Cost and Fuel Efficient SCR-only Solution for post-2010 HD Emission Standards

29-1-915 Cost and Fuel Efficient SCR-only Solution for post-21 HD Emission Standards Copyright 29 SAE International Robert Cloudt, Frank Willems, Peter van der Heijden TNO Automotive ABSTRACT A promising SCR-only solution is presented to meet post-21 NO x emission targets for heavy duty applications. The proposed concept is based on an engine from a EURO IV SCR application, which is considered optimal with respect to fuel economy and costs. The addition of advanced SCR after treatment comprising a standard and a close-coupled SCR catalyst offers a feasible emission solution, especially suited for EURO VI. In this paper, results of a simulation study are presented. This study concentrates on optimizing SCR deno x performance. Simulation results of cold start FTP and WHTC test cycles are presented to demonstrate the potential of the close-coupled SCR concept. Comparison with measured engine out emissions of an EGR engine shows that a close-coupled SCR catalyst potentially has NO x reduction performance as good as EGR. Practical issues regarding the use of an SCR catalyst in closecoupled position will be addressed, as well as engine and exhaust layout. For comparison, the requirements of a US 21/EURO VI compliant high EGR engine are discussed: base engine design, heat rejection, fuel injection equipment, turbo charging and fuel economy. From this study, it is concluded that the SCR-only approach leads to a less expensive engine design with better fuel economy and lower PM emissions. INTRODUCTION During the past decade, heavy duty truck and engine manufacturers have been facing challenges in complying with more and more stringent emission standards, culminating in the US EPA 21 standard and EURO VI in Europe. Table 1 gives an overview of the current and future NO x and Particulate Matter (PM) limits for the US and Europe. Table 1. US and European Heavy Duty emission targets Emission legislation Year Test cycle NO x limit () PM limit () US 27 27 FTP / SET (1.5) 1.13 US 21 21 FTP / SET.27.13 EURO V 28 EURO VI 2 213 ESC 2..2 ETC 2..3 ESC / ETC.4.1 WHSC.4.1 WHTC.5-.7.1 1 There is no crisp limit for US 27. The 21 NO x limit is phased in from 27 to 29 on a 5% of sale basis. The EPA Averaging, Banking and Trading scheme further allows postponing and compensation of emission reduction, based on other engine families or saved credits. According to [1], most 27 model year HD truck engines have been certified at a NO x level of 1.6 to 2.. 1.5 is based on the mean of 24 and 21 targets. 2 Expected values, legislation is not fixed yet Details of the EURO VI legislation have not been consolidated yet, though emission targets based on ETC and ESC cycles have been published. Correlation factors for emissions on the WHTC are being determined. The European Automobile Manufacturers Association suggests a WHTC offset of +.3 relative to the ETC, for NO x emissions only [2]. An expected WHTC NO x target of.5 has also been reported. The The Engineering Meetings Board has approved this paper for publication. It has successfully completed SAE s peer review process under the supervision of the session organizer. This process requires a minimum of three (3) reviews by industry experts. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. ISSN 148-7191 Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solely responsible for the content of the paper. SAE Customer Service: Tel: 877-66-7323 (inside USA and Canada) Tel: 724-776-497 (outside USA) Fax: 724-776-79 Email: CustomerService@sae.org SAE Web Address: http://www.sae.org Printed in USA

WHTC will be tested twice in a cold start - soak - hot start order. The expected weights for the cold and hot start cycles are 1% and 9% respectively. To further reduce the impact of heavy duty vehicles on health and air quality in densely populated areas, the final EURO VI legislation may include limits on the particle number emission and NO 2 emission. By limiting the particle numbers instead of particulate mass, the emission of fine particulates is more heavily penalized. EURO VI may also include a limit on NH 3 slip for after treatment systems using urea Selective Catalytic Reduction (SCR). Besides emission targets for test cycles, requirements for real-world performance will be introduced during the next stages: On-Board Diagnostics (OBD) and In-Use Compliance (IUC). Figure 1 gives an overview of the applied and expected emission reduction technologies during the different emission legislation stages. With the introduction of EURO IV and V emission limits, urea SCR after treatment systems were introduced to the heavy duty truck market to reduce NO x emissions. A few European truck brands have opted for a solution which does not require an additional reagent to be carried on board. These non-scr solutions generally apply Exhaust Gas Recirculation (EGR) and a Diesel Particulate Filter (DPF) to reduce the increased raw PM emission. developing an EGR based engine solution for the US 21 emission target [3, 4]. However, growing concerns about CO 2 emission contributing to the greenhouse effect, energy security and especially fuel prices, have caused a shift in attention towards fuel efficiency. EGR is well known to cause an increase in fuel consumption, and clearly is not the favourable solution when fuel economy is of concern [5]. In this paper, the potential of an alternative SCR-only solution is examined. Starting from a basic non-egr engine, SCR after treatment with a close-coupled SCR catalyst is added to comply with post-21 emission standards. This approach will result in an inexpensive engine design with better fuel economy and lower PM emissions, especially suited for EURO VI. This paper starts with a comprehensive discussion on design and performance considerations for the high EGR engine concept. Secondly, the SCR-only approach is introduced. This approach requires very high NO x conversion efficiencies of the after treatment system. For this concept, issues and trade-offs in achieving optimal SCR performance are discussed. Focus is on low temperature and cold start performance. Finally, possible after treatment configurations for EURO VI will be dsicussed. HIGH EGR ENGINE CONCEPT SCR + DPF SCR EGR + SCR + DPF EGR + DPF EGR 24 25 27 28 21 213 EURO IV EURO V EURO VI US 24 US 27 US 21 Figure 1. Overview of emission reduction technologies for HD applications In Europe, the urea distribution network is gradually becoming mature. In North America on the other hand, the urea infrastructure is still rather sparse. Concerns about the availability of AdBlue, lower fuel prices and advances in EGR, turbocharging and fuel injection equipment technology, have caused 27 model year heavy duty engines for the US market to be equipped with EGR and DPF systems. US 27 engine platforms can be made US 21 or EURO VI compliant, by either adding urea-scr after treatment, or by further developing EGR technology. Some parties announced that they will pursue in Figure 2. High EGR engine concept Figure 2 shows the general layout of a high EGR engine concept. Such an engine can accomplish low engine-out NO x emission (well below 2 ) by applying high EGR rates: over 2% at full load and over 4% at some partial load conditions. A DPF is incorporated in the exhaust line to reduce the PM emissions. A fuel vaporizer and Diesel Oxidation Catalyst (DOC) facilitate active regeneration of the DPF. An already performed study [6] into EGR trade-offs on a two-stage turbocharged 12 litre heavy duty diesel engine provides general trends for NO x and PM emission and

Break Specific Fuel Consumption (BSFC) as a function of the EGR percentage. Figure 3 depicts the NO x, PM and BSFC trade-offs for the A25 and C1 ESC modes., PM ().9.8.7.6.5.4.3.2.1.5% C1 A25.% 25 3 35 4 45 5 55 EGR (%) PM BSFC 4.% 3.5% 3.% 2.5% 2.% 1.5% 1.% Figure 3. NO x and PM emission and BSFC penalty as a function EGR percentage. BSFC penalty is relative to lowest EGR percentage. The next sections will present a discussion on the subsystems of the high EGR engine concept. TURBOCHARGING The high EGR engine concept as described requires a high performance turbocharging system in order to establish the required in-cylinder Air Fuel Ratio (AFR) in combination with high EGR rates. The turbo charging system needs to deliver higher inlet manifold pressures compensating the increased in-cylinder mass flow demand. As a result of this, both pre-intercooler temperature and intake manifold temperature will increase significantly. The pressure ratios required for a high EGR engine push the envelope for single-stage turbocharging compressor. Two-stage turbocharging can deliver the required boost pressure, but is more costly. Moreover, the additional pumping work will cause an increase in fuel consumption [7]. Achieving a US 21 engine-out NO x level of.27 requires EGR rates of 35% at full load and 5% at partial load. For these EGR rates, it is expected that the required boost pressure causes the compressed air temperature at the outlet of the low pressure compressor to be too high for a standard material high pressure turbo compressor. Advanced materials like a Titanium compressor wheel and a die-cast compressor housing will be required to withstand the more severe conditions. Alternatively, a second intercooler can be placed between the low and high pressure compressors. BSFC penalty (%) COOLING The exhaust gas diverted back to the engine intake is cooled by an EGR cooler for lower NO x emission and to prevent over heating of the engine. With increasing EGR rates, the amount of heat rejected to the cooling circuit gradually increases. For an EGR rate of 3% at full load on a 35 kw engine, the EGR cooler needs to have cooling capacity of 1 kw [6]! Taken into account the additional heat form the intercoolers, the total cooling capacity for a high EGR engine needs almost to be doubled in comparison with a conventional 35 kw class engine. Recent improvements in (EGR) cooling technology may relieve the impact on the cooling system [8], but still EGR cooler design poses challenges with respect to performance, durability and sizing aspects. The EGR cooler is suspect to fouling and corrosion. Corrosion is promoted by condensation, which is of concern especially during low temperature load conditions. The cooling efficiency of the EGR cooler drops, once it is fouled. Catalysts and filters fitted upstream of the EGR cooler can alleviate this issue, but will also increase the cooler back pressure [9]. FUEL INJECTION EQUIPMENT To comply with 27 PM targets and beyond, the engine out PM emission needs to be below.2, when an overall DPF filtration efficiency of 95% is assumed. An even lower engine out PM level of about.1 is desired to reduce the DPF regeneration frequency and according fuel penalty, prevent filter plugging, and reduce EGR cooler fouling. When high EGR rates are applied to reduce engine out NO x emission, low PM levels can only be accomplished by high pressure fuel injection equipment (up to 24 bar) [7, 1]. These advanced high pressure diesel injection systems introduce additional costs and parasitic losses affecting the fuel economy. BASE ENGINE DESIGN Apart from all the requirements a high EGR engine is posing on the subsystems, the base engine design is also affected. Depending on how far one is pushing to lower NO x emissions through EGR, requirements on the mechanical components tighten. If the engine power output is kept constant, engine Peak Firing Pressures (PFP) will increase, due to higher injection pressures and high EGR rates. High Peak Firing Pressures (up to 25 bar) are demanding for the cylinder head, gaskets and bearings, and may require different materials for the piston (steel). High boost pressures (up to 4 bar) challenges the intake manifold fastening, construction and seals. Oil change intervals will have to be shortened due to interference of the high pressure fuel injection jet with the

cylinder wall oil film, and increased exposure of oil to exhaust gas due to high EGR rates In summary, EGR is a successful method of NO x abatement. With application of a DPF and recent improvements in cooling, turbocharging and fuel injection equipment, it appears even possible to accomplish US 21 and EURO VI emission targets without using NO x after treatment like SCR. However, by adopting such an EGR-only approach for post-21 emission targets, the solution compromises on: Specific power output Fuel economy / CO 2 emission Costs The increased costs are due to the additional cooling equipment and turbocharger, advanced fuel injection system and the extra expenses for the improved base engine design. It will be a challenge to design an EGR-only engine platform that is robust and durable. It pushes the limits of vehicle cooling capacity, and has to accomplish compliant emission results under off-cycle conditions and a broad range of ambient temperatures and altitudes. In the next section a promising alternative approach is presented. Starting from an engine that is optimised for cost and fuel consumption, the requirements for SCR NO x after treatment are investigated, necessary for post- 21 emission standards. SCR-ONLY CONCEPT Heavy-duty engines used in EURO IV applications with SCR after treatment are relatively inexpensive and are optimised with respect to fuel economy. They generally apply a straightforward single stage turbocharging system and no EGR. The fuel injection equipment knows maximum pressures of 12 to 18 bar. Fuel injection timing is mainly optimised for fuel consumption. For these systems, engine out NO x emission is between 6 and 9. PM emissions are very low ranging from.1 to.3. Using this engine as a starting point, SCR deno x after treatment is added in an attempt to comply with post- 21 emission targets. It is evident from the high raw NO x emissions, that very high NO x conversion efficiencies are required over the life time of the SCR catalyst: 9% to 97%. Before discussing possible after treatment configurations, the aspects contributing to the required optimal SCR performance are addressed first. OPTIMAL SCR PERFORMANCE Catalyst manufacturers are continuously attempting to improve their catalyst s performance. There is a strong focus on low temperature performance, because of the required cold start testing for US legislation and EURO VI, and on-road performance. The choice of the SCR catalyst is strongly influenced by the exhaust line layout. If the SCR catalyst is applied downstream of a DPF, it has to withstand the high temperatures encountered during active DPF regeneration. Zeolite based catalysts are applied in these configurations. Cu-Zeolite SCR catalysts are attracting a lot of attention for their low temperature performance. Combinations of Fe-Zeolite and Cu-Zeolite catalysts can benefit from a broad temperature activity window and less NH 3 slip [11]. Vanadium based catalysts have a temperature limit of about 5 ºC, which prevents them from application downstream of a DPF. However, a Vanadium based SCR catalyst performs better than a Zeolite catalyst in the absence of NO 2, and has improved tolerance for fuel quality [12, 13]. Moreover, a Vanadium catalyst is generally less expensive than a Zeolite catalyst. Given the choice of a certain catalyst type, the SCR NO x conversion performance can be optimised by: 1. Optimisation of the catalyst operating temperature 2. Optimisation of the NO 2 :NO ratio 3. Increasing the SCR catalyst volume 4. Effective use of the available SCR volume NO 2 / ratio.7.6.5.4.3.2 45 o C 4 o C 2 o C 25 o C 275 o C 3 o C.1 325 o C 35 o C 1 2 3 4 5 SCR Catalyst Volume (litre) Figure 4. Trade-off curves for 8% steady-state NO x conversion on a Vanadium type SCR catalyst at 9 kg/h exhaust flow, 5 ppm NO x and stoichiometric NH 3 injection Figures 4 and 5 present the relations between NO x conversion, catalyst temperature, NO 2 :NO ratio and catalyst volume. The figures are generated for a modeled Vanadium catalyst in TNO s SimCat simulation tool [14]. Such a simulation environment offers a valuable tool in studying trade-offs and determining the most cost-effective exhaust after treatment configuration.

The model has been used in prior publications. Background information on the SCR model is provided in [14, 15, 16]. NO x conversion (%) 1 9 8 7 6 5 4 3 2 1 5 1 15 2 25 3 SCR Catalyst Volume (litre) Figure 5. Steady-state NO x conversion at stoichiometric NH 3 injection vs. Vanadium SCR catalyst volume for several conditions: 25 ºC (blue solid), 3 ºC (red dashed), 35 ºC (green dotted), NO 2 /NO x =.1 (circles), NO 2 /NO x =.33 (triangles), NO 2 /NO x =.5 (plusses), NO 2 /NO x =.7 (squares). Exhaust flow = 9 kg/h, pre- SCR NO x = 5 ppm. In Figure 4 the trade-off between temperature, NO 2 /NO x ratio and SCR volume is visualized, for achieving 8% steady-state NO x conversion on the modelled Vanadium catalyst, at stoichiometric NH 3 injection. The figure shows that for this particular catalyst and a exhaust flow of 9 kg/h, 8% NO x conversion is infeasible for NO 2 /NO x ratios larger than.7 and below 2 ºC when the NO 2 /NO x ratio is not between.5 and.6. Judging from Figure 5, 98% NO x conversion can be obtained for about any temperature, as long as the NO 2 /NO x ratio is close to.5. At an NO 2 /NO x ratio of.7, the maximum NO x conversion under the studied operating conditions is limited to 75%. When NO 2 only accounts for 1% of the NO x, an SCR temperature of at least 325 ºC is required, but an SCR volume of 1 litres is yet sufficient for 8% conversion. If the SCR temperature is increased to 4 ºC, a 5 litre SCR catalyst will achieve at least 8% conversion at the simulated exhaust flow over a wide range of NO 2 /NO x ratios up to.6. The fourth mentioned way of optimizing the SCR performance, is based on better utilization of the available SCR volume. Allowing steady-state NH 3 to NO x dosing ratios in excess of one can improve NO x conversion, as for example is demonstrated in [17]. The increased NH 3 supply results in a higher NH 3 surface coverage towards the end of the catalyst. This improves NO x conversion at the expense of increased NH 3 slip. The same principle can also be applied under transient conditions. This method requires a catalyst downstream of the SCR where excess NH 3 reacts; an Ammonia Oxidation (AMOX) catalyst, for instance. A concept that adds extra SCR volume to the after treatment setup, and benefits from elevated exhaust gas temperatures, is the addition of a close-coupled SCR catalyst. Similar to close-coupled catalysts in light-duty applications, this close-coupled SCR catalyst needs to be mounted as close to the engine as possible for rapid heat up. The potential of this approach for US 21 and EURO VI applications is investigated through simulation. CLOSE-COUPLED SCR CATALYST Figure 6 shows simulated temperatures in a cold start WHTC cycle from a 12 litre 35 kw class engine used in a EURO IV SCR application. Three scenarios are simulated: 5.6 litre close-coupled SCR catalyst (ø6 x 12 ) 34 litre SCR catalyst upstream of a DOC/DPF 34 litre SCR catalyst downstream of a 41 litre DOC/DPF combination. All substrates are assumed to be cordierite, with a simulated heat capacity of 15 J/kg/K. Substrate between the turbine outlet and the SCR catalyst s backside provides a temperature buffer and smoothens the course of the temperature. For the 34 litre SCR catalyst downstream of the DOC/DPF, it takes 669 s before the whole catalyst is above 2 ºC, as opposed to 495 s for the SCR catalyst upstream of the DPF. The smaller close-coupled SCR catalyst shows a fluctuating temperature, but can benefit from the faster heat up and the high temperature periods. Turbine outlet and post-scr Temperature ( o C) 5 45 4 35 3 25 2 15 1 5 Temperatrures in cold start WHTC Turbine outlet 5.6 litre close-coupled SCR 34 litre SCR downstream of 41 litre DOC/DPF 34 litre SCR upstream of DOC/DPF 2 4 6 8 1 12 14 16 18 Time (s) Figure 6. Temperatures in cold start WHTC

Temperature (ºC) To investigate the NO x reduction potential of the closecoupled SCR catalyst, cold start test cycle data from two engines is used; 1. Cold start FTP of a 12 litre 35 kw class engine equipped with EGR. This engine is calibrated to comply with US 21 targets using a DOC/DPF and downstream Fe-Zeolite SCR. 2. Cold start WHTC of 12 litre 35 kw class non-egr engine calibrated for EURO IV using Vanadium SCR after treatment. For the EGR engine, it is possible to compare the simulated close-coupled SCR NO x reduction potential to that of EGR. 45. 4. 35. 3. 25. 2. 15. 1. 5. T post-turbine EGR disabled EGR EGR + downstream SCR 5.6 litre Vanadium cc-scr in cold start FTP. 2 4 6 8 1 12 Time (s) Figure 7. Cumulative NO x emission in cold start FTP Figure 7 presents the cumulative NO x emissions for the cold start FTP cycle of the EGR engine. When EGR is disabled, the particular engine emits 15 g NO x in the cold start FTP cycle. With EGR enabled, the raw NO x emission drops to 58 g. The Fe-Zeolite SCR catalyst downstream of the DPF reduces the NO x to a tailpipe level of 27 g over the cold FTP cycle. 16 14 12 1 8 6 4 2 Cumulative (g) Temperature (ºC) 45. 4. 35. 3. 25. 2. 15. 1. 5. T post-turbine Engine-out 5.6 litre Vanadium cc-scr in cold start WHTC. 2 4 6 8 1 12 14 16 18 Time (s) Figure 8. Cumulative NO x emission in cold start WHTC Based on the impression of the NO x reduction performance of a close-coupled SCR catalyst, targets for NO x reduction in the second, large SCR can be established. Table 2 and 3 illustrate the NO x conversion scenarios for US 21 and EURO VI. The engine from the EURO IV SCR application emits about 6.3 NO x both in the FTP and WHTC cycles. When the engine has warmed up, the raw cycle emissions can be as high as 9. The US 21 scenario in Table 2 requires a very high NO x conversion efficiency for the second SCR catalyst. 95% conversion over the life time of an SCR catalyst is very ambitious. In order to successfully apply closecoupled SCR for US 21 applications, the raw engine out NO x emission needs to be lowered. Either through adjusting the injection timing or through modest EGR. Both solutions compromise on fuel economy. Hence they cancel the fuel saving targeted by opting for a SCR-only NO x emission solution. The combination of EGR and SCR NO x after treatment turns out to be most feasible for US 21. It is well accepted, and most manufacturers seem to choose for this route. 2 15 1 5 Cumulative (g) The measured raw NO x emission with EGR enabled is compared to the NO x emission downstream of the simulated 5.6 litre (ø6 x 12 ) Vanadium close-coupled catalyst. The NH 3 storage in the close-coupled SCR catalyst is assumed high enough not to limit the NO x conversion. This is not unrealistic as the close-coupled SCR catalyst will be followed downstream by a second SCR or DOC/DPF combination. The simulation shows that the NO x conversion obtained with the 5.6 litre closecoupled SCR catalyst is as good as the NO x reduction accomplished by EGR. A similar simulation is conducted for a cold start WHTC test of the non-egr engine. Figure 8 shows the measured cumulative raw NO x emission, and the simulated post close-coupled SCR NO x emission. The simulated close-coupled SCR NO x conversion is on the same order as for the cold start FTP: 69% on the WHTC, 63% on the FTP. Table 2. US 21 NO x conversion targets Cold start FTP Hot FTP engine-out 6.3 9. ηcc-scr ηscr 63% 53% 73% 95% tailpipe 1.1.13 1/7 6/7 weight Cycle result.16.11.27

Table 3. EURO VI NO x conversion targets Cold start WHTC Hot WHTC engine-out 6.3 9. ηcc-scr ηscr 69% 44% 75% 81% tailpipe 1.1.43 weight 1/1 9/1 Cycle result.11.39.5 The EURO VI scenario presented in Table 3 is much more favourable for the close-coupled SCR case. The lower weight on the cold start cycle, and the higher NO x target both relieve the pressure on the required NO x conversion in the second SCR catalyst. Even when the WHTC NO x emission limit is decided to be.4, the required NO x conversion for the second SCR catalyst will only be 87%, with still some opportunities left to optimize raw NO x emissions through engine design and fuel injection timing. This makes the EURO VI scenario viable. It appears that it will be possible to design an SCR-only EURO VI engine and after treatment concept that will be cost and fuel efficient. First investigations have proven that application of a close-coupled SCR can be very attractive. However, there are still issues that need to be addressed with regard to practical application. Urea injection for the close-coupled SCR catalyst can be challenging because of the short mixing length. Mixers, possibly in combination with a hydrolysis catalyst, can improve the distribution and decomposition of urea upstream of the close-coupled SCR [18]. The mechanical design of the close-coupled SCR catalyst and possible mixer has to fit in a confined space under the bonnet or overhead cabin. When used in a close-coupled position, the SCR catalyst has to withstand the exhaust environment encountered right behind the turbine. The catalyst will be exposed to elevated temperatures, temperature fluctuations and the wet urea spray. The bulk material needs to be tolerant to these conditions without causing too much backpressure. The catalytic material itself needs to be long-term stable up to the maximum temperature encountered at the turbine outlet. Furthermore, it has to be resistant to Hydro Carbon (HC) poisoning (especially at cold start conditions, HC emission can be significant). Control and On-Board Diagnostics (OBD) for the closecoupled SCR catalyst is not trivial. The urea dosing strategy has to be robust and ensure maximal NO x conversion. A model-based NH 3 storage based urea dosing strategy can offer this combination of desirabilities [15]. A study into an OBD algorithm concept has to point out where NO x and/or NH 3 sensors are required. AFTER TREATMENT CONFIGURATION FOR EURO VI The close-coupled SCR concept offers an innovative approach for HD EURO VI applications. It relies on a proven, robust, cost effective and fuel efficient engine design from EURO IV SCR applications. With small improvements of the engine design and fuel injection equipment, it will be possible to accomplish engine out EURO VI compliant particulate mass emission, without PM after treatment. However, having particle filter technology available, a DPF with a certain minimum filtration efficiency might become mandatory in the near future. PM after treatment requirements are very modest, because of the low engine out PM emission. A partial flow filter is an attractive option [19]. It can be placed downstream of the large SCR, which is beneficial for NO x conversion, and does not require active regeneration, as long as there is enough NO x (or actually NO 2 ) available for passive soot oxidation. It is yet unclear whether the partial flow filter will provide enough reduction of particulate numbers, possibly required for EURO VI. A wall flow DPF provides excellent PM reduction. In combination with a non-egr engine, near zero PM emission seems feasible. DPFs are usually applied upstream of a Zeolite SCR catalyst. This position is mainly dictated by thermal requirements for frequent active DPF regeneration. With raw PM emissions of a non-egr engine 3 to 1 times lower than for an EGR engine, the regeneration frequency and according fuel penalty is drastically reduced. This may allow to apply the DOC/DPF combination downstream of the SCR catalyst, and use thermal management to improve regen conditions when required. Excess NH 3 slip can be oxidized in the DOC and DPF, if the DOC/DPF is placed downstream of the main SCR catalyst. This helps improving the NO x conversion of the SCR catalyst. When the main SCR catalyst is placed at the tailpipe position, an AMOX is desired to prevent NH 3 slip and promote NO x conversion. In absence of an AMOX, the urea dosing has to be conservative in order to prevent NH 3 slip. Urea has to be supplied to both the close-coupled and the main, downstream SCR catalyst. If the close-coupled catalyst is directly succeeded by the main SCR catalyst, all urea can be supplied by the injector upstream of the close-coupled SCR catalyst. When the main SCR volume is located downstream of a DPF, a second urea injector will be necessary to provide the reagent. NH 3 slipping through the close-coupled SCR catalyst will then be oxidized in the DOC/DPF combination. This option adds extra costs to the after treatment setup.

The decision on the after treatment layout and catalyst sizes and types has to be based on technological, economical and legal considerations. Within the boundaries defined by catalyst technology, the costs for the after treatment solution are optimised. Trade-offs can be made based on precious metal prices (Pt, Pd, present in DOC DPF and AMOX) and the cost of other catalyst technology (Vanadium, Cu and Fe Zeolite). CONCLUSION An original approach for complying with HD post-21 emission targets has been presented. The application of a close-coupled SCR catalyst allows a cost and fuel efficient engine and after treatment solution, especially suited for EURO VI. The US 21 scenario is less viable because of too ambitious NO x conversion targets. In contrary to the generally adopted approach for post- 21 emission targets, the presented concept does not rely on EGR. Instead, it is based on a EURO IV engine, that is considered optimal in terms of costs and fuel consumption. Simulation results of cold start FTP and WHTC cycles prove the feasibility of the close-coupled SCR concept. Future work will focus on the practical aspects of the application of a close-coupled SCR, and the demonstration of the concept on an experimental set-up. Control and OBD strategies for the close-coupled SCR are being developed. REFERENCES 1. www.dieselnet.com 2. ACEA comments concerning the European Commission s public consultation on future Euro VI emission limits for heavy-duty vehicles, http://ec.europa.eu/enterprise/automotive/pagesback ground/pollutant_emission/heavy_duty/public_consul tation/acea.pdf 3. International Trucks and Engines Will Comply with 21 Emissions Standards without SCR, Navistar press release dd. 31 Oct. 27, www.navistar.com 4. Cummins Announces Right Technology for 21, Cummins press release dd. 23 Sept. 27, www.cummins.com 5. Cummins Announces Recent Technology Development Achieves Significant Fuel Economy Benefits, Cummins press release dd. 13 Aug. 28, www.cummins.com 6. Van Aken, Willems and de Jong, Appliance of high EGR rates with a short and long route EGR system on a Heavy Duty diesel engine, SAE 27-1-96 7. Moser, Sams and Dreisbach, Lowest Engine-Out Emissions as the Key to the Future of the Heavy- Duty Diesel Engine New Development Results, Proceedings JSAE Annual Congress, pp. 11-14, 25 8. Krüger, Edwards, Pantow, Lutz, Dreisbach and Glensvig, High-Performance Cooling and EGR Systems as a Contribution to Meeting Future Emission Standards, SAE 28-1-1199 9. Zhan, Eakle, Miller and Anthony, EGR System Fouling Control, SAE 28-1-66 1. Scania XPI the fuel injection system of the future, Scania press release dd. 12 Dec. 23, www.scania.com 11. Girard, Cavataio, Snow, Lambert, Combined Fe-Cu SCR Systems with Optimized Ammonia to Ratio for Diesel Control, SAE 28-1-1185 12. Chatterjee, Burkhardt, Weibel, Nova, Grossale, Tronconi, Numerical Simulation of Zeolite- and V- Based SCR Catalytic Converters, SAE 27-1-1136 13. Girard, Montreuil, Kim, Cavataio and Lambert, Technical Advantages of Vanadium SCR Systems for Diesel Control in Emerging Markets, SAE 28-1-129 14. Van den Eijnden, Cloudt, Willems and van der Heijden, Automated model fit tools for SCR control and OBD development, SAE 29-1-1285 15. Willems, Cloudt, van den Eijnden, van Genderen, Verbeek, de Jager, Boomsma, van den Heuvel, Is closed-loop SCR control required to meet future emission targets?, SAE 27-1-1574 16. Van Helden, Verbeek, Willems and van der Welle, Optimization of Urea SCR de Systems for HD Diesel Engines SAE 24-1-154 17. Hϋnnekes, van der Heijden, Patchett, Ammonia Oxidation Catalyst for Mobile SCR Systems, SAE 26-1-64 18. Jacob, Müller, Scheeder, Cartus, Dreisbach, Gotre, Mai, Paulus and Spengler, High performance SCR catalyst system: Elements to guarantee the lowest emission of, Proceedings of 27th International Vienna Motor Symposium, pp. 24-264, 26 19. Maus and Brück, Exhaust Gas Aftertreatment Systems for Commercial Vehicles Technologies and Strategies for the Future, ICPC 27-2.1, 4 th AVL International Commercial Powertrain Conference CONTACT Robert Cloudt TNO Automotive Steenovenweg 1 P.O. Box 756 57 AT Helmond The Netherlands E-mail: robert.cloudt@tno.nl Tel: +31 15 269 676