Exhaust gas aftertreatment system and method for exhaust gas aftertreatment of an internal combustion engine

Abstract

An exhaust gas aftertreatment system for an internal combustion engine (10), comprising an exhaust system (20) in which a first oxidation catalyst (28) is arranged in the flow direction of an exhaust gas flow of the internal combustion engine (10), and a particulate filter (36) and an exhaust gas aftertreatment component (38, 40) for reducing nitrogen oxide emissions are arranged downstream of the first oxidation catalyst (28), wherein the first oxidation catalyst (28) has a catalytic coating (64) for carrying out continuous soot oxidation, wherein the first oxidation catalyst (28) is heatable by means of an electrically heatable catalyst (66) in order to be brought to and maintained at a temperature level at which continuous oxidation of the soot particles takes place by means of a CRT process on the catalytically active coating (64), characterized in thatthat the electrically heatable catalyst (66) is arranged immediately downstream of the first oxidation catalyst (28) with the catalytically active coating (64).

Description

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

[0002] Current emissions legislation, which will become increasingly stringent in the future, places high demands on raw engine emissions and exhaust aftertreatment in internal combustion engines. The demands for further reduced fuel consumption and the further 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 usual way via a three-way catalytic converter and additional catalysts upstream and downstream of the three-way catalytic converter. Diesel engines currently use exhaust aftertreatment systems that include an oxidation catalyst or 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 possibly additional catalysts.To meet the stringent requirements for minimal nitrogen oxide emissions, exhaust gas aftertreatment systems are known that feature two SCR catalysts connected in series, with a metering element for metering a reducing agent upstream of each SCR catalyst. The reducing agent used is preferably a synthetic, aqueous urea solution, which is mixed with the hot exhaust stream in a mixing device upstream of the SCR catalyst. This mixing heats the aqueous urea solution, releasing ammonia into the exhaust duct. A commercially available aqueous urea solution generally consists of 32.5% urea and 67.5% water.

[0003] EP 3 192 991 B1 discloses an exhaust aftertreatment system for a diesel engine, which comprises an electrically heatable oxidation catalyst and a diesel particulate filter arranged downstream of the electrically heatable oxidation catalyst, as well as an SCR catalyst arranged downstream of the diesel particulate filter. The diesel oxidation catalyst is heated by the electrical heating element to a temperature of 320°C to 380°C in order to establish an ideal NO to NO2 ratio for the selective catalytic reduction of nitrogen oxides.

[0004] DE 10 2011 109 761 A1 discloses a method for purifying exhaust gases from an internal combustion engine with an exhaust aftertreatment system comprising an electrically heatable catalyst, an oxidation catalyst, a particulate filter, a DeNOx catalyst, and a heat exchanger for heat recovery. During a cold start of the internal combustion engine, the exhaust gas is passed through the exhaust aftertreatment components, where it undergoes an exothermic reaction. This heat is utilized via the heat exchanger.

[0005] The invention is based on the object of reducing the soot loading of a particle filter in the exhaust system of an internal combustion engine in order to extend the regeneration intervals of the particle filter and thus minimize the exhaust back pressure and the fuel consumption of the internal combustion engine.

[0006] According to the invention, this object is achieved by an exhaust gas aftertreatment system for an internal combustion engine, comprising an exhaust system in which an oxidation catalyst is arranged in the flow direction of an exhaust gas flow of the internal combustion engine through the exhaust system, and downstream of the oxidation catalyst, a particulate filter and an exhaust gas aftertreatment component for reducing nitrogen oxide emissions, in particular an SCR catalyst, are arranged. According to the invention, the oxidation catalyst has a coating for carrying out continuous soot oxidation, wherein the oxidation catalyst can be heated by means of an electrically heatable catalyst in order to be brought to and maintained at a temperature level at which continuous oxidation of the soot particles takes place by means of a continuous regeneration trap process (CRT process) on the catalytically active coating.

[0007] The features listed in the dependent claims enable advantageous improvements and non-trivial further developments of the exhaust gas aftertreatment system for an internal combustion engine mentioned in the independent claim.

[0008] In a preferred embodiment of the invention, the particulate filter comprises a catalytically active coating for the selective catalytic reduction of nitrogen oxides (SCR coating) or a catalytically active coating for implementing a CRT process. This can provide additional catalyst capacity. A particulate filter with a catalytically active coating for the continuous oxidation of soot particles can slow the loading of the particulate filter, since a portion of the soot particles is always broken down again by oxidation. An SCR coating can also reduce the nitrogen oxide emissions of the combustion engine.The particulate filter with the SCR coating is preferably supported by a further SCR catalyst, which is arranged downstream of the particulate filter, in order to expand the characteristic map range of the internal combustion engine, in which at least one of the SCR catalysts is operated in a temperature range required for the selective catalytic reduction of nitrogen oxides.

[0009] In an embodiment of the exhaust gas aftertreatment system not forming part of the invention, the electrically heatable catalyst is arranged directly upstream of the oxidation catalyst with the catalytically active coating. "Directly upstream" refers to an arrangement in which no further exhaust gas aftertreatment components are arranged between the electrically heatable catalyst and the oxidation catalyst. Preferably, the electrically heatable catalyst is spaced from the oxidation catalyst at a distance of less than 5 cm to enable efficient heat transfer from the electrically heatable catalyst to the oxidation catalyst by means of thermal conduction, thermal radiation, and convection.

[0010] According to the invention, the electrically heatable catalyst is arranged directly downstream of the oxidation catalyst with the catalytically active coating. The electrically heatable catalyst is preferably arranged at a distance of less than 5 cm downstream of the oxidation catalyst, so that the heat from the electrically heatable catalyst is transferred to the oxidation catalyst via thermal conduction and thermal radiation.

[0011] In a further advantageous embodiment of the invention, the electrically heatable catalyst is arranged downstream of a first oxidation catalyst and upstream of a second oxidation catalyst. This allows the thermal radiation of the electrically heatable catalyst to heat the first oxidation catalyst, and heat conduction, thermal radiation, and convection to heat the second oxidation catalyst. The catalyst volume can be divided between the two oxidation catalysts, allowing the respective oxidation catalysts to be smaller and more compact than a single oxidation catalyst with the same performance, thus allowing them to warm up to operating temperature more quickly.

[0012] In a preferred embodiment of the exhaust gas aftertreatment system, the electrically heatable catalyst is integrated into the oxidation catalyst or arranged in a common housing with the oxidation catalyst. This allows for particularly simple installation of the exhaust gas aftertreatment system, as the number of components to be installed in the exhaust system can be reduced.

[0013] According to the invention, a method for exhaust gas aftertreatment of an internal combustion engine with such an exhaust gas aftertreatment system is proposed, wherein the oxidation catalyst is heated by the electrically heatable catalyst to a temperature of at least 350°C, preferably at least 400°C, to enable continuous oxidation of the soot particles in the exhaust stream of the internal combustion engine using a CRT process. This ensures that the soot particles contained in the exhaust stream of the internal combustion engine oxidize with the nitrogen oxides contained in the exhaust stream of the internal combustion engine upon impact with a surface of the oxidation catalyst.

[0014] An improvement to the process provides for the electrically heated catalyst to be activated in engine map regions with increased raw particulate emissions. Continuous oxidation of the soot particles creates a second exhaust aftertreatment component in addition to the particulate filter to reduce soot particles. Thus, even higher raw particulate emissions can be reduced by exhaust aftertreatment, so that an increase in raw particulate emissions does not lead to an unacceptable increase in tailpipe emissions or an unacceptably high loading of the particulate filter.

[0015] A further improvement to the process provides for the electrically heated catalyst to be activated when conversion of nitrogen oxide emissions in the exhaust stream of the combustion engine is limited or impossible. In principle, the design of a diesel combustion process involves a trade-off between low nitrogen oxide emissions and low particulate emissions (NOx-particulate trade-off). If efficient exhaust aftertreatment of nitrogen oxide emissions cannot be guaranteed, the combustion process can be designed with a view to low raw NOx emissions and increased raw particulate emissions. These increased raw particulate emissions can be reduced through the continuous oxidation of soot on the catalytically active surface of the oxidation catalyst, thereby achieving minimal tailpipe emissions of nitrogen oxides and particulates.

[0016] It is particularly preferred if the combustion process, essentially consisting of the injection quantity and / or the injection time of the fuel into the combustion chambers of the internal combustion engine, boost pressure and temperature and the proportion of recirculated exhaust gas, is selected such that the raw nitrogen oxide emissions are reduced and at the same time the raw particulate emissions are increased.

[0017] The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless otherwise stated in the individual case.

[0018] The invention is explained below in exemplary embodiments with reference to the accompanying drawings. Identical components or components with the same function are provided with the same reference numerals in the different figures. They show: Fig. 1 shows a first embodiment of an exhaust aftertreatment system of an internal combustion engine for the continuous oxidation of soot particles; Fig. 2 a preferred embodiment of an internal combustion engine with an exhaust aftertreatment system for the continuous oxidation of soot particles by means of a CRT process. Fig. 3 a preferred catalyst arrangement of an exhaust gas aftertreatment system according to the invention for the continuous oxidation of soot particles; Fig. 4 shows a further catalyst arrangement of an exhaust gas aftertreatment system according to the invention for the continuous oxidation of soot particles; Fig. 5 shows a further catalyst arrangement of an exhaust gas aftertreatment system according to the invention for the continuous oxidation of soot particles;

[0019] Fig. 1 shows an internal combustion engine 10, which is connected by its outlet 16 to an exhaust system 20. The internal combustion engine 10 has a plurality of combustion chambers 12, each of which has a fuel injector 14 arranged thereon for injecting a combustible fuel into the respective combustion chambers 12 of the internal combustion engine 10. The outlet 16 of the internal combustion engine 10 includes an exhaust manifold 18, in which the exhaust gases from the combustion chambers 12 of the internal combustion engine 10 are collected and fed to a common exhaust duct 22 of the exhaust system 20.

[0020] In the exhaust system 20, an electrically heatable catalyst 66 is arranged downstream of a turbine 26 of an exhaust gas turbocharger 24 in the flow direction of an exhaust gas flow from the internal combustion engine 10. A first oxidation catalyst 28 is arranged downstream of the electrically heatable catalyst 66, and a particulate filter 36 is arranged downstream of the first oxidation catalyst 28. The particulate filter 36 preferably has a catalytically active coating 38, in particular a coating 38 for the selective catalytic reduction of nitrogen oxides (SCR coating) or a coating for the catalytic oxidation of soot particles. Alternatively, the particulate filter 36 can also be uncoated. The electrically heatable catalyst 66 comprises an electrical heating element 68, in particular an electrical heating disk.Downstream of the first oxidation catalyst 28 and upstream of the particulate filter 36, a first metering valve 58 is arranged for introducing a reducing agent 52 into the exhaust duct 22. A first exhaust mixer 34 is arranged downstream of the first metering valve 58 to enable homogeneous mixing of the reducing agent 52 with the exhaust gas flow of the internal combustion engine 10.

[0021] Downstream of the particulate filter 36, a branch 46 is formed, at which an exhaust gas recirculation channel of a low-pressure exhaust gas recirculation system 48 branches off from the exhaust channel 22 of the exhaust system 20. Downstream of the branch 46, an exhaust flap 44 is provided, with which the amount of exhaust gas recirculated via the low-pressure exhaust gas recirculation system 48 can be controlled. The first metering valve 58 is connected via a first reducing agent line 54 to a reducing agent tank 50, in which the reducing agent 52, in particular an aqueous urea solution, is stored.

[0022] A control unit 70 is assigned to the internal combustion engine 10, via which the fuel injection by the injectors 14 into the combustion chambers 12 of the internal combustion engine 10 is controlled. Furthermore, the control unit 70 controls the metering of the reducing agent 52 through the first metering valve 58 and the heating output of the electrically heatable catalyst 66.

[0023] During operation of the internal combustion engine 10, the electrically heatable catalyst 66 is controlled by the control unit 70 and heated by the electrical heating element 68. The electrical heating element 68 is controlled such that the oxidation catalyst 28 heats up to a temperature of at least 350°C, preferably at least 400°C, particularly preferably at least 450°C, so that the soot particles can be oxidized with the nitrogen oxides present in the exhaust gas stream using a CRT process. This makes it possible to reduce the loading of the particulate filter 36, thereby extending the period between two necessary regenerations of the particulate filter 36. The lower loading of the particulate filter 36 reduces the exhaust backpressure, thereby reducing the consumption of the internal combustion engine 10.

[0024] In Fig. Figure 2 shows a further embodiment of an internal combustion engine 10, which is connected by its outlet 16 to an exhaust system 20. The internal combustion engine 10 has a plurality of combustion chambers 12, each of which has a fuel injector 14 arranged thereon for injecting a combustible fuel into the respective combustion chamber 12 of the internal combustion engine 10. The outlet 16 of the internal combustion engine 10 includes an exhaust manifold 18, in which the exhaust gases from the combustion chambers 12 of the internal combustion engine 10 are collected and fed to a common exhaust duct 22 of the exhaust system 20.

[0025] In the exhaust system 20, an electrically heatable catalyst 66 is arranged downstream of a turbine 26 of an exhaust gas turbocharger 24 in the flow direction of an exhaust gas flow of the internal combustion engine 10. Downstream of the electrically heatable catalyst 66 is a first catalyst, preferably a first oxidation catalyst 28 or a NOx storage catalyst 30. Downstream of the first catalyst 28, 30 is a particulate filter 36. The particulate filter 36 preferably has a catalytic coating 38, in particular a coating for the selective catalytic reduction of nitrogen oxides (SCR coating). Alternatively, the particulate filter 36 can also be coated with a coating 38 for reducing the oxidation temperature of soot particles. Alternatively, the particulate filter 36 can also be uncoated. The electrically heatable catalyst 66 comprises an electrical heating element 68, in particular an electrical heating disk.Downstream of the first catalyst 28, 30 and upstream of the particulate filter 36, a first metering valve 58 is arranged for introducing a reducing agent 52 into the exhaust duct 22. A first exhaust mixer 34 is arranged downstream of the first metering valve 58 to enable homogeneous mixing of the reducing agent 52 with the exhaust gas flow of the internal combustion engine 10.

[0026] Downstream of the particulate filter 36, a branch 46 is formed, at which a non-illustrated exhaust gas recirculation channel of a low-pressure exhaust gas recirculation system 48 branches off from the exhaust duct 22 of the exhaust system 20. Downstream of the branch 46, a further SCR catalyst 40 is arranged. An ammonia blocking catalyst 42 is preferably arranged downstream of this catalyst to prevent uncontrolled escape of ammonia and the associated odor nuisance. Downstream of the branch 46 and upstream of the further SCR catalyst 40, a second metering valve 60 is arranged for introducing a reducing agent 52 into the exhaust duct 22 of the internal combustion engine 10. A further exhaust gas mixer 62 is connected downstream of the second metering valve 60 in order to improve the mixing of the exhaust gas flow with the reducing agent 52 before entering the SCR catalyst 40.The first metering valve 58 and the second metering valve 60 are each connected via a reducing agent line 54, 56 to a reducing agent container 50 in which the reducing agent 52, in particular aqueous urea solution, is stored.

[0027] A control unit 70 is assigned to the internal combustion engine 10, via which the fuel injection by the injectors 14 into the combustion chambers 12 of the internal combustion engine 10 is controlled. Furthermore, the control unit 70 controls the metering of the reducing agent 52 through the two metering valves 58, 60 and the heating output of the electrically heatable catalyst 66.

[0028] In Fig. 3 shows a catalyst arrangement of an exhaust gas aftertreatment system not belonging to the invention. Arranged in the flow direction of an exhaust gas flow from the internal combustion engine 10 through the exhaust system 20 are a first oxidation catalyst 28 with a catalytically active coating 64, and downstream of the oxidation catalyst 28 is a particulate filter 36 with a catalytically active coating 38. Downstream of the oxidation catalyst 28 is a first metering valve 58 for metering a reducing agent 52 into the exhaust duct 22 of the exhaust system 20. An exhaust mixer 34 is connected downstream of the metering valve 58 to achieve better mixing of the reducing agent 52 with the exhaust gas flow from the internal combustion engine 10 before it enters the particulate filter 36.In this preferred catalyst arrangement, the waste heat of the electrically heatable catalyst 66 is transferred to the first oxidation catalyst 28 via heat conduction, heat radiation and convection, whereby the latter can be heated to its operating temperature particularly quickly and effectively.

[0029] In Fig. 4 shows a catalyst arrangement for an exhaust gas aftertreatment system according to the invention. With essentially the same structure as Fig. 3, the first oxidation catalyst 28 in this embodiment is divided into a first oxidation catalyst 28 and a second oxidation catalyst 32, wherein the electrically heatable catalyst 66 is arranged downstream of the first oxidation catalyst 28 and upstream of the second oxidation catalyst.

[0030] In Fig. 5 shows a further embodiment of a catalyst arrangement of an exhaust gas aftertreatment system according to the invention. With essentially the same structure as Fig. 3, in this exemplary embodiment, the electrically heatable catalyst 66 is arranged downstream of the first oxidation catalyst 28. The first oxidation catalyst 28 is heated by heat conduction and heat radiation from the electrically heatable catalyst 66. Convective heat transfer through the exhaust gas flow is eliminated, and the waste heat from the electrically heatable catalyst 66 can be used in this case to electrically heat the exhaust mixer 34 and the particulate filter 36. Convective heat transfer from the electrically heatable catalyst 66 to the exhaust mixer 34 can prevent deposits of the reducing agent 52 from forming on the exhaust mixer 34 and thus increasing the exhaust backpressure. List of reference symbols 10 Combustion engine 12 combustion chamber 14 Fuel injector 16 Outlet 18 exhaust manifold 20 Exhaust system 22 exhaust duct 24 exhaust gas turbochargers 26 turbines 28 first oxidation catalyst 30 NOx storage catalyst 32 second oxidation catalyst 34 first exhaust gas mixer 36 particle filters 38 catalytically active coating (SCR coating) 40 SCR catalyst 42 Ammonia slip catalyst 44 Exhaust flap 46 Branching 48 Low-pressure exhaust gas recirculation 50 reducing agent containers 52 reducing agents 54 first reducing agent line 56 second reducing agent line 58 first dosing valve 60 second dosing valve 62 second exhaust mixer 64 catalytic coating 66 electrically heated catalyst 68 electric heating element 70 Control unit

Claims

[1] Exhaust aftertreatment system for an internal combustion engine (10), comprising an exhaust system (20) in which a first oxidation catalyst (28) is arranged in the direction of flow of an exhaust gas stream of the internal combustion engine (10) and a particulate filter (36) and an exhaust aftertreatment component (38, 40) for reducing nitrogen oxide emissions are arranged downstream of the first oxidation catalyst (28), wherein the first oxidation catalyst (28) has a catalytic coating (64) for carrying out continuous soot oxidation, wherein the first oxidation catalyst (28) can be heated by means of an electrically heated catalyst (66) in order to be brought to and kept at a temperature level at which continuous oxidation of the soot particles takes place on the catalytically active coating (64) by means of a CRT process, characterized by, that the electrically heated catalyst (66) is arranged immediately downstream of the first oxidation catalyst (28) with the catalytically active coating (64). [2] Exhaust aftertreatment system according to claim 1, characterized by , that the particle filter (36) has a catalytically effective coating (38) for the selective catalytic reduction of nitrogen oxides or a catalytically effective coating (38) for carrying out a CRT process. [3] Exhaust aftertreatment system according to claim 1 or 2, characterized by , that the electrically heated catalyst (66) is arranged downstream of a first oxidation catalyst (28) and upstream of a second oxidation catalyst (32). [4] Exhaust aftertreatment system according to any one of claims 1 to 3, characterized by , that the electrically heated catalyst (66) is integrated into the first oxidation catalyst (28). [5] Method for exhaust aftertreatment of an internal combustion engine (10) with an exhaust aftertreatment system according to one of claims 1 to 4, wherein the oxidation catalyst (28) is heated by the electrically heated catalyst (66) to a temperature of at least 350°C in order to enable continuous oxidation of the soot particles in the exhaust stream of the internal combustion engine (10) by means of a CRT process. [6] Method for exhaust aftertreatment of an internal combustion engine (10) according to claim 5, characterized by , that the electrically heated catalyst (66) is activated in operating ranges of the combustion engine (10) with increased raw particulate emissions. [7] Method for exhaust aftertreatment of an internal combustion engine (10) according to one of claims 5 or 6, characterized by, that the electrically heated catalyst (66) is activated when conversion of nitrogen oxide emissions in the exhaust stream of the internal combustion engine (10) is limited or impossible. [8] Method for exhaust aftertreatment of an internal combustion engine (10) according to claim 7, characterized by , that the combustion process of the internal combustion engine (10) is selected such that raw NOx emissions are reduced and raw particulate emissions are increased at the same time.

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