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

Abstract

A method for the exhaust aftertreatment of an internal combustion engine (10) which is connected at its outlet (18) to an exhaust system (40), wherein an SCR catalyst or particulate filter (48) with a coating (50) for the selective catalytic reduction of nitrogen oxides is arranged in the exhaust system (40) downstream of a turbine (44) of an exhaust gas turbocharger (36), and wherein a first NOx sensor (62) is arranged upstream of the SCR catalyst or particulate filter (48), with a metering element (56) by which a reducing agent (78) can be metered into the exhaust channel (42) upstream of the SCR catalyst or particulate filter (48), and wherein a second NOx sensor (64) is arranged downstream of the SCR catalyst or particulate filter (48), and the method comprises the following steps: - Dosing the reducing agent (78) into the exhaust channel (42) of the internal combustion engine (10); - Determining an initial ammonia concentration (N1); - Measuring an initial NOx concentration (NOx_1) using the first NOx sensor (62); - Measuring a second NOx concentration (NOx_2) using the second NOx sensor (64); - Determining the second ammonia concentration (N2) based on the first NOx concentration (NOx_1) and based on the second NOx concentration (NOx_2); - Determining an aging factor C, wherein the aging factor is proportional to the quotient (N2 / N1) of the second ammonia concentration (N2) and the first ammonia concentration (N1), wherein - the dosage of the reducing agent (78) is adjusted based on the calculated aging factor C.

Description

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

[0002] Current and increasingly stringent emissions legislation 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 catalyst, as well as additional catalysts upstream and downstream of the three-way catalyst. Diesel engines currently employ 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. Ammonia is the preferred reducing agent.Because handling pure ammonia is complex, vehicles typically use 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] Exhaust gas aftertreatment systems are known from the prior art, which comprise a first SCR catalyst located close to the engine, in particular a particulate filter with an SCR coating, and a second SCR catalyst located in the underbody of the motor vehicle, away from the engine. The close-to-engine particulate filter with the SCR coating can be heated to its operating temperature more quickly after a cold start of the combustion engine and can thus be used promptly after a cold start of the combustion engine for efficient conversion of nitrogen oxide emissions.

[0004] The second SCR catalyst, located remote from the engine, is used at high engine loads and / or during regeneration of the particulate filter, where the close-coupled particulate filter with the SCR coating operates above the temperature range required for selective catalytic reduction and thus only ensures insufficient conversion of the nitrogen oxides. The close-coupled arrangement of the particulate filter with the SCR coating also results in only a short mixing path for mixing the reducing agent and exhaust gas before entering the particulate filter, thus limiting even distribution across the cross-section of the particulate filter. This effect is amplified at high exhaust gas mass flows and the associated high flow velocities in the exhaust duct.This can lead to the fact that, regardless of the temperature, the conversion performance of the particulate filter is insufficient and the conversion performance of the second SCR catalyst is also necessary.

[0005] Furthermore, it is known that the conversion efficiency of the two SCR catalysts can be increased if a defined amount of ammonia is stored, in particular adsorbed, on the catalytically active surface of the respective SCR catalyst. Depending on the current operating state of the combustion engine, reducing agent is metered into the exhaust duct upstream of the coated particulate filter, upstream of the underbody SCR catalyst, or upstream of both SCR catalysts.

[0006] However, the disadvantage of the state-of-the-art solutions is that aging and the associated changes in the conversion performance and NH3 storage capacity of the SCR catalysts can only be inadequately simulated by modeling, resulting in ammonia slip. This leads to increased consumption of reducing agent and can cause an increase in ammonia emissions.

[0007] Furthermore, prior art methods for determining ammonia slip require calibration prior to determining the ammonia slip. To do this, the injected ammonia quantity is first increased to such an extent that the catalyst is completely filled or loaded with ammonia, after which the injected ammonia quantity is slowly reduced. Using this method, the resulting measurement curve can be used to determine the presence of ammonia slip and the aging of the catalyst. A disadvantage of this method, however, is that the calibration described above is complex, consumes additional reducing agent, and can lead to ammonia emissions.

[0008] DE 10 2010 026 317 A1 discloses an exhaust aftertreatment system for a diesel engine, in which an oxidation catalyst and an SCR catalyst are arranged in the exhaust system. A first NOx sensor is provided upstream of the oxidation catalyst and a second NOx sensor is provided downstream of the SCR catalyst to assess the efficiency of the SCR catalyst and thus prevent ammonia slip.

[0009] From DE 10 2010 026 373 A1 an exhaust gas aftertreatment system and a method for determining the ammonia slip are known, in which the nitrogen oxide concentration in the exhaust duct is determined upstream of the SCR catalyst and downstream of the SCR catalyst and the amount of reducing agent is adjusted in such a way that the nitrogen oxide emissions are minimized.

[0010] Furthermore, DE 10 2014 019 483 A1 discloses another method for determining ammonia slip in an exhaust aftertreatment system of an internal combustion engine with an SCR catalyst. The nitrogen oxide concentration in the exhaust duct is determined upstream and downstream of a particulate filter with an SCR coating, and the ammonia slip is calculated based on the nitrogen oxide concentration in conjunction with the metered amount of reducing agent.

[0011] DE 10 2011 086 625 A1 describes a method for operating an SCR catalyst. This method involves determining a correction value, which represents the aging state of the SCR catalyst, and metering a reducing agent solution into the SCR catalyst, with the metered amount being determined depending on the correction value. Calibration is required to determine the ammonia slip. A disadvantage of this method is that the calibration is complex and consumes additional reducing agent.

[0012] DE10 2016 219 555 A1 describes a method for operating an SCR catalyst. A control unit is designed to instruct the ammonia dosing system to inject ammonia into the exhaust system at different dosing rates in order to determine the optimal amount of ammonia. The disadvantage of this method is that such calibration with different dosing rates is complex and consumes additional reducing agent.

[0013] DE 10 2017 201 400 A1 discloses a method whereby calibration requires the catalyst to be completely filled or loaded with ammonia. The disadvantage of this method is that the calibration described above is complex and consumes reducing agent.

[0014] Furthermore, DE 10 2016 209 533 A1 discloses a method for determining an ageing state of an SCR catalyst arranged in an exhaust system of an internal combustion engine, wherein a propagation of electromagnetic waves within the catalyst is continuously measured, which correlates with a mass balance ΔmNH3 of the stored ammonia.

[0015] The invention is based on the object of converting nitrogen oxide emissions into nitrogen as efficiently as possible in an exhaust aftertreatment system with multiple SCR catalysts, while simultaneously minimizing the use of reducing agents to avoid ammonia emissions. In particular, the actual aging of the catalyst is to be determined at any given time. This allows ammonia slip to be determined even after replacing the control unit intended for monitoring the catalyst.

[0016] To achieve this object, a method for exhaust gas aftertreatment of an internal combustion engine is proposed, which is connected by its outlet to an exhaust system, wherein a particulate filter with a coating for the selective catalytic reduction of nitrogen oxides is arranged in the exhaust system downstream of a turbine of an exhaust gas turbocharger, and wherein a first NOx sensor is arranged upstream of the particulate filter, as well as a metering element with which a reducing agent can be metered into the exhaust duct upstream of the particulate filter, which comprises the following steps: - Dosing the reducing agent into the exhaust duct of the internal combustion engine, wherein a first NOx concentration NOx_1 is measured upstream of the particulate filter by means of the first NOx sensor in the exhaust duct; - Determining a first ammonia concentration N1; - determining a second ammonia concentration N2 based on the first NOx concentration NOx_1; - Determining an ageing factor C, wherein the ageing factor is proportional to the quotient of the second ammonia concentration N2 and the first ammonia concentration N1.

[0017] In the context of the present invention, a catalyst is provided that enables the selective catalytic reduction of nitrogen oxides to ammonia. This can, in particular, be either a "pure" SCR catalyst and / or an SCR-coated particulate filter. If a (coated) particulate filter is mentioned in the context of the present invention, it is to be assumed, unless expressly stated otherwise, that a "pure" SCR catalyst, i.e., an SCR catalyst without a particulate filter, can also be used additionally or alternatively.

[0018] In a further embodiment of the method, an aging factor for the SCR catalyst (or the SCR-coated particulate filter) is determined using the efficiency model. Modeling ammonia slip involves significant uncertainties due to the different aging stages of the exhaust aftertreatment components. However, modeling aging of the SCR coating is necessary to determine the SCR coating's ammonia storage capacity. By combining the measurement of the nitrogen oxide concentrations upstream and downstream of the particulate filter with the efficiency of the particulate filter's NOx conversion, the mass of nitrogen oxides converted by the particulate filter and the ammonia slip can be determined.The ammonia loading of the SCR coating of the particulate filter can be adjusted depending on the aging state of the coating to minimize ammonia slip and thus optimize the use of reducing agent. Above all, it also facilitates fill level control.

[0019] The SCR-coated particulate filter or SCR catalyst is preferably a close-coupled particulate filter. In this context, a close-coupled particulate filter is understood to be a particulate filter whose inlet, with an exhaust gas path length of no more than 80 cm, preferably no more than 50 cm, is spaced from the exhaust of the combustion engine. A method according to the invention makes it possible to determine the aging of the particulate filter / SCR catalyst at any time.

[0020] According to the invention, the dosage of the reducing agent is adjusted on the basis of the calculated aging factor C.

[0021] This makes it possible to reduce the consumption of reducing agent because the particulate filter does not have to operate at its ammonia storage limit, resulting in less ammonia slip. Furthermore, less ammonia is recirculated into the combustion chambers of the combustion engine via the low-pressure exhaust gas recirculation system, which minimizes raw emissions, especially nitrogen oxide emissions. Furthermore, it is possible to reduce NOx emissions by understanding aging.

[0022] The features listed in the dependent claims enable advantageous improvements and non-trivial further developments of the method for exhaust gas aftertreatment of the internal combustion engine listed in the independent claim.

[0023] According to a further embodiment, a method is described, wherein a second NOx sensor is arranged downstream of the particulate filter and the method further comprises the following further steps: - measuring a second NOx concentration NOx_2 downstream of the particulate filter by means of the second NOx sensor; - Determining the second ammonia concentration N2 based on the first NOx concentration NOx_1 and on the basis of the second NOx concentration NOx_2.

[0024] According to a further embodiment, a method is described, characterized in that the second ammonia concentration N2 is determined on the basis of the first NOx concentration NOx_1, on the basis of the second NOx concentration NOx_2 and on the basis of an efficiency model for the selective catalytic reduction of nitrogen oxides of the particulate filter.

[0025] In a further embodiment of the method, a difference in the nitrogen oxide concentration upstream and downstream of the particulate filter is determined, and the mass of nitrogen oxides converted by the particulate filter is calculated from the difference. By measuring the nitrogen oxide concentration upstream and downstream of the particulate filter, the conversion performance of the particulate filter with regard to nitrogen oxide emissions can be determined in a relatively simple manner. Thus, the amount of reducing agent can be adjusted to achieve the most efficient conversion of nitrogen oxide emissions.

[0026] An efficiency model can also be used to model other influencing factors, in particular aging of the SCR coating of the particulate filter, which further improves the accuracy of the model for calculating ammonia slip.

[0027] According to a further embodiment, a method for exhaust gas aftertreatment is described, characterized in that the second ammonia concentration N2 is determined according to the following formula: N2∗x=NOx_2−(NOx1∗(1−efficiency model SDPF)) x represents a factor for the cross-sensitivity of the NOx sensor to NH3.

[0028] When NOx sensor(s) are mentioned in the context of the present invention, this also includes ammonia sensors. In the above equation, this scenario can be taken into account via the sensor's cross-sensitivity x.

[0029] According to a further embodiment, a method for exhaust gas aftertreatment is described, wherein the first ammonia concentration N1 models a typically expected aging of the catalyst.

[0030] According to a preferred embodiment, the ammonia concentration N1 and N2 is determined by integration over time, these integrated values ​​then being used to determine the aging factor.

[0031] According to a further embodiment, a method for exhaust gas aftertreatment is described, wherein the first ammonia concentration N1 is determined without taking into account the first NOx concentration NOx_1.

[0032] Unlike the second ammonia concentration N2, the first ammonia concentration N1 is not determined based on the actual nitrogen oxide concentration. Rather, the first ammonia concentration N1 models the concentration in an SCR catalyst aged to a reference value. The ammonia concentration N1 merely represents a model of the NH3 slip without taking into account the actually measured nitrogen oxide values. The method according to the invention accounts for the actual catalyst aging by determining the quotients of the second ammonia concentration N2 and the first ammonia concentration N1.

[0033] According to a further embodiment, a method is described, characterized in that the first ammonia concentration N1 is determined without taking into account the second NOx concentration NOx_2.

[0034] As already described, unlike the second ammonia concentration N2, the first ammonia concentration N1 is not determined based on the actual nitrogen oxide concentration. Rather, the first ammonia concentration N1 models the concentration in an SCR catalyst aged to a reference value. The ammonia concentration N1 merely represents a model of the NH3 slip without taking into account the actually measured nitrogen oxide values. The method according to the invention accounts for the actual catalyst aging by calculating the quotient of the second ammonia concentration N2 and the first ammonia concentration N1. The quotient is preferably calculated using the corresponding time integrals of both concentrations.

[0035] According to a further embodiment, a method is described, wherein a second SCR catalyst is arranged downstream of the second NOx sensor and a third NOx sensor is arranged downstream of the second SCR catalyst, the method further comprising the following further steps: - measuring a third NOx concentration NOx_3 downstream of the second SCR catalyst by means of the third NOx sensor; - Determining a third ammonia concentration N3 based on the second NOx concentration NOx_2 and the third NOx concentration NOx_3.

[0036] To enable complete nitrogen oxide conversion, a further SCR catalyst can be provided according to the invention. This enables essentially complete conversion of the nitrogen oxides, despite an efficiency of less than 1 for the first SCR catalyst. The third NOx sensor advantageously serves to determine the ammonia concentration downstream of the second catalyst. Thus, it can also be determined for the second catalyst whether ammonia slip is occurring.

[0037] A further aspect of the invention relates to an exhaust gas aftertreatment system for an internal combustion engine which is connected to an exhaust system via an outlet, wherein a particulate filter with a coating for the selective catalytic reduction of nitrogen oxides is arranged in the exhaust system downstream of a turbine of an exhaust gas turbocharger, and wherein a first NOx sensor is arranged upstream of the particulate filter, and wherein the exhaust gas aftertreatment system has a metering element with which a reducing agent can be metered into the exhaust duct upstream of the particulate filter, and wherein the exhaust gas aftertreatment system is designed to carry out a method according to the method according to the invention.

[0038] According to a preferred embodiment, an exhaust gas aftertreatment system for an internal combustion engine is described, wherein a diesel oxidation catalyst and / or a nitrogen oxide storage catalyst is arranged downstream of the first NOx sensor and upstream of the particulate filter.

[0039] An oxidation catalyst can change the ratio of nitrogen monoxide and nitrogen dioxide, thereby increasing the efficiency of the selective catalytic reduction of nitrogen oxides.

[0040] According to a further embodiment, an exhaust gas aftertreatment system for an internal combustion engine is described, wherein a second SCR catalyst is arranged downstream of the second NOx sensor and a third NOx sensor is arranged downstream of the second SCR catalyst, and wherein the exhaust gas aftertreatment system is designed to carry out the following further steps: - measuring a third NOx concentration NOx_3 downstream of the second SCRF catalyst by means of the third NOx sensor; - Determining a third ammonia concentration N3 based on the second NOx concentration NOX_2 and the third NOx concentration NOx_3.

[0041] To enable complete nitrogen oxide conversion, a further SCR catalyst can be provided according to the invention, as already described. This enables essentially complete conversion of the nitrogen oxides despite an efficiency of less than 1 for the first SCR catalyst. The third NOx sensor advantageously serves to determine the ammonia concentration downstream of the second catalyst. Thus, it can also be determined for the second catalyst whether ammonia slip is occurring.

[0042] In a preferred embodiment of the exhaust aftertreatment system, the second SCR catalyst comprises an ammonia blocking catalyst, or an ammonia blocking catalyst is connected downstream of the second SCR catalyst. An ammonia blocking catalyst prevents the metered addition of reducing agent from leading to an increase in ammonia tailpipe emissions. This ensures operational reliability that the ammonia present in the exhaust duct downstream of the particulate filter is converted, even in the event of ammonia slippage via the particulate filter.

[0043] In a further embodiment of the exhaust gas aftertreatment system, a low-pressure exhaust gas recirculation system branches off from the exhaust duct downstream of the particulate filter and upstream of the second SCR catalyst. The particulate filter filters out soot particles and other solid particles from the exhaust gas flow, preventing them from being recirculated into the air supply system via the low-pressure exhaust gas recirculation system and causing damage there, particularly to the compressor of the exhaust gas turbocharger. Furthermore, exhaust gas recirculation via the low-pressure exhaust gas recirculation system can minimize the raw emissions of the combustion engine in a known manner, which can also reduce the use of reducing agents in the exhaust gas aftertreatment system.

[0044] According to a preferred embodiment of the exhaust gas aftertreatment system, a second metering element is arranged downstream of the particulate filter and upstream of the second SCR catalyst. A second metering element can expand the operating range of the exhaust gas aftertreatment system. This makes it possible, particularly at high exhaust gas temperatures at the particulate filter, for example, during a high-load phase of the combustion engine or during regeneration of the particulate filter, to enable efficient conversion of the nitrogen oxides by the second SCR catalyst and to prevent thermal decomposition of the ammonia obtained from the reducing agent.

[0045] It is particularly preferred if the first dosing element and the second dosing element are supplied with reducing agent from a common reducing agent container. A common reducing agent container enables a particularly simple and cost-effective supply of the reducing agent to both dosing elements.

[0046] In a further embodiment of the invention, an exhaust gas mixer is arranged in the exhaust duct downstream of the first metering element and upstream of the particulate filter with the coating for the selective catalytic reduction of nitrogen oxides. An exhaust gas mixer can achieve a homogeneous distribution of the reducing agent in the exhaust gas stream before it enters the particulate filter. The exhaust gas mixer can shorten the length of the mixing section to achieve such a homogeneous distribution. This allows the particulate filter to be arranged closer to the exhaust of the internal combustion engine, which promotes heating of the particulate filter after a cold start of the internal combustion engine or for regeneration of the particulate filter.

[0047] The various embodiments of the invention mentioned in this application can be combined with one another, unless otherwise stated in the individual case.

[0048] 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 in the different figures. They show: Fig. 1 shows an internal combustion engine with an exhaust gas aftertreatment system according to the invention according to a first embodiment; Fig. 2 shows an internal combustion engine with an exhaust aftertreatment system according to the invention according to a second embodiment; and Fig. 3 a diagram with a driving profile of a motor vehicle, wherein a first ammonia concentration N1 and a second ammonia concentration N2 are shown as a function of time, wherein the integrals are additionally shown.

[0049] Fig. 1 shows a schematic representation of an internal combustion engine 10 with an air supply system 20 and an exhaust system 40. The internal combustion engine 10 is designed as a direct-injection diesel engine and has a plurality of combustion chambers 12. A fuel injector 14 is arranged at each of the combustion chambers 12 for injecting fuel into the respective combustion chamber 12. The internal combustion engine 10 is connected by its inlet 16 to an air supply system 20 and by its outlet 18 to an exhaust system 40. The internal combustion engine 10 further comprises a high-pressure exhaust gas recirculation system with a high-pressure exhaust gas recirculation valve, via 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.

[0050] The air supply system 20 comprises an intake duct 28, in which, in the direction of flow of fresh air through the intake duct 28, an air filter 22, downstream of the air filter 22 an air mass meter 24, in particular a hot-film air mass meter, downstream of the air mass meter 24 a compressor 26 of an exhaust gas turbocharger 36, downstream of the compressor 26 a throttle valve 30, and further downstream 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 an assembly. Downstream of the air filter 22 and upstream of the compressor 26, an inlet 34 is provided, at which an exhaust gas recirculation line 86 of a low-pressure exhaust gas recirculation 80 opens into the intake duct 28.

[0051] The exhaust system 40 comprises an exhaust duct 42, in which a turbine 44 of the exhaust gas turbocharger 36 is arranged in the flow direction of an exhaust gas from the internal combustion engine 10 through the first exhaust duct 42. This turbine 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 44, via which the flow of the exhaust gas onto the blades of the turbine 44 can be varied. Downstream of the turbine 44, a plurality of exhaust gas aftertreatment components 46, 48, 50, 52, 54 are provided. An oxidation catalyst 46 or a NOx storage catalyst is arranged immediately downstream of the turbine 44 as the first component of the exhaust gas aftertreatment.Downstream of the oxidation catalyst 46 or the NOx storage catalyst, a particulate filter 48 with a coating 50 for the selective catalytic reduction of nitrogen oxides (SCR coating) is arranged. Downstream of the particulate filter 48, a second SCR catalyst 52 is arranged in the exhaust duct 42, preferably in the underbody position of a motor vehicle. The second SCR catalyst 52 has an ammonia barrier catalyst 54. Downstream of the oxidation catalyst 46 and upstream of the particulate filter 48 with the SCR coating 50, a first metering element 56 is provided for metering a reducing agent 78 into the exhaust duct 42. Downstream of the particulate filter 48, an exhaust gas recirculation line 86 of a low-pressure exhaust gas recirculation 80 branches off from the exhaust duct 42 at a branch 72.Downstream of the branch 72 and upstream of the second SCR catalyst 52, a second metering element 58 is arranged to meter the reducing agent 78 into the exhaust duct 42. The first metering element 56 and the second metering element 58 are each connected via a reducing agent line 74 to a common reducing agent tank 76, in which the reducing agent 78 is stored. Furthermore, the exhaust system 40 includes an exhaust flap 60, with which the exhaust gas recirculation can be controlled via the low-pressure exhaust gas recirculation 80.

[0052] In addition to the exhaust gas recirculation line 86, the exhaust gas recirculation system 80 comprises an exhaust gas recirculation cooler 82 and an exhaust gas recirculation valve 84, via which the exhaust gas recirculation can be controlled through the exhaust gas recirculation line 86. A temperature sensor 88 is provided on the exhaust gas recirculation line 86 of the exhaust gas recirculation system 80. This temperature sensor can be used to determine an exhaust gas temperature in the exhaust gas recirculation system 80 in order to activate the exhaust gas recirculation system 80 as soon as the exhaust gas temperature in the 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 out and causing damage or deposits in the exhaust gas recirculation system 80 or in the air supply system 20.Downstream of the branch and upstream of the exhaust gas recirculation cooler 82, a filter may be provided to minimize the entry of particles into the exhaust gas recirculation 80. The exhaust gas recirculation duct 86 opens into the intake line 28 of the air supply system 20 at an inlet 34.

[0053] A first NOx sensor 62 is arranged in the exhaust system 40 downstream of the oxidation catalyst 46 and upstream of the first dosing element 56. A second NOx sensor 64 is arranged downstream of the particulate filter 48 and upstream of the branch 72. Furthermore, the particulate filter 48 has a differential pressure sensor 66, which determines a pressure difference Δp across the particulate filter 48. In this way, the loading state of the particulate filter 52 can be determined, and if a defined loading level is exceeded, regeneration of the particulate filter 52 can be initiated. Furthermore, a temperature sensor 38 is provided in the exhaust system 40 to determine the exhaust gas temperature.

[0054] Downstream of the first metering element 56 and upstream of the particulate filter 48, a first exhaust mixer 68 may be provided to improve the mixing of the exhaust gas flow of the internal combustion engine 10 and the reducing agent 78 before entering the particulate filter 48 with the SCR coating 50 and to shorten the length of the mixing section. Downstream of the second metering element 58 and upstream of the second SCR catalyst 52, a second exhaust mixer 70 may be arranged to improve the mixing of the exhaust gas flow and the reducing agent 78 and to assist the evaporation of the reducing agent 78 in the exhaust passage 42.

[0055] The internal combustion engine 10 is connected to an engine control unit 90, which is connected via signal lines not shown to the NOx sensors 62, 64, the differential pressure sensor 66, the temperature sensors 38, 88 as well as to the fuel injectors 14 of the internal combustion engine 10 and the metering elements 56, 58.

[0056] In Fig. 2 shows an internal combustion engine with an exhaust aftertreatment system according to the invention according to a second embodiment. As in the case of the embodiment shown in Fig. As shown in Figure 1, the exhaust aftertreatment system of the internal combustion engine 10 includes a particulate filter 48 with a coating 50 for the selective catalytic reduction of nitrogen oxides. A first NOx sensor 62 is arranged upstream of the particulate filter 48, as well as a first metering element 56, with which a reducing agent 78 can be metered into the exhaust passage 42 upstream of the particulate filter 48. An exhaust mixer 68 is arranged downstream of the metering element.

[0057] Furthermore, a second NOx sensor 64 is arranged downstream of the particulate filter 48. Furthermore, a second SCR catalyst 52 is arranged downstream of the second NOx sensor 64, and a third NOx sensor 110 is arranged downstream of the second SCR catalyst 52. A second dosing element 58 and a second exhaust gas mixer 70 are arranged downstream of the second NOx sensor 64.

[0058] A first NOx concentration NOx_1 is measured upstream of the particulate filter 48, 50 by means of the first NOx sensor 62. A second NOx concentration NOx_2 is also measured downstream of the particulate filter 48, 50 by means of the second NOx sensor 64.

[0059] Optionally, a third NOx concentration NOx_3 is measured downstream of the second SCR catalyst 52 by means of the third NOx sensor 110.

[0060] A second ammonia concentration N2 is now determined based on the first NOx concentration NOx_1 and on the second NOx concentration NOx_2.

[0061] A third ammonia concentration N3 is optionally determined based on the second NOx concentration NOx_2 and the third NOx concentration NOx_3 by calculating the difference, analogous to the determination of the second ammonia concentration. The efficiency of the second SCR catalyst can also be included, analogous to the method described for calculating the second ammonia concentration N2.

[0062] In Fig. 3 shows a diagram with a driving profile of a motor vehicle, wherein a first ammonia concentration N1 and a second ammonia concentration N2 are shown as a function of time.

[0063] To determine the ammonia concentration N2, the NOx concentration is measured upstream of the particulate filter / nitrogen oxide catalyst 48, 50 using the first NOx sensor 62. Since the measurement of the first NOx sensor 62 takes place upstream of the particulate filter / nitrogen oxide catalyst 48, 50, and the metering of the reducing agent and the reduction of the nitrogen oxides take place downstream of the first NOx sensor 62, the exhaust gas is enriched with nitrogen oxides because a reduction of the nitrogen oxides has not yet occurred. The first NOx sensor 62 thus measures the concentration of nitrogen oxides before the catalytic reduction. Downstream of the first NOx sensor 62, reducing agent, which serves as an ammonia source, is metered in. The exhaust gas then enters the particulate filter / nitrogen oxide catalyst 48, 50, where the nitrogen oxides contained in the exhaust gas are reduced by the ammonia now contained in the exhaust gases.After exiting the particulate filter / nitrogen oxide catalyst 48, 50, the now largely purified exhaust gas reaches the second NOx sensor 64. It should be noted that NOx sensors are sensitive to both nitrogen oxides and ammonia. The second NOx sensor 64 thus measures the total concentration of ammonia and any remaining nitrogen oxides. The second ammonia concentration N2 is determined based on the following equation: N2∗x=Nox_2−NOx1∗(1−efficiency model SDPF)

[0064] The factor x represents the cross sensitivity of the sensor.

[0065] When NOx sensor(s) are mentioned in the context of the present invention, this also includes ammonia sensors. In the above equation, this scenario can be taken into account via the sensor's cross-sensitivity x.

[0066] The efficiency of the particulate filter in the particulate filter / nitrogen oxide catalyst 48, 50 is included in the equation. This efficiency takes into account the fact that the particulate filter / nitrogen oxide catalyst 48, 50 generally does not allow for complete conversion of the nitrogen oxides. In particular, the particulate filter / nitrogen oxide catalyst 48, 50 ages over time, causing the efficiency of the particulate filter / nitrogen oxide catalyst 48, 50 to decrease over time.

[0067] In the case of complete conversion, an efficiency of 1 would be assumed, meaning that the nitrogen oxides previously contained in the exhaust gas are completely converted by the particulate filter / nitrogen oxide catalyst 48, 50. Consequently, the purified exhaust gas, which is measured by the second NOx sensor 64, no longer contains any nitrogen oxides, and the value measured by the second NOx sensor 64 corresponds to the ammonia concentration still contained in the exhaust gas, i.e., the excess ammonia, i.e., the ammonia slip. In this case, the second ammonia concentration N2 corresponds to the nitrogen oxide concentration NOx_2 measured by the second nitrogen oxide sensor 64.

[0068] If the nitrogen oxides are only partially converted by the particulate filter / nitrogen oxide catalyst 48, 50, the efficiency is less than 1. The purified exhaust gas, which is measured at the second NOx sensor 64, still contains nitrogen oxides. Since, as already mentioned, nitrogen oxide sensors are sensitive to both nitrogen oxides and ammonia, the value measured at the second NOx sensor 64 corresponds to the total concentration of ammonia and nitrogen oxides. The above formula takes this into account by subtracting the remaining nitrogen oxide content based on the efficiency and the nitrogen oxide concentration NOx_1 measured by the first nitrogen oxide sensor 62.

[0069] The second ammonia concentration, N2, is plotted as an integral measured value over time. This averages out background noise, i.e., isolated deviations in the measured values ​​caused by measurement inaccuracies. In this sense, a sufficiently long time interval is considered for the determination of N2.

[0070] The first ammonia concentration N1 is determined without considering the first NOx concentration NOx_1 and the second NOx concentration NOx_2. In other words, unlike the second ammonia concentration N2, the first ammonia concentration N1 is not determined based on the actual nitrogen oxide concentration. Rather, the first ammonia concentration N1 models a typically expected concentration of a catalyst artificially aged to a reference value. To account for catalyst aging when calculating the first ammonia concentration N1, various influencing factors are known from the state of the art.

[0071] For example, it is conceivable to use the number of regeneration cycles of the particulate filter as a measure of catalyst aging in order to determine an aging factor purely in advance. Furthermore, the total reducing agent introduced into the particulate filter can serve as the basis for calculating the aging of the particulate filter. Another possibility is to use the total amount of heat introduced as the basis for calculating the aging of the particulate filter.

[0072] In this embodiment, an NH3 slip expected for a FUL (full useful life) was assumed, which was compared with the actual slip to determine the aging factor. Fig. In Figure 3, the ammonia concentration N1, as in the case of the ammonia concentration N2, is represented as an integral over time. It should be emphasized again that the first ammonia concentration N1 only describes the aging of the catalyst in a model manner.

[0073] Therefore, the invention provides for determining an aging factor C that takes into account the actually measured nitrogen oxide concentration and the NH3 concentration determined from it. This is done by taking into account the second ammonia concentration N2, with the aging factor being proportional to the quotient N2 / N1 of the second ammonia concentration N2 and the first ammonia concentration N1.

[0074] In the Fig.In the driving profile shown in Figure 3, the ammonia concentration N2, as an integral over time, is smaller than the ammonia concentration N1. In the example shown, the catalyst has therefore aged less than would be expected based on an ammonia concentration of 1. Consequently, the aging factor is less than 1.

[0075] The aging factor obtained by the method described here can now be used to adjust the dosage of the reducing agent. List of reference symbols 10 Combustion engine 12 combustion chamber 14 Fuel injector 16 Entrance 18 Outlet 20 Air supply system 22 air filters 24 air mass meters 26 compressors 28 intake duct 30 Throttle valve 32 intercooler 34 Junction 36 exhaust gas turbochargers 38 Temperature sensor 40 Exhaust system 42 exhaust duct 44 turbines 46 Oxidation catalyst / NOx storage catalyst 48 particle filters 50 Coating for the selective catalytic reduction of nitrogen oxides 52 second SCR catalyst 54 Ammonia slip catalyst 56 first dosing element 58 second dosing element 60 exhaust flap 62 first NOx sensor 64 second NOx sensor 66 differential pressure sensor 68 first exhaust gas mixer 70 second exhaust mixer 72 branching 74 Reducing agent line 76 reducing agent tanks 78 reducing agents 80 Low-pressure exhaust gas recirculation 82 exhaust gas recirculation cooler 84 Exhaust gas recirculation valve 86 Exhaust gas recirculation line 88 Temperature sensor 90 Engine control unit 105 Diesel oxidation catalyst 110 third NOx sensor

Claims

[1] A method for exhaust gas aftertreatment of an internal combustion engine (10) which is connected by its outlet (18) to an exhaust system (40), wherein an SCR catalyst or particulate filter (48) with a coating (50) for the selective catalytic reduction of nitrogen oxides is arranged in the exhaust system (40) downstream of a turbine (44) of an exhaust gas turbocharger (36), and wherein a first NOx sensor (62) is arranged upstream of the SCR catalyst or particulate filter (48), with a metering element (56) with which a reducing agent (78) can be metered into the exhaust duct (42) upstream of the SCR catalyst or particulate filter (48), and wherein a second NOx sensor (64) is arranged downstream of the SCR catalyst or particulate filter (48), and the method comprises the following steps: - Dosing the reducing agent (78) into the exhaust duct (42) of the internal combustion engine (10); - Determining a first ammonia concentration (N1); - measuring a first NOx concentration (NOx_1) by means of the first NOx sensor (62); - measuring a second NOx concentration (NOx_2) by means of the second NOx sensor (64); - determining the second ammonia concentration (N2) based on the first NOx concentration (NOx_1) and on the basis of the second NOx concentration (NOx_2); - Determining an ageing factor C, wherein the ageing factor is proportional to the quotient (N2 / N1) of the second ammonia concentration (N2) and the first ammonia concentration (N1), wherein - the dosage of the reducing agent (78) is adjusted on the basis of the calculated ageing factor C. [2] Method for exhaust gas aftertreatment according to claim 1, characterized by that the first ammonia concentration N1 and the second ammonia concentration N2 are formed by integration over time. [3] Method for exhaust gas aftertreatment according to one of the preceding claims, characterized by that the second ammonia concentration (N2) is determined on the basis of the first NOx concentration (NOx_1), on the basis of the second NOx concentration (NOx_2), and on the basis of an efficiency model for the selective catalytic reduction of nitrogen oxides of the SCR catalyst or particulate filter (48). [4] Method for exhaust gas aftertreatment according to claim 3, characterized by that the second ammonia concentration (N2) is determined according to the following formula: N2∗x=Nox_2−(NOx1∗(1−efficiency wheel model SPDF)) where x takes into account the cross-sensitivity of the sensor for ammonia. [5] Method for exhaust gas aftertreatment according to one of the preceding claims, characterized by that the first ammonia concentration (N1) models the ammonia slip of a catalyst aged on the basis of a reference value. [6] Method for exhaust gas aftertreatment according to one of the preceding claims, characterized bythat the first ammonia concentration (N1) is determined without taking into account the first NOx concentration (NOx_1). [7] Method for exhaust gas aftertreatment according to one of the preceding claims, characterized by that the first ammonia concentration (N1) is determined without taking into account the second NOx concentration (NOx_2). [8] Method according to one of the preceding claims, wherein a second SCR catalyst (58) is arranged downstream of the second NOx sensor (64) and a third NOx sensor (110) is arranged downstream of the second SCR catalyst (52), the method further comprising the following further steps: - measuring a third NOx concentration (NOx_3) downstream of the second SCR catalyst by means of the third NOx sensor (110); - Determining a third ammonia concentration (N3) based on the second NOx concentration (NOx_2) and the third NOx concentration (NOx_3). [9] Exhaust gas aftertreatment system for an internal combustion engine (10) which is connected to an exhaust system (40) via an outlet (18), wherein a particle filter (48) with a coating (50) for the selective catalytic reduction of nitrogen oxides is arranged in the exhaust system (40) downstream of a turbine (44) of an exhaust gas turbocharger (36), and wherein a first NOx sensor (62) is arranged upstream of the particle filter (48), and wherein the exhaust gas aftertreatment system has a metering element (56) with which a reducing agent (78) can be metered into the exhaust gas duct (42) upstream of the particle filter (48), and wherein a second NOx sensor (64) is arranged downstream of the particle filter (48), and wherein the exhaust gas aftertreatment system is designed to carry out a method according to one of the preceding claims.

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