Method for operating an exhaust aftertreatment system comprising two SCR catalysts of an internal combustion engine of a motor vehicle and motor vehicle

Abstract

The invention relates to a method for operating an exhaust aftertreatment device (14) of an internal combustion engine (12) designed as a diesel engine of a motor vehicle, in which the exhaust aftertreatment device (14) through which exhaust gas of the internal combustion engine (12) flows comprises a first SCR catalyst (16), a second SCR catalyst (18) arranged downstream of the first SCR catalyst (16), a metering device (26) by means of which a reducing agent for removing nitrogen oxides from the exhaust gas can be introduced into the exhaust gas at an injection point (E1) arranged upstream of the first SCR catalyst (16), at least one first nitrogen oxide sensor (36) arranged upstream of the first SCR catalyst (16), at least one second nitrogen oxide sensor (38) arranged downstream of the second SCR catalyst (18) and at least one temperature sensor.The first nitrogen oxide sensor (36) measures an initial quantity that characterizes a first amount of nitrogen oxides contained in the exhaust gas. The second nitrogen oxide sensor (38) measures a second quantity that characterizes a second amount of nitrogen oxides contained in the exhaust gas. An electronic computing device (48) determines the expected efficiency of the second SCR catalyst (18).

Description

The invention relates to a method for operating an exhaust gas aftertreatment device of an internal combustion engine of a motor vehicle, which is designed as a diesel engine.DE 10 2023 005 120 B3 discloses a method for operating an exhaust gas aftertreatment device of an internal combustion engine of a motor vehicle designed as a diesel engine, in which the exhaust gas aftertreatment device through which exhaust gas from the internal combustion engine can flow has a first SCR catalyst, a second SCR catalyst arranged downstream of the first SCR catalyst and a metering device. An electronic computing device is used to determine an expected efficiency of the second SCR catalytic converter.DE 10 2017 201 400 A1 discloses a method for fault detection in an SCR system of an internal combustion engine in a motor vehicle, which has two SCR catalytic converters and at least two nitrogen oxide sensors, a first nitrogen oxide sensor being arranged between the two SCR catalytic converters and a second nitrogen oxide sensor being arranged downstream of the two SCR catalytic converters. In the method, a continuous detection of a signal is carried out at the first nitrogen oxide sensor and an ammonia mass is determined therefrom at the first nitrogen oxide sensor.DE 10 2021 202 965 A1 discloses a method for determining the ammonia content and / or nitrogen oxide content in the exhaust gas of an exhaust tract of an internal combustion engine as well as an exhaust tract for an internal combustion engine, which includes a catalytic converter device with selective catalytic reduction, a reducing agent injection device, and an exhaust gas sensor arranged downstream of the catalytic converter device. The exhaust gas sensor is configured to generate an exhaust signal indicative of an exhaust value indicative of ammonia content and / or nitrogen oxide content in the exhaust gas.DE 10 2021 212 868 A1 discloses a method for determining an exhaust gas composition of an exhaust gas of an internal combustion engine with respect to an ammonia and a nitrogen oxide fraction in an exhaust system having at least one SCR catalyst.DE 10 2014 205 434 A1 discloses a method for detecting ammonia slip in an exhaust gas aftertreatment system of an internal combustion engine. Furthermore, DE 10 2018 121 938 A1 discloses an emission control system for treating exhaust gas in a motor vehicle having an internal combustion engine. DE 10 2018 104 385 A1 discloses a method for monitoring an ammonia slip catalyst. Furthermore, DE 10 2019 118 526 A1 discloses a method for exhaust gas aftertreatment of an internal combustion engine.It is the object of the present invention to provide a method and a motor vehicle, so that particularly advantageous operation of an exhaust gas aftertreatment device of an internal combustion engine can be realized in a particularly advantageous manner.This object is achieved by a method having the features of patent claim 1 and by a motor vehicle having the features of patent claim 7. Advantageous embodiments with expedient developments of the invention are specified in the other claims.A first aspect of the invention relates to a method for operating an exhaust gas aftertreatment device of an internal combustion engine, which is designed as a diesel engine and is also referred to as an internal combustion engine or internal combustion engine or engine, of a motor vehicle, which is also referred to simply as a vehicle. This means that the motor vehicle, which is preferably designed as a motor vehicle, in particular as a passenger vehicle, has the internal combustion engine in its completely produced state and can be driven by means of the internal combustion engine. During a fired operation of the internal combustion engine, combustion processes take place in the internal combustion engine, in particular in at least one combustion chamber of the internal combustion engine. During the respective combustion process, a respective fuel-air mixture, also referred to simply as a mixture, is burned, resulting in exhaust gas of the internal combustion engine. The respective mixture comprises air and a liquid fuel, which is designed as a diesel fuel. In particular, the method is carried out during the fired operation of the internal combustion engine. In the method, the exhaust gas can flow or is passed through the exhaust gas aftertreatment device. In the method, the exhaust gas aftertreatment device through which the exhaust gas can flow has a first SCR catalyst and a second SCR catalyst, wherein the second SCR catalyst is arranged downstream of the first SCR catalyst in the flow direction of the exhaust gas flowing through the exhaust gas aftertreatment device and thus through the SCR catalysts. In particular, it is provided that the second SCR catalytic converter is spaced apart from the first SCR catalytic converter, in particular completely, in the flow direction of the exhaust gas flowing through the exhaust gas aftertreatment device, so that, for example, no other, further SCR catalytic converter is arranged between the SCR catalytic converters in the flow direction of the exhaust gas flowing through the exhaust gas aftertreatment device. In other words, at least one longitudinal region of the exhaust gas aftertreatment device is preferably provided between the SCR catalytic converters in the flow direction of the exhaust gas flowing through the exhaust gas aftertreatment device, the longitudinal region of said longitudinal region being free of an SCR catalytic converter. The respective SCR catalyst is catalytically active for selective catalytic reduction (SCR), so that the selective catalytic reduction can be catalytically effected or supported by the respective SCR catalyst. During the respective selective catalytic reduction, any nitrogen oxides (NOx) present in the exhaust gas react with ammonia to form water and nitrogen, wherein the ammonia is provided or made available, for example, by an in particular liquid reducing agent and is present in the exhaust gas. In the method, the exhaust gas aftertreatment device has at least one metering device, by means of which the reducing agent for denitrogenating the exhaust gas can be introduced into the exhaust gas at at least or exactly one introduction point. In this case, the introduction point is arranged upstream of the first SCR catalytic converter and therefore also upstream of the second SCR catalytic converter in the flow direction of the exhaust gas flowing through the exhaust gas aftertreatment device. Denitrogenating the exhaust gas is understood to mean that at least some of the nitrogen oxides contained in the exhaust gas are removed or are removable from the exhaust gas by the respective SCR (selective catalytic reduction), in that, as described above, the nitrogen oxides contained in the exhaust gas react with the ammonia to form water and nitrogen at the respective SCR. Thus, denitrification is understood to mean the at least partial removal of the nitrogen oxides from the exhaust gas, and since the reducing agent can provide or provides the ammonia, the reducing agent is used for denitrification of the exhaust gas. In particular, for example, the reducing agent is thermally decomposed by being introduced into the exhaust gas by means of the metering device, whereby the reducing agent provides the ammonia. Most preferably, the reducing agent is an aqueous urea solution. Preferably, the reducing agent is a component of the exhaust gas aftertreatment device. Preferably, it is provided that during the method, the reducing agent is introduced into the exhaust gas flowing through the exhaust gas aftertreatment device at the introduction point arranged upstream of the first SCR catalytic converter by means of the metering device. The introduction of the reducing agent into the exhaust gas is also referred to as metering or metering.In the method, the exhaust gas aftertreatment device has at least one first nitrogen oxide sensor arranged upstream of the first SCR catalytic converter and at least one second nitrogen oxide sensor arranged downstream of the second SCR catalytic converter and therefore also downstream of the first SCR catalytic converter. This means that a first quantity of nitrogen oxides contained in the exhaust gas, thus a first quantity characterizing the first quantity, can be detected by means of the first SCR catalyst at a first measurement point arranged upstream of the first SCR catalyst, and that a second quantity of nitrogen oxides contained in the exhaust gas, thus a second quantity characterizing the second quantity, can be detected by means of the second nitrogen oxide sensor at a second measurement point arranged downstream of the second SCR catalyst. The respective nitrogen oxide sensor is also referred to as a respective NOx sensor.In the method, the first variable is detected, i.e. measured, by means of the first nitrogen oxide sensor, in particular at the first measurement point, which characterizes, i.e. indicates, the first quantity of the nitrogen oxides contained in the exhaust gas. In the method, the second variable is detected by means of the second nitrogen oxide sensor, in particular at the second measurement point, which characterizes, i.e. indicates, the second quantity of nitrogen oxides contained in the exhaust gas. In particular, it is conceivable that the first variable and the second variable are the same variable, in particular the measured variable, and therefore that the first variable corresponds to the second variable and vice versa, wherein, for example, the ordinal number words "first" and "second" are then used with respect to the variables in order to be able to conceptually distinguish between the variable which is detected by means of the first nitrogen oxide sensor and the second variable which is detected by means of the second nitrogen oxide sensor. In particular, the respective variable is a respective measured variable, wherein it is conceivable that the first variable and the second variable are the same variable.In the method, the exhaust gas aftertreatment device also has at least one temperature sensor, by means of which a temperature of the exhaust gas flowing through the exhaust gas aftertreatment device can be detected, in particular at a temperature measurement point. For example, the temperature sensor, in particular the temperature measuring point, is arranged downstream of the second SCR catalytic converter. It is conceivable for the temperature sensor, in particular the temperature measuring point, to be arranged upstream of the first SCR catalytic converter. It is furthermore conceivable for the temperature sensor, and therefore the temperature measuring point, to be arranged downstream of the first SCR catalytic converter and upstream of the second SCR catalytic converter.In the method, an expected efficiency of the second SCR catalytic converter is determined, in particular calculated, by means of an electronic computing device, by means of which the method is carried out. In the method, in particular by means of the electronic computing device, the second set is determined from the second variable and the first set from the first variable. In particular, it is conceivable that, in particular by means of the electronic computing device, the second set is determined, in particular calculated, from the second variable and the first set is determined, in particular calculated, from the first variable at regular first time intervals and / or at equidistant first times. For example, the second nitrogen oxide sensor is a so-called end-of-line sensor (end-of-pipe sensor), since, for example, no other, further exhaust gas aftertreatment component for, in particular targeted, aftertreatment of the exhaust gas is arranged downstream of the second nitrogen oxide sensor.In the method, the temperature of the exhaust gas, also referred to as the exhaust gas temperature, is detected by means of the temperature sensor. In the method, the temperature sensor provides an in particular electrical signal, which characterizes the temperature detected, that is to say measured, by means of the temperature sensor, in particular at the temperature measurement point, and which is also referred to as a temperature signal or sensor signal.Depending on the signal of the temperature sensor, a change, in particular a gradient, of the temperature of the exhaust gas is determined, in particular by means of the electronic computing device, wherein the gradient is also referred to as a temperature gradient. It is conceivable that the change in temperature is determined, in particular calculated, at regular second time intervals and / or at equidistant second times, in particular by means of the electronic computing device, as a function of the signal of the temperature sensor. It is conceivable that the second time intervals correspond to the first time intervals. It is conceivable that the second points in time may be the first points in time and vice versa.In the method, and in particular only when it is determined by means of the electronic computing device that the change is greater than a predefined threshold value, a current third quantity of ammonia contained in the exhaust gas downstream of the second SCR catalyst is determined, in particular calculated, by means of the electronic computing device as a function of the second variable and as a function of the determined expected efficiency. The third amount is also referred to as ammonia amount. The threshold value is 25° Celsius.A background on which the invention is based is in particular that monitoring ammonia (NH 3) possibly contained in the exhaust gas is particularly advantageous and / or desired in order to be able to detect, for example, an excessively large amount of ammonia contained in the exhaust gas, and therefore an excessively large value of the third amount. Consequently, countermeasures can be initiated, for example, in order to counteract this excessively large quantity of ammonia contained in the exhaust gas, so that particularly low-emission operation can be produced. At present, however, no ammonia sensor is available or can be used economically, by means of which an amount of, in particular pure, ammonia possibly contained in the exhaust gas could be measured. Therefore, it is provided to calculate the third quantity (ammonia quantity) from the first quantity and the second quantity or from the first quantity and the second quantity, i.e. depending on the first quantity and on the second quantity or depending on the first quantity and on the second quantity. The method according to the invention now makes it possible to be able to determine, in particular calculate, the ammonia quantity advantageously and sufficiently precisely and in a simple manner, such that the method according to the invention makes low-emission operation possible in a particularly advantageous manner. Furthermore, the invention is also based in particular on the assumption or finding that, in the case of at least almost any change in the temperature detected by means of the temperature sensor, in particular to the effect that the exhaust gas temperature increases, a probability that ammonia slip and thus an excessively large amount of ammonia absorbed in the exhaust gas downstream of the second SCR catalyst increases. This finding is used and determined, in particular calculated, by the invention, the third quantity then and in particular only when the change in the temperature is greater than the threshold value. In other words, the change, also referred to or formed as a temperature change or temperature gradient, in the temperature of the exhaust gas detected by means of the temperature sensor is used as an indicator for possible ammonia slip events, as a result of which an excessively large amount of, in particular pure, ammonia could be contained in the exhaust gas downstream of the second SCR catalytic converter. As a result, the third quantity can be determined, in particular calculated, in a targeted and efficient manner.In order to be able to determine an excessively large amount of, in particular pure, ammonia, in particular, absorbed in the exhaust gas downstream of the second SCR catalytic converter advantageously and in particular precisely and in a targeted manner and subsequently to realize particularly low-emission operation, it is provided in one embodiment of the invention that the third amount is determined by means of the electronic computing device then and preferably only when it is determined by means of the electronic computing device that the change in the temperature detected by means of the temperature sensor is greater than the threshold value for a predefined time period, also referred to as a first time period, of at least one second, in particular of at least two seconds and very particularly of at least or exactly five seconds, in particular continuously. In other words, said one debouncing time is also referred to as first debouncing time. For example, the change is determined, in particular calculated, at or for a first point in time and for a or at a second point in time, wherein the second point in time follows the first point in time and wherein the first point in time and the second point in time are apart from one another by the mentioned time period and thus by at least one second, in particular by at least two seconds and very particularly by at least or exactly five seconds. If it is determined both at the first point in time and at the second point in time, in particular by means of the electronic computing device, that the change is greater than the threshold value, then and preferably only then is the third quantity determined, in particular calculated. In other words, the time interval is taken into account as the debouncing time in order to ascertain, in particular calculate, the third set. This makes it possible to prevent short-term fluctuations in the temperature from leading to misinterpretations. If it is determined, for example, at the first point in time that the change in the temperature is greater than the threshold value, then, for example, at the second point in time the change in the temperature is determined again. If it is then detected or determined that the change in the temperature is greater than the threshold value both at the first point in time and at the second point in time, then and preferably only then the third quantity is determined.It can be seen that the third quantity is determined as a function of at least or exactly one release condition also referred to as a first release condition, in particular in such a way that the third quantity is then and preferably only determined, in particular calculated, when the, in particular first, release condition is fulfilled. The first release condition comprises, i.e. the first release condition is fulfilled if it is determined by means of the electronic computing device that the change is greater than the predetermined threshold value.Furthermore, it is preferably provided that the third quantity is determined depending on a second release condition, i.e. only when the second release condition is fulfilled. The second release condition is and comprises, and therefore the second release condition is satisfied if it is determined by means of the electronic computing device that the change for the predefined time period is greater than the threshold value.In order to be able to advantageously determine an excessively large amount of, in particular pure, ammonia, possibly contained in the exhaust gas, and subsequently to be able to ensure particularly low-emission operation, it is provided in a further embodiment of the invention that, by means of the electronic computing device, a difference, which is also referred to as a further difference, between the first amount and the second amount and / or between the first amount and the second amount is also determined by means of the electronic computing device. Furthermore, it is preferably provided that the third set is determined, in particular calculated, as a function of the further difference. For example, the third quantity is then and preferably only determined if the further difference is negative. The further difference is determined, for example, in such a way that the second variable or the second set is subtracted from the first variable or from the first set. Thus, it is preferably provided that the third quantity is determined and in particular only when the first quantity or the first quantity is smaller than the second quantity or the second quantity. Thus, for example, it is provided that the third quantity is determined as a function of a third release condition, thus and preferably only when the third release condition is fulfilled. The third release condition is or comprises, thus the third release condition is fulfilled if the said further difference is negative, thus if the first variable or the first quantity is smaller than the second variable or the second quantity. The background of this embodiment is that, if the further difference mentioned is negative, that is to say if the first variable or the first quantity is less than the second variable or the second quantity, ammonia slip can be assumed. It has proven particularly advantageous if a gas transit time between the nitrogen oxide sensors is taken into account for the determination of the further difference. For example, the gas transit time is at least one second, in particular at least two seconds and very particularly at least or exactly five seconds.In other words, it is preferably provided that the third set is determined then and preferably only when the further difference is greater or less than a comparison value. Thus, the third release condition is or comprises, for example, and therefore the third release condition is met and preferably only if the further difference is greater or less than the comparison value.Furthermore, it is preferably provided that the third quantity is determined, and preferably only, when the further difference is greater than or less than the comparison value for a predefined time duration of at least one second, in particular of at least two seconds and in particular of at least or exactly five seconds. Thus, for example, the third quantity is determined as a function of a fourth release condition, and therefore the third quantity is determined only when the fourth release condition is fulfilled and preferably only when the fourth release condition is fulfilled. The fourth release condition is or comprises, and therefore the fourth release condition is met and preferably only if the further difference for the time duration of at least one second, in particular of at least two seconds and in particular at least or exactly five seconds, is greater than or less than the comparison value. The time duration is thus the gas transit time mentioned. By taking into account the gas transit time, it is possible to avoid conclusions being drawn about ammonia slip, although in fact no ammonia slip is present or present. Thus, for example, the gas transit time is a second debouncing time, by taking account of short-term fluctuations, which do not lead to erroneous detections of ammonia slip events. For example, the gas transit time is determined, in particular calculated, via a characteristic curve in a development period as a function of a mass flow formed by the exhaust gas, and taken into account accordingly.A further embodiment of the invention is characterized in that the third quantity is determined by means of the electronic computing device when it is determined by means of the electronic computing device that the first quantity and / or the second quantity is greater than a predefined limit value. Thus, for example, the third quantity is determined as a function of a fifth release condition, in particular in such a way that the third quantity is determined only when the fifth release condition is fulfilled and preferably only when the fifth release condition is fulfilled. The fifth release condition is or comprises, thus the fifth release condition is met then and preferably only if the first quantity and / or the second quantity is greater than the limit value. This allows the third set to be determined in a meaningful manner.It has been found to be particularly advantageous if the third quantity is determined by means of the electronic computing device when and preferably only when it is determined by means of the electronic computing device that the first quantity and / or the second quantity is continuously greater than the limit value for a predefined second time period of at least one second, in particular of at least two seconds and very preferably of at least or exactly five seconds. The second time interval is a third debouncing time, by taking account of short-term fluctuations, does not lead to ammonia slip being detected incorrectly, although there is actually no ammonia slip present. Thus, for example, the third quantity is determined as a function of a sixth release condition, in particular in such a way that the third quantity is determined only when the sixth release condition is fulfilled and preferably only when the sixth release condition is fulfilled. The sixth release condition is or comprises, and therefore the sixth release condition is met then and preferably only if the first quantity and / or the second quantity is greater than the limit value for the predefined second time period.In order to be able to determine the third quantity in a particularly advantageous manner, it is provided according to the invention that the change in the temperature of the exhaust gas is determined as a function of the signal of the temperature sensor in such a way that the signal is filtered by means of a filter. For example, as the fixed, a PT1 filter is filtered. The difference between the filtered signal and the unfiltered signal is determined by means of the electronic computing device. In this case, the change in the temperature of the exhaust gas is determined by means of the electronic computing device as a function of the difference. In particular, the difference between the signals is determined in such a way that respective signal values of the signals are subtracted from one another. As a result, the third quantity can be determined in a targeted and precise manner, as a result of which particularly low-emission operation can be produced.Finally, it has been shown to be particularly advantageous if the third quantity corresponds to a product of the second quantity and the determined expected efficiency of the second SCR catalyst. As a result, the third quantity can be determined in a targeted and precise manner, so that operation with particularly low emission can subsequently be performed.A second aspect of the invention relates to a motor vehicle which is also referred to simply as a vehicle and is preferably designed as a motor vehicle, in particular as a passenger vehicle, and which is designed to carry out a method according to the first aspect of the invention. Advantages and advantageous configurations of the first aspect of the invention are to be regarded as advantages and advantageous configurations of the second aspect of the invention and vice versa.The invention makes it possible to determine the third quantity without having to use for this purpose special ammonia sensors which are designed to measure any, in particular pure, ammonia present in the exhaust gas. As a result, particularly low-emission operation can be realized in a cost-effective manner. In addition, a complexity of the exhaust gas aftertreatment device can advantageously be kept low. At the same time, it is possible to determine the third quantity with sufficient precision, so that, for example, the internal combustion engine, in particular the motor vehicle, can be operated as a function of the third quantity. This ensures particularly low-emission operation. In particular, the specific and precise determination of the third set makes it possible to be able to identify and / or limit potential sources of error, as a result of which simpler maintenance and maintenance can be realized in comparison with conventional solutions. The invention enables a cost effective solution to determine the third set and to represent low emission operation.Further advantages, features and details of the invention will become apparent from the following description of a preferred exemplary embodiment and with reference to the drawing. The features and combinations of features mentioned above in the description and the features and combinations of features mentioned below in the description of the figures and / or shown alone in the figures can be used not only in the respectively specified combination but also in other combinations or alone without departing from the scope of the invention.The drawing shows in: FIG. 1 shows a schematic illustration of a drive device of a motor vehicle, having an internal combustion engine and an exhaust gas aftertreatment device for aftertreatment of exhaust gas of the internal combustion engine; FIG. 2 is a diagram illustrating a method for operating the exhaust gas aftertreatment device; FIG. 3 shows a further diagram for further illustrating the method; and FIG. 4 shows a further diagram for further illustrating the method.In the figures, identical or functionally identical elements are provided with the same reference numerals.FIG. 1 shows a schematic illustration of a drive device 10 of a motor vehicle, also referred to simply as a vehicle, which is preferably designed as a motor vehicle, in particular as a passenger vehicle. The drive device 10 has an internal combustion engine 12 by means of which the motor vehicle can be driven. The drive device 10 also has an exhaust gas aftertreatment device 14, which can be flown through by exhaust gas of the internal combustion engine 12 and is also referred to as an exhaust system and by means of which the exhaust gas can be post-treated. With reference to FIGS. 1, 2, 3 to 4, a method for operating the exhaust gas aftertreatment device 14 is described below. In the method, the internal combustion engine 12 is designed as a diesel engine. The exhaust gas aftertreatment device 14 has a first SCR catalytic converter 16 and a second SCR catalytic converter 18, which is arranged downstream of the SCR catalytic converter 16 in the flow direction of the exhaust gas flowing through the exhaust gas aftertreatment device 14, in such a way that at least one length region L of the exhaust gas aftertreatment device 14 is arranged between the SCR catalytic converters 16 and 18 in the flow direction of the exhaust gas flowing through the exhaust gas aftertreatment device 14, wherein the length region L is free of an SCR catalytic converter. The exhaust gas aftertreatment device 14 also has a particle filter 20, a third SCR catalytic converter 22 and an oxidation catalytic converter 24 designed as a diesel oxidation catalytic converter. It can be seen that the SCR catalytic converter 22 is arranged upstream of the SCR catalytic converter 18 and downstream of the SCR catalytic converter 16. The particle filter 20, which is designed as a diesel particle filter, is arranged downstream of the SCR catalytic converter 16 and upstream of the SCR catalytic converter 18, in the present case in such a way that the particle filter 20 is arranged downstream of the SCR catalytic converter 16 and upstream of the SCR catalytic converter 22. In the exemplary embodiment shown in FIG. 1, the particle filter 20 is designed as a catalytically active particle filter 20, which is catalytically active for selective catalytic reduction (SCR). The SCR catalytic converters 16, 18 and 22 are also catalytically active for selective catalytic reduction (SCR), it being possible for any nitrogen oxides present in the exhaust gas to be at least partially removed from the exhaust gas during the respective selective catalytic reduction, in that the nitrogen oxides (NOx) present in the exhaust gas react with ammonia (NH 3) present in the exhaust gas to form water and nitrogen. The ammonia is or can be provided or is provided by a reducing agent. The at least partial removal of the nitrogen oxides from the exhaust gas is also referred to as denitrification of the exhaust gas, so that the reducing agent is used for denitrification of the exhaust gas. The oxidation catalyst 24 is disposed upstream of the SCR catalysts 16, 18, and 22 and upstream of the particulate filter 20.The exhaust gas aftertreatment device 14 has a first metering device 26, by means of which the reducing agent, which is preferably designed as an aqueous urea solution, can be introduced or is introduced into the exhaust gas and thus into the exhaust gas aftertreatment device 14 at a first introduction point E 1. The exhaust gas aftertreatment device 14 has a second metering device 28, by means of which the reducing agent can be introduced or is introduced into the exhaust gas and thus into the exhaust gas aftertreatment device 14 at a second introduction point E 2. It can be seen that the first introduction point E 1 is arranged downstream of the oxidation catalytic converter 24 and upstream of the SCR catalytic converter 16 and thus also upstream of the particle filter 20 and upstream of the SCR catalytic converters 18 and 22 and upstream of the second introduction point E 2. The second introduction point E 2 is arranged upstream of the second SCR catalytic converter 18 and downstream of the oxidation catalytic converter 24, downstream of the introduction point E 1, downstream of the SCR catalytic converters 16 and 22 and downstream of the particle filter 20.The exhaust gas aftertreatment device 14 has a branching device 30, by means of which at a branching point A at least some of the exhaust gas flowing through the exhaust gas aftertreatment device 14 can be branched off from the exhaust gas aftertreatment device 14. This is illustrated in FIG. 1 by an arrow 32. The exhaust gas branched off from the exhaust gas aftertreatment device 14 can be recirculated to an intake tract of the internal combustion engine 12, which is also referred to as an intake tract, and introduced into the intake tract. It can be seen that the branch point A is arranged upstream of the SCR catalytic converter 18, in particular upstream of the introduction point E 2 and downstream of the oxidation catalytic converter 24 and downstream of the SCR catalytic converter 16, in particular downstream of the particle filter 20, downstream of the SCR catalytic converter 22 and downstream of the introduction point E 1. In FIG. 1, arrows 34 illustrate the exhaust gas flowing through the exhaust after-treatment device 14.The exhaust gas aftertreatment device 14 has a first nitrogen oxide sensor 36, a second nitrogen oxide sensor 38 and a third nitrogen oxide sensor 40. The nitrogen oxide sensor 36 is arranged upstream of the SCR catalytic converters 16, 18 and 22, and in the present case also upstream of the particle filter 20, the oxidation catalytic converter 24, the introduction points E 1 and E 2 and the branch point A. The second nitrogen oxide sensor 38 is disposed downstream of the SCR catalyst 18. The nitrogen oxide sensor 40 is arranged upstream of the SCR catalytic converter 18 and downstream of the SCR catalytic converter 16 and in the process also downstream of the SCR catalytic converter 22. By means of the first nitrogen oxide sensor 36, a first variable can be measured at a first measurement point, which characterizes, i.e. indicates, a first quantity of nitrogen oxides contained in the exhaust gas at the first measurement point. A second variable can be detected at a second measurement point by means of the second nitrogen oxide sensor 38, which second variable characterizes, i.e. specifies, a second quantity of nitrogen oxides contained in the exhaust gas at the second measurement point. A third variable can be detected, i.e. measured, at a third measurement point by means of the third nitrogen oxide sensor 40, which third variable specifies a third quantity of nitrogen oxides contained by means of exhaust gas at the third measurement point. The first measurement point is arranged upstream of the SCR catalytic converters 16, 18 and 22 and in the present case also upstream of the oxidation catalytic converter 24. The second measurement point is arranged downstream of the SCR catalytic converter 18, and the third measurement point is arranged upstream of the second SCR catalytic converter 18 and downstream of the SCR catalytic converter 16, in particular downstream of the SCR catalytic converter 22.The SCR catalytic converters 16 and 22 and, for example, also the particle filter 20 are components of a first SCR system, which is preferably a system close to the engine. Thus, the first SCR catalyst 16 is preferably a catalyst close to the engine. The first SCR system and, for example, also the oxidation catalytic converter 24 are arranged, for example, in an engine compartment of the motor vehicle, wherein the internal combustion engine 12 is also arranged in the engine compartment. In contrast, the second SCR catalytic converter 18 is, for example, an underbody catalytic converter (UB catalytic converter), which is arranged outside the engine compartment and, in the vehicle vertical direction of the motor vehicle, below an underbody of the motor vehicle, the interior of which, for example, is also referred to as a passenger compartment or passenger compartment, is formed by a structure which is in particular designed as a self-supporting body. In this case, for example, the structure has the underbody, wherein, for example, the SCR catalytic converter 18 (underbody catalytic converter) is arranged below the underbody in the vehicle vertical direction in such a way that the SCR catalytic converter 18 is arranged outside the engine compartment and is in particular completely covered by the underbody in the vehicle vertical direction of the motor vehicle upwards.The exhaust gas aftertreatment device 14 has a first temperature sensor 42, a second temperature sensor 44 and a third temperature sensor 46. By means of the respective temperature sensor 42, 44, 46, a respective temperature of the exhaust gas is measured, that is to say detected, at a respective temperature measurement point. The temperature measurement point, by means of which the temperature can be detected or is detected by means of the temperature sensor 42, is also referred to as the first temperature measurement point and is arranged in the present case upstream of the SCR catalytic converter 16 and in particular upstream of the oxidation catalytic converter 24. For example, the first temperature measurement point coincides with the first measurement point. The temperature measurement point at which the temperature can be detected or is detected by means of the second temperature sensor 44 is also referred to as a second temperature measurement point, which is arranged upstream of the SCR catalytic converter 16 and downstream of the oxidation catalytic converter 24, in particular the introduction point E 1. The temperature measurement point at which the temperature can be detected or is detected by means of the third temperature sensor 46 is also referred to as a third temperature measurement point which is arranged downstream of the SCR catalytic converter 16, in particular downstream of the SCR catalytic converter 22, and downstream of the particle filter 20 and upstream of the SCR catalytic converter 18. In the present case, the third temperature measuring point is arranged downstream of the branch point A and downstream of the introduction point E 2. The second temperature measurement point is arranged downstream of the introduction point E 1. The temperature sensor 46 is also referred to as a T7 sensor, since the temperature that can be detected or is detected by means of the T7 sensor is also referred to as T7 or as T7. An electronic computing device 48, which is shown particularly schematically in FIG. 1 and is also referred to as a control device, is also provided. The electronic computing device 48 is, for example, a component of the internal combustion engine 12 or of the exhaust gas aftertreatment device 14 or of the drive device 10. The method is carried out by means of the electronic computing device 48.Preferably, the first variable, the second variable and the third variable are the same variable, in particular the measured variable. In the method, the first variable is detected by means of the first nitrogen oxide sensor 36 and the second variable is detected by means of the nitrogen oxide sensor 38 and the third variable is detected by means of the nitrogen oxide sensor 40, for example. The respective nitrogen oxide sensor 36, 38, 40 provides, for example, a respective, in particular electrical, sensor signal which characterizes, for example, the respective variable detected by means of the respective nitrogen oxide sensor 36, 38, 40. The electronic computing device 48 receives the sensor signals, for example, and thus determines the quantities mentioned, for example, and thus the quantities mentioned. Since the respective amount is a respective amount of nitrogen oxides contained in the exhaust gas, the respective amount is also referred to as the amount of nitrogen oxide, so that the first amount is referred to as the first amount of nitrogen oxide, the second amount is referred to as the second amount of nitrogen oxide, and the third amount is referred to as the third amount of nitrogen oxide.By means of the electronic computing device 48, an expected efficiency of the second SCR catalytic converter 18 is determined, in particular calculated. The second quantity is determined from the second variable and the first quantity from the first variable by means of the electronic computing device 48. The temperature of the exhaust gas, also referred to as exhaust gas temperature or T 7, is detected by means of the T 7 sensor. The T7 sensor provides an, in particular electrical, signal characterizing the temperature detected by means of the T7 sensor, which signal is also referred to as a temperature signal. By means of the electronic computing device 48, a change, in particular a gradient, of the exhaust gas temperature detected by means of the T7 sensor is determined as a function of the temperature signal of the T7 sensor, also referred to as a T7 signal. If reference is made below to the exhaust gas temperature, this is to be understood as meaning, unless otherwise stated, the temperature of the exhaust gas detected by means of the T7 sensor.If it is determined by means of the electronic computing device 48 that the change in the exhaust gas temperature is greater than a predefined threshold value of, in particular, 25° C., then and in particular only then by means of the electronic computing device 48 a current fourth quantity of ammonia, which is contained in the exhaust gas downstream of the second SCR catalytic converter 18 and is also referred to as ammonia quantity, is determined as a function of the second variable and as a function of the determined expected efficiency. In other words, for example, the ammonia quantity is determined, in particular calculated, by means of the electronic computing device 48 then and in particular only when a first release condition is fulfilled. The first release condition is then and in particular only fulfilled if the change in the exhaust gas temperature, also referred to as a temperature change, is greater than the threshold value, which is, for example, 25° C.Preferably, it is provided that the ammonia quantity is determined by means of the electronic computing device 48 when and in particular only when it is determined by means of the electronic computing device 48 that the change in the exhaust gas temperature, also referred to as a temperature change or temperature change, is greater than the threshold value at least for a predefined time period of at least one second, in particular of at least two seconds and in particular of at least or exactly five seconds. Thus, for example, the third quantity is determined, in particular calculated, only when, in particular simultaneously, the first release condition and a second release condition are fulfilled. The second release condition is then and in particular only fulfilled if the change in the exhaust gas temperature is greater than the threshold value at least for the mentioned time period, which is also referred to as the first time period.For example, in the method, a difference between the first quantity and the second quantity and / or between the first quantity and the second quantity is determined by means of the electronic computing device 48, which difference is referred to as a further difference compared to a signal difference described below. For example, the fourth quantity (ammonia quantity) is determined as a function of the further difference. For example, it is provided that the ammonia quantity is determined by means of the electronic computing device 48 when it is determined by means of the electronic computing device 48 that the named further difference is smaller than a comparison value, which is zero, for example. The further difference is determined, for example, by subtracting the second variable from the first variable or the second set from the first set. In other words, the ammonia quantity is thus determined, for example, and in particular only when the first quantity or the first variable is smaller than the second quantity or the second variable. Thus, for example, the ammonia quantity is determined, in particular calculated, only when, in particular simultaneously, the first release condition, the second release condition and a third release condition are fulfilled. The third release condition is fulfilled, for example, only when the further difference is smaller than the comparison value, in particular smaller than zero, and therefore when the first variable or first set is smaller than the second variable or second set.For example, the ammonia quantity is determined and in particular only when the first quantity or first quantity is smaller than the second quantity or the second quantity for a second time period. In other words, for example, the ammonia quantity is determined, in particular calculated, when, in particular simultaneously, the first release condition, the second release condition, the third release condition and a fourth release condition are fulfilled. The fourth release condition is fulfilled, for example, and in particular only when the first variable or first quantity is smaller than the second variable or second quantity for a predefined second time period. The first time interval amounts, for example, to at least one second, in particular at least two seconds and very preferably at least or exactly five seconds. The second time period amounts to, for example, at least one second, in particular at least two seconds and very particularly at least or exactly five seconds.Furthermore, it is preferably provided that the ammonia quantity is determined and in particular only when the second quantity is greater than or equal to a limit value which is, for example, 10 parts per million (ppm). Thus, for example, the ammonia quantity is determined, in particular calculated, when, in particular simultaneously, the first release condition, the second release condition, the third release condition, the fourth release condition and a fifth release condition are fulfilled. The fifth release condition is then and in particular only fulfilled when the second quantity is greater than or equal to the limit value.Furthermore, it is provided, for example, that the ammonia quantity is determined when, and in particular only when, the second quantity is greater than the limit value for a third time period. In other words, for example, the ammonia quantity is determined when, in particular simultaneously, the first release condition, the second release condition, the third release condition, the fourth release condition, the fifth release condition and a sixth release condition are fulfilled. The sixth release condition is met, for example, when and only when the second quantity for the second time period is greater than or equal to the limit value, which is 100 ppm, for example. The first time interval amounts, for example, to at least one second, in particular at least two seconds and very preferably at least or exactly five seconds. The second time interval amounts, for example, to at least one second, in particular at least two seconds and very preferably at least or exactly five seconds. The third time interval amounts, for example, to at least one second, in particular at least two seconds and very preferably at least or exactly five seconds.FIG. 2 shows a diagram on whose abscissa 50 the time is plotted, namely increasingly viewed from left to right with respect to the image plane of FIG. 2. The exhaust gas temperature is plotted on the ordinate 52 of the diagram shown in FIG. 2, which exhaust gas temperature can be detected or is detected by means of the temperature sensor 46 at the third measurement point, namely increasingly from bottom to top with respect to the image plane of FIG. 2. A time curve V 1 is a time curve of the unfiltered temperature signal provided by the T 7 sensor. In the method, the temperature signal provided by the temperature sensor 46 (T7 sensor) is filtered by means of a PT1 filter, wherein a curve V2 is a time curve of the filtered temperature signal in FIG. 2. The change in the exhaust gas temperature measured by means of the T7 sensor, also referred to as a gradient or embodied as a gradient, is determined as a function of the temperature signal of the T7 sensor in such a way that a difference, also referred to as a signal difference, between the filtered temperature signal of the T7 sensor and the unfiltered temperature signal of the T7 sensor is determined by means of the electronic computing device 48. The signal difference is determined, in particular calculated, for example, by subtracting respective signal values of the unfiltered temperature signal and of the filtered temperature signal from one another. FIGS. 3 and 4 show diagrams, on the abscissa 54 of which the time is plotted. The first quantity and the second quantity are plotted on the ordinates 56 of the diagrams shown in FIGS. 3 and 4, in particular rising from bottom to top. A respective temporal profile V 3 is a respective temporal profile of the first set, and a respective temporal profile V 4 is a respective temporal profile of the second set. In this case, FIG. 3 shows the time curves of the first quantity and of the second quantity in a traction mode of the internal combustion engine 12, and FIG. 4 shows the time curves V 3 and V 4, and therefore the time curves of the first quantity and of the second quantity in a traction mode of the internal combustion engine 12.List of reference characters10 Drive device 12 Internal combustion engine 14 Exhaust gas aftertreatment device 16 First SCR catalyst 18 Second SCR catalyst 20 Particle filter 22 Third SCR catalyst 24 Oxidation catalyst 26 First metering device 28 Second metering device 30 Branch device 32 Arrow 34 Arrow 36 First nitrogen oxide sensor 38 Second nitrogen oxide sensor 40 Third nitrogen oxide sensor 42 First temperature sensor 44 Second temperature sensor 46 Third temperature sensor 48 Electronic computing device 50 Abscissa 52 Ordinate 54 Abscissa 56 Ordinate A Branch point E 1 First introduction point E 2 Second introduction point V 1 Course V 2 Course V 3 Course V 4 Course

Claims

Method for operating an exhaust gas aftertreatment device (14) of an internal combustion engine (12) of a motor vehicle, which is designed as a diesel engine, in which: - the exhaust gas aftertreatment device (14) through which exhaust gas from the internal combustion engine (12) can flow has: ◯ a first SCR catalytic converter (16); ◯ a second SCR catalytic converter (18) arranged downstream of the first SCR catalytic converter (16); ◯ at least one metering device (26), by means of which a reducing agent for denitrogenating the exhaust gas can be introduced into the exhaust gas at an introduction point (E1) arranged upstream of the first SCR catalytic converter (16); ◯ at least one first nitrogen oxide sensor (36) arranged upstream of the first SCR catalytic converter (16); ◯ at least one second nitrogen oxide sensor (38) arranged downstream of the second SCR catalytic converter (18); and ◯ at least one temperature sensor; a first variable characterizing a first quantity of nitrogen oxides contained in the exhaust gas is detected by means of the first nitrogen oxide sensor (36); a second variable characterizing a second quantity of nitrogen oxides contained in the exhaust gas is detected by means of the second nitrogen oxide sensor (38); an expected efficiency of the second SCR catalyst (18) is determined by means of an electronic computing device (48); the second quantity is determined from the second variable and the first quantity is determined from the first variable; a temperature of the exhaust gas is detected by means of the temperature sensor (46); the temperature sensor (46) provides a signal characterizing the temperature detected by means of the temperature sensor (46); a change in the temperature of the exhaust gas is determined as a function of the signal of the temperature sensor (46); if it is determined by means of the electronic computing device (48) that the change is greater than a predefined threshold value, a current third quantity of ammonia contained in the exhaust gas downstream of the second SCR catalytic converter (18) is determined by means of the electronic computing device (48) as a function of the second variable and as a function of the determined expected efficiency, wherein the change in the temperature of the exhaust gas is determined as a function of the signal of the temperature sensor (46) in such a way that: the signal is filtered by means of a filter; a difference between the filtered signal and the unfiltered signal is determined by means of the electronic computing device (48); and the change in the temperature of the exhaust gas is determined as a function of the difference by means of the electronic computing device (48).Method according to Claim 1, characterized in that the third quantity is determined by means of the electronic computing device (48) if it is determined by means of the electronic computing device (48) that the change is greater than the threshold value for a predefined time period of at least one second.Method according to Claim 1 or 2, characterized in that: - a further difference between the first quantity and the second quantity and / or between the first quantity and the second quantity is determined by means of the electronic computing device (48); and - the third quantity of ammonia is determined only if the first quantity is smaller than the second quantity and / or the first quantity is smaller than the second quantity.Method according to Claim 3, characterized in that a gas transit time between the nitrogen oxide sensors (36, 38) is taken into account for the ascertainment of the further difference.Method according to one of the preceding claims, characterized in that the third quantity is determined by means of the electronic computing device (48) if it is determined by means of the electronic computing device (48) that the first quantity and / or the second quantity is greater than a predefined limit value.Method according to Claim 5, characterized in that the third quantity is determined by means of the electronic computing device (48) if it is determined by means of the electronic computing device (48) that the first quantity and / or the second quantity is greater than the limit value for a predefined time period of at least one second.Motor vehicle which is designed to carry out a method according to one of the preceding claims.

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