Dipl.-Ing. R. Brück, Dr.- Ing. M. Presti, Dipl.-Ing. O. Holz, Continental, Lohmar; Dr. A. Geisselmann, Dr.-Ing. A. Scheuer, Umicore AG & Co KG, Hanau-Wolfgang The Way to Achieve CARB post 2023 Emission Legislation for Commercial Vehicles Abstract The current discussion on tightening of NO X limits in the commercial vehicle sector in Europe as well as especially in the Unites States presents a new challenge for the engineand catalyst-manufacturer. Lowering of 90 % (down to 0,02 g/bhp-hr) requires NO X reduction at all engine operating conditions. Especially cold start and load points with low exhaust temperatures demand high activity of the exhaust aftertreatment system, particularly the amount and preparation of the reductant. In passenger car application well proven close coupled catalyst configuration is deemed to be a major step to reach the future limits. The use of additional heating measures seems unavoidable to ensure the perfect preparation of the reductant on the one hand side and to reach the needed conversion efficiency of the SCR catalysts on the other hand. Particularly an electrical heated catalyst with hydrolyses coating was installed just downstream of the reductant injection. An improvement of almost 60 % could be demonstrated for the NO X values in the FTP cycle coming along with reduction of muffler size of 50 %. For this reason the considerable smaller exhaust system can be positioned closer to the engine and make use of the thermodynamical advantages of the hotter exhaust. This paper describes the development of a close coupled exhaust aftertreatment system utilizing CAD and by testing on a dynamical HD engine dyno.
1 Introduction The subject "does the internal combustion engine, and especially the diesel engine in the future, still has a chance or will it be replaced by the electric drive" is controversially discussed. However, most of the studies assume that the pure electric drive in passenger cars is only a small percentage in 2025. Most vehicles will be electrified, meaning the number of hybrids, from mild hybrid to plug-in hybrid, will increase. In the case of commercial vehicles, the per-kilometer and / or operating hourly power requirements are significantly higher in comparison to the passenger car. Further aspects such as operating times, necessary recharging times for electric drive and reduced payload will continue to receive the diesel engine in the future. The legislature, starting in the USA, responds accordingly and is working on further reductions in the emission limit values. The likely tightening of the NO X - limits in the commercial vehicle sector in the USA poses new challenges for the engine manufacturers. The discussed reduction of up to 90% (to 0.02 g / bhp-hr) - without negative effects on CO 2 - emissions (greenhouse gas emissions) - requires a reduction in NO X under all operating conditions. In particular, the cold start and the load points with low exhaust gas temperatures place increased demands on the activity of the exhaust gas aftertreatment system and, in particular, the amount and preparation of the reducing agent. NO X - engine out reduction by means of engine management (engine heating measures) is not a viable solution because of the associated increased CO 2 -emissions. Temperature management can take a wide variety of methods. Starting with the insulation of the exhaust system to the reduction of the heat losses to active heating measures by burner, HC-injectiong, an electrically heated catalyst or the combination of the mentioned possibilities. The position of the catalytic converters to the engine and the order of the arrangement of the catalytic converters in the exhaust system are also included. In order to get a high NO X -conversion after cold start as quickly as possible, it is useful to move the AdBlue dosing and the SCR catalyst as close as possible to the engine. This arrangement also helps during low load operation. Although a close coupled oxidation catalyst helps in the conversion of CO and HC and, if necessary, the NO 2 formation, which is helpful for low-temperature SCR, the heating-up behavior of the SCR system is already negatively influenced. Since the fuel economy plays a very important role in the commercial vehicle, the aim is to regenerate the particle filter only passively. This means, however, that the particle filter should also be arranged before the AdBlue dosing and the SCR Kat. This further worsens the SCR effectiveness. In principle, it can be said that all arrangements and / or combinations of exhaust gas aftertreatment systems have advantages and disadvantages. In the further course of the paper, the different arrangements are discussed and partly tested.
2 System Configurations 2.1 Components used Typical exhaust gas aftertreatment systems for commercial vehicles comprise the following: Diesel oxidation catalyst (DOC): Here, engine HC and CO emissions are oxidized and, if necessary, any additional fuel is used for increasing the temperature in the exhaust system. Nitrogen oxide in the engine is converted to NO 2 and used for passive DPF regeneration. Diesel particulate filter (DPF): This system filters out the particulates. Regeneration takes place either passively with NO 2 (formed via the DOC) or through an active increase in temperature via the additional injection of fuel. Particulate filters often have an oxidation coating comparable to that in DOCs. Reducing agent dosage (watery urea solution: AdBlue, DEF): Provision of ammonia for reducing nitrogen oxide levels in the downstream SCR catalyst. Nitrogen oxide reduction (Selective Catalytic Reduction=SCR): Selective reduction of the nitrogen oxide takes place with the ammonia (NH3) formed from the watery urea solution. The potential breakthrough of excessive ammonia is prevented by the ammonia slip catalyst. Figure 1: Conventional system: all components in the muffler on the vehicle frame 2.2 Arrangement of the components used Figure 1 shows the setup of what is today the most common configuration for EU VI and US 2010 systems: all the components are located in a muffler. The arrangement of the components governs the heat-up performance and temperature distribution in the exhaust system: Large thermal masses result in delayed heating of the downstream components during a cold engine start. This also causes a delay in the catalytic activity of the emission
cleanup system. Insulating the (long) pipes to the muffler is a complex task and, during cold engine starts, offers only minimal benefits because the pipe itself absorbs energy along its length during heating. Reducing the pipe length by positioning the catalysts directly downstream of the turbocharger is a method already used in passenger car applications and has also been confirmed in [1] and [2]. Due to the high temperature level with identical conversion, this close-coupled arrangement means that the catalytic converter volume can be reduced (by up to 30% [1]) and the conversion rate increased. One advantage of this smaller volume is the smaller heat capacity, which means that the downstream components can heat up more quickly. In the conventional arrangement depicted in Figure 1 as well as when the DOC [1] is located near the engine, the start of NO X conversion is greatly delayed during a cold engine start due to the DPF, which acts as a heat sink. Figure 2: Optimized system: cceat configuration with close-coupled DOC and SCR-coated DPF The use of a close-coupled system including diesel particulate filter with an SCR coating and advance reducing agent dosing (cceat: close-coupled exhaust aftertreatment, Figure 2) significantly increases the temperature level upstream of the first SCR catalyst:
Figure 3: Temperature comparison upstream of the first SCR catalyst (FTP cycle, cold) AdBlue dosing can then with the identical dosing strategy also take place earlier: around 150 seconds earlier with the same DOC volume. Figure 4: Temperatures at the dosing position and start of dosing in the cold FTP cycle (0 600 s) In a further step, variations in the required electrical heating energy are evaluated in order to determine an optimum compromise between temperature increase and energy consumption. Numeric modeling is ideal for this purpose.
3 Modeling the Influence of an Electrically Heated Catalyst on Emission Cleanup in a Truck Engine 3.1 Experimental approach On the engine test bench, emission and temperature data from a 15 l, 6-cylinder, EPA 2010 engine was measured in the following cycles: WHTC (warm), WHTC (cold), Heavy Duty FTP (warm), Heavy Duty FTP (cold). This input data was then used to calculate the emissions from different exhaust systems. Catalysts manufactured by Umicore were used for this purpose ([8]-[10]). The models map the behavior of thermally pre-aged catalysts (16 h at 700 C under hydrothermal conditions). The catalyst technologies are all used in production. Table 1: Components in the modeled catalytic converter system In the model, urea dosing was optimized under the general condition of a maximum ammonia slip value of 200 vppm/10 vppm in the middle of the cycle. A standard strategy was used here with a temperature-dependent specification for the ammonia fill level on the SCR catalyst [11]. The precious metal load on the oxidation catalyst (DOC) and diesel particulate filter (DPF) was adjusted such that, in the hot WHTC cycle without additional heating, an NO 2 / NO X ratio of 42.5% on the SCR input results in the middle of the cycle and is maintained for all heating strategies. For the thermal influence of the heated catalyst, an adiabatic, instant temperature increase with a thermal efficiency of 100% was assumed. The temperature at the exhaust system inlet, as determined on the test bench, was thus modified as a function of the electrical power applied. The performance and position of the heated catalyst were varied here (see below). The temperatures across the entire exhaust system were modeled. In each case, a test sequence comprising two heating cycles with subsequent cooling to room temperature followed by the actual cold/heat cycle was modeled. To evaluate the emission behavior, only the results from the final cold/heat cycle with the legally required weighting of the individual cycles were used; the initial cycles are used only for ensuring the presence of ammonia in the system.
Figure 5: Exhaust gas temperatures at the DOC inlet with an upstream heated catalyst element with variable performance in WHTC-cold (left) and WHTC-hot (right). Unmodified EO (blue line) is the unmodified temperature. According to this, the inlet temperature at the DOC without heating in 42% of the time is less than 200 C in WHTC-cold. 3.2 Results The advantage of positioning the heated catalyst at the exhaust system inlet is that all catalyst components benefit from the energy input. As a result, the NO 2 / NO X level at the DPF outlet increases from 42.8% in the reference case to 47.5% with an electrical power input of 4 kw. However, the drawback here is that the heat takes a long time to reach the component that is most sensitive regarding the emissions result: the SCR catalyst. Figure 6 shows the effect of the applied heating energy on the cycle emissions result in the WHTC and FTP. In both the cold and hot cycle, a significant reduction in emissions can be observed; the effect increases when the heating energy is applied. In the weighted overall cycle result, and with heating energy of 3 kw, emissions can be reduced by 44% in WHTC and 37% in FTP. Figure 6: Emissions as a function of the applied heating energy in WHTC (left) and FTP (right)
A detailed analysis of the emission reductions reveals that these are not more or less constant over the cycle, but instead that they can be achieved with respect to the reference case only when the SCR inlet temperature is less than around 220 C. If a load requirement then occurs, the heated systems have much lower emissions. Heating at other points in the cycle does not yield any significant advantages and is thus not necessary. The efficient application of a heated catalyst should therefore take into account the SCR inlet temperature as a control value in any form. This is why it is also recommended to position the heating element immediately upstream of the SCR catalyst to ensure direct coupling with the control value. Another advantage of this arrangement is that it allows the conditioning of the injected urea to be optimized by spraying the urea solution directly onto the heated surface. A simple digital variant of this strategy was used in another level of emission modeling. The heating energy is, in turn, introduced constantly (1, 2, 3, 4 kw); as soon as a defined minimum temperature at the filter is reached, however, the heating element is shut down. This shutdown temperature was varied for each heating energy level: 180 C, 210 C, 220 C and 240 C. Figure 7: Emissions as a function of heating energy introduced in the combined cold/hot FTP cycle for a constant heating strategy with a heating element immediately upstream of the DOC and a variable heating strategy with different shutdown temperatures and a heating element immediately upstream of the SCR. Both the emissions and energy consumption were weighted with 1:6 (cold:hot) Figure 7 shows significantly improved efficiency for the variable heating strategy compared with the constant strategy in that the emission reduction in relation to the quantity of energy introduced is much bigger. Higher shutdown temperatures lead to further emission reductions, even if the efficiency is slightly less. Figure 8 gives a sense of the amount of energy required. The heating energy introduced is set in relation to the engine work in the cycle. In the heating cycle, which probably represents the biggest component of driving operation, the energy consumption is 0% 1.1% higher. With an appropriately designed electrical infrastructure, a significant portion of this energy could be derived through recuperation. Interestingly, this
recuperation energy is available especially in combination with cold exhaust gas temperatures (e.g. in urban traffic). Additionally, the actual energy consumption should be slightly lower than assumed because the feedback of the higher engine load to the exhaust gas temperatures in the model was not taken into account. Figure 8: Heating energy introduced in relation to the engine cycle work, for different heating energy levels and shutdown temperatures 4 Test Program on the Engine Test Bench To assess the potential of close-coupled systems compared with today s conventional exhaust gas systems, a test program was performed on a 15 l, 6-cylinder EPA 2010 engine. A setup similar to that shown in Figure 1 and, for cceat (= optimized system), similar to that shown in Figure 2 was used as a reference. The total volume of the optimized system was first reduced by 20% (14.4 l) compared with the reference (Table 2). With the transfer of the DOC to the close coupled position the volume, which would be accommodated in the muffler, was reduced by around 40%: Additionally, the volume for AdBlue preparation (UDP: universal decomposition pipe [3]) is located also outside of the muffler. This reduced the required muffler volume to around 50% of that in the reference. For reasons of flexibility, the test bench setup (Figure 9) featured large deflection chambers and individually flanged carriers. US FTP cycle and WHTC tests were performed. To make it easier to compare the initial measurements, the ammonia store in the catalyst was emptied after each test, which meant that each test was started without the presence of stored ammonia in the catalyst.
Figure 9: Illustration showing the test bench setup; the short feed pipe is 700 mm shorter than the reference Table 2: Components used in the test program 4.1 Results in the US FTP cold test The challenge in the cold FTP cycle lies in the long idle running phase after initial acceleration: The system cannot be heated up all the way through due to the low exhaust
gas temperatures and low mass flow. The 150 s earlier start of dosing for cceat (see Figure 4) increases the conversion power by just 3 percentage points (from 62% to 65%; without ammonia already present in the catalyst). At the still low temperatures in the cold system, not enough DEF deposit-free for ensuring that a high conversion rate can be achieved in the second acceleration phase can be added. A significant improvement of the NO X tailpipe emissions can be achieved with a heated evaporator in the UDP [3] (see Figure 10) and a modified dosing strategy. The benefits of an electrically heated catalyst (EHC) on the efficiency of a SCR-catalyst have beed described in detail in chapter 3. Further advantages in combination with locally urea decomposition have already been demonstrated in multiple applications [6, 7]. Figure 10: UDP (universal decomposition pipe) with Emicat The DEF is sprayed directly onto the heated disc. Because of the higher surface temperature of the heating disc, dosing can begin at a lower exhaust gas temperature; the maximum quantity can also be increased without the risk of deposits. The electrical energy introduced also increases the temperature of the downstream SCR catalyst, which means that this operates in a more effective range.
Figure 11: cceat: increased temperature upstream of the first SCR catalyst through energizing of the Emicat (4 kw heating, first 600 s in the FTP cold cycle) When the temperatures upstream of the first SCR catalyst (reference; and upstream of the S-DPF for cceat) are compared with and without a heated Emicat (Figure 11), the heated, close-coupled system is on average around 50 to 70 K higher than the reference. With the modifications (and no outgassing at the end of the test), the conversion rate was increased from 62% to 77% with respect to the reference in the cold test. Any further increase in the conversion rate during cold engine start is limited by the temperature in the SCR catalysts (or S-DPF). 4.2 Results in the US FTP hot test In the hot test, too, the cceat system shows improvements over the reference. With the heated cceat system, for example, the conversion rate in the FTP hot test was increased to more than 97%: Figure 12.
Figure 12: Tailpipe emissions reference and cceat with heated Emicat ( FTP hot cycle) 4.3 Conclusions derived from the US FTP results Figure 13: Standardized tailpipe emissions reference and cceat (combined cold/hot FTP) The weighted results of the US FTP test on the reference and cceat are shown in Figure 13. It is clear that further measures will have to be implemented to reach 0,02 g/bhp-hr what is assumed to be the NO X limits in California for 2023 (crosshatched area). A proportion of this will be achieved by means of a thermally optimized muffler design (compared with the flanged test bench setup). Another significant proportion of this will
also be achieved through heating measures during cold engine starts and at low loads. The OEMs should take this into account when refining and enhancing the engines. It may be useful here to upgrade existing, tried-and-tested parts featuring new components to include new functions. With the typically already available HC dosing system and Emicat as a light-off support catalyst for the DOC ( match ), for example, the exhaust system could be used for heating during cold engine starts and/or at low loads as a type of catalytic heater without the need to influence the combustion process in the engine. The energy from the fuel is then converted directly and highly efficiently to heat and increases without any loss of performance the temperature of the exhaust gas catalysts. The cceat concept with the close-couple arrangement of components helps to keep the necessary additional outlay within reasonable limits. Component volumes and thermal masses can be reduced. 5 Summary and Outlook It has been shown that the higher temperature with the close-coupled arrangement of catalysts can have a positive effect on their conversion rate in commercial vehicles, too. This can help to fulfill the new emission requirements with less overall outlay compared with standard vehicle framework systems. Active heating of the injection line by means of an EHC leads to a significant reduction in NO X levels in the test. By modelling the capability of basic control strategies for efficient energy utilization could be demonstrated. However, additional heating measures appear to be necessary for complying with the limit values discussed here, even under consideration of the series tolerances. One promising approach for bringing about a significant temperature increase involves using an electrically heated catalyst combined with an HC dosing system. In this case fuel is the primary heat source, the EHC is acting only for ignition. The first promising results were achieved on a burner test bench with propane and propane injection (1000 ppm). The achievable temperature increase was measured at the catalyst inlet and outlet. Figure 14 shows how the measured support catalyst temperature (orange line) remains autothermically above the light-off temperature (200 C) after the EHC has been switched on for a short period (red line). The measured rear exhaust pipe HC concentration (propane green line) thus falls to zero due to the active conversion in the catalyst.
Figure 14: Example of a temperature increase through combined HC injection and an electrically heated catalyst When one considers the long-term future of trucks beyond 2025 2030, significant changes to the cabin layout will be necessary (e.g. longer wheel base and cabins). The significant reduction in the volume of catalysts in the exhaust gas system described here (cceat) results in more compact muffler boxes, which means that a thermodynamically efficient exhaust gas system can be positioned near the engine and optimally integrated in future cabin designs (see Figure 15). Figure 15: Example of a future truck cabin design with a compact, close-coupled exhaust gas system
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