treatment system increases particle number concentrations by an order of magnitude relative to the DPF out number concentrations. The particle formation mechanism from advanced aftertreatment equipped engines could be attributed to sulfur oxidation over the catalyst. The fact that no distinguished particle size distribution is observed during low load, low exhaust temperature conditions, we can rule out that hydrocarbon precursors as a contributor to any significant particle formation. However, the nucleation mode distribution observed could be sulfuric acid based particles formed as a result of sulfur oxidation over the catalyst surfaces at exhaust temperatures over 380 °C. Herner et al. have shown in their recent study that onset of nucleation is characterized by a critical exhaust temperature of 373 °C. Also, the study points to the fact that catalyzed after-treatment systems are necessary for formation of nucleation mode particles.12 Studies have shown the ability of lubrication oil sulfur to undergo oxidation reactions over catalytic surfaces to form SO3, that can readily form sulfuric acid particles in the presence of water.6,7 Kittelson et al. have shown previously that at exhaust temperatures over 360 °C sulfur conversion is close to 100% and below 300 °C falls to less than 1%.13,14 It is to be noted that the results of the current study points to the fact that SO3 formation could be more dominant with the DPF and SCR after-treatment systems placed in series compared to the presence of only the DPF system. Although,


average exhaust temperatures of 425 °C. As a result, a single mode distribution peaking between 6 and 15 nm for both DPF out and DPF-SCR out conditions was observed. Again, the SCR out number concentrations in the nucleation mode are observed to be an order of magnitude greater than DPF out concentrations. Figure 6 shows the particle size distribution for Modes 2 and 4 at DPF out and DPF-SCR out conditions. Modes 2 and 4

Figure 6. SMPS particle size distributions for Modes 2 and 4.

resulted in average exhaust temperatures of 410 and 397 °C. The results again show DPF out particle concentrations near detection limits of the instrument for all low temperature

Table 3. Exhaust Temperatures and Particle CMDs for the Bi-Modal Distribution

mode mode mode mode mode mode mode

1 2 3 4 5 6 7

exhaust temperature (°C)

DPF out nucleation CMD (nm)

SCR out nucleation CMD (nm)

DPF out accumulation CMD (nm)

SCR out accumulation CMD (nm)

445.4 409.2 364.7 393.1 366.9 327.9 424.4

8.2 7.9 68.5 6.2

10.2 8.5 41.4 6.9

63.8 44.5 46.1 47.0

68.5 40.0 41.4 42.9

7.9

9.1

46.1

46.1

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Figure 7. DMS transient particle size distribution and concentration over the PA3 in-use cycle.

production of isocyanide, volatile nitro-organic compounds and ammonia as result of reduced SCR efficiencies.9,15−17 However, their role in particle formation is still not clearly understood. Species such as ammonia may not contribute to tailpipe nanoparticle emissions, but can be attributed to secondary PM formation in the atmosphere.

regulatory agencies have mandated ultra low sulfur diesel, contribution of sulfur from lubrication oil has proven significant toward producing precursors for sulfuric acid nucleation in the presence of exhaust after-treatment systems. Results shown in Figure 6 show the possible effect of multiple catalytic systems placed in series that could contribute to nanoparticle formation as a result of successive sulfur oxidation. While humidity of dilution air was maintained constant during the entire test procedure, the DPF out conditions for Modes 2 and 4 did not produce nucleation, whereas the SCR out conditions resulted in orders of magnitude higher nucleation mode particle counts. Figure 7 shows the transient particle size distribution variation over the PA3 in-use NTE cycle. The subchart shows the temperature variation downstream of the DPF and SCR catalyst. The transient results clearly illustrate the gradual formation of the nucleation mode particles with continuous increase in exhaust temperatures. The onset of particle formation occurs at about 400 s, with DPF out and SCR out temperatures at about 380 and 350 °C, respectively. The PA3 cycle contains a short idle period at around the 1000 s set point. Due to low exhaust flow rate and closed rack fuelling during idle the nucleation mode momentarily ceases during the idling period. Figure S5 of the SI shows the total particle number concentration with respect to engine exhaust temperatures. The bulk of the total particle emissions during the in-use operation of the engine results at exhaust temperatures over 380 °C. This again illustrates the fact that an engine operation characterized by NTE operation at higher load conditions results in increased nanoparticle emissions. Results of particle concentrations over the low exhaust temperature refuse truck cycle resulted in levels close to the detection levels of the instrument, due to which particle size distribution with high variations in particle number count was measured. Exhaust temperatures below the threshold limits during the entire cycle did not appear to contribute to sulfur oxidation and hence no particle formation was observed. The study also attempted to investigate the possibility of other particle formation mechanisms, which could result due to incomplete urea decomposition and possible ammonia slip from the SCR catalyst. Previous studies have shown the

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 shows the arrangement of the after-treatment system downstream of the engine. Figure S2 shows the engine dynamometer speed and torque trace of the refuse truck cycle. Figure S3 shows the engine dynamometer speed and torque trace of the PA3 cycle. Table S4 lists the speed and torque set points of the 7 steady state modes identified within the NTE zone. Figure S5 shows the scatter plot of exhaust temperature vs total particle number count for the PA3 transient cycle testing. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS We acknowledge South Coast Air Quality Management District for providing the funding for this study. We also thank Volvo power train in providing technical expertise and ECU software support for their engine.

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REFERENCES

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with Different Engine-out Calibrations to meet the 2010 Emission Limits. SAE 2009, 2009−01−1183. (4) Herner, D. J.; Hu, S.; Robertson, H. W.; Huai, T.; Collins, F. J.; Dwyer, H.; Ayala, A. Effect of advanced aftertreatment for PM and NOx control on heavy-duty diesel truck emissions. Environ. Sci. Technol. 2009, 43, 5928−5933. (5) Vouitsis, E.; Ntziachristos, L.; Samaras, Z. Theoretical investigation of the nucleation mode formation downstream of diesel after-treatment devices. Aerosol Air Qual. Res. 2008, 8 (1), 37−53. (6) Kittelson, D. B.; Watts, W. F.; Johnson, J. P.; Thorne, C.; Higham, C.; Payne, M.; Goodier, S.; Warrens, C.; Preston, H.; Zink, U.; Pickles, D.; Goersamnn, C.; Twigg, M. V.; Walker, A. P.; Boddy, R. Effect of fuel and lube oil sulfur on the performance of a diesel exhaust gas regenerating trap. Environ. Sci. Technol. 2008, 42, 9276−9282. (7) Vaaraslahti, K.; J, K.; Giechaskiel, B.; A, S.; T, M.; H, V. Effect of Lubricant on the formation of Heavy-Duty Diesel Exhaust Nanoparticles. Environ. Sci. Technol. 2005, 39, 8497−8504. (8) Ardanese, M.; Ardanese, R.; Besch, C. M.; Adams, R. T.; Sathi, V.; Shade, C. B.; Gautam, M., Emissions of NOx, NH3 and fuel consumption using high and low engine-out NOx calibrations to meet 2010 heavy-duty diesel engine emissions standards. SAE 2009, 2009−01−0909. (9) Strots, O. V.; Santhanam, S.; Adelman, J. B.; Griffin, A. G.; Derybowski, M. E. Deposit formation in urea-SCR systems. SAE 2009, 2009-, 01−2780. (10) Krishnamurthy, M.; Gautam, M. Development of a heavy-duty engine test cycle representative of on-highway Not-To-Exceed operation. Proc. Inst. Mech. Engrs. D: J. Auto. Eng. 2006, 220 (6), 837−848. (11) Code of Federal Regulations- Title 40: Protection of Environment. 2007; Vol. Part 86.1370-Not-To-Exceed Test Procedures. (12) Herner, D. J.; Hu, S.; Robertson, H. W.; Huai, T.; Chang, O. M. C.; Reiger, P. L.; Ayala, A. Effect of advanced aftertreatment for PM and NOx reduction on heavy-duty diesel engine ultrafine particle emissions. Environ. Sci. Technol. 2011, 45 (6), 2413−2419. (13) Kittelson, D. B.; Watts, W. F.; Johnson, J. P.; Rowntree, C. J.; Goodier, S. P.; Payne, M. J.; Preston, W. H.; Warrens, C. P.; Ortiz, M.; Zink, U.; Goersmann, C.; Twigg, M. V.; Walker, A. P. Driving down on-highway particulate emissions. SAE 2006, 2006−01−0916. (14) Kittelson, D. B.; Watts, W. F.; Johnson, J. P.; Rowntree, C. J.; Payne, M.; Goodier, S.; Warrens, C.; Preston, H.; Zink, U.; Ortiz, M.; Goersamnn, C.; Twigg, M. V.; Walker, A. P. On-road evaluation of two diesel exhaust aftertreatment devices. J. Aerosol Sci. 2006, 37, 1140− 1151. (15) Sluder, S. C.; Storey, J. M. E.; Lewis, S. A.; Lewis, L. A. Low temperature urea decomposition and SCR performance. SAE 2005, 2005−01−1858. (16) Way, P.; Viswanathan, K.; Preethi, P.; Gilb, A.; Zambon, N.; Blaisdell, J. SCR performance optimization through advancements in aftertreatment packaging. SAE 2009, 2009−01−0633. (17) Heeb, V. N.; ZImmerli, Y.; Czerwinski, J.; Schmid, P.; Zennegg, M.; Haag, R.; Seiler, C.; Wichser, A.; Ulrich, A.; Honegger, P.; Zeyer, K.; Emmenegger, L.; Mosimann, T.; Kasper, M.; Mayer, A. Reactive nitrogen compounds (RNCs) in exhaust of advanced PM-NOx abatement technologies for future diesel applications. Atmos. Environ. 2011, 45 (18), 3203−3209.

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Influence of Real-World Engine Load Conditions on Nanoparticle

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