Exhaust aftertreatment system and methods for exhaust aftertreatment of an internal combustion engine

Abstract

Exhaust aftertreatment system for an internal combustion engine (10), comprising an exhaust system (40) with an exhaust duct (42) in which a first passive NOx adsorber (48) located close to the engine, a first NOx-reducing exhaust gas purification device (56, 58) is arranged downstream of the first passive NOx adsorber (48), a NOx storage unit (64) with a second passive NOx adsorber (66) is arranged downstream of the first NOx-reducing exhaust gas purification device (56, 58), and a second NOx-reducing exhaust gas purification device (70) is arranged downstream of the NOx storage unit (64), wherein a first electric heating element (74) is connected downstream of the first passive NOx adsorber (48) and wherein at least one further upstream or downstream electric heating element (94, 96) is associated with the NOx storage unit (64), characterized in that the NOx storage unit (64) has a main channel (106) and a bypass (108),wherein the second passive NOx adsorber (66) is arranged in the bypass (108).

Description

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

[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 challenges for engine developers. In gasoline engines, exhaust gas purification is achieved in the familiar manner using 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 an oxidation 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.

[0003] Furthermore, it is known to incorporate a passive NOx adsorber in an exhaust aftertreatment system, which temporarily stores the nitrogen oxides contained in the exhaust stream during the cold start phase of the combustion engine until a NOx storage catalyst or an SCR catalyst has reached its operating temperature. During the cold start phase of the exhaust aftertreatment system, the exhaust gases from the combustion engine flow through the exhaust system almost untreated and are emitted into the environment. These cold start emissions constitute a significant proportion of the total emissions from a diesel engine.

[0004] Every catalytic exhaust gas purification system requires exceeding a minimum temperature, the so-called light-off temperature, to become effective. During a cold start of a vehicle, the combustion engine and the exhaust aftertreatment components are at approximately ambient temperature. Even with a high energy input into the exhaust system, the thermal inertia of the exhaust system must first be overcome and radiation and convection losses compensated for in order to achieve at least partial effectiveness of the exhaust aftertreatment components. During this time, the raw emissions from the combustion engine are emitted largely untreated. Depending on the energy input into the exhaust system, this period can be shortened, but it can never be reduced to zero.

[0005] In diesel engines, it is known to install a NOx storage catalyst upstream of an SCR exhaust gas purification system. This catalyst achieves good conversion performance even in the 120°C - 200°C range, whereas the SCR catalyst only enables the conversion of nitrogen oxide emissions from approximately 180°C. The exhaust aftertreatment components can be individually or collectively supported by electric heating elements or thermal exhaust gas burners during their warm-up phase, particularly up to the respective light-off temperature.

[0006] WO 2018 / 057 170 A1 discloses an exhaust aftertreatment system for an internal combustion engine with an exhaust system in which, in the direction of flow of an exhaust stream through the exhaust system, a first passive NOx adsorber, downstream of the first passive NOx adsorber a diesel oxidation catalyst, downstream of the diesel oxidation catalyst an exhaust aftertreatment module with a diesel particulate filter and an SCR catalyst arranged downstream of the diesel particulate filter, is provided, wherein a further passive NOx adsorber is arranged upstream of the diesel particulate filter and / or downstream of the SCR catalyst.

[0007] From US patent 2013 / 0111886A1, an exhaust aftertreatment system for an internal combustion engine is known, in which, in the direction of exhaust gas flow through the exhaust system, a first oxidation catalyst is arranged, downstream of the first oxidation catalyst a first SCR catalyst, downstream of the first SCR catalyst an electrically heated catalyst, further downstream a second oxidation catalyst, still further downstream a second SCR catalyst, and finally a particulate filter. Both oxidation catalysts have NOx adsorption capacity and can also be designed as passive NOx adsorbers.

[0008] DE 10 2018 009 233 A1 discloses an exhaust aftertreatment system for an internal combustion engine with an exhaust system in which, in the direction of flow of an exhaust stream of the internal combustion engine, a diesel oxidation catalyst, downstream of the diesel oxidation catalyst a first SCR catalyst, downstream of the first SCR catalyst a diesel particulate filter, downstream of the particulate filter a passive NOx adsorber, downstream of the passive NOx adsorber a second SCR catalyst and subsequently an ammonia barrier catalyst are arranged.

[0009] Furthermore, DE 10 2018 129 683 A1 discloses an exhaust aftertreatment system comprising an exhaust pipe in which, in the direction of flow, a passive NOx adsorber with a downstream electric heating element, an injector for metering a reducing agent, an exhaust gas mixer, and a further downstream SCR catalyst are arranged. The electric heating element is designed such that it can heat the SCR catalyst to its operating temperature before the nitrogen oxides stored in the passive NOx adsorber are released again.

[0010] German patent DE 10 2018 122 875 A1 describes an exhaust aftertreatment system for an internal combustion engine. In the exhaust system of the internal combustion engine, a first catalyst is arranged in the direction of exhaust gas flow. Downstream of the first catalyst, a first exhaust aftertreatment component for the selective catalytic reduction of nitrogen oxides is located, and further downstream, a second exhaust aftertreatment component for the selective catalytic reduction of nitrogen oxides is also arranged. A low-temperature NOx storage catalyst is arranged downstream of the first exhaust aftertreatment component and upstream of the second exhaust aftertreatment component.

[0011] From DE 10 2018 101 929 A1, an exhaust aftertreatment system for an internal combustion engine is known. The exhaust aftertreatment system comprises an exhaust system in which a close-coupled NOx storage catalyst or an oxidation catalyst is arranged. Downstream of this first close-coupled catalyst, two SCR catalysts are arranged, each of which is assigned a metering valve for dosing a reducing agent. The NOx storage catalyst or the oxidation catalyst has an electric heating element with which the catalyst can be heated independently of the exhaust gas flow of the internal combustion engine. It is provided that the close-coupled NOx storage catalyst or the close-coupled oxidation catalyst is electrically heated from the moment the internal combustion engine is started in order to reach an operating temperature as quickly as possible at which conversion or intermediate storage of emissions is possible.

[0012] A disadvantage of the solutions known from the state of the art is that you cannot reduce cold start emissions to an extremely low residual value or require a corresponding start delay so that the exhaust aftertreatment system can ensure efficient conversion of pollutants from the start of the combustion engine.

[0013] The invention is based on the objective of further reducing the cold start emissions of an internal combustion engine and overcoming the disadvantages known from the prior art.

[0014] This task is accomplished by an exhaust aftertreatment system for an internal combustion engine, comprising an exhaust system with an exhaust duct in which a first passive NOx adsorber close to the engine, a first NOx-reducing exhaust gas purification device, in particular a particulate filter close to the engine with an SCR coating, and a second NOx-reducing exhaust gas purification device, in particular an SCR catalyst, are arranged downstream of the first NOx-reducing exhaust gas purification device.According to the invention, a NOx storage unit with a second passive NOx adsorber is arranged downstream of the first NOx-reducing exhaust gas purification device and upstream of the second NOx-reducing exhaust gas purification device. A first electric heating element is connected downstream of the first passive NOx adsorber, and at least one further upstream or downstream electric heating element is assigned to the NOx storage unit. In this context, a position close to the engine is understood to be a position in the exhaust system with an exhaust gas flow length of less than 80 cm, preferably less than 50 cm, from the exhaust outlet of the internal combustion engine. The exhaust aftertreatment according to the invention can, in particular, further reduce the cold-start emissions of an internal combustion engine.The second passive NOx adsorber, in combination with the first electric heating element located near the engine, allows nitrogen oxide emissions to be stored immediately after a cold start of the combustion engine. The electric heating elements ensure that the NOx-reducing exhaust gas purification devices located downstream of the passive NOx adsorbers, in particular the SCR catalysts, have reached their operating temperature, specifically a temperature of at least 180°C, preferably at least 200°C, before the nitrogen oxides can desorb from the passive NOx adsorbers. This prevents large quantities of unconverted nitrogen oxides from being emitted into the environment immediately after a cold start of the combustion engine.

[0015] According to the invention, the NOx storage unit has a main channel and a bypass, with the second passive NOx adsorber being located in the bypass. This reduces the exhaust backpressure, as the exhaust gas flow is not routed through the second passive NOx adsorber during normal operation. In this context, normal operation refers to operation of the exhaust aftertreatment system in which all exhaust aftertreatment components have reached their operating temperature.

[0016] A particularly advantageous feature is that the bypass is designed as a ring catalyst, which surrounds the main channel. This allows for a particularly compact design of the NOx storage unit. Furthermore, the exhaust gas flow through the main channel can be used to support the thermal desorption of nitrogen oxides stored in the ring catalyst.

[0017] The features mentioned in the dependent claims enable advantageous further developments and non-trivial improvements to the exhaust aftertreatment system specified in the independent claim.

[0018] In a further improvement to the exhaust aftertreatment system, an actuator, in particular a control valve, is provided on the NOx storage unit. This actuator allows the exhaust gas flow from the combustion engine to be selectively directed through the main channel and / or the bypass. Switching the exhaust gas flow between the main channel and the bypass is easily accomplished with this actuator. A control valve is a particularly simple and cost-effective actuator that, when open, does not lead to a significant increase in flow resistance and thus no increase in exhaust backpressure. This allows the fuel consumption of the combustion engine to remain largely constant.

[0019] In an advantageous embodiment of the exhaust aftertreatment system, the additional electric heating element is arranged downstream of the second passive NOx adsorber. This additional electric heating element downstream of the second passive NOx adsorber assists in heating the second NOx-reducing exhaust gas purification device, particularly the second SCR catalyst, enabling it to reach its operating temperature more quickly. The aim is for the second SCR catalyst to reach its operating temperature before thermal desorption of the nitrogen oxides stored in the second passive NOx adsorber occurs.

[0020] Alternatively or additionally, a further electric heating element is provided for upstream of the second passive NOx adsorber. This additional electric heating element upstream of the second passive NOx adsorber can support thermal desorption of the nitrogen oxides stored in the second passive NOx adsorber. This allows for complete regeneration of the second passive NOx adsorber even at low exhaust gas temperatures.

[0021] In a further improvement of the invention, it is provided that the first electric heating element and at least one further electric heating element can be controlled via a common heating control unit. This allows the heating current to be distributed to the two electric heating elements as needed. In particular, a method for operating the electric heating elements can be designed such that only one electric heating element is active at any given time. This eliminates the need for an additional control unit for the electric heating element, thus reducing the costs of the exhaust aftertreatment system.

[0022] In an advantageous embodiment of the exhaust aftertreatment system, an exhaust channel is provided directly upstream of the NOx storage unit, featuring an inlet for a secondary air supply. This allows a carrier air flow to be generated even when the combustion engine is switched off, enabling the expulsion of the nitrogen oxides stored in the second passive NOx adsorber. The expelled nitrogen oxides can then be converted on the second NOx-reducing exhaust gas purification device, particularly the second SCR catalyst, so that the second passive NOx adsorber is largely empty upon restarting the combustion engine and exhibits a high storage capacity for nitrogen oxides.

[0023] Another aspect of the invention relates to a method for the exhaust aftertreatment of an internal combustion engine with such an exhaust aftertreatment system, which comprises the following process steps: - Starting the combustion engine, whereby the nitrogen oxide emissions in the exhaust stream of the combustion engine are stored in the first passive NOx adsorber located near the engine, - Heating the exhaust gas stream downstream of the first passive NOx adsorber by the first electric heating element, - Heating the first NOx-reducing exhaust gas purification device to its operating temperature, in particular to an operating temperature of the SCR coating of a particulate filter, wherein the nitrogen oxides are stored in the second passive NOx adsorber during heating, - Regenerating the first passive NOx adsorber when the first NOx-reducing exhaust gas purification device has reached its operating temperature and the nitrogen oxides are converted by selective catalytic reduction by the first NOx-reducing exhaust gas purification device, in particular by the SCR coating of the particulate filter.

[0024] A method according to the invention can further reduce the cold-start emissions of an internal combustion engine. The second passive NOx adsorber, in combination with the first electric heating element located close to the engine, enables the storage of nitrogen oxide emissions immediately after the cold start of the internal combustion engine. The electric heating elements ensure that, before the nitrogen oxides can desorb from the passive NOx adsorbers, the NOx-reducing exhaust aftertreatment components located downstream of the passive NOx adsorber, in particular the SCR catalysts, have reached their operating temperature, specifically a temperature of at least 180°C, preferably at least 200°C. This prevents large quantities of unconverted nitrogen oxides from being emitted into the environment immediately after the cold start of the internal combustion engine.

[0025] In an advantageous embodiment of the method, the second passive NOx adsorber is conditioned when the combustion engine is switched off. This is achieved by introducing secondary air into the exhaust system upstream of the second passive NOx adsorber to create a carrier air flow. This airflow carries away the nitrogen oxides stored in the second passive NOx adsorber and converts them using the second NOx-reducing exhaust gas purification device, in particular the second SCR catalyst. The carried-away nitrogen oxides can then be converted on the second SCR catalyst, so that the second passive NOx adsorber is largely empty when the combustion engine is restarted and has a high storage capacity for nitrogen oxides. This ensures that sufficient storage capacity for nitrogen oxides is available when the combustion engine is restarted.

[0026] Further preferred embodiments of the invention result from the other features mentioned in the dependent claims.

[0027] Unless otherwise specified in individual cases, the various embodiments of the invention mentioned in this application can be advantageously combined with one another.

[0028] The invention is explained below in exemplary embodiments with reference to the accompanying drawings. Identical components or components with the same function are identified by the same reference numerals. The drawings show: Fig. 1 an internal combustion engine with an air supply system and an exhaust system with an exhaust aftertreatment system according to the invention; Fig. 2 a preferred embodiment of a NOx storage unit with a passive NOx adsorber; and Fig. 3 Another preferred embodiment of a NOx storage unit with a passive NOx adsorber for arrangement in an underbody position of a motor vehicle.

[0029] Fig. Figure 1 shows a schematic representation of an internal combustion engine 10. The internal combustion engine 10 is designed as a direct-injection diesel engine. The internal combustion engine 10 has several combustion chambers 12. A fuel injector 14 is arranged at each combustion chamber 12 for injecting fuel into the respective combustion chamber 12. The internal combustion engine 10 is connected to an air supply system 20 via its inlet 16 and to an exhaust system 40 via its outlet 18. The internal combustion engine 10 also includes a high-pressure exhaust gas recirculation system 30 with an exhaust gas recirculation line 36 and a high-pressure exhaust gas recirculation valve 38, through which exhaust gas from the internal combustion engine 10 can be recirculated from the outlet 18 to the inlet 16.Inlet valves and outlet valves are arranged on the combustion chambers 12, with which a fluidic connection from the air supply system 20 to the combustion chambers 12 or from the combustion chambers 12 to the exhaust system 40 can be opened or closed.

[0030] The air supply system 20 comprises an intake duct 28 in which, in the direction of fresh air flow through the intake duct 28, an air filter 22, an air mass meter 24 (in particular a hot-film air mass meter), a compressor 26 of an exhaust gas turbocharger 44, and a charge air cooler 32 are arranged. The air mass meter 24 can also be arranged in a filter housing of the air filter 22, so that the air filter 22 and the air mass meter 24 form a single assembly. An inlet 34 is provided downstream of the air filter 22 and upstream of the compressor 26, at which an exhaust gas recirculation line 86 of a low-pressure exhaust gas recirculation system 80 opens into the intake duct 28.

[0031] The exhaust system 40 comprises an exhaust duct 42 in which, in the direction of flow of exhaust gas from the internal combustion engine 10 through the exhaust duct 42, a turbine 46 of the exhaust gas turbocharger 44 is arranged, which drives the compressor 26 in the air supply system 20 via a shaft. The exhaust gas turbocharger 36 is preferably designed as an exhaust gas turbocharger 36 with variable turbine geometry. For this purpose, adjustable guide vanes are arranged upstream of a turbine wheel of the turbine 46, by means of which the flow of exhaust gas onto the blades of the turbine 46 can be varied. Several exhaust gas aftertreatment components 48, 50, 52, 54, 56, 58, 64, 66, 70, 72 are provided downstream of the turbine 46.In this system, a first passive NOx adsorber 48 is arranged immediately downstream of the turbine 46 as the first component of the exhaust aftertreatment system. Downstream of this adsorber, in the direction of flow of an exhaust gas stream from the combustion engine 10 through the exhaust system 40, a first electric heating element 74, in particular an electrically heated catalyst, is connected. Downstream of the first passive NOx adsorber 48, a first catalyst 50, preferably an oxidation catalyst 52 or a NOx storage catalyst 54, is arranged, which comprises an oxidation catalyst 52. Downstream of the first catalyst 50, a particulate filter 56 with a coating 58 for the selective catalytic reduction of nitrogen oxides is arranged, forming a first NOx-reducing exhaust gas purification device. An SCR disc 114 can be arranged upstream of the particulate filter 56 on the inlet side.Downstream of the particulate filter 56, a branch 60 is provided, at which an exhaust gas recirculation line 86 of a low-pressure exhaust gas recirculation system 80 branches off from the exhaust duct 42. Downstream of the branch, an exhaust gas backpressure valve 62 is provided, with which the cross-section of the exhaust duct 42 can be at least partially blocked in order to increase the exhaust backpressure in the exhaust duct 42. Downstream of the exhaust gas backpressure valve 62, a NOx storage unit 64 is arranged, which comprises a second passive NOx adsorber 66. The NOx storage unit 64 has a second electric heating element 94, which is preferably arranged downstream of the second passive NOx adsorber 66. Downstream of the NOx storage unit 64, a further SCR catalyst 70 (second NOx-reducing exhaust gas purification device) is arranged in the exhaust system 40, to which an ammonia barrier catalyst 72 is connected.

[0032] The low-pressure exhaust gas recirculation system 80 comprises, in addition to the exhaust gas recirculation line 86, a low-pressure exhaust gas recirculation cooler 82 and an exhaust gas recirculation valve 84, by which the exhaust gas recirculation through the exhaust gas recirculation line 86 can be controlled. A temperature sensor 98 is provided on the exhaust gas recirculation line 86 of the low-pressure exhaust gas recirculation system 80, by which an exhaust gas temperature in the low-pressure exhaust gas recirculation system 80 can be determined in order to activate the low-pressure exhaust gas recirculation system 80 as soon as the exhaust gas temperature in the low-pressure exhaust gas recirculation system 80 has exceeded a defined threshold value. This prevents water vapor or reducing agent contained in the exhaust gas for the selective catalytic reduction of nitrogen oxides, in particular liquid urea solution, from condensing and causing damage or deposits in the low-pressure exhaust gas recirculation system 80 or in the air supply system 20.

[0033] In the exhaust system 40, a first metering element 76 is arranged downstream of the first catalyst 50 and upstream of the particulate filter 56 with the SCR coating 58. This metering element allows a reducing agent, preferably liquid, in particular an aqueous urea solution, to be metered into the exhaust channel 42. A second metering element 78 is arranged downstream of the NOx storage unit 64 and upstream of the second SCR catalyst 70. This element also allows the reducing agent to be metered into the exhaust system 40. Downstream of the exhaust flap 62 and upstream of the NOx storage unit 64, an inlet 92 for a secondary air supply 90 is provided on the exhaust channel 42. This inlet allows a carrier air flow to be generated independently of the exhaust flow when the internal combustion engine 10 is stationary.

[0034] Furthermore, a temperature sensor 98 is provided in the exhaust gas channel 42, preferably on one of the exhaust aftertreatment components 48, 50, 52, 54, 56, 58, 64, 66, 70, 72, in particular on one of the passive NOx adsorbers 48, 66, on the particulate filter 56 or on the SCR catalyst 70, with which an exhaust gas temperature in the exhaust system 40 can be monitored in order to enable effective and efficient exhaust aftertreatment of the exhaust gas of the combustion engine 10. Differential pressure sensors 110 are also provided to determine a pressure difference across the particulate filter 56. In this way, the loading state of the particulate filter 56 can be determined and regeneration of the particulate filter 56 can be initiated if a defined loading level is exceeded. At least one sensor 112 for determining the nitrogen oxide concentration is also arranged in the exhaust gas channel 42.

[0035] The internal combustion engine 10 is connected to an engine control unit 100, which is connected via signal lines not shown to the temperature, pressure and nitrogen oxide sensors 98, 110, 112 as well as to the fuel injectors 14 of the internal combustion engine 10 and the control devices of the air supply system 20 and the exhaust system 40.

[0036] In Fig. Figure 2 shows a preferred embodiment of a NOx storage unit 64. The NOx storage unit 64 has a second passive NOx adsorber 66 and at least one electric heating element 94, 96. The electric heating element 94, 96 is preferably arranged downstream of the second passive NOx adsorber 66 in order to heat the exhaust gas flow of the combustion engine 10 before it enters the second SCR catalyst 70 and to bring the second SCR catalyst 70 to its operating temperature essentially independently of the exhaust gas flow. Alternatively or additionally, a further electric heating element 96 can be provided upstream of the second passive NOx adsorber 66 in order to initiate regeneration of the passive NOx adsorber 66.

[0037] After the combustion engine 10 is started, the raw nitrogen oxide emissions are initially temporarily stored in the first passive NOx adsorber 48 located near the engine. The downstream particulate filter 56 with the SCR coating 58 is heated by the first electric heating element 74. Once the SCR coating 58 has reached its operating temperature, reducing agent is metered into the exhaust system 40 upstream of the particulate filter 56 by the first metering element 76. The particulate filter 56 with the SCR coating 58 is typically heated more quickly than the first passive NOx adsorber 48 reaches its desorption temperature. If this is not the case due to dynamic driving, the nitrogen oxides not converted by the particulate filter 56 are temporarily stored in the second passive NOx adsorber 66.

[0038] Due to the high thermal inertia of the exhaust system 40 located upstream of the second passive NOx adsorber 66, which is located further from the engine, the exhaust aftertreatment components 64, 66, 70, 72 in the underbody position heat up significantly more slowly than the exhaust aftertreatment components 48, 50, 56, 58 located closer to the engine. The second electric heating element 94 upstream of the second SCR catalyst 70 is operated in parallel with the first electric heating element 74 located closer to the engine. Alternatively, the second electric heating element 94 can also be activated only after a predetermined temperature threshold for the particulate filter 56 has been reached. Once the second SCR catalyst 70 has also reached its operating temperature, the second metering element 78 is also activated. The reducing agent is then metered either by the first metering element 76 and / or the second metering element 78.The desorption of the nitrogen oxides stored in the second passive NOx adsorber 66, located further from the engine, occurs either due to increasing exhaust gas temperatures during the driving cycle or, preferably, by switching on the second electric heating element 94 upstream of the second passive NOx adsorber 66. For maximum protection during short-distance operation of the vehicle, it is advantageous to connect a secondary air supply 90 upstream of the NOx storage unit 64. This allows desorption of the nitrogen oxides stored in the second passive NOx adsorber 66, located further from the engine, and conversion by the second SCR catalyst 70 to take place even when the combustion engine 10 is switched off. Thus, the second passive NOx adsorber 66 can be preconditioned in such a way that, at least upon each restart of the combustion engine 10, the second passive NOx adsorber 66, located further from the engine, has a defined loading state.

[0039] In Fig.Figure 3 shows a further embodiment of a NOx storage unit 64 with a second passive NOx adsorber 66. In this embodiment, the storage unit 64 is designed as a ring catalyst 102 and has a central main channel 106 and a bypass 108 coaxially surrounding the central main channel 106, in which a second passive NOx adsorber 66, designed as a ring, is arranged. The NOx storage unit 64 also has an actuating element 104 in the form of a control flap 104, with which the exhaust gas flow of the combustion engine 10 can be selectively directed either through the main channel 106 and / or through the bypass 108.

[0040] When the combustion engine 10 is started, the control valve 104 is initially closed and the exhaust gas flow is directed over the second passive NOx adsorber 66, which is designed as a ring. As soon as the engine-mounted exhaust aftertreatment components 48, 56, 58 have reached their operating temperature, the control valve 104 is opened and the second electric heating element 94 at the inlet of the NOx storage unit 64 is activated. The heated exhaust gas flows predominantly through the main channel 106 and acts on the second SCR catalyst 70. As soon as the second SCR catalyst 70 has reached its operating temperature, the control valve 104 is closed again and, with the second electric heating element 94 activated, the exhaust gas flow is directed through the bypass to achieve thermal desorption of the nitrogen oxides stored in the second passive NOx adsorber 66.These nitrogen oxides are converted into molecular nitrogen and water vapor by means of selective catalytic reduction by the second SCR catalyst. After the second passive NOx adsorber 66 is emptied, the second electric heating element 94 is deactivated and the control flap 104 is reopened, so that the exhaust gas flow from the combustion engine 10 is again directed through the main channel 106. Reference symbol list 10 Internal combustion engine 12 Combustion chamber 14 Fuel injector 16 Admission 18 Outlet 20 Air supply system 22 air filters 24 air mass meters 26 compressors 28 Intake manifold 30 High-pressure exhaust gas recirculation 32 Intercoolers 34 Junction 36 Exhaust gas recirculation line 38 High-pressure exhaust gas recirculation valve 40 Exhaust system 42 Exhaust duct 44 exhaust gas turbochargers 46 Turbine 48 first passive NOx adsorber 50 first catalyst 52 Oxidation catalyst 54 NOx storage catalyst 56 particulate filters 58 SCR coating 60 branching 62 Exhaust gas flap 64 NOx storage unit 66 second passive NOx adsorber 68 Heating control 70 SCR catalyst 72 Ammonia barrier catalyst 74 first electric heating element 76 first dosing element 78 second dosing element 80 Low-pressure exhaust gas recirculation 82 Low-pressure exhaust gas recirculation coolers 84 Exhaust gas recirculation valve 86 Exhaust gas recirculation line 88 filters 90 Secondary air supply 92 Induction point 94 second electric heating element 96 third electric heating element 98 Temperature sensor 100 control unit 102 Ring catalyst 104 Actuator 106 Main Channel 108 Bypass 110 Pressure sensor 112 NOx sensor 114 SCR disc

Claims

[1] Exhaust aftertreatment system for an internal combustion engine (10), comprising an exhaust system (40) with an exhaust duct (42) in which a first passive NOx adsorber (48) located close to the engine, a first NOx-reducing exhaust gas purification device (56, 58) is arranged downstream of the first passive NOx adsorber (48), a NOx storage unit (64) with a second passive NOx adsorber (66) is arranged downstream of the first NOx-reducing exhaust gas purification device (56, 58), and a second NOx-reducing exhaust gas purification device (70) is arranged downstream of the NOx storage unit (64), wherein a first electrical heating element (74) is connected downstream of the first passive NOx adsorber (48) and wherein at least one further upstream or downstream electrical heating element (94, 96) is associated with the NOx storage unit (64), characterized by, that the NOx storage unit (64) has a main channel (106) and a bypass (108), wherein the second passive NOx adsorber (66) is arranged in the bypass (108). [2] Exhaust aftertreatment system according to claim 1, characterized by , that the bypass (108) is designed as a ring catalyst (102) which surrounds the main channel (106). [3] Exhaust aftertreatment system according to claim 1 or 2, wherein an actuating element (104) is arranged on the NOx storage unit (64) with which the exhaust gas flow can be directed either through the main channel (106) and / or the bypass (108). [4] Exhaust aftertreatment system according to any one of claims 1 to 3, characterized by , that the further electrical heating element (94, 96) is arranged downstream of the second passive NOx adsorber (66). [5] Exhaust aftertreatment system according to any one of claims 1 to 4, characterized by, that the further electrical heating element (94, 96) is arranged upstream of the second passive NOx adsorber (66). [6] Exhaust aftertreatment system according to any one of claims 1 to 5, characterized by , that the first electric heating element (74) and at least one further electric heating element (94, 96) can be controlled via a common heating control (68). [7] Exhaust aftertreatment system according to any one of claims 1 to 6, characterized by , that an inlet point (92) for a secondary air supply (90) is formed on the exhaust duct (42) immediately upstream of the NOx storage unit (64). [8] Method for exhaust aftertreatment of an internal combustion engine (10) with an exhaust aftertreatment system according to any one of claims 1 to 7, comprising the following method steps: - Starting the combustion engine (10), wherein the nitrogen oxide emissions in the exhaust stream of the combustion engine (10) are stored in the first passive NOx adsorber (48) located near the engine, - Heating the exhaust gas stream downstream of the first passive NOx adsorber (48) by the first electric heating element (74), - Heating the first NOx-reducing exhaust gas purification device (56, 58) to an operating temperature, wherein the nitrogen oxides are stored in the second passive NOx adsorber (66) during heating, - Regenerating the first passive NOx adsorber (48) when the first NOx-reducing exhaust gas purification device (56, 58) has reached its operating temperature and the nitrogen oxides are converted by the first NOx-reducing exhaust gas purification device (56, 58). [9] Method according to claim 8, characterized by, that the second passive NOx adsorber (48) is conditioned when the combustion engine (10) is switched off by injecting secondary air into the exhaust system (40) upstream of the second passive NOx adsorber (48) to form a carrier air flow and to carry away the nitrogen oxides stored in the second passive NOx adsorber (66) and convert them with the second NOx-reducing exhaust gas purification device (70).

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