Methods for exhaust aftertreatment of an internal combustion engine and exhaust aftertreatment system

Abstract

A method for exhaust aftertreatment of an internal combustion engine (10) with an exhaust system (20) in which a NOx storage catalyst (28) with a low-temperature storage component (48) made of cerium oxide and a high-temperature storage component (52) made of barium carbonate as well as an SCR catalyst (34) and / or particulate filter (30) with an SCR coating (32) arranged downstream of the NOx storage catalyst (28) in the flow direction of an exhaust gas of the internal combustion engine (10), comprising the following steps: - Modeling the loading state of the NOx storage catalyst (28), wherein - the charging of the low-temperature storage component (48) is determined by a first model for the charging of the low-temperature storage component (48), and - the loading of the high-temperature storage component (52) is determined by a second model for the loading of the high-temperature storage component (52), - Regenerating the NOx storage catalyst (28) by introducing unburned hydrocarbons into the exhaust system (20) of the combustion engine (10) when the high-temperature storage component (48) has reached a defined loading level and the SCR catalyst (34) or the particulate filter (30) with the SCR coating (32) has an operating temperature at which selective catalytic reduction of nitrogen oxides is possible, wherein - depending on whether the SCR catalyst (34) or the particulate filter (30) with the SCR coating (32) has an operating temperature at which selective catalytic reduction of nitrogen oxides is possible, at least two threshold values ​​are used, wherein - a first threshold value is provided at which a regeneration of the NOx storage catalyst (28) is carried out when the SCR catalyst (34) or the particulate filter (30) with the SCR coating (32) has reached its operating temperature and wherein - a second threshold is provided at which regeneration of the NOx storage catalyst (28) takes place independently of the temperature of the SCR catalyst (34) or the particulate filter (30) with SCR coating (32).

Description

[0001] The invention relates to a method for exhaust aftertreatment of an internal combustion engine and to an exhaust aftertreatment system for such an internal combustion engine according to the preamble of the independent claim.

[0002] Current and increasingly stringent emissions legislation places high demands on raw engine emissions and exhaust aftertreatment in combustion engines. The requirements for further reductions in fuel consumption and the tightening of emissions standards regarding permissible nitrogen oxide emissions pose a challenge for engine developers. In gasoline engines, exhaust gas purification is achieved in the familiar manner via a three-way catalytic converter, as well as additional catalysts upstream and downstream of the three-way converter. Diesel engines currently employ exhaust aftertreatment systems that include a NOx storage catalyst, a catalyst for the selective catalytic reduction of nitrogen oxides (SCR catalyst), a particulate filter for the separation of soot particles, and, if necessary, further catalysts. Ammonia is preferably used as the reducing agent.Because handling pure ammonia is complex, vehicles typically use a synthetic, aqueous urea solution, which is mixed with the hot exhaust gas stream in a mixing unit upstream of the SCR catalyst. This mixing heats the aqueous urea solution, causing it to release ammonia into the exhaust system. A commercially available aqueous urea solution generally consists of 32.5% urea and 67.5% water. However, before the SCR catalyst reaches its light-off temperature, the NOx emissions cannot be converted into harmless exhaust components by the catalyst. Therefore, especially during the cold start phase, it is essential to minimize raw NOx emissions from the combustion engine and / or to ensure rapid heating of the exhaust aftertreatment components.

[0003] The NOx storage catalyst can have several different storage components in order to store nitrogen oxides at different exhaust gas temperatures and thus extend the operating range of the NOx storage catalyst.

[0004] From DE 10 2010 046 747 A1, an internal combustion engine is known in whose exhaust system a catalyst, in particular a three-way catalyst or a NOx storage catalyst, is arranged in the direction of flow of an exhaust gas through the exhaust system and a particulate filter is arranged downstream of the catalyst.

[0005] German patent application DE 10 2016 222 012 A1 describes a method for controlling a NOx storage catalyst during the operation of a vehicle with an internal combustion engine. The method uses a model to determine the NOx storage capacity of the NOx storage catalyst, which includes submodels for low-temperature and high-temperature storage capacity, in order to define an advantageous regeneration process. This involves analyzing a driving profile and, based on this analysis, defining and implementing a suitable regeneration process.

[0006] From DE 10 2017 110 065 A1, a method for modeling the NOx storage behavior of a NOx storage catalyst is known, which has at least two different NOx storage elements with different temperature activities. The method comprises at least two NOx storage catalyst models, each with at least one model function for NOx adsorption, NOx desorption, or NOx regeneration. Using these at least two NOx storage catalyst models, the NOx storage behavior is selectively modeled for the different NOx storage elements of the NOx storage catalyst with respect to NOx adsorption, NOx desorption, or NOx regeneration.

[0007] DE 10 2014 105 210 A1 describes an exhaust aftertreatment system for an internal combustion engine with a NOx storage catalyst and a particulate filter downstream of the NOx storage catalyst with a coating for the selective catalytic reduction of nitrogen oxides.

[0008] Furthermore, DE 10 2016 118 309 A1 discloses an internal combustion engine with an exhaust aftertreatment system in which a NOx storage catalyst, a particulate filter with an SCR coating, and an SCR catalyst are arranged in the direction of exhaust gas flow through the exhaust system. A reducing agent is metered into the exhaust system downstream of the NOx storage catalyst and upstream of the particulate filter with the SCR coating to enable selective catalytic reduction of nitrogen oxides.

[0009] From DE 10 2016 200 207 A1 an exhaust gas purification system for combustion engines is known, which has at least two catalysts with the task of reducing NOx in the exhaust gas.

[0010] DE 103 61 793 A1 describes a NOx storage catalyst for an internal combustion engine with the storage catalyst components cerium oxide and a barium salt and / or barium oxide. In the catalyst outlet area, the proportion of the barium component is greater than the proportion of the cerium oxide component.

[0011] Furthermore, a method and an arrangement for regenerating a NOx storage catalyst in the exhaust system of an internal combustion engine are known from the subsequently published document DE 10 2018 203 086 A1.

[0012] A disadvantage of known solutions, however, is that the NOx storage catalyst has a limited storage capacity and must be regenerated periodically. This regeneration of the NOx storage catalyst can negatively impact the conversion performance of the SCR catalyst.

[0013] The purpose of the invention is to make exhaust aftertreatment even more efficient and effective, and also to minimize pollutant emissions during the regeneration of the NOx storage catalyst.

[0014] According to the invention, this problem is solved by a method for the exhaust aftertreatment of an internal combustion engine, in particular a diesel engine, with an exhaust system in which a NOx storage catalyst with a low-temperature storage component made of cerium oxide and a high-temperature storage component made of barium carbonate, as well as an SCR catalyst and / or a particulate filter with an SCR coating arranged downstream of the NOx storage catalyst in the direction of flow of exhaust gas from the internal combustion engine through the exhaust system, is arranged. The method comprises the following steps: - Modeling the loading state of the NOx storage catalyst, wherein ◯ the charging of the low-temperature storage component is determined by a first model for the charging of the low-temperature storage component, and ◯ the loading of the high-temperature storage component is determined by a second model for the loading of the high-temperature storage component, and - Regenerating the NOx storage catalyst by introducing unburned hydrocarbons into the exhaust system of the combustion engine when the high-temperature storage component has reached a defined loading level and the SCR catalyst or the particulate filter with the SCR coating has an operating temperature at which selective catalytic reduction of nitrogen oxides is possible.

[0015] The aim of combining the two catalyst technologies is to extend the temperature range of NOx aftertreatment and to minimize the cross-coupling of the two systems.

[0016] Since the SCR system exhibits higher NOx conversion at higher temperatures, the NOx storage catalyst should preferably only be active at low temperatures. A disadvantage is the large overlap between the two systems and current NOx storage catalyst technologies, as these also have a high-temperature active component to provide the necessary low-temperature storage capacity.

[0017] The NOx storage catalyst comprises at least two different storage components capable of storing nitrogen oxides at different temperatures and influencing each other. To model this behavior, at least two storage models are used, divided into a low-temperature range and a high-temperature range. A low-temperature range is defined as a temperature range below 250°C. The low-temperature storage component is, for example, a cerium oxide in which nitrogen oxides are stored in the form of cerium nitrate. A high-temperature range is defined as a temperature range above 200°C, preferably between 200°C and 500°C. A barium carbonate, for example, can be used as the storage component for the high-temperature range, in which nitrogen oxides are stored in the form of barium nitrate.By dividing the system into multiple temperature ranges, it is possible to adapt the regeneration strategy of the NOx storage catalyst to these ranges. Using at least two storage models to characterize the different storage components of the NOx storage catalyst allows for adjustment of the regeneration strategy of the NOx storage catalyst in conjunction with the SCR catalyst. This enables a reduced conversion rate of the NOx storage catalyst in the high-temperature range without compromising the conversion rate in the low-temperature range. As a result, fuel consumption of the combustion engine and thus CO2 emissions can be reduced. Furthermore, the negative impact of NOx storage catalyst regeneration on the conversion rate of the SCR system can be minimized.

[0018] The features mentioned in the dependent claims enable advantageous improvements and further developments of the method specified in the independent claim.

[0019] In a preferred embodiment of the invention, the regeneration of the NOx storage catalyst is performed when the low-temperature storage component reaches a defined loading level. Since the high-temperature storage component is particularly active in a temperature range where efficient exhaust aftertreatment by the SCR catalyst is possible, the low-temperature storage component is crucial for reducing nitrogen oxide emissions. Therefore, regeneration of the NOx storage catalyst is initiated when the low-temperature storage component reaches a defined loading level. In combination with the SCR catalyst, the high-temperature range of the NOx storage catalyst is no longer required for NOx conversion once the SCR conversion rate has reached a sufficient level.Otherwise, regeneration of the NOx storage catalyst is triggered if a defined loading level of the high-temperature storage component of the NOx storage catalyst is exceeded. This regeneration of the NOx storage catalyst occurs independently of the exhaust gas temperature, meaning it also occurs even if the SCR catalyst has not yet reached its operating temperature.

[0020] According to the invention, regeneration of the high-temperature storage component occurs when it reaches a defined loading level. Depending on whether the SCR storage catalyst or the particulate filter with the SCR coating operates at a temperature at which selective catalytic reduction of nitrogen oxides is possible, at least two loading level thresholds are used. A first threshold is provided at which regeneration of the NOx storage catalyst occurs when the SCR catalyst or the particulate filter with the SCR coating reaches its operating temperature. Furthermore, a second threshold is provided at which regeneration of the NOx storage catalyst occurs regardless of the temperature of the SCR catalyst or the particulate filter with the SCR coating.

[0021] In a preferred embodiment of the method, the threshold for the high-temperature storage component is higher when the SCR storage catalyst or the particulate filter with the SCR coating operates at a temperature at which selective catalytic reduction of nitrogen oxides is possible. The regeneration of the high-temperature storage component of the NOx storage catalyst is achieved using substoichiometric exhaust gas and the unburned hydrocarbons it contains. In a diesel engine, substoichiometric exhaust gas can be achieved, in particular, by late post-injection into the combustion chambers of the internal combustion engine or by fuel injection into the exhaust manifold.

[0022] According to the invention, an exhaust aftertreatment system for an internal combustion engine is proposed, comprising an exhaust system which can be connected to an outlet of the internal combustion engine, wherein a NOx storage catalyst with a low-temperature storage component and a high-temperature storage component is arranged in the exhaust system in the direction of exhaust gas flow through the exhaust system. Downstream of the NOx storage catalyst, at least one SCR catalyst or a particulate filter with an SCR coating is arranged, wherein the exhaust aftertreatment system includes a control unit which is configured to carry out an exhaust aftertreatment method according to the invention when a machine-readable program code is executed by the control unit.With such an exhaust aftertreatment system and a correspondingly programmed control unit, a method according to the invention for the exhaust aftertreatment of an internal combustion engine can be implemented in a simple manner.

[0023] In an advantageous embodiment of the invention, the NOx storage catalyst is arranged close to the engine, with a particulate filter with a coating for the selective catalytic reduction of nitrogen oxides arranged downstream of the engine-located NOx storage catalyst, and a further SCR catalyst arranged downstream of the particulate filter. In this context, an engine-located arrangement of the NOx storage catalyst and the particulate filter is understood to mean an arrangement with an exhaust gas flow length of less than 100 cm, preferably less than 80 cm, from an exhaust outlet of the combustion engine. The NOx storage catalyst and the particulate filter with the SCR coating can be designed as a single assembly and, in particular, arranged in a common housing.The close proximity of the SCR catalyst to the engine facilitates the heating of the NOx storage catalyst and the particulate filter, particularly after a cold start of the combustion engine. Additionally, the energy required to heat the particulate filter for regeneration can be minimized, as less waste heat is dissipated between the combustion engine exhaust and the particulate filter via the exhaust manifold walls compared to a particulate filter located further from the engine. By placing the SCR catalyst downstream of the particulate filter, the operating range of the SCR system can be extended, as the operating time increases during which at least one of the SCR catalysts operates within the temperature range necessary for the selective catalytic reduction of nitrogen oxides.

[0024] It is preferred that a first dosing module is provided for introducing a reducing agent into the exhaust system, with which the reducing agent is injected downstream of the NOx storage catalyst and upstream of the particulate filter. Dosing the reducing agent downstream of the NOx storage catalyst prevents the ammonia recovered from the reducing agent from being oxidized to NOx and thus increasing NOx emissions. By dosing upstream of the particulate filter, the reducing agent flows through both the SCR coating of the particulate filter and the SCR catalyst, thereby increasing the conversion efficiency and ensuring complete conversion of the reducing agent.

[0025] It is particularly advantageous if a second dosing module is provided, with which the reducing agent is metered into the exhaust system downstream of the particulate filter with the SCR coating and upstream of the additional SCR catalyst. This second dosing module allows the selective catalytic reduction of nitrogen oxides to take place either on the SCR coating of the particulate filter and / or on the SCR catalyst. Depending on the temperature of the exhaust aftertreatment component, the reducing agent is metered into the exhaust system at the point where particularly efficient reduction of nitrogen oxides is expected.

[0026] A further improvement to the exhaust aftertreatment system is the inclusion of a low-pressure exhaust gas recirculation (EGR) system. This EGR system branches off from the exhaust manifold downstream of the particulate filter and upstream of the SCR catalyst. With a fully warmed-up engine, particularly at exhaust gas temperatures of 250°C and above, the low-pressure EGR system allows exhaust gas to be recirculated into the intake manifold to reduce raw emissions during combustion. An EGR cooler can be integrated into the low-pressure EGR system to cool the recirculated exhaust gas and further reduce raw NOx emissions.

[0027] Further preferred embodiments of the invention result from the other features mentioned in the dependent claims.

[0028] Unless otherwise stated in individual cases, the various embodiments of the invention mentioned in this application can be advantageously combined with one another.

[0029] The invention is explained below using exemplary embodiments with reference to the accompanying drawings. These show: Fig. 1 a first embodiment of an internal combustion engine with an exhaust aftertreatment system according to the invention; Fig. 2 a further embodiment of an exhaust aftertreatment system according to the invention; and Fig. 3 a diagram of the conversion line of the storage components of the NOx storage catalyst.

[0030] Fig. Figure 1 shows an embodiment of an internal combustion engine 10 with an exhaust aftertreatment system according to the invention. The internal combustion engine 10 is designed as a self-igniting internal combustion engine based on the diesel principle. The internal combustion engine 10 comprises a plurality of combustion chambers 12, each equipped with a fuel injector 14 for injecting fuel into the combustion chambers 12. The internal combustion engine 10 has an inlet 16, through which the internal combustion engine 10 can be connected to an air supply system (not shown), through which the air necessary for fuel combustion is supplied to the combustion chambers 12. The internal combustion engine 10 also has an outlet 18, through which the internal combustion engine 10 is connected to an exhaust system 20.In the exhaust system 20, a turbine 26 of an exhaust gas turbocharger 24, a NOx storage catalyst 28, a particulate filter 30 with an SCR coating 32 and another SCR catalyst 34 are arranged in the direction of flow of an exhaust gas from the combustion engine 10 through an exhaust channel 22 of the exhaust system 20.

[0031] Upstream and downstream of the particulate filter 30, a pressure sensor 36, 38 is provided, which can be used to determine the differential pressure across the particulate filter 30. The loading state of the particulate filter 30 can be estimated from this pressure difference, so that if the pressure difference increases above a defined threshold, regeneration of the particulate filter 30 can be initiated. Furthermore, a temperature sensor 35 is provided in the exhaust system 20, which can be used to estimate the exhaust gas temperature and the temperature of the exhaust aftertreatment components 28, 30, 32, 34.

[0032] Downstream of the NOx storage catalyst 28 and upstream of the particulate filter 30, a first metering module 40 is provided for metering a reducing agent 44, in particular an aqueous urea solution. Downstream of the first metering module 40 and upstream of the particulate filter 30, a mixing element can be arranged in the exhaust gas channel 22 to improve the mixing of the exhaust gas and the reducing agent 44. Downstream of the particulate filter 30 and upstream of the SCR catalyst 34, a second metering module 42 is provided to meter the reducing agent 44 into the exhaust gas channel 22. The two metering modules 40 and 42 are connected to a reducing agent tank 46 in which the reducing agent 44 is stored.

[0033] The NOx storage catalyst 28 has a low-temperature storage component 48, namely a cerium oxide, with which nitrogen oxides can be temporarily stored at low exhaust gas temperatures. The NOx storage catalyst 28 also has a high-temperature storage component 52, namely a barium carbonate, with which nitrogen oxides can be temporarily stored at high exhaust gas temperatures. The NOx storage catalyst 28 can be emptied by a regeneration process in which the nitrogen oxides are reacted with a reducing agent, in particular with unburned hydrocarbons, and converted into harmless exhaust gas components.

[0034] The combustion engine 10 is connected to a control unit 50, which, among other things, controls the fuel quantity and the injection timing of the fuel into the combustion chambers 12. Furthermore, the control unit 50 is connected to the metering modules 40 and 42 to control the metering of the reducing agent 44 into the exhaust gas channel 22.

[0035] In Fig. Figure 2 shows a further embodiment of an exhaust aftertreatment system according to the invention. The exhaust aftertreatment system comprises an exhaust system 20 and a control unit 50, which controls the metering of fuel and reducing agent for exhaust aftertreatment. The exhaust system 20 comprises an exhaust channel 22 in which, in the direction of exhaust gas flow through the exhaust channel 22, a NOx storage catalyst 28 located close to the engine, a particulate filter 30 also located close to the engine with an SCR coating 32, and a further downstream SCR catalyst 34 located further away from the engine are arranged. Immediately downstream of an outlet from the NOx storage catalyst 28, a first metering module 40 for metering a reducing agent 44 is provided. Between the exit from the NOx storage catalyst 28 and the entry of the exhaust gas into the particulate filter 30, the exhaust gas flow is deflected by approximately180° to achieve improved mixing of the exhaust gas and the reducing agent 44. Downstream of the particulate filter 30, a branch 56 is provided, at which an exhaust gas recirculation channel of a low-pressure exhaust gas recirculation system 54 branches off from the exhaust gas channel 22. Downstream of the branch 56 and upstream of the SCR catalyst 34, a second metering module 42 is provided, with which the reducing agent 44 can be metered into the exhaust gas channel 22 at a position far from the engine and reacted on the SCR catalyst 34. Due to the different positions in the exhaust system 20, the temperature of the SCR coating 32 on the particulate filter 30 is higher than the temperature of the SCR catalyst 34, thus extending the operating range of the exhaust aftertreatment system in which at least one of the SCR catalysts 32, 34 ensures efficient conversion of nitrogen oxides.Due to the position of the particulate filter 30 close to the engine, the SCR coating 32 on the particulate filter 30 reaches its operating temperature in a comparatively short time after a cold start of the combustion engine 10.

[0036] In Fig. Figure 3 shows a diagram illustrating the conversion performance of the storage components 48, 52 of the NOx storage catalyst 28. The conversion performance KNOx of the NOx storage catalyst 28 is shown as a function of the temperature T of the NOx storage catalyst 28. As in Fig. As can be seen in Figure 3, the areas in which the low-temperature storage component 48 and the high-temperature storage component 52 are active overlap. Zone I represents the operating range of the low-temperature storage component 48, and Zone II represents the operating range of the high-temperature storage component 52. In Zone III, i.e., in the overlapping area of ​​Zones I and II, nitrogen oxides are stored in both components 48 and 52 of the NOx storage catalyst 28. During operation of the combustion engine 10 after a cold start, the low-temperature component of the NOx storage catalyst 28 first reaches its operating temperature. In this phase, in which the catalysts 32 and 34 for the selective catalytic reduction of nitrogen oxides have not yet reached their operating temperature, the NOx emissions are temporarily stored in this way.As the exhaust gas temperature increases during further operation of the combustion engine, the high-temperature storage component 52 of the NOx storage catalyst 28 and the SCR coating 32 of the particulate filter 30 also reach their operating temperature. Once the SCR coating 32 has reached its operating temperature, the conversion of NOx emissions is carried out primarily by this SCR coating 32, and subsequently, with further increases in temperature, by the SCR coating 32 and / or the SCR catalyst 34. During this process, the NOx storage catalyst 28 continues to be loaded with nitrogen oxides and must be periodically regenerated to maintain its storage capacity.

[0037] A process diagram for the regeneration of the NOx storage catalyst 28 according to the invention is shown in Fig.Figure 4 illustrates this. In a first process step, the loading state of the NOx storage catalyst 28 is modeled. For this purpose, the NOx storage catalyst is divided into a first model for the loading of the low-temperature component 48 and a second model for the loading of the high-temperature component 52. Once the low-temperature storage component 48 has reached a defined loading level, the NOx storage catalyst 28 is regenerated. The loading state of the high-temperature storage component 52 is generally ignored for triggering regeneration of the NOx storage catalyst 28. Reference symbol list 10 Internal combustion engine 12 Combustion chamber 14 Fuel injector 16 Admission 18 Outlet 20 Exhaust system 22 Exhaust duct 24 exhaust gas turbochargers 26 Turbine 28 NOx storage catalyst 30 particulate filters 32 SCR coating 34 SCR catalyst 35 Temperature sensor 36 Pressure sensor 38 Pressure sensor 40 first dosing module 42 second dosing module 44 Reducing agents 46 Reducing agent tank 48 low-temperature storage components 50 control unit 52 high-temperature storage components 54 Low-pressure exhaust gas recirculation 56 branching

Claims

A method for the exhaust aftertreatment of an internal combustion engine (10) with an exhaust system (20) in which a NOx storage catalyst (28) with a low-temperature storage component (48) made of cerium oxide and a high-temperature storage component (52) made of barium carbonate, as well as an SCR catalyst (34) and / or a particulate filter (30) with an SCR coating (32) arranged downstream of the NOx storage catalyst (28) in the flow direction of an exhaust gas of the internal combustion engine (10), comprising the following steps: - modeling the loading state of the NOx storage catalyst (28), wherein - the loading of the low-temperature storage component (48) is determined by a first model for the loading of the low-temperature storage component (48), and - the loading of the high-temperature storage component (52) is determined by a second model for the loading of the high-temperature storage component (52).- Regenerating the NOx storage catalyst (28) by introducing unburned hydrocarbons into the exhaust system (20) of the combustion engine (10) when the high-temperature storage component (48) has reached a defined loading level and the SCR catalyst (34) or the particulate filter (30) with the SCR coating (32) has an operating temperature at which selective catalytic reduction of nitrogen oxides is possible, wherein - depending on whether the SCR catalyst (34) or the particulate filter (30) with the SCR coating (32) has an operating temperature at which selective catalytic reduction of nitrogen oxides is possible, at least two threshold values ​​are used, wherein - a first threshold value is provided at which regeneration of the NOx storage catalyst (28) is carried out,when the SCR catalyst (34) or the particulate filter (30) with the SCR coating (32) has reached its operating temperature and wherein a second threshold is provided at which regeneration of the NOx storage catalyst (28) takes place independently of the temperature of the SCR catalyst (34) or the particulate filter (30) with SCR coating (32). Method according to claim 1, characterized in that a regeneration of the NOx storage catalyst (28) is carried out when the low temperature storage component (48) has reached a defined loading level. Method according to one of claims 1 or 2, characterized in that the operating temperature is in the range of 200°C - 500°C. Exhaust aftertreatment system for an internal combustion engine (10) with an exhaust system (20) which can be connected to an outlet (18) of the internal combustion engine (10), wherein in the exhaust system (20) in the flow direction of an exhaust gas of the internal combustion engine (10) through the exhaust system a NOx storage catalyst (28) with a low-temperature storage component (48) and a high-temperature storage component (52), and wherein downstream of the NOx storage catalyst (28) at least one SCR catalyst (34) or a particulate filter (30) with an SCR coating (32) are arranged, characterized in that the exhaust aftertreatment system comprises a control unit (50) which is configured to carry out a method for exhaust aftertreatment according to one of claims 1 to 3 when a machine-readable program code is executed by the control unit (50). Exhaust aftertreatment system according to claim 4, characterized in that the NOx storage catalyst (28) is arranged close to the engine, wherein downstream of the NOx storage catalyst (28) a particulate filter (30) close to the engine with a coating (32) for the selective catalytic reduction of nitrogen oxides and downstream of the particulate filter (30) a further SCR catalyst (34) are arranged. Exhaust aftertreatment system according to claim 4 or 5, characterized in that a first metering module (40) is provided with which a reducing agent (44) is metered into the exhaust system (20) downstream of the NOx storage catalyst (28) and upstream of the particulate filter (30). Exhaust aftertreatment system according to claim 6, characterized in that a second metering module (40) is provided, with which the reducing agent (44) is metered into the exhaust system (20) downstream of the particulate filter (30) and upstream of the SCR catalyst (34). Exhaust aftertreatment system according to one of claims 4 to 7, characterized in that the exhaust system (20) comprises a low-pressure exhaust gas recirculation (54), wherein the low-pressure exhaust gas recirculation branches off from the exhaust duct (22) of the exhaust system (20) downstream of the particulate filter (30) and upstream of the SCR catalyst (34).

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