new catalyst substrate innovation for achieving rde and sulev

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- 1 - Dipl. Ing. Rolf Brück, Dipl. Ing. Peter Hirth, Dipl. Ing. Bin Hu, Dipl. Ing. Christian Schorn Continental Emitec GmbH New Catalyst Substrate Innovation for Achieving RDE and SULEV 30 Emission Legislation Abstract Exhaust emissions for passenger cars and trucks have been reduced considerably over the past decade. The rising requirements have been met by means of catalytic converter systems located close to the engine and progressive catalytic converters in both modern gasoline engines and diesel engines. Thanks to the development of turbulent-acting catalytic converter structures, it was also possible to make the catalytic converter systems smaller and more compact. The introduction of the RDE (Real Driving Emissions) passenger car legislation and of the stricter In-Use- Compliance regulation for commercial vehicles will require exhaust-gas aftertreatment to operate over the entire engine map in the future. As a result, catalytic converters must be designed for low-load and high-load operation in such a way that very high conversion rates can be achieved without a major increase in pressure loss. In the U.S.A. (California, for example), additional requirements are being formulated, such as a further reduction in the NOx limit values by 90% within the SULEV legislation or the commercial vehicle limit values currently under discussion for California from 2020 onward. Particularly with regard to gasoline engines, implementing these requirements makes it necessary to avoid single-cylinder lambda effects and to further mix flow zones of different exhaust gas concentrations together in order to guarantee a high NOx conversion rate in particular. Solutions such as installing underbody catalytic converters as a subsequent NOx cleaning stage downstream of a mixing pipe in addition to the catalytic converters located close to the engine increase both the complexity, the exhaust gas back pressure, and the costs. This publication presents, for the first time, innovative metallic catalyst substrate concepts in a ring-shaped design that meet the above-mentioned requirements. In addition to the structure and design of the catalyst systems, comprehensive calculation and test results of the aftertreatment concepts are presented.

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Dipl. Ing. Rolf Brück, Dipl. Ing. Peter Hirth, Dipl. Ing. Bin Hu, Dipl. Ing. Christian Schorn Continental Emitec GmbH

New Catalyst Substrate Innovation for Achieving RDE and SULEV 30 Emission Legislation

Abstract Exhaust emissions for passenger cars and trucks have been reduced considerably over the past decade. The rising requirements have been met by means of catalytic converter systems located close to the engine and progressive catalytic converters in both modern gasoline engines and diesel engines. Thanks to the development of turbulent-acting catalytic converter structures, it was also possible to make the catalytic converter systems smaller and more compact. The introduction of the RDE (Real Driving Emissions) passenger car legislation and of the stricter In-Use- Compliance regulation for commercial vehicles will require exhaust-gas aftertreatment to operate over the entire engine map in the future. As a result, catalytic converters must be designed for low-load and high-load operation in such a way that very high conversion rates can be achieved without a major increase in pressure loss. In the U.S.A. (California, for example), additional requirements are being formulated, such as a further reduction in the NOx limit values by 90% within the SULEV legislation or the commercial vehicle limit values currently under discussion for California from 2020 onward. Particularly with regard to gasoline engines, implementing these requirements makes it necessary to avoid single-cylinder lambda effects and to further mix flow zones of different exhaust gas concentrations together in order to guarantee a high NOx conversion rate in particular. Solutions such as installing underbody catalytic converters as a subsequent NOx cleaning stage downstream of a mixing pipe in addition to the catalytic converters located close to the engine increase both the complexity, the exhaust gas back pressure, and the costs. This publication presents, for the first time, innovative metallic catalyst substrate concepts in a ring-shaped design that meet the above-mentioned requirements. In addition to the structure and design of the catalyst systems, comprehensive calculation and test results of the aftertreatment concepts are presented.

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1. Introduction The future of mobility, and in particular what it could potentially look like, has been under discussion for many years, with conflicting views being expressed in some cases. Does the combustion engine have a future? Will the electric vehicle become a reality? Are hybrid drives the method of choice for the time being? Or is the fuel cell the concept for the future after all? Significantly, all drive concepts can be reduced to two basic questions: Where does the energy come from, and what pollutants are emitted? Even though electricity comes from the socket, it still has to be generated. CO2-free electricity production by means of photovoltaics or wind power appears to yield results once the electricity generators have been produced and commissioned. However, it is known that the considerable fluctuation in electricity production depending on weather conditions presents an obstacle. Storing the excess electricity produced still appears difficult, depending on the location – at present, at least. In some cases, peak power cannot be reduced, and storage power stations are not available in many cases. A widely discussed variant is to produce hydrogen as an energy carier, which can be used to generate electricity in fuel cells. A comparison of the energy density of hydrogen (at 700 bar pressure) with today's conventional liquid fuel (Figure 1) clearly illustrates one of the problems. Even tank systems produced with a comparatively high outlay are in no way comparable with a conventional gasoline tank in terms of weight or size.

Figure 1: Energy density of different gasous and liquid energy carriers Further processing the hydrogen with CO2 to create a diesel-like fuel is another option. In this case, CO2 from combustion procedures (coal-fired power plant) or industrial processes (steel industry) is used more or less regeneratively, and is converted with hydrogen from electrolysis systems operated with regenerated electricity. Over several subsequent process steps, a fuel customized precisely to the application in question can be generated in this way. Thanks to its chemical uniformity, this fuel is an ideal basis for an emission-free combustion engine.

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If such a fuel is then used in the combustion engine, the combustion process is therefore CO2-neutral. This would counter one of the arguments against the combustion engine, i.e. the CO2 emissions. There is a further challenge in designing the combustion engine and its exhaust-gas aftertreatment so effectively that the composition of the exhaust gas does not contain pollutant concentrations higher than the ambient air drawn in for the purpose of combustion. Emission legislation is becoming stricter and stricter throughout the world. In particular, the Real Driving Emissions (RDE) are currently under discussion and have already been resolved in some cases. While these two factors constitute a major challenge for the automotive industry, they also may cause the passenger car with a combustion engine to no longer be regarded as an environmental polluter. The current SULEV 20 and SULEV 30 limit values in California as part of the LEV III legislation in force there once again require NMOG + NOx emissions to be reduced to 20 mg and 30 mg respectively. At the same time, the RDE legislation is being implemented in Europe, and this will require emission measurements not just in the test cycle but also in real driving situations in the future. This means new technical challenges for engine developers as well as for catalytic converter and filter technology, but will also bring the passenger car one step closer to achieving a positive environmental image.

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2. The three-way catalytic converter in the gasoline engine drive train The regulated three-way catalytic converter was launched by Volvo in 1976. With the help of the oxygen sensor and lambda control system, it became possible to adjust the exhaust gas composition in such a way that oxidation of carbon monoxide and hydrocarbons and a reduction in nitrogen oxides take place simultaneously. Today's catalytic converter concepts achieve conversion rates of over 99% in test cycles. If further increases in effectiveness are now to be achieved, the requirements placed on the engine's mixture control will increase accordingly with regard to the uniformity of the exhaust gas mixture in the individual cylinders. A similar demand exists for using the entire catalytic converter volume as much as possible and as homogeneously as possible. It is particularly important here that the correct fuel-air mixture is present throughout the entire catalytic converter volume. To achieve this goal, the exhaust gas from every single cylinder really must impact the entire catalyst cross-section with a high level of uniformity instead of just individual areas; this can lead to lambda deviations in the catalytic converter due to different individual cylinder lambdas. The use of turbo engines is already leading to considerable improvements as the charger virtually works as a large mixer. However, since most gasoline engine turbochargers are wastegate chargers, i.e. have a switchable bypass valve, achieving a good exhaust gas mixture depends heavily on the load point and the wastegate aperture angle, particularly if the catalyst is located close to the engine. Figure 2 shows the flow distribution of a catalyst located close to the engine with the wastegate fully open and closed.

Figure 2: Flow distribution of a close coupled catalyst with the wastegate fully

open and closed

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A major influence of the wastegate on the uniformity index of the catalyst’s flow objection can be observed. A low level of uniformity leads to a local overload of the catalytic converter. It is known that radially permeable catalytic converters with turbulent-acting structures can lead to significant improvements here. For example, using the PE structure [1, 2] facilitates radial equalization of differing flow speeds and pollutant concentrations of the exhaust gas components. This effect has been known for many years [3] and relevant products have been manufactured in mass production for a long time. Figure 3 shows a Lambda measurement by means of four oxygen sensors distributed across the catalytic converter's cross-section, with a standard catalytic converter and a catalytic converter with a PE structure. Despite the existence of the PE converter technology, and particularly in applications that do not use such solutions, it is often difficult, in modern catalytic converters located close to the engine, to achieve perfect distribution of the individual cylinder flows due to installation space restrictions. .

Figure 3: Measurement of the Lambda signals from oxygen sensors distributed across the catalytic converter’s cross section of a standard converter and a PE catalyst

3. An additional catalyst as a quick solution Using a second, additional catalytic converter in the underbody, i.e. downstream of a certain length of the exhaust pipe, that acts as a mixing section constitutes an easy and

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readily available solution to increase the catalytic converter's efficiency. Typical solutions are based on a catalyst volume of around one liter. Figure 4 shows such an exhaust system, with a main catalytic converter located close to the engine and an additional one in the underbody position.

Figure 4: An exhaust system with a close coupled main catalytic converter and an

additional catalyst in the underbody position If we talk about the need to increase the conversion rate in this regard, we should not neglect the fact that a standard catalytic converter located close to the engine already converts 98–99% of all pollutants. As such, the additional underbody catalytic converter "only" has to do the remaining work, i.e. giving a final clean to the exhaust gas that has already been well mixed in this catalyst position. The additional absolute efficiency that is needed here thus comes to only 1–2%; the reduction in residual emissions, and hence tailpipe emissions, is naturally much higher. Since the absolute conversion rate is so low, the pollutant transport to the channel wall [4, 5] primarily determines the catalytic effectiveness, i.e. the diffusion of the pollutants to the catalytically active surfaces of the catalytic converter. This means that the required catalytic converter volume is necessary "only" to provide the required channel length for the diffusion process. Just like the PE converter design described above, additional metallic catalytic converter structures are offered. In particular, their property is their ability to intensify the pollutant transport and the diffusion of the pollutants to the channel wall. E.g. the LS structure "relocates" part of the channel wall into the channel's core flow, and thus shortens the diffusion paths considerably. To determine an optimum cost-benefit ratio of the catalytic converters, a test program was conducted on a 1.6 liter medium-sized vehicle with 110 kW. In addition to the standard catalytic converter located close to the engine, an additional catalyst was also installed in the underbody position. Table 1 describes these tested catalytic converters. First of all, the pressure loss of the catalytic converters were measured (Figure 5).

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Table 1: Properties of the tested additional catalytic converters in

the underbody position

Figure 5: Pressure loss of the tested additional catalytic converters in the

underfloor The pressure loss shown is to be understood as additional to the standard catalytic converter system that has been installed close coupled. The goal was to achieve a pressure loss as low possible in order to prevent negative effects on engine power and fuel consumption. As expected, the short catalytic converters exhibit a pressure loss that is around 50% lower than that of the additional catalytic converter in the standard size. All catalytic converters were equipped with an identical catalytic coating and a specific precious metal load of 4 g/ft³ with Pt/Pd/Rh = 0:2:2. The tests were conducted in the FTP cycle as well as the US06 Highway Test cycle. Figure 6 shows the comparison of NOx emissions of all catalysts in the US06 Highway Test Cycle.

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Figure 6: NOx emissions in the US06 test cycle with the standard system and

various additional underbody catalysts The measurements were conducted without any additional change to engine management or the mixture control. The best result was achieved using a 40 mm catalytic converter disk in the 400/800 cpsi LS structure. Compared with the additional catalyst in standard size, the volume, the absolute precious metal load, and hence the costs could be reduced by 60%. In addition, the tailpipe NOx emissions were reduced by around 45%. This clearly demonstrated that short catalytic converters with a turbulent-acting structure are the optimum solution for a clean-up catalytic converter in terms of effectiveness, pressure loss, and costs.

4. The ring-shaped catalytic converter: a new concept for close-coupled three-way catalysts

The short additional catalytic converter shown is a very good solution, particularly when the goal is to improve existing vehicles and exhaust gas concepts. However, the medium-term goal was to avoid any additional catalytic converter solution and to find a over all concept close to the engine. The ring-shaped catalytic converter is a possible solution here. This substrate design concept has already been launched as a solution in SCR non-road applications. As shown in Figure 7, in those SCR applications the exhaust gas initially flows through the cat structure in the outer ring, is deflected at the end of the catalytic converter after passing a swirl generator, and fed into the central pipe. An SCR injection nozzle is installed in the deflection chamber, and injects the urea into the central pipe. Since the hot gas pre-heats the curved outlet pipe at the catalytic converter's inlet, AdBlue drops that have not yet evaporated may hit a hot pipe wall there, whereby deposits are avoided.

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Figure 7: Ring-shaped catalytic converter as a DOC with central

SCR injection [6, 7] This concept has been changed as follows for use of the ring-shaped catalyst in close-coupled gasoline engine applications: The exhaust gas flows out of the turbocharger outlet through the central pipe, is mixed-up intensively in the deflection chamber, and flows back through the catalyst structure on the way back. Figure 8 shows the ring-shaped catalyst in this configuration.

Figure 8: Concept of the close-coupled ring-shaped catalytic converter for

use in gasoline engines The swirl flow at the turbocharger outlet, which is also to be found in the ring catalyst’s inner pipe, results in an outstanding good mixture. In particular, the wastegate opening angle has no more influence on the flow distribution upstream of the catalyst as the wastegate flow passes into the swirl flow in the inner pipe. Figure 9 shows the flow distribution at the outlet of a ring-shaped catalyst with the wastegate closed and fully open. It becomes clear that, unlike the normal catalytic converter layout (Figure 2), the new configuration is barely influenced by the wastegate opening at all.

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Figure 9: Flow distribution in the ring-shaped catalyst depending on the

wastegate opening angle In both applications, the entire catalytic converter system was measured, including the inlet and outlet cones. As shown in Figure 8, using the ring-shaped catalyst offers even more potential. In the tested version, the cross-section of the standard turbocharger outlet was too large. As a result, a confusor had to be installed. Adjusting the diameter of the turbocharger outlet and of the ring catalyst’s inner pipe would guaranty a flow with lower losses, as a result of which the pressure loss is reduced. Table 2 shows the standard system's pressure losses compared with those of the ring catalyst system with different exhaust gas mass flows.

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Table 2: Pressure loss of the standard catalyst system compared with the ring

catalyst with different exhaust gas mass flows Thanks to the very uniform flow distribution, the catalytic converter is used to full capacity, even at high loads, and the catalytic converter aging is equalized in real driving situations and thus positively influenced. A further advantage is that, particularly at high loads, a significant temperature loss between the turbocharger outlet and the catalyst inlet can be adjusted if needed thanks to the flexible design. As a result of this, either the thermal catalyst aging can be reduced or a lambda 1-strategy can be applied in an expanded engine map range. The goal of the next step was to determine whether the longer inflow pipe and the deflection upstream of the catalytic converter leads to a deterioration in the light-off behavior. These tests were conducted on a 2.0 l four-cylinder engine with 130 kW. Figure 10 shows the accumulated HC, CO and NOx emissions in the entire NEDC cycle of the standard close-coupled catalyst system flanged directly to the turbocharger in comparison with the ring-shaped catalyst system.

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Figure 10: Accumulated HC, CO and NOx emissions in the entire NEDC cycle

of a close-coupled catalytic converter system in comparison with thering-shaped catalyst system

Within the limits of measuring accuracy, the result shows identical tailpipe emissions of HC, CO, and nitrogen oxides, both in the cold start as well as throughout the entire NEDC test cycle. During cold start the wastegate is normally closed, therefore at both systems – standard system as well as the ring catalyst system – the catalyst front face will be objected in a similar good manner. The smaller frontal face of the ring catalyst does compensate hereby for the slightly higher distance to the turbocharger outlet. In the next step, a complete test program was planned in which the individual cylinder lambda values were also deliberately disrupted in order to test their influence on the standard system and on the ring-shaped catalyst. In addition, test cycles such as the new worldwide harmonized cycle, higher constant load points, and the RDE "dynamic" test were conducted in order to test the potential of the ring-shaped catalyst solution. Unfortunately, the test results were not available at the time this paper was submitted, but they will be shown in the presentation.

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4.1 Ring-shaped catalytic converter as an electrically heated catalyst (EHC)

It has already been shown multiple times in the past [8, 9] that the electrically heated catalytic converter offers an effective solution in the gasoline engine as well as the diesel engine for conducting thermal management independently of the engine. In gasoline engine applications, the EHC primarily supports the cold engine start. The goal here is to completely do without engine-based catalyst heating. It has been proven that systems with an EHC show advantages in fuel consumption, and hence in CO2 emissions. There is also evidence of advantages regarding a reduction in particle number emissions during the cold engine start phase. Figure 11 shows the particle number emissions in the cold start of the NEDC cycle with and without engine-based catalyst heating. The heating power and heating time of the EHC were set in such a way that the HC, CO, and NOx emissions were comparable with the results in engine-based heating.

Figure 11: Particle number emissions in the cold start of the NEDC cycle with and without engine-based catalyst heating measures, and with/without an EHC

In addition to the advantages of the EHC in gasoline engine applications, it shows major potential particularly in combination with a 48 V mild hybrid [10]. For this reason, the ring-shaped catalyst has also been developed in a version as a heated catalyst (Figure 12).

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Figure 12: Ring-shaped electrically heated catalyst for a gasoline and hybrid applications

The ring-shaped catalytic converter thus constitutes a flexible tool for varies applications in the future.

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5. Summary Emissions legislation such as the current SULEV 20 or SULEV 30 limits in California or even the current Real Driving Emissions (RDE) requirements are becoming stricter and stricter throughout the world. These represent a major challenge for the automotive industry and the manufacturers of catalytic converter and filter technology. Today's catalytic converter concepts achieve conversion rates close to 100%. In order to achieve further increases, the requirements placed on the engine's mixture control will increase dramatically, as will the requirements regarding using the entire catalytic converter volume as much as possible and as homogeneously as possible. Short-term and simple solutions for increasing the catalytic converter's efficiency include using a second, additional catalyst in the underfloor position, preferably with small volumes of around one liter. This allows the tailpipe NOx emissions to be reduced by around 45%. In this work, it could be shown, that short catalytic converters with a turbulent-acting structure represent very good solutions for a clean-up catalyst in terms of efficiency, pressure loss and costs. In the longer term, over-all concepts located close to the engine such as the ring-shaped catalyst are to be preferred. The swirl flow at the turbocharger outlet results in an outstanding good mixture prior to the entry into the catalyst structure. Thanks to this, the influence of the wastegate opening angle was eliminated completely. The result shows identical tailpipe emissions of HC, CO, and nitrogen oxides, both in the cold start as well as throughout the entire NEDC test cycle, compared with a close-coupled standard catalyst system. Since the electrically heated catalyst shows major potential, particularly in combination with a 48 V mild hybrid, the ring-shaped catalyst has also been developed in an electrically heated version.

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