DEVElOPMENT
Exhaust Aftertreatment
SCR-only Concept for Heavy-duty Euro VI Applications To meet Euro VI emission targets for heavy-duty applications, truck manufacturers concentrate on Exhaust Gas Recirculation (EGR) and its combination with urea-based Selective Catalytic Reduction (SCR). TNO developed a concept that opens the route for an alternative solution which relies on SCR as the main technology for NOx abatement. This concept offers potential fuel benefits in combination with low impact on engine design and cooling equipment. Together with Haldor Topsøe, Yara and Grundfos, TNO examined the achievable NOx emission reduction on an engine dynamometer.
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1 Introduction Heavy-duty truck and engine manufacturers face enormous challenges to find a fuel and cost-efficient solution to meet the more and more stringent emission targets. Euro VI NOx and Particulate Matter (PM) limits of 0.4 and 0.01 g/kWh, respectively, have been set for the European Stationary Cycle (ESC) and European Transient Cycle (ETC). Besides these tighten ing emission limits, a new test cycle (World Harmonized Test Cycle, WHTC) in combination with a cold start procedure will be introduced for Euro VI legislation. Based on correlation factors with ETC, the actual WHTC emission targets will be specified in the near future. Irrespective of the test cycle and corresponding limits, further developments of emission reduction technologies are required to meet these new requirements.
2 Possible Concepts for Euro VI Figure 1 summarizes possible routes to meet European and US emission targets for heavy-duty applications. With the introduction of Euro IV and Euro V emission legislation, the vast majority of truck manufacturers opted for urea-based SCR technology. To avoid the need for an additional reagent, a few manufacturers applied EGR in combination with a Diesel Particulate Filter (DPF). In North America, all 2007 model year heavy-duty engines were equipped with EGR and DPF systems. This choice was partly driven by concerns about the availability of urea reagent for SCR solutions. To meet post-2010 emission targets, some parties pursue the development of an EGR-only engine solution. This concept relies on high EGR rates: up to 40 % at full load. To maintain the desired incylinder air-fuel ratio, high boost pressures are required that go beyond the capabilities of single-stage turbo charging. The elevated boost and peak fire pressures strongly affect the mechanical design of the engine. Furthermore, the high rates of exhaust gas diverted back to the engine pose demanding requirements on the cooling equipment: increased cooling capacity and advanced EGR cooling will be required [1]. High pressure common rail fuel injection equipment is
used to partly compensate for the offset in PM emission due to the increased EGR rates, but still the PM reduction will be demanding for the DPF system. With growing concerns about CO2 emission, energy security and rising fuel prices, fuel efficiency becomes more and more a crucial factor. In that respect, current data indicate that an EGR-only approach is not likely to be the favourable solution. The combination of EGR, DPF and SCR technology is a well accepted choice for US 2010 applications, and will be an evident option for Euro VI. This route towards post-2010 emission targets can be seen as a further development of US 2007 engine platforms through addition of SCR deNOx technology. The added SCR after treatment system opens opportunities to improve the fuel economy with respect to US 2007 engines. The engine can be calibrated for higher NOx and low fuel consumption, by changing the EGR rate and fuel injection. In fact, there is a possibility that – if sufficiently high conversion efficiency can be achieved under all conditions – the best solution could be a SCR-only concept that does not require EGR to lower raw NOx emissions. Such a concept would combine an inexpensive engine design with relatively high fuel economy and relatively low PM emissions. The latter will also reduce the requirements for PM after treatment. In this article, the potential of this alternative SCR focused concept to meet Euro VI requirements is presented.
The Authors Ir. Robert Cloudt is Research Engineer at the Diesel Emission Control Group of TNO Automotive in Helmond (Netherlands).
Prof. Dr. Ir. Rik Baert is Senior Research Scientist at the Diesel Emission Control Group of TNO Automotive in Helmond (Netherlands).
Dr. Ir. Frank Willems is Senior Research Engineer and responsible for Powertrain Control Developments at TNO Automotive in Helmond (Netherlands).
Ing. Marco Vergouwe is Manager of the Diesel Emission Control Group of TNO Automotive in Helmond (Netherlands).
Figure 1: Overview of emission reduction technologies for heavy-duty applications in Europe and US MTZ 09I2009 Volume 70
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Exhaust Aftertreatment
Figure 2: Artist impression of close-coupled SCR concept
Figure 3: Proposed after treatment configuration based on close-coupled SCR catalyst concept
3 Close-coupled SCR Concept A feasible SCR-only Euro VI solution requires very high NOx conversion in the order of 95 %. Furthermore, high NOx conversion is challenged by cold start and low temperature conditions in transient test cycles, like the WHTC. A concept that improves NOx conversion under these conditions is the close-coupled SCR concept [2]. It is based on the addition of a small volume SCR catalyst that benefits
Figure 4: SCR ammonia storage control structure 60
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from rapid heat up and high temperatures through placement close to the turbine outlet. Figure 2 shows an artist impression of this concept. A second larger SCR catalyst is placed downstream and will reduce the larger part of the NOx emissions. The technology is essentially based on a Euro IV SCR-based engine platform which is optimized for fuel economy. Addition of a close-coupled SCR catalyst improves SCR NOx conversion efficiency,
potentially to the levels required to meet Euro VI limits. The envisioned setup is portrayed in Figure 3. Full body corrugated Vanadium based catalysts are selected for the closecoupled and downstream SCR catalyst. This type of catalyst is chosen because of its relatively low price, tolerance for fuel quality and performance at low temperature and in absence of NO2. Due to costs and packaging considerations, urea is only dosed at a single point upstream of the close-coupled SCR (cc-SCR) catalyst. The ammonia slip of the close-coupled SCR is used as reagent in the downstream SCR catalyst. Reliable urea injection is provided through a compact air assisted dosing system. A particulate filter with upstream Diesel Oxidation Catalyst (DOC) is installed for PM reduction. The filter is regenerated by oxidation of injected fuel on the DOC. It is noted that compared to an EGR engine, engine-out PM emission levels can be considerably lower. Consequently, regeneration frequency, corresponding fuel penalty and DPF precious metal loading can be reduced. A DPF can be avoided when PM reduction can be accomplished by engine measures.
4 Experimental Demonstration of the Close-coupled SCR Concept The necessary NOx conversions required to meet Euro VI standards have been demonstrated on a 6.7 litre 165 kW engine with a standard non-optimized Euro IV engine calibration. The engine-out NOx emissions are 10.9 g/kWh over a ETC cycle. A 17.1 litre Vanadium SCR catalyst lowers the NOx emission below the 3.5 g/kWh Euro IV level. A 5 litre close-coupled SCR (cc-SCR) catalyst is added to the standard Euro IV configuration to improve NOx conversion. A urea dosing strategy has been developed for the system with close-coupled SCR catalyst. The new model-based control strategy relies on control of the total ammonia storage of the integrated SCR system, Figure 4. Ammonia storage control has proven to yield optimal trade-off between NOx conversion and tailpipe NH3 slip under steady-state and transient conditions [3]. To estimate the ammonia storage, an in-house developed SCR model is used that is capable of real-time im-
Figure 5: WHTC SCR model fit
plementation on an automotive type ECU. This model is first calibrated for the close-coupled and downstream SCR catalyst using a dedicated engine dynamo meter test procedure [4]. Figure 5 illustrates the model fit for the downstream SCR catalyst on the cold start WHTC cycle. The accuracy is well suited for use in an ammonia storage control strategy. The two SCR models running on the controller serve as a virtual sensor for on-line estimation of the NH3 storage on the cc-SCR and down-
Figure 6: Demonstration of NH3 storage control
stream SCR catalyst. Figure 6 demonstrates how the urea dosing is controlled to track the temperature dependent NH3 storage setpoint. Figure 7 shows the resulting NOx conversion and temperature traces during the cold start and hot WHTC cycle. Thermal management has been applied to improve close-coupled SCR heat up. In the cold start WHTC the cc-SCR catalyst reduces the NOx concentrations from 11.6 g/kWh to 3.8 g/kWh. The second SCR catalyst brings the NOx emission down to
1.2 g/kWh. The hot part of the WHTC test is tested after a 10 minute soak period. For this part of the test the cumulative tailpipe NOx emission curve can be kept virtually flat, culminating in 0.58 g/kWh. It is worth mentioning that in these experiments also a 0.40 g/kWh weighted cold/hot WHTC cycle result could be obtained. This resulted however in a too high ammonia slip. During the WHTC cycle, the urea consumption was measured to be 8 % of the fuel consumption on a volume basis. It was found that the close-coupled catalyst has little effect on the fuel consumption: at the C100 ESC mode, the measured fuel penalty corresponding to an increased backpressure is less than 0.1 %. Given the measured NOx conversion, an engine-out NOx emission budget can be set up for an SCR-only Euro VI solution. This emission budget is presented in Table 1 for an ambitious scenario with WHTC limits equal to ETC emission limits.
5 Discussion The close-coupled SCR concept allows a strategy with an engine optimized for fuel economy, whereas emission compliance is completely realised through exhaust gas after treatment. The engine-out NOx emission target level of about 7 g/kWh can be realized simply by retarding the injection process. The corresponding engine design is compact and straight-forward and injection system and turbo charging requirements are minimal. This concept is compared with alternative Euro VI concepts for the important long haul truck applications. These trucks typically have an engine with a displacement in the order of 12 to13 litre and a maximum power rating of 370 kW The principal characteristics of the different concepts are summarized in Table 2. The second concept is similar to the mainstream US 2010 configuration: a DPF for PM reduction followed by a Zeolite SCR system which together with EGR ensures low NOx emission. The engineout NOx emissions is expected to be in the range of 2.5 g/kWh. Experience [5] has shown that such levels can be realized with moderate rates of cooled EGR, varying from 15 % at full load until 30 % at lower loads. The corresponding engine is MTZ 09I2009 Volume 70
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more complex because of the added EGR system with its control valves and cooling circuit. Because of the moderate EGR levels, single-stage turbo charging suffices. A Variable Geometry Turbocharger (VGT) is used to control the EGR rate. The third concept considered avoids the use of NOx after treatment. The implicit very low engine-out NOx levels are achieved by a combination of high amounts of cooled EGR and fuelling stra tegies that result in a large portion of the injected fuel being mixed to lean air-fuel ratio levels prior to the start of combustion. This is typically realized by early injection of (part of ) the fuel and by a reduction in effective compression ratio. At high engine loads, the compression ratio can be further reduced through Variable Valve Actuation (VVA) technol ogy. At the same time, overall air-fuel ratios should remain high enough to avoid excessive particulate formation. This requires very high boost pressures that go beyond the capacity of single-stage turbocharging. Intercooling and after-cooling is required to limit intake manifold and compressor exit temperatures. This, and the need to cool high EGR flows, considerably increases the complexity of the cooling system as well as the requested cooling power.
Figure 7: NOx reduction cold start and hot WHTC
NOx Engine-out
cc-SCR reduction
SCR reduction
NOx tailpipe
weight
NOx cycle result
Cold-start WHTC
7.4 g / kWh
67 %
68 %
0.78 g / kWh
x 1 / 10
0.08 g / kWh
Hot WHTC
7.4 g / kWh
70 %
84 %
0.36 g / kWh
x 9 / 10
0.32 g / kWh 0.40 g / kWh
Table 1: WHTC NOx emission budget
Table 2: Principal characteristics of different Euro VI engine concepts for a 13 l 370 kW class engine 1 SCR-only
2 SCR+EGR
3 EGR-only
Turbo charging
Single stage Air-to-air charge cooler
Single stage, Aftercooled Variable turbo geometry
Two stage Intercooled Aftercooled Variable turbo geometry
Valve timing
Conventional
Conventional
Variable valve timing
Fuel injection equipment
Pump-line-nozzle Low pressure (2500 bar)
Piston
Conventional
Conventional
New dedicated design
NOx aftertreatment
(Vanadium) SCR + cc-SCR
Zeolite SCR
None
PM aftertreatment
DPF
DPF
DPF
EGR [%]
0
15 – 30
40 – 60
Cooling power [kW]
220
260
410
Exhaust gas temperature
0
–
–
Design complexity
+
0
–
Engine Packaging
+
0
–
Fuel quality sensitivity
0
–
–
Service intervals, Oil degradation
0
–
–
Challenges
Low temperature exhaust gas aftertreatment
Low temperature exhaust gas aftertreatment
Transient control Cold start
Estimated additional costs to OEM*
€ 3300
€ 5200
€ 5400 ) relative to Euro IV engine with EGR or SCR costs excluded
*
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nents). Initial total cost of ownership levels would be higher especially for the EGR-only concept, since it requires the largest amount of new engine components.
6 Conclusion
Figure 8: Predicted brake specific fuel consumption and urea consumption converted to fuel consumption on a cost equivalent basis (urea solution price assumed 45 % of of diesel price)
Clearly, the EGR-only concept requires the highest additional development effort (both from the engine manufacturer and from its suppliers). In particular, this concept requires substantial efforts to guarantee robust control solutions that compensate for the sensitivity of the combustion process to fuel quality variation and small changes in trapped gas pressure, temperature and composition. To deal with these issues, the use of incylinder sensors for closed-loop combustion control has been suggested. How ever, it is not expected that this sensor technology will be applied in the first generation of Euro VI compliant engines. In terms of complexity and development effort, the SCR-based concept clearly comes out best, closely followed by the SCR+EGR concept. Ultimately, the total cost of ownership will decide which concept will be most successful. Figure 8 shows a fuel and urea consumption comparison for the three concepts. A Euro IV SCR-type engine with a brake specific fuel consumption of 198 g/kWh was taken as a reference. It is generally conceived that Euro VI emission requirements cause a fuel penalty of 3.5 to 5 % relative to the Euro IV baseline [6], based on an approach with a combination of EGR and SCR technology. Based on measured trade-offs, the fuel penalty of the concept without NOx after treatment is estimated on the order of 7 % relative to the Euro IV baseline. The SCR approach with close-coupled SCR requires virtually no adjustments to the Euro IV engine. It is expected though, that the recalibration of the engine to a
7 g/kWh NOx level, thermal management and the added close-coupled SCR backpressure can result in a mild fuel penalty of 1 to 2 %. The urea consumption for such a platform will be 7 % of the fuel consumption (on a volume basis). The urea consumption of EGR+SCR concept is rather low due to the low engine-out NOx emission: 2 % of the fuel consumption. The comparison shows that the SCRbased concept is competitive to the EGR+SCR concept with respect to operation costs. Of course, the results of the comparison are dependent on the urea – diesel price ratio. Conservative estimates of the additional costs to the engine manufacturer are presented in Table 2. These estimates are based confidential information and discussions with suppliers and engine manufacturers. These costs are relative to Euro IV engine platform with costs of EGR or SCR systems excluded. The SCR based concept prevails when it comes to base engine and development costs, and CO2 emission. This last aspect would become very relevant should CO2 reduction incentives, for instance in terms of tax benefits, be implemented in the near future in Europe. Taking into account development costs, OEM profit margins and fleet owner capital expenditure costs, the SCR-only concept comes out very well. It should be pointed out further that initially cost differences will be higher than mentioned in Table 2 (the component prices mentioned assume large production numbers of the different new engine compo-
A new SCR-only approach for a heavyduty Euro VI platform has been presented. It relies on the application of a closecoupled SCR catalyst. Based on the presented engine dynamometer results, it is concluded that this approach is a promising alternative to the currently studied EGR-only and EGR+SCR based concepts. The main benefits of this SCR-only solution are the low development costs, low costs of ownership, and low CO2 emissions. In terms of operating costs, the SCR concept is competitive with the efficient EGR+SCR strategy.
References
[1] Edwards, S.; Eitel, J.; Pantow, E.; Lutz, R.; Dreisbach, R.; Glensvig, M.: Emissionskonzepte und Kühlsysteme für Euro 6 bei schweren Nutz fahrzeugen. In: MTZ 69 (2008), Nr. 9, P. 690-700 [2] Cloudt, R.; Willems, F.; van der Heijden, P.: Cost and Fuel Efficient SCR-only Solution for post-2010 HD Emission Standards. SAE paper 2009-01-0915 [3] Willems, F.; Cloudt, R.; van den Eijnden, E.; van Genderen, M.; Verbeek, R.; de Jager, B.; Boomsma, W.; van den Heuvel, I.: Is closed-loop SCR control required to meet future emission targets? SAE paper 2007-01-1574 [4] Van den Eijnden, E.; Cloudt, R.; Willems, F.; van der Heijden, P.: Automated model fit tools for SCR control and OBD development. SAE paper 200901-1285 [5] Baert, R.; Beckman, D.; Veen, A.: Efficient EGR technology for future HD Diesel engine emission targets. SAE paper 1999-01-0837 [6] Gense, N.; Riemersma, I.; Such, C.; Ntziachristos, L.: Euro VI technologies and costs for Heavy Duty vehicles – The expert panels summary of stakeholders responses. TNO report 06.OR.PT.023.2/NG. (ec.europa.eu/environment/air/pdf/euro_6.pdf)
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