Method for operating a drive device and corresponding drive device

Abstract

The invention relates to a method for operating a drive device (1), which has an exhaust-gas-generating drive unit and at least one vehicle catalytic converter (4) designed as an SCR catalytic converter for aftertreatment of the exhaust gas, wherein a reducing agent is added to the exhaust gas at least intermittently upstream of the vehicle catalytic converter (4). According to the invention, a first nitrogen oxide value, which describes the nitrogen oxide content of the exhaust gas upstream of the vehicle catalytic converter (4), and a second nitrogen oxide value, which describes the nitrogen oxide content of the exhaust gas downstream of the vehicle catalytic converter (4), are determined and, at least intermittently, on the basis of the nitrogen oxide values, a first slip indicator is ascertained using a first reducing agent slip model (10) and a second slip indicator is ascertained using a second reducing agent slip model (17), wherein an overall slip indicator is ascertained from the first slip indicator and the second slip indicator. The invention further relates to a drive device (1).

Description

[0001] Method for operating a drive device and corresponding drive device

[0002] DESCRIPTION:

[0003] The invention relates to a method for operating a drive device, in particular for a motor vehicle, which has an exhaust-generating drive unit and at least one vehicle catalyst configured as an SCR catalyst for aftertreating the exhaust gas, wherein a reducing agent is added to the exhaust gas at least temporarily upstream of the vehicle catalyst. The invention further relates to a drive device, in particular for a motor vehicle.

[0004] From the prior art, for example, the document DE 10 2005 031 720 B4 is known. This describes a method for metering a reducing agent in the form of an aqueous urea solution into an exhaust line of an internal combustion engine with an exhaust gas purification system, comprising a valve arranged in the exhaust line for adding the reducing agent to the exhaust gas and a nitrogen oxide reduction catalyst arranged downstream of the metering valve in the exhaust line, designed as an SCR catalyst, at which a selective reduction of nitrogen oxides contained in the exhaust gas can take place with ammonia, and a control device for controlling the exhaust gas purification system, wherein the control device sets a metering rate of the reducing agent to be added to the exhaust gas via the metering valve.

[0005] It is provided that the control device determines changes in a wall film mass of reducing agent deposited on the inner wall of the exhaust pipe and takes them into account when setting the dosing rate, wherein the control device determines an accumulation rate of reducing agent accumulating in the wall film and a desorption rate of reducing agent desorbing from the wall film and the amount of the wall film mass is continuously determined by balancing the accumulation rate and the desorption rate.

[0006] Furthermore, the document DE 10 2009 034 622 B4 discloses a dosing control system and a dosing control method with continuous storage estimation for catalysts for selective catalytic reduction.

[0007] It is an object of the invention to propose a method for operating a drive device, in particular for a motor vehicle, which has advantages over known methods, in particular reliably detects and indicates a breakthrough of the reducing agent through the vehicle catalyst with low latency.

[0008] This is achieved according to the invention with a method for operating a drive device, in particular for a motor vehicle, having the features of claim 1. It is provided that, in particular by means of a first nitrogen oxide sensor, a first nitrogen oxide value describing the nitrogen oxide content of the exhaust gas upstream of the vehicle catalytic converter is determined and, in particular by means of a second nitrogen oxide sensor, a second nitrogen oxide value describing the nitrogen oxide content of the exhaust gas downstream of the vehicle catalytic converter is determined and, at least temporarily, on the basis of the nitrogen oxide values, a first slip indicator is determined using a first reducing agent slip model and a second slip indicator is determined using a second reducing agent slip model, wherein an overall slip indicator is determined from the first slip indicator and the second slip indicator.

[0009] This is done by setting the total slip indicator to the second status over a specific period of time when one of the slip indicators changes status from a first status corresponding to no reductant slip to a second status corresponding to reductant slip, and resetting it to the first status when the other of the slip indicators does not change status by the end of the first period of time, and / or when one of the slip indicators changes status from the second status to the first status while the other of the slip indicators remains the same, the total slip indicator is only set to the first value after a specific second period of time has elapsed, and when the other slip indicator changes status before the end of the second period of time.

[0010] Advantageous embodiments with useful further developments of the invention are specified in the dependent claims. It should be noted that the exemplary embodiments explained in the description are not limiting; rather, any variations of the features disclosed in the description, the claims, and the figures are feasible.

[0011] The drive device preferably serves to drive the motor vehicle, i.e. to provide a drive torque directed towards driving the motor vehicle. To provide the drive torque, the drive device comprises the drive unit. During operation of the drive device, fuel and fresh gas are supplied to the drive unit at least temporarily, wherein the fresh gas contains fresh air at least temporarily. In addition, the fresh gas can comprise exhaust gas, provided that exhaust gas recirculation is implemented, in which the exhaust gas generated by the drive unit is at least partially recirculated into the drive unit, namely as a component of the fresh gas. The fuel and the fresh gas supplied to the drive unit form a fuel-fresh gas mixture with a specific composition, which is reacted in the drive unit.

[0012] During operation of the drive unit, the chemical reaction between fuel and fresh gas produces exhaust gas, which is discharged to the environment outside the drive unit or the vehicle. Since the exhaust gas generated by the drive unit contains pollutants, particularly nitrogen oxides, the exhaust gas is first fed to an exhaust gas aftertreatment system before being released into the environment. In the exhaust gas aftertreatment system, the pollutants are at least partially converted into less hazardous products. Only after passing through the exhaust gas aftertreatment system is the exhaust gas discharged to the environment outside.

[0013] The exhaust gas aftertreatment system has at least one vehicle catalyst configured as an SCR catalyst. The SCR catalyst is provided and configured to perform a selective catalytic reduction of pollutants, in particular nitrogen oxides. To perform the reduction, the reducing agent is added to the exhaust gas, namely upstream of the vehicle catalyst or, in other words, fluidically between the drive unit and the vehicle catalyst, preferably between the first nitrogen oxide sensor—if present—and the vehicle catalyst. This means that the reducing agent passes through the vehicle catalyst together with the exhaust gas, thereby effecting or at least promoting the reduction of the pollutants present in the exhaust gas. Ammonia, for example, is used as a reducing agent and is preferably introduced into the exhaust gas in the form of an aqueous urea solution.Ammonia is produced from the urea solution by thermolysis in the exhaust gas.

[0014] In addition to the SCR catalyst, the exhaust gas aftertreatment system may include at least one additional vehicle catalyst and / or a particulate filter. The additional vehicle catalyst may be, for example, a three-way catalyst, an oxidation catalyst, a NO x A storage catalyst or the like is present. The particulate filter is preferably designed as a gasoline particulate filter or a diesel particulate filter. The particulate filter can be manufactured with an integrated vehicle catalyst and, for example, have a catalytic coating for this purpose.

[0015] A specific amount of reducing agent is required to convert pollutants into less hazardous products in the vehicle's catalytic converter. Excess reducing agent passes through the vehicle's catalytic converter, at least partially chemically unchanged, or is desorbed from the vehicle's catalytic converter. This is referred to as reducing agent slip and leads to a deterioration in vehicle emissions. While concepts already exist for detecting reducing agent slip and adjusting the amount of reducing agent introduced into the exhaust gas or the reducing agent flow rate, these are not sufficiently accurate and / or only provide the required information with a time delay.

[0016] The latter also applies to measuring the reducing agent concentration downstream of the vehicle's catalytic converter. As soon as the measured reducing agent concentration exceeds a certain threshold, an unacceptably large amount of reducing agent is released into the outside environment. This means that a reaction to exceeding the threshold only occurs once the reducing agent has already passed through the vehicle's catalytic converter. The goal, however, is to detect reducing agent slip before or immediately upon its actual occurrence and to initiate appropriate countermeasures. These include, for example, adjusting the amount of reducing agent introduced or the reducing agent mass flow.

[0017] To detect reducing agent slip or impending reducing agent slip, the nitrogen oxide content of the exhaust gas is first determined, namely upstream of the vehicle's catalytic converter and downstream of the vehicle's catalytic converter. For this purpose, for example, the first nitrogen oxide value is measured using the first nitrogen oxide sensor and the second nitrogen oxide value is measured using the second nitrogen oxide sensor. Alternatively, the first nitrogen oxide value can be determined using a model. In this case, the first nitrogen oxide sensor can be omitted, and the second nitrogen oxide sensor can simply be referred to as the nitrogen oxide sensor.

[0018] The first nitrogen oxide value describes the nitrogen oxide content of the exhaust gas upstream of the vehicle's catalytic converter, and the second nitrogen oxide value describes the nitrogen oxide content of the exhaust gas downstream of the vehicle's catalytic converter. In other words, the first nitrogen oxide value is determined fluidically between the drive unit and the vehicle's catalytic converter, and the second nitrogen oxide value is determined fluidically between the vehicle's catalytic converter and a tailpipe through which the exhaust gas from the drive unit is released into the outside environment.

[0019] The nitrogen oxide sensor(s) react not only to nitrogen oxide but also cross-sensitively to the reducing agent. Therefore, the first nitrogen oxide level and the second nitrogen oxide level do not, at least temporarily, correspond to the actual nitrogen oxide level of the exhaust gas at the respective location, but are distorted by the reducing agent content of the exhaust gas. Therefore, the nitrogen oxide sensors or the nitrogen oxide values ​​can be used to assess the reducing agent penetration.

[0020] Based on the two nitrogen oxide values, slip indicators are determined using two different reductant slip models. Slip indicators are status indicators that indicate whether or not the respective reductant slip model detects reductant slip. If the respective slip indicator has a first value, the respective reductant slip model assumes that no reductant slip is present. However, with the second status, the respective reductant slip model assumes the presence of reductant slip. In total, two slip indicators are determined based on the two nitrogen oxide values: the first slip indicator and the second slip indicator. This is done using different reductant slip models, namely the first reductant slip model and the second reductant slip model.The slip indicators each show either the first value or the second value; intermediate values ​​are not provided.

[0021] By using the two different reductant slip models, redundancy is achieved and the quality of the results is improved. However, the two reductant slip models determine the respective slip indicator independently of each other, so a subsequent evaluation of the slip indicators is necessary to determine whether reductant slip actually exists. This evaluation should be as robust as possible and enable a reliable assessment of whether reductant slip is present.

[0022] During the evaluation, the total slip indicator is determined, which ultimately indicates whether or not reductant slip is present. The total slip indicator can also assume the first and second statuses, in particular only the first and second statuses. This means that the first slip indicator, the second slip indicator, and the total slip indicator each temporarily correspond to the first status and temporarily to the second status.

[0023] In order to determine the total slip indicator from the two slip indicators, when the slip indicator changes status from the first to the second, the total slip indicator is initially set to the second status, at least for the first time period. In the first status, no reductant slip is detected, meaning that according to the models there is no reductant slip. In the second status, however, reductant slip is detected. If the other slip indicator does not change status by the end of the first time period, the total slip indicator is reset to the first status. If, on the other hand, the other slip indicator also changes to the second status, the total slip indicator remains in the second status and is therefore not reset.

[0024] This approach, on the one hand, ensures high sensitivity in detecting reductant slip. On the other hand, it prevents false detection of reductant slip by requiring that, after the first time period, both slip indicators must be in the second state for the overall slip indicator to continue to indicate the second state and thus reductant slip. The first time period is, for example, at least 0.1 s, at least 0.5 s, or at least 1 s.

[0025] Additionally or alternatively, if the slip indicator changes from the second status to the first status and the other slip indicator remains in the second status, the total slip indicator is only set to the first value after the second time period has elapsed. However, if the other slip indicator also changes from the second status to the first status, in particular before the second time period has elapsed, the total slip indicator is immediately set to the first value, i.e., before the second time period has elapsed. The second time period can correspond to the first time period.

[0026] The total slip indicator is set from the second value to the first value as soon as at least one of two conditions is met. According to the first condition, the slip indicator changes status from the second to the first, whereas the other slip indicator remains in the second status, but the second time period has already elapsed since the slip indicator's change. According to the second condition, both the first slip indicator and the second slip indicator change status from the second to the first. In this case, the total slip indicator is set from the second value to the first value, regardless of the second time period.

[0027] The described procedure ensures reliable detection of the reducing agent slip, since the overall slip indicator does not only depend on one reducing agent slip model, but two reducing agent slip models are included in it, and moreover, not just a simple OR connection is made between the two slip indicators.

[0028] A further development of the invention provides that the first reducing agent slip model has a frequency model and / or a correlation model, wherein a frequency model concentration value is determined for the frequency model by means of a frequency analysis of the nitrogen oxide values, in particular using at least one recursive filter, and a first partial indicator is set to the first value when the frequency model concentration value falls below a first frequency model threshold value and to the second value when the frequency model concentration value exceeds a second frequency model threshold value, and / or wherein a correlation coefficient is determined from the nitrogen oxide values ​​for the correlation model,wherein a second sub-indicator is set to the first value when a first correlation model threshold is undershot by a distance of the correlation coefficient from a target value corresponding to a match of the nitrogen oxide values, and to the second value when a second correlation model threshold is exceeded by the distance of the correlation coefficient from the target value, and / or wherein the first slip indicator is set to the second value when the first sub-indicator corresponds to the second value and / or the second sub-indicator corresponds to the second value, and is set to the first value when the first sub-indicator corresponds to the first value and / or the second sub-indicator corresponds to the first value.

[0029] The first reducing agent slip model thus contains at least one submodel or several submodels, namely the frequency model and the correlation model. The frequency analysis of the nitrogen oxide values ​​is performed for the frequency model. The frequency analysis is implemented, for example, as a Fourier analysis, in particular using a discrete Fourier transform or a fast Fourier transform (FFT). Preferably, a recursive filter is also used, i.e., a filter whose output values ​​are also used as input values. In other words, an output of the recursive filter is connected to an input of the recursive filter. The recursive filter has specific filter parameters.

[0030] As part of the frequency analysis, the frequency model concentration value is determined, which describes a reducing agent concentration downstream of the vehicle catalyst. In particular, the frequency model concentration value is determined from the low-pass filtered first nitrogen oxide value, the low-pass filtered second nitrogen oxide value, the high-pass filtered first nitrogen oxide value, and the high-pass filtered second nitrogen oxide value. The relationship used. TP stands for low-pass filtering, HP for high-pass filtering; NOxIS is the first nitrogen oxide value, NOxDS the second nitrogen oxide value. For filtering, values ​​of the respective measured value, which are temporarily stored in a buffer memory, are preferably used.

[0031] The low-pass filtered first nitrogen oxide value describes both the reducing agent concentration and the nitrogen oxide concentration downstream of the vehicle's catalytic converter. The ratio of the high-pass filtered second nitrogen oxide value to the high-pass filtered first nitrogen oxide value corresponds to a conversion ratio for the nitrogen oxide in the vehicle's catalytic converter. The low-pass filtered first nitrogen oxide value, in turn, describes the nitrogen oxide concentration of the exhaust gas upstream of the vehicle's catalytic converter. The term

[0032] HP NOxDS)

[0033] ■ TP (NOxUS)

[0034] HP NOxUS describes the nitrogen oxide concentration present downstream of the vehicle's catalytic converter. The nitrogen oxide concentration calculated using this term is also referred to as the model nitrogen oxide concentration; the term itself is used in a nitrogen oxide concentration model.

[0035] If the frequency model concentration value is less than the first frequency model threshold, the first sub-indicator is set to the first value. If, however, the frequency model concentration value is greater than the second frequency model threshold, the first sub-indicator is set to the second value. The first frequency model threshold and the second frequency model threshold can be identical. However, they are preferably different from each other to achieve a hysteresis-like behavior of the frequency model.

[0036] Within the correlation model, the correlation coefficient is determined, which describes the degree of agreement between the nitrogen oxide values. The correlation coefficient is, for example, the empirical correlation coefficient, which is calculated according to the relationship is determined. Here, x is the first nitrogen oxide value and y is the second nitrogen oxide value. The correlation coefficient is determined based on values ​​temporarily stored in the buffer memory(s).

[0037] A separate buffer memory is preferably provided for each of the nitrogen oxide values, i.e., a first buffer memory for the first nitrogen oxide value and a second buffer memory for the second nitrogen oxide value. The most recent nitrogen oxide values ​​are stored in the buffer memory, preferably in a FIFO buffer format. The value n denotes the number of values ​​from the buffer memory for which the calculation is performed. It can correspond to a maximum of the number of values ​​stored in the buffer memory. The index i is the specific value from the buffer memory used for the calculation.

[0038] The average values ​​are calculated based on the nitrogen oxide values ​​temporarily stored in the buffer memory. Therefore, preferably in determining the mean values, whereby the nitrogen oxide values ​​temporarily stored in the buffer memories are used.

[0039] If the correlation coefficient r x , ytowards 1 , then there is a high probability that there is no reducing agent slip. However, if it approaches 0, then there is probably reducing agent slip. Accordingly, the distance between the correlation coefficient and the target value is checked within the framework of the correlation model. The target value is in particular equal to 1 . If the distance between the correlation coefficient and the target value is smaller than the first correlation model threshold value, the second sub-indicator is set to the first value. If, on the other hand, it is greater than the second correlation model threshold value, the second sub-indicator is set to the second value. The first correlation model threshold value and the second correlation model threshold value can in turn be identical. However, they are preferably different from one another in order to achieve the hysteresis-like behavior.

[0040] It can be provided that the first reducing agent slip model only contains the frequency model or the correlation model. In this case, the first slip indicator is set equal to the first sub-indicator or the second sub-indicator. If, on the other hand, both the frequency model and the correlation model are used, both sub-indicators are included in the first slip indicator and are linked accordingly. Preferably, the first slip indicator is set to the second value if only one of the two sub-indicators has the second value. Conversely, the first slip indicator is only set to the first value once both sub-indicators have the first value. However, it can also be provided that the first slip indicator is only set to the second value if both sub-indicators have the second value.In this case, the first slippage indicator is already set to the first value, provided that only one of the two sub-indicators has this value.

[0041] The use of the frequency model, the correlation model, or both the frequency model and the correlation model, and the combination of their results, enables the achievement of particularly good forecast quality. Accordingly, the first slip indicator is highly accurate.

[0042] A further development of the invention provides that the first slip indicator is also set to the second value if a second nitrogen oxide concentration determined from the second nitrogen oxide value is greater than a first nitrogen oxide concentration determined from the first nitrogen oxide value and the second nitrogen oxide concentration is greater than a model nitrogen oxide concentration calculated from the first nitrogen oxide value and the second nitrogen oxide value using a nitrogen oxide concentration model, and / or if the second nitrogen oxide concentration is greater than the first nitrogen oxide concentration during overrun of the drive device.

[0043] The first slip indicator therefore depends on variables other than the first reducing agent slip model or the frequency model and / or the correlation model. First, the nitrogen oxide concentrations are determined from the two nitrogen oxide values, namely directly from the nitrogen oxide values ​​and without taking into account any possible cross-influences from any reducing agent contained in the exhaust gas. Furthermore, the model nitrogen oxide concentration is calculated from the nitrogen oxide value, namely in accordance with the previous explanations. If both the second nitrogen oxide concentration is greater than the first nitrogen oxide concentration and the second nitrogen oxide concentration is greater than the model nitrogen oxide concentration, it is assumed that the second nitrogen oxide value is influenced by the reducing agent present downstream of the vehicle catalytic converter and, consequently, reducing agent slip is present.

[0044] Additionally or alternatively, the first slip indicator is also set to the second value if, on the one hand, the drive system's overrun mode is detected and, at the same time, the second nitrogen oxide concentration is higher than the first nitrogen oxide concentration. "School mode" means that the drive system or drive unit is towed, i.e., driven by the kinetic energy of the vehicle. This means that the drive system or drive unit provides no drive torque or only a low drive torque.

[0045] For example, overrun operation is determined by determining, in particular measuring, an oxygen concentration in the exhaust gas upstream of the vehicle catalytic converter. If the oxygen concentration is above a first oxygen concentration threshold, overrun operation is detected; if it is below a second oxygen concentration threshold, overrun operation is not detected. The two oxygen concentration thresholds can again be identical, but are preferably different from one another to achieve a hysteresis-like behavior. The additional influence of one or both of the aforementioned conditions on the first slip indicator further improves the quality of the detection of reducing agent slip.

[0046] A further development of the invention provides that, for a first slip indicator corresponding to the second value, a first reducing agent mass flow is determined from at least one of the nitrogen oxide values ​​and at least one state variable of the drive device, in particular an exhaust gas temperature, using a reducing agent mass flow model. If the first slip indicator has the second value, it can be assumed that reducing agent slip is present. In this case, the reducing agent mass flow should also be determined in order to determine the extent of the reducing agent slip. This is done using the reducing agent mass flow model, which has at least one of the nitrogen oxide values ​​and at least one state variable of the drive device as input variables, and the reducing agent mass flow as output variable.

[0047] For example, an intermediate value is calculated using the second measured value and the state variable, preferably using a mathematical relationship, a characteristic map, or a table. The exhaust gas temperature is preferably used as the state variable. It can be provided that the reducing agent mass flow is set equal to the intermediate value. However, it is particularly preferred that the intermediate value is first multiplied by a factor resulting from subtracting the model nitrogen oxide concentration from the second nitrogen oxide concentration. In other words, the factor is equal to the second nitrogen oxide concentration minus the model nitrogen oxide concentration. The described procedure enables the reducing agent mass flow to be determined with high accuracy.

[0048] A further development of the invention provides that, within the framework of the second reducing agent slip model, at least one of the nitrogen oxide values ​​and at least one state variable of the drive device are used as input variables of a characteristic map, in particular filtered by means of a further recursive filter, from which a second reducing agent mass flow results as an output variable, wherein the second slip indicator is set to the first value when the second reducing agent mass flow falls below a first reducing agent slip model threshold value and is set to the second value when the second reducing agent mass flow exceeds a second reducing agent slip model threshold value.

[0049] The second reducing agent slip model is map-based in this respect. The first nitrogen oxide value, the second nitrogen oxide value, or both nitrogen oxide values, as well as at least one state variable, serve as input variables for the characteristic map. Using the characteristic map, the output variable—namely, the second reducing agent mass flow—is determined from the input variables. If the second reducing agent mass flow is less than the first reducing agent slip model threshold, the second slip indicator is set to the first value. If, however, the second reducing agent mass flow is greater than the second value, the second slip indicator is set to the second value. The first reducing agent slip model threshold and the second reducing agent slip model threshold can be identical. However, they are preferably different from one another to achieve a hysteresis-like behavior.Preferably, both the first nitrogen oxide value and the second nitrogen oxide value are used as input variables for the characteristic map, with the first nitrogen oxide value being used directly and the second nitrogen oxide value in the form of a difference between the second nitrogen oxide value and the model nitrogen oxide concentration. In other words, the input variables are formed at least from the first nitrogen oxide value and the difference between the second nitrogen oxide value minus the model nitrogen oxide concentration. In addition to the at least one nitrogen oxide value, the at least one state variable is used as an input variable. The state variable is in particular a temperature, preferably an exhaust gas temperature. Furthermore, a temperature gradient, in particular a temporal gradient of the exhaust gas temperature, can be used as an input variable. Additionally or alternatively, the exhaust gas mass flow is used as an input variable.Particularly preferably, at least one of the input variables is subjected to low-pass filtering, in particular recursive low-pass filtering. The filter parameters of the filtering preferably correspond to the filter parameters already mentioned above.

[0050] Such a procedure is known, for example, from published patent application DE 10 2020 111 204 A1, published patent application DE 10 2020 111 206 A1 and published patent application DE 10 2020 111 208 A1, the disclosure of which is fully incorporated into the present description by reference. Published patent application DE 102020 111 204 A1 describes a method for operating a control unit for a motor vehicle, wherein n-dimensional reference input vectors are each assigned a reference output vector, from which output vectors are determined for an n-dimensional input vector, wherein the following steps are carried out: a. Repeating the following steps until a reference input vector is present in an n-dimensional space around the input vector whose distance to the input vector is below a threshold value and / or a maximum number of iterations is reached: i.Selecting at least one neighboring vector from the reference input vectors; ii. Determining an additional reference input vector from the at least one neighboring vector and adding the additional reference input vector to the reference input vectors; b. Selecting at least one calculation vector closest to the input vector from the reference input vectors and calculating the output vector based on the at least one selected calculation vector.

[0051] The n-dimensional reference input vectors and the reference output vectors define the characteristic map or are stored in the characteristic map. The n-dimensional input vector is defined by the input variables, with each of the input variables representing a dimension of the input vector. In the case of the procedure described above, a five-dimensional input vector is present, which contains the first nitrogen oxide value, the temperature, the temperature gradient, the exhaust gas mass flow, and the difference between the second nitrogen oxide value and the model nitrogen oxide concentration. The output vector contains the second reducing agent mass flow, so the output vector is one-dimensional in this case.

[0052] The control unit of published patent application DE 10 2020 111 204 A1 is a component of the drive device and serves, in particular, to control the drive device or the drive unit. At least during operation of the drive device, it determines the second reducing agent mass flow. Particularly preferably, the control unit also carries out the first reducing agent slip model and determines the total slip indicator from the two slip indicators. When determining the second reducing agent slip model threshold value, the procedure is preferably according to claim 1 of published patent application DE 10 2020 111 204 A1, namely according to the definitions already mentioned. The procedure can be further developed according to one or more of claims 2 to 10 of published patent application DE 10 2020 111 204 A1.Further advantageous refinements of the procedure can be found in the description of published patent application DE 10 2020 11 1 204 A1, which is incorporated in its entirety by reference. The described procedure enables a particularly precise determination of the reducing agent mass flow and thus of the second slip indicator.

[0053] A further development of the invention provides that a reducing agent concentration present downstream of the vehicle catalytic converter is measured by means of a reducing agent sensor and used to adapt the characteristic map. The reducing agent sensor is purely optional. If present, it serves to measure the reducing agent concentration downstream of the vehicle catalytic converter and thus to check the reducing agent slip models. It is particularly advantageously used to adapt the characteristic map of the second reducing agent slip model in order to improve its accuracy during operation of the drive system. In this case, the corresponding reducing agent mass flow is preferably determined from the reducing agent concentration, in particular using the exhaust gas mass flow.

[0054] The reducing agent mass flow resulting from the reducing agent concentration is compared with the second reducing agent mass flow. If it deviates, in particular by more than a permissible tolerance, the reducing agent mass flow determined from the reducing agent concentration and the associated input variables are included in the characteristic map. This is particularly preferably carried out according to the procedure explained in the published application DE 10 2020 111 206 A1. This describes a method for operating a control unit for a motor vehicle, wherein n-dimensional reference input vectors are each assigned a reference output vector, from which an output vector can be determined for at least one n-dimensional input vector, and wherein, if a new reference input vector with an associated new reference output vector is present, the following steps are carried out: a. Determining an individual error in the reference input vectors; b.Buffering the reference input vector with the smallest single error and removing this reference input vector from the reference input vectors; c. Calculating the output vector with the new reference input vector as the input vector; d. Determining the single error from a difference between the output vector and the reference output vector associated with the new reference input vector; e. Adding the removed reference input vector to the reference input vectors; f. Replacing the reference input vector with the smallest single error with the new reference input vector if the single error of the new reference input vector is greater than the smallest single error.

[0055] The control unit preferably corresponds to the previously mentioned control unit. The n-dimensional reference input vectors and the associated reference output vectors in turn define the characteristic map; the input vector is composed of the input variables, and the output vector contains the second reducing agent mass flow. The characteristic map is preferably adapted according to patent claim 1 of published patent application DE 10 2020 111 206 A1. The procedure can be further developed according to one or more of patent claims 2 to 10 of published patent application DE 10 2020 111 206 A1. Additional advantageous developments are contained in the description of published patent application DE 10 2020 111 206 A1 and can each optionally be used in addition to the further development of the described method. The content of published patent application DE 10 2020 111 206 A1 is hereby incorporated in its entirety by reference into the present description.

[0056] The described procedure serves to improve the characteristic map directly during vehicle operation; therefore, no intermediate storage of values ​​or subsequent optimization of the characteristic map is necessary. Rather, the computing power of the control unit is completely sufficient to both determine the second reducing agent mass flow and adapt the characteristic map.

[0057] A further development of the invention provides that one of the following operating modes is used: a. a first operating mode, if at least one of the nitrogen oxide values ​​lies outside a predetermined value range and / or a number of values ​​temporarily stored in a buffer memory for at least one of the nitrogen oxide values ​​is less than a predetermined minimum number; b. a second operating mode, if 1) the second nitrogen oxide value corresponds within a tolerance range of the model nitrogen oxide concentration and the first nitrogen oxide concentration is greater than the second nitrogen oxide concentration, and / or 2) the second nitrogen oxide value is less than a threshold value, and / or thirdly the nitrogen oxide value is greater than the threshold value and is transient and the distance of the correlation coefficient determined from the nitrogen oxide values ​​from the target value, in particular taking into account a hysteresis, is less than a correlation threshold value; and / or c.a third operating mode if 1) the first nitrogen oxide value is non-stationary and the distance between the correlation coefficient determined from the nitrogen oxide values ​​and the target value, in particular taking into account the hysteresis, is smaller than the correlation threshold value, and / or 2) the second nitrogen oxide concentration determined from the second nitrogen oxide value is greater than the first nitrogen oxide concentration determined from the first nitrogen oxide value and the second nitrogen oxide concentration is greater than the model nitrogen oxide concentration calculated from the first nitrogen oxide value and the second nitrogen oxide value using the nitrogen oxide concentration model, and / or 3) the drive device is in overrun mode and the second nitrogen oxide concentration is greater than the first nitrogen oxide concentration; d. a fourth operating mode if the first nitrogen oxide value is stationary.

[0058] The drive device is operated in one of the stated operating modes, preferably always in exactly one of the stated operating modes. Based on the stated conditions, the operating mode to be used is determined and subsequently used to operate the drive device until the condition or conditions for another of the operating modes are met. Preferably, the fourth operating mode is a fallback operating mode, which is used if none of the stated conditions for the operating modes are met. The respective operating mode is used to operate the drive device provided that at least one of the stated conditions for it is met. It is therefore not necessary for several or even all of the conditions for the respective operating mode to be met. Of course, it can also be provided that several of the stated conditions must be met for the corresponding operating mode to be used.

[0059] The first operating mode is used if one of the nitrogen oxide values ​​lies outside the specified value range. This indicates a faulty measurement, based on which a reliable assessment of the reducing agent slip cannot be made. The value range is set such that the nitrogen oxide values ​​occurring during normal operation of the drive system lie within the value range. Additionally or alternatively, the first operating mode is used if a sufficient number of nitrogen oxide values ​​are not stored in the buffer memory. The buffer memory is used to temporarily store the nitrogen oxide values; in particular, there is a separate buffer memory for each of the nitrogen oxide values. The first nitrogen oxide values ​​are therefore temporarily stored in a first buffer memory and the second nitrogen oxide values ​​in a second buffer memory.The buffer memory is preferably a FIFO buffer memory, in which the most recently measured values ​​of the respective measured value are stored. The buffer memory is used in particular for averaging the respective nitrogen oxide values, as well as for filtering and / or frequency analysis. If the buffer memory does not contain a sufficient number of values, a meaningful evaluation of the first reductant slip model and / or the second reductant slip model is not possible.

[0060] The second operating mode is used if the second nitrogen oxide value is within the tolerance range of the model nitrogen oxide concentration and, at the same time, the first nitrogen oxide concentration is greater than the second nitrogen oxide concentration. In this case, it is assumed that the nitrogen oxide values ​​are caused solely by the nitrogen oxide and that no reducing agent can be present downstream of the vehicle's catalytic converter. Additionally or alternatively, the second operating mode is used if the second nitrogen oxide value is below the threshold value. The threshold value is selected such that if the second nitrogen oxide value falls below the threshold value, there is definitely no reducing agent present in the exhaust gas downstream of the vehicle's catalytic converter.

[0061] Additionally or alternatively, the second operating mode is used if the first nitrogen oxide value is greater than the threshold value and is simultaneously transient. In addition, the distance between the correlation coefficient and the target value must be smaller than the correlation threshold. Whether the first nitrogen oxide value is stationary or transient is preferably assessed based on the buffer memory for the first nitrogen oxide value. If the values ​​stored in the buffer memory for the first nitrogen oxide value deviate sufficiently significantly from an average of the values, it is assumed that the first nitrogen oxide value is sufficiently transient. The third operating mode is used if one or more of the aforementioned conditions apply. In the third operating mode, the reducing agent slip can be determined particularly well with the help of the reducing agent slip models.

[0062] The fourth operating mode is used if the first nitrogen oxide value is sufficiently steady. In this case, a meaningful evaluation of the reductant slip using the reductant slip models is usually not possible. Using the described procedure, the accuracy of determining the reductant slip can be significantly improved.

[0063] A further development of the invention provides that when the first operating mode is used, the total slip indicator is continuously calculated using frozen nitrogen oxide values ​​and the total slip indicator is frozen; and / or when the second operating mode is used, the total slip indicator is set to and held at the first value, and / or an adaptation of the nitrogen oxide concentration model is carried out based on a difference between the second nitrogen oxide value and the model nitrogen oxide concentration; and / or when the third operating mode is used, the total slip indicator is determined from the first slip indicator and the second slip indicator using the current nitrogen oxide values; and / or when the fourth operating mode is used, the determination of the total slip indicator is suspended until a minimum number of different values ​​have been recorded for at least one of the nitrogen oxide values.

[0064] In the first operating mode, the current nitrogen oxide values, particularly those from the nitrogen oxide sensors, are not used to run the reductant slip models, but rather values ​​that were present immediately before the initiation of the first operating mode. Accordingly, in the first operating mode, the total slip indicator is also frozen at the level used immediately before the initiation of the first operating mode. Since no meaningful statement about the actual total slip indicator is possible in the first operating mode, it is assumed that the value of the total slip indicator present before the initiation of the first operating mode continues to apply.

[0065] In the second operating mode, it is assumed that no reducing agent slip can occur (“no-slip”). Accordingly, the total slip indicator is set to the first value and held at this value. Additionally or alternatively, in the second operating mode, the nitrogen oxide concentration model is adapted or its accuracy improved. For this purpose, the difference between the second nitrogen oxide value and the model nitrogen oxide concentration is determined. From the difference, a correction factor for the nitrogen oxide concentration model or the model nitrogen oxide concentration determined using the nitrogen oxide concentration model is then determined and subsequently used to determine the model nitrogen oxide concentration. In the second operating mode, it is assumed that no reducing agent slip is present, i.e. the second nitrogen oxide value is due solely to the nitrogen oxide.The difference or correction factor is preferably filtered to avoid any influence of measurement errors on the nitrogen oxide concentration model.

[0066] In the third operating mode, the procedure is as described. First, the two slip indicators are determined, and from these, the total slip indicator is calculated.

[0067] In the fourth operating mode, the determination of the total slip indicator is suspended and thus frozen at its value immediately before the initiation of the fourth operating mode. This continues until it is detected that the nitrogen oxide value is again non-stationary, i.e., a sufficient number of different values ​​for the respective nitrogen oxide value are available. The described procedure again serves to improve the quality of the total slip indicator.

[0068] A further development of the invention provides that an error signal is generated when the total slip indicator corresponds to the second value. The total slip indicator indicates that reducing agent slip is present, i.e., that reducing agent is passing through the vehicle's catalytic converter. The error signal is generated accordingly. For example, it is provided that, when the error signal occurs, this is indicated to a driver of the motor vehicle, in particular visually and / or acoustically. Additionally, it can be provided to adapt an operating parameter of the drive device such that the reducing agent slip is reduced; for example, a rated power, a rated torque, or the like is reduced for this purpose. This prevents an impermissibly high amount of reducing agent from escaping into the external environment.

[0069] The invention further relates to a drive device, preferably for a motor vehicle, in particular for carrying out the method according to the explanations in the context of this description, wherein the drive device has a drive unit generating exhaust gas and at least one vehicle catalyst designed as an SCR catalyst for aftertreating the exhaust gas, wherein a reducing agent is added to the exhaust gas at least temporarily upstream of the vehicle catalyst.

[0070] The drive device is provided and designed to determine, in particular by means of a first nitrogen oxide sensor, a first nitrogen oxide value describing the nitrogen oxide content of the exhaust gas upstream of the vehicle catalytic converter and, in particular by means of a second nitrogen oxide sensor, a second nitrogen oxide value describing the nitrogen oxide content of the exhaust gas downstream of the vehicle catalytic converter and, at least temporarily, on the basis of the nitrogen oxide values, to determine a first slip indicator using a first reducing agent slip model and a second slip indicator using a second reducing agent slip model, wherein an overall slip indicator is determined from the first slip indicator and the second slip indicator.

[0071] For this purpose, the total slip indicator is set to the second status over a specific first period of time when one of the slip indicators changes status from a first status corresponding to no reductant slip to a second status corresponding to reductant slip, and is reset to the first status when the other of the slip indicators does not change status by the end of the first period of time, and / or is set to the first value when one of the slip indicators changes status from the second status to the first status while the other of the slip indicators remains the same only after a specific second period of time has elapsed, and when the other slip indicator changes status before the end of the second period of time.

[0072] The advantages of such a drive device design or such a procedure have already been pointed out. Both the drive device and the method for its operation can be further developed according to the explanations in this description, so reference is made to these in this regard.

[0073] The features and feature combinations described in the description, in particular the features and feature combinations described in the following description of the figures and / or shown in the figures, can be used not only in the respective combination specified, but also in other combinations or on their own, without departing from the scope of the invention. Thus, embodiments are also considered to be encompassed by the invention that are not explicitly shown or explained in the description and / or the figures, but which follow from or can be derived from the explained embodiments.

[0074] The invention will be explained in more detail below with reference to the exemplary embodiments shown in the drawings, without limiting the invention.

[0075] Figure 1 shows a schematic representation of a drive device, preferably for a motor vehicle. Figure 1 shows a schematic representation of a drive device 1, preferably for a motor vehicle. The drive device 1 has a drive unit (not shown here), which generates exhaust gas during operation, which is discharged via an exhaust line 2 towards an external environment of the drive device 1. In the exhaust line 2 there is an exhaust gas aftertreatment device 3, which in the exemplary embodiment shown here has at least one vehicle catalytic converter 4 designed as an SCR catalytic converter and an - optional - further vehicle catalytic converter 5.

[0076] A first nitrogen oxide sensor 6 is located upstream of the vehicle catalytic converter 4, and a second nitrogen oxide sensor 7 is located downstream of the vehicle catalytic converter 4. The first nitrogen oxide sensor 6 measures a first nitrogen oxide value describing the nitrogen oxide content of the exhaust gas upstream of the vehicle catalytic converter 4, and a second nitrogen oxide sensor measures a second nitrogen oxide value describing the nitrogen oxide content of the exhaust gas downstream of the vehicle catalytic converter 4. In an alternative embodiment, the first nitrogen oxide sensor 6 can be omitted. In this case, the first nitrogen oxide value is determined using a model.

[0077] For example, the first nitrogen oxide sensor 6 is fluidically arranged between the vehicle catalytic converter 4 and the additional black catalyst 5. Fluidically arranged between the first nitrogen oxide sensor 6 and the vehicle catalytic converter 4 is an introduction point 8, at which a reducing agent can be introduced into the exhaust gas by means of an injector 9, namely upstream of the vehicle catalytic converter 4.

[0078] The two nitrogen oxide values ​​are fed to a first reducing agent slip model 10, which contains a frequency model 11 and a correlation model 12. A sub-indicator is determined using each of these models 11 and 12, namely a first sub-indicator using the frequency model 11 and a second sub-indicator using the correlation model 12. The two sub-indicators are fed to a computing device 13, which combines them into a first slip indicator. The computing device 13 preferably also performs a plausibility check of the first slip indicator using the first measured value, the second measured value, and a nitrogen oxide value from a reducing agent sensor 14.

[0079] The first slip indicator is subsequently fed to a further computing device 15, as is at least one state variable, for example, an exhaust gas temperature. Additionally, the second nitrogen oxide value and / or the nitrogen oxide value of the reducing agent sensor 14 can be fed to the further computing device. The further computing device 15 calculates a first reducing agent mass flow from at least one of the nitrogen oxide values ​​of the nitrogen oxide sensors 6 and 7 and the at least one state variable, namely using a reducing agent mass flow model. The further computing device 15 transmits both the first slip indicator and the first reducing agent mass flow to an evaluation device 16.

[0080] In addition, the drive device 1 has a second reducing agent slip model 17. Several state variables are fed to this at an input 18, in particular the first nitrogen oxide value, an exhaust gas temperature, an exhaust gas temperature gradient, an exhaust gas mass flow, and a difference between the second sensor value and a model nitrogen oxide concentration. The input variables 18 are filtered using a low-pass filter 19, at which specific filter parameters 20 are set. The low-pass filtered input variables are fed to a calculation unit 21, which, using a characteristic map 22, determines a second reducing agent mass flow from the input variables. This is transmitted to a calculation unit 23, which determines a second slip indicator from the second reducing agent mass flow. The second slip indicator and the second reducing agent mass flow are transmitted to the evaluation unit 16.

[0081] If the reducing agent sensor 14 is present, the nitrogen oxide value detected by it is provided at an input 24. A calculation unit 25 adapts the characteristic map 22 based on the measured value of the reducing agent sensor 14 and the low-pass filtered input variable, so that the data stored therein is optimized.

[0082] The evaluation device 16 determines a total slip indicator from the first slip indicator and the second slip indicator and provides it at an output. For example, if the total slip indicator corresponds to a specific value, an error signal is generated. The described configuration of the drive device 1 and the explained method serve to determine the total slip indicator extremely precisely. Accordingly, it is determined with good accuracy whether or not reducing agent slip is occurring through the vehicle catalytic converter 4.

[0083] LIST OF REFERENCE SYMBOLS:

[0084] 1 drive device

[0085] 2 exhaust pipe

[0086] 3 Exhaust aftertreatment system

[0087] 4 Vehicle catalytic converter

[0088] 5 additional vehicle catalysts

[0089] 6 1 . Nitrogen oxide sensor

[0090] 7 2. Nitrogen oxide sensor

[0091] 8 Place of entry

[0092] 9 Injector

[0093] 10 1. Reductant slip model

[0094] 11 Frequency model

[0095] 12 Correlation model

[0096] 13 Computing device

[0097] 14 Reductant sensor

[0098] 15 additional computing devices

[0099] 16 Evaluation device

[0100] 17 2. Reductant slip model

[0101] 18 Entrance

[0102] 19 low-pass filters

[0103] 20 filter parameters

[0104] 21 Calculation device

[0105] 22 map

[0106] 23 Calculation device

[0107] 24 entrance

[0108] 25 Calculation device

Claims

PATENT CLAIMS:

1. A method for operating a drive device (1) having an exhaust-generating drive unit and at least one vehicle catalyst (4) configured as an SCR catalyst for aftertreating the exhaust gas, wherein a reducing agent is added to the exhaust gas at least temporarily upstream of the vehicle catalyst (4), characterized in that a first nitrogen oxide value describing the nitrogen oxide content of the exhaust gas upstream of the vehicle catalyst (4) and a second nitrogen oxide value describing the nitrogen oxide content of the exhaust gas downstream of the vehicle catalyst (4) are determined, and at least temporarily, based on the nitrogen oxide values, a first slip indicator is determined using a first reducing agent slip model (10) and a second slip indicator is determined using a second reducing agent slip model (17), wherein a total slip indicator is determined from the first slip indicator and the second slip indicator.by setting the total slip indicator a. to the second status upon a status change of one of the slip indicators from a first status corresponding to a lack of reductant slip to a second status corresponding to reductant slip over a specific first period of time and resetting it to the first status upon a status change of the other of the slip indicators not occurring by the end of the first period of time, and / or b. upon a status change of one of the slip indicators from the second status to the first status, if the other of the slip indicators remains unchanged, the total slip indicator is only set to the first value upon expiration of a specific second period of time and upon a status change of the other slip indicator before the expiration of the second period of time.

2. The method according to claim 1, characterized in that the first reducing agent slip model (10) has a frequency model (11) and / or a correlation model (12), wherein a. a frequency model concentration value is determined for the frequency model (11) by means of a frequency analysis of the nitrogen oxide values ​​and a first partial indicator is set to the first value when the frequency model concentration value falls below a first frequency model threshold value and to the second value when the frequency model concentration value exceeds a second frequency model threshold value, and / or wherein b.a correlation coefficient is determined from the nitrogen oxide values ​​for the correlation model (12), wherein a second partial indicator is set to the first value when a first correlation model threshold value is undershot by a distance of the correlation coefficient from a target value corresponding to a match of the nitrogen oxide values, and is set to the second value when a second correlation model threshold value is exceeded by the distance of the correlation coefficient from the target value, and / or wherein c. the first slip indicator is set to the second value when the first partial indicator corresponds to the second value and / or the second partial indicator corresponds to the second value, and is set to the first value when the first partial indicator corresponds to the first value and / or the second partial indicator corresponds to the first value.

3. Method according to one of the preceding claims, characterized in that the first slip indicator is also set to the second value if a. a second nitrogen oxide concentration determined from the second nitrogen oxide value is greater than a first nitrogen oxide concentration determined from the first nitrogen oxide value and the second nitrogen oxide concentration is greater than a model nitrogen oxide concentration calculated from the first nitrogen oxide value and the second nitrogen oxide value using a nitrogen oxide concentration model, and / or if b. during overrun operation of the drive device (1), the second nitrogen oxide concentration is greater than the first nitrogen oxide concentration.

4. Method according to one of the preceding claims, characterized in that, for the first slip indicator corresponding to the second value, a first reducing agent mass flow is determined from at least one of the nitrogen oxide values ​​and at least one state variable of the drive device (1) by means of a reducing agent mass flow model.

5. Method according to one of the preceding claims, characterized in that, within the framework of the second reducing agent slip model (17), at least one of the nitrogen oxide values ​​and at least one state variable of the drive device (1) are used as input variables of a characteristic map (22), from which a second reducing agent mass flow results as an output variable, wherein the second slip indicator is set to the first value when the second reducing agent mass flow falls below a first reducing agent slip model threshold value and is set to the second value when the second reducing agent mass flow exceeds a second reducing agent slip model threshold value.

6. Method according to one of the preceding claims, characterized in that a reducing agent concentration present downstream of the vehicle catalyst (4) is measured by means of a reducing agent sensor (14) and used to adapt the characteristic map (22).

7. Method according to one of the preceding claims, characterized in that one of the following operating modes is used: a. a first operating mode if at least one of the nitrogen oxide values ​​lies outside a predetermined value range and / or a number of values ​​temporarily stored in a buffer memory for one of the nitrogen oxide values ​​is less than a predetermined minimum number; b. a second operating mode if i. the second nitrogen oxide value is within a tolerance range of the model nitrogen oxide concentration calculated using the nitrogen oxide concentration model from the first nitrogen oxide value and the second nitrogen oxide value and the first nitrogen oxide concentration is greater than the second nitrogen oxide concentration, and / or ii. the second nitrogen oxide value is less than a threshold value, and / or iii. the first nitrogen oxide value is greater than the threshold value and is transient and the distance between the correlation coefficient determined from the nitrogen oxide values ​​and the target value is less than a correlation threshold value; and / or c. a third operating mode if i.the first nitrogen oxide value is non-stationary and the distance of the correlation coefficient determined from the nitrogen oxide values ​​from the target value is smaller than the correlation threshold value, and / or ii. the second nitrogen oxide concentration determined from the second nitrogen oxide value is greater than the first nitrogen oxide concentration determined from the first nitrogen oxide value and the second nitrogen oxide concentration is greater than the one determined from the. Nitrogen oxide concentration model calculated from the first nitrogen oxide value and the second nitrogen oxide value, and / or iii. the propulsion device (1) is in overrun mode and the second nitrogen oxide concentration is greater than the first nitrogen oxide concentration; d. a fourth operating mode if the first nitrogen oxide value is stationary.

8. Method according to one of the preceding claims, characterized in that a. when using the first operating mode, the total slip indicator is continuously calculated with frozen nitrogen oxide values ​​and the total slip indicator is frozen; and / or b. when using the second operating mode i. the total slip indicator is set and maintained at the first value, and / or ii. an adaptation of the nitrogen oxide concentration model is carried out based on a difference between the second nitrogen oxide value and the model nitrogen oxide concentration; and / or c. when using the third operating mode, the total slip indicator is determined from the first slip indicator and the second slip indicator using the current nitrogen oxide values; and / or d. when using the fourth operating mode, the determination of the total slip indicator is suspended until at least for one of the Nitrogen oxide values ​​a minimum number of different values ​​were recorded.

9. Method according to one of the preceding claims, characterized in that an error signal is generated when the total slip indicator corresponds to the second value.

10. Drive device (1), in particular for carrying out the method according to one or more of the preceding claims, wherein the drive device (1) has a drive unit generating exhaust gas and at least one vehicle catalyst (4) designed as an SCR catalyst for aftertreating the exhaust gas, wherein a reducing agent is added to the exhaust gas at least temporarily upstream of the vehicle catalyst (4), characterized in that the drive device (1) is provided and designed toto determine a first nitrogen oxide value describing the nitrogen oxide content of the exhaust gas upstream of the vehicle catalytic converter (4) and a second nitrogen oxide value describing the nitrogen oxide content of the exhaust gas downstream of the vehicle catalytic converter (4), and to determine, at least temporarily, a first slip indicator using a first reducing agent slip model (10) and a second slip indicator using a second reducing agent slip model (17) based on the nitrogen oxide values, wherein a total slip indicator is determined from the first slip indicator and the second slip indicator,by setting the total slip indicator a. to the second status upon a status change of one of the slip indicators from a first status corresponding to a lack of reducing agent slip to a second status corresponding to a reducing agent slip over a certain first period of time and resetting it to the first status upon a status change of the other of the slip indicators not occurring until the end of the first period of time, and / or b. in the event of a status change of one of the slip indicators from the second status to the first status, if the other of the slip indicators remains the same, the value is only set to the first value after a certain second period of time has elapsed and in the event of a status change of the other slip indicator before the end of the second period of time.

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