Method for operating a drive device for a motor vehicle, corresponding drive device and computer program product

Abstract

The invention relates to a method for operating a drive device for a motor vehicle, which has an exhaust-generating drive unit, an exhaust aftertreatment device configured as a vehicle catalyst for aftertreating the exhaust gas, and a lambda probe arranged downstream of the exhaust aftertreatment device for determining a combustion air ratio in the exhaust gas. It is provided that, upon detection of a specific combustion air ratio by the lambda probe, a gradient of the combustion ratio is detected and used to determine a state variable describing a state of the exhaust aftertreatment device. The invention further relates to a drive device for a motor vehicle and a computer program product.

Description

[0001] The invention relates to a method for operating a drive device for a motor vehicle, which has an exhaust-generating drive unit, an exhaust aftertreatment device configured as a vehicle catalyst for aftertreating the exhaust gas, and a lambda probe arranged downstream of the exhaust aftertreatment device for determining a combustion air ratio in the exhaust gas. The invention further relates to a drive device for a motor vehicle and a computer program product.

[0002] For example, US 2021 / 0404368 A1 is known from the prior art. This describes a degradation diagnosis device comprising a downstream air-fuel ratio sensor and a control device. The control device is configured to alternately and repeatedly perform a rich process and a lean process in a degradation diagnosis process to diagnose degradation of the exhaust catalyst. The control device is further configured to determine that the exhaust catalyst has deteriorated in the degradation diagnosis process when the lean process is performed and the frequency with which an air-fuel ratio measured by the sensor is equal to a lean air-fuel ratio is equal to or greater than a predetermined frequency.

[0003] Furthermore, document US 10,072,553 B2 discloses a degradation diagnosis device for an SCR catalyst in which, when an air-fuel ratio of a mixture to be combusted in an internal combustion engine is a lean air-fuel ratio, induction processing is carried out to induce a water-gas shift reaction in a pre-catalyst by changing the air-fuel ratio of the mixture from the lean air-fuel ratio to a predetermined rich air-fuel ratio, and diagnosis processing is carried out to diagnose deterioration of the SCR catalyst based on an output difference between two air-fuel ratio sensors at the time of execution of the induction processing.However, the diagnostic processing is not executed if the SCR catalyst is in a state where it is subject to sulfur poisoning resulting from the execution of a pre-stage catalyst cleaning process.

[0004] Another degradation diagnostic device is described in the document US 10,072,553 B2.

[0005] The object of the invention is to propose a method for operating a drive device for a motor vehicle, which has advantages over known methods, in particular enabling a particularly reliable and accurate diagnosis of the exhaust gas aftertreatment device or the vehicle catalyst.

[0006] This is achieved according to the invention with a method for operating a drive device for a motor vehicle having the features of claim 1. It is provided that when a specific combustion air ratio is detected by means of the lambda probe, a gradient of the combustion air ratio is detected and used to determine a state variable describing a state of the exhaust gas aftertreatment device.

[0007] 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.

[0008] The method is provided for operating the drive device. The drive device serves to drive the motor vehicle, thus providing a drive torque directed towards driving the motor vehicle. The drive unit provides the drive torque. The drive unit is preferably in the form of an internal combustion engine, in particular a gasoline internal combustion engine or a diesel internal combustion engine. During operation of the drive device, fuel and fresh gas are supplied to the drive unit at least temporarily, wherein the fresh gas at least temporarily contains fresh air. 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 back into the drive unit, namely as a component of the fresh gas.The fuel and fresh gas supplied to the drive unit form a fuel-fresh gas mixture with a specific composition, which is reacted in the drive unit.

[0009] During operation of the drive unit, the chemical reaction between fuel and fresh gas produces exhaust gas, which is discharged to the outside environment of the drive system or motor vehicle. Since the exhaust gas generated by the drive unit contains pollutants, the exhaust gas is first fed to the exhaust aftertreatment system before being released into the outside environment. In the exhaust aftertreatment system, the pollutants are at least partially converted into less hazardous products. Only after passing through the exhaust aftertreatment system is the exhaust gas discharged to the outside environment, in particular through a tailpipe of the drive system.

[0010] The exhaust gas aftertreatment device is in the form of a vehicle catalyst, in particular a three-way catalyst, oxidation catalyst, NOx storage catalyst, or SCR catalyst, or at least comprises one. Particularly preferably, the vehicle catalyst is integrated into a particulate filter, in particular a gasoline particulate filter or a diesel particulate filter. For this purpose, the particulate filter is provided with a catalytic coating, for example. The conversion rate and thus the conversion performance of the exhaust gas aftertreatment device, with which the pollutants are converted into less hazardous products, depend in particular on the composition of the exhaust gas supplied to the exhaust gas aftertreatment device, the temperature of the exhaust gas aftertreatment device, and the condition of the exhaust gas aftertreatment device.

[0011] The components of the exhaust gas produced by the drive unit are also referred to as raw emissions. Raw emissions describe the composition of the exhaust gas upstream of the exhaust aftertreatment device, or in terms of flow between the drive unit and the exhaust aftertreatment device. The substances contained in the exhaust gas are partially converted as the exhaust gas passes through the exhaust aftertreatment device, changing the composition of the exhaust gas. The substances present in the exhaust gas downstream of the exhaust aftertreatment device, which make up the exhaust gas, are also referred to as tailpipe emissions, since the exhaust gas with this composition is released into the outside environment through the tailpipe of the drive unit.

[0012] The quantity of pollutants contained in the tailpipe emissions depends, as already mentioned, on the raw emissions, but also on the conversion efficiency of the exhaust aftertreatment system. This is temperature-dependent. In particular, the further the temperature of the exhaust aftertreatment system is from the operating temperature of the exhaust aftertreatment system, i.e., the greater the absolute value of the difference between the temperatures, the lower the conversion efficiency. The temperature of the exhaust aftertreatment system is understood to mean, in particular, the temperature of a ceramic honeycomb body provided with the catalytic coating. In addition, the conversion efficiency depends on the condition of the exhaust aftertreatment system, which results in particular from the age or operating time of the exhaust aftertreatment system.

[0013] Downstream of the exhaust gas aftertreatment device is the lambda probe, which serves to determine the combustion air ratio in the exhaust gas, i.e., to determine the combustion air ratio present downstream of the exhaust gas aftertreatment device. To this end, the lambda probe measures, in particular, a residual oxygen content in the exhaust gas and uses this to determine the combustion air ratio. The lambda probe is preferably designed as a step-down lambda probe.

[0014] To determine the conversion efficiency of the exhaust aftertreatment system, it is important to determine the condition of the exhaust aftertreatment system with the greatest possible accuracy. The condition of the exhaust aftertreatment system can be determined, for example, based on its oxygen capacity, i.e., its capacity for temporarily storing oxygen. The higher the oxygen capacity, the better the condition of the exhaust aftertreatment system. The oxygen capacity decreases with increasing operating time of the exhaust aftertreatment system, so this is reflected in the resulting condition of the exhaust aftertreatment system.

[0015] However, the applicant has surprisingly discovered that the water-gas shift reaction is also an indicator of the condition of the exhaust gas aftertreatment system. The reaction can be described by the molecular formula CO + H 2 O → CO 2 + H 2. If the fuel-fresh gas mixture is rich, there is an excess of carbon monoxide in the raw emissions. The vehicle's catalytic converter extracts oxygen from the water also contained in the exhaust gas, which reacts with the carbon monoxide and optionally with hydrocarbons or oxidizes them. The combustion air ratio in the exhaust gas is not changed as a result; instead, a reduction in the carbon monoxide concentration and an increase in the concentration of molecular hydrogen can be observed. While hydrogen emissions are not subject to any restrictions, carbon monoxide emissions are limited by law.

[0016] The lambda sensor is cross-sensitive to hydrogen, meaning it also reacts to it. This means that if carbon monoxide components are replaced by hydrogen components in equal proportions, meaning the combustion ratio remains unchanged, the lambda sensor measures a richer mixture than actually present. The combustion air ratio is determined by: λ = m Luft m Kraftstoff ⋅ 14 , 7 = 2 C O 2 + 2 C CO 2 + C H 2 O + C CO + C NO + 2 C NO 2 2 C CO + C H 2 + 9 C C 3 H 6 + 10 C C 3 H 8 + 2 C CO 2 + C H 2 O

[0017] The diffusion rate can be expressed as follows: v x ¯ = 8 k B T πm x

[0018] This results in the relationship: λ gemessen = 2 1 32 C O 2 + 2 1 44 C CO 2 + 1 18 C H 2 O + 1 28 C CO + 1 30 C NO + 2 1 46 C NO 2 2 1 28 C CO + 1 2 C H 2 + 9 1 42 C C 3 H 6 + 10 1 44 C C 3 H 8 + 2 1 44 C CO 2 + 1 18 C H 2 O

[0019] The intensity of the water-gas shift reaction changes with the condition of the exhaust aftertreatment system and its temperature. At low temperatures, the decrease is particularly pronounced with increasing operating time, i.e., with deteriorating condition. However, it is not possible to measure this directly, as this would require operating the powertrain with a rich mixture for an extended period. This would lead to high emissions and high fuel consumption.

[0020] For this reason, the combustion air ratio gradient is used as soon as the specific combustion air ratio is detected using the lambda sensor. If the lambda sensor measures a combustion air ratio that corresponds to the specific combustion air ratio, the combustion air ratio gradient is evaluated, namely at the exact point in time at which the combustion air ratio is equal to the specific combustion air ratio. From the gradient, it can be deduced how the combustion air ratio would further change if the drive unit were operated with a constant composition of the fuel / fresh gas mixture. Accordingly, the combustion air ratio can be deduced from the gradient with a combustion air ratio deviating from λ = 1.Consequently, the state of the exhaust gas aftertreatment system can be reliably derived from the gradient and the state variable describing it can be determined.

[0021] Preferably, the determined combustion air ratio is selected such that, when it is reached by the combustion air ratio measured by the lambda probe, the oxygen reservoir of the exhaust gas aftertreatment device is completely or at least almost completely emptied. For example, the determined combustion air ratio is at least 0.992, at least 0.996, or at least 0.998 and / or at most 0.999, at most 0.998, or at most 0.997. Particularly preferably, the state variable is determined from both the oxygen capacity of the exhaust gas aftertreatment device and the gradient, so that both variables are incorporated into the state variable, or the state variable is available as a function of the oxygen capacity and the gradient. This achieves particularly high accuracy.

[0022] A further development of the invention provides that the drive unit is operated with a first composition of the fuel-fresh gas mixture corresponding to a rich fuel-fresh gas mixture until the determined combustion air ratio is detected by the lambda probe, and / or that after the determined combustion air ratio has been detected by the lambda probe, the drive unit is operated with a second composition of the fuel-fresh gas mixture that differs from the first composition. The first composition is selected such that the combustion air ratio measured by the lambda probe changes towards the determined combustion air ratio. The first composition corresponds to the rich fuel-fresh gas mixture, so that the measured combustion air ratio also changes towards a rich combustion air ratio.The drive unit is operated with the first composition at least until the measured combustion air ratio corresponds to the determined combustion air ratio. At the point in time at which this is the case for the first time, the gradient of the combustion air ratio is recorded or calculated and subsequently used to determine the state of the exhaust gas aftertreatment system.

[0023] Additionally or alternatively, the drive unit is operated with the second composition after the determined combustion air ratio has been detected using the lambda probe. Preferably, it is provided to switch from the first composition to the second composition as soon as the measured combustion air ratio corresponds to the determined combustion air ratio. Thus, the drive unit is operated with the first composition until the determined combustion air ratio is reached by the measured combustion air ratio; from the moment the determined combustion air ratio is reached by the measured combustion air ratio, it is operated with the second composition.

[0024] The second composition differs from the first composition; in particular, it corresponds to a lean fuel-fresh gas mixture. For example, the first composition corresponds to a combustion air ratio of at most 0.98, at most 0.96, or at most 0.94. Additionally or alternatively, the second composition corresponds to a combustion air ratio of at least 1.02, at least 1.04, or at least 1.06. This achieves a high degree of accuracy in determining the state variable.

[0025] For example, the drive unit is operated with the second composition until the combustion air ratio measured by the lambda probe corresponds to another specific combustion air ratio. This further specific combustion air ratio is selected in particular such that, when it is reached by the measured combustion air ratio, it can be assumed that the oxygen storage of the exhaust gas aftertreatment system is completely filled. The period over which the drive unit is operated with the second composition can be used to determine the oxygen capacity of the exhaust gas aftertreatment system and thus its condition.

[0026] A further development of the invention provides that, prior to operation with the first composition, the drive unit is operated with a third composition of the fuel-fresh gas mixture, wherein the third composition is selected such that a combustion air ratio measured by the lambda probe changes away from the determined combustion air ratio. In other words, the measured combustion air ratio should change during operation of the drive unit with the third composition such that its absolute distance from the determined combustion air ratio increases. Preferably, the third composition corresponds to a lean fuel-fresh gas mixture; particularly preferably, the third composition is the same as the second composition.

[0027] A further development of the invention provides that the drive unit is operated with the first composition of the fuel-fresh gas mixture as part of a diagnosis of the exhaust gas aftertreatment system and / or after an overrun phase. Diagnosing the exhaust gas aftertreatment system primarily serves to determine its condition. Diagnosing the exhaust gas aftertreatment system preferably includes completely emptying the oxygen reservoir of the exhaust gas aftertreatment system and then completely filling it. The oxygen reservoir is emptied by operating the drive unit with the first composition, and filled by operating it with the second composition.

[0028] Additionally or alternatively, operation with the first composition can also be carried out after the overrun phase of the drive unit. The overrun phase is understood to be an operating state of the drive unit during which it is towed by an external torque without fuel supply, i.e. with an interrupted fuel supply. During the overrun phase, the drive unit delivers fresh gas and therefore oxygen towards the exhaust gas aftertreatment system so that the oxygen reservoir of the exhaust gas aftertreatment system is filled. Since the conversion performance of the exhaust gas aftertreatment system also depends on the fill level of the oxygen reservoir, it is necessary to at least partially empty it again after the overrun phase in order to set the fill level to a target fill level, which is, for example, at least 40% and at most 60%, preferably approximately or exactly 50%.

[0029] Particularly preferably, after the overrun phase, operation with the first composition is carried out until the oxygen reservoir is completely empty. Subsequently, however, it is not completely filled; instead, the fill level is adjusted to the target level, thus only partially filling the oxygen reservoir. Thus, the condition of the exhaust gas aftertreatment system can be determined not only during its diagnosis, but significantly more frequently, namely after the overrun phase, preferably after each overrun phase. This significantly improves the accuracy of the determined state variable.

[0030] A further development of the invention provides that the specific combustion air ratio is determined as a function of the temperature of the lambda sensor and / or the temperature of the lambda sensor is used to correct the gradient. The combustion air ratio measured by the lambda sensor during operation of the drive unit with the rich fuel-fresh gas mixture, i.e., with the first composition, depends not only on the composition but also on the temperature of the lambda sensor. Thus, the lambda sensor measures a leaner combustion air ratio at a lower temperature than at a higher temperature.

[0031] For this reason, it is beneficial to the accuracy in determining the state of the exhaust gas aftertreatment system if the specific combustion air ratio is determined as a function of the temperature of the lambda probe, in particular if the higher the temperature, the lower the value is selected. Additionally or alternatively, the gradient is corrected using the temperature. With such a procedure, it is provided, for example, that the specific combustion air ratio is selected independently of the temperature and that the gradient of the measured combustion air ratio present when the specific combustion air ratio is reached by the measured combustion air ratio is adjusted based on the temperature. In any case, a particularly high level of accuracy of the gradient and thus of the state of the exhaust gas aftertreatment system is achieved.

[0032] A further development of the invention provides that the temperature is determined from the electrical resistance of the lambda sensor. The electrical resistance is, in particular, the electrical resistance of a Nernst cell of the lambda sensor. This resistance changes with the temperature of the lambda sensor and is easy to evaluate using circuitry. Consequently, accuracy is increased using simple means.

[0033] A further development of the invention provides that a first lambda probe is located upstream of the exhaust gas aftertreatment device for measuring a first combustion air ratio in the exhaust gas, and the lambda probe is used as a second lambda probe for measuring the combustion air ratio present as the second combustion air ratio, wherein a lambda control of the drive unit is carried out based on the first combustion air ratio and the second combustion air ratio. Therefore, preferably, several lambda probes are present; in particular, the first lambda probe is located upstream of the exhaust gas aftertreatment device, and the second lambda probe is located downstream of the exhaust gas aftertreatment device. The second lambda probe represents the aforementioned lambda probe.

[0034] The first lambda sensor is used to determine a first combustion air ratio, and the second lambda sensor is used to determine a second combustion air ratio. The first combustion ratio corresponds to a combustion air ratio in the exhaust gas upstream of the exhaust aftertreatment device, i.e., in terms of flow, between the drive unit and the exhaust aftertreatment device. The second combustion air ratio is a combustion air ratio present in the exhaust gas downstream of the exhaust aftertreatment device. The two combustion air ratios, i.e., the first combustion air ratio and the second combustion air ratio, are preferably used to operate the drive device. For example, they are used to implement lambda control.In particular, the composition of the fuel-fresh gas mixture is adjusted based on the first combustion air ratio, whereas the second combustion air ratio is used to correct the first combustion air ratio as part of a trim control. Ultimately, the lambda control is based on both the first combustion air ratio and the second combustion air ratio.

[0035] Preferably, the first lambda sensor is a broadband lambda sensor, and the second lambda sensor is a step-type lambda sensor. The broadband lambda sensor enables the residual oxygen content or the corresponding combustion air ratio to be measured over a wider measuring range than the step-type lambda sensor. The broadband lambda sensor is preferably used to implement the aforementioned lambda control and, accordingly, to adjust the composition of the fuel-fresh gas mixture with which the drive unit is operated.

[0036] The step-off lambda sensor has a narrower measuring range than the broadband lambda sensor; in particular, it is used (only) to detect a combustion air ratio of λ = 1. However, the measurement accuracy of the step-off lambda sensor is higher than that of the broadband lambda sensor. Deviations and errors of the broadband lambda sensor are preferably at least partially compensated using the trim control or by using the step-off lambda sensor. This allows the composition of the fuel-fresh gas mixture to be adjusted with high precision.

[0037] A further development of the invention provides that if the state variable leaves a setpoint range, a fault in the exhaust gas aftertreatment system is detected. The state variable describing the state of the exhaust gas aftertreatment system has a specific value that is determined from the gradient of the combustion air ratio. If the value lies within the setpoint range, it is assumed that the exhaust gas aftertreatment system is OK or functional. If, however, the value of the state variable leaves the setpoint range, it is assumed that a fault is present and, accordingly, the exhaust gas aftertreatment system is not functional. In this case, the fault is detected and, preferably, the fault is displayed.

[0038] For example, if a fault occurs, the operation of the drive system is adjusted, for example, by limiting the drive unit's power to a level lower than its rated power. Alternatively, the drive system is shut down when a fault occurs, thus no longer permitting operation of the drive system. This reliably prevents permissible emission limits from being exceeded.

[0039] The invention further relates to a drive device for a motor vehicle, in particular for carrying out the method according to the embodiments within the scope of this description, wherein the drive device has a drive unit generating exhaust gas, an exhaust aftertreatment device designed as a vehicle catalyst for aftertreating the exhaust gas, and a lambda probe arranged downstream of the exhaust aftertreatment device for determining a combustion air ratio in the exhaust gas. The drive device is provided and configured to detect a gradient of the combustion air ratio when a specific combustion air ratio is detected by means of the lambda probe and to use this gradient to determine a state variable describing a state of the exhaust aftertreatment device.

[0040] The advantages of such a design of the drive device 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.

[0041] The invention also relates to a computer program product comprising instructions that cause the drive device to execute the explained method according to the embodiments of this description. Regarding the advantages and possible advantageous developments, reference is made to the entire description.

[0042] 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.

[0043] The invention will be explained in more detail below with reference to the exemplary embodiments shown in the drawings, without limiting the invention. Figure 1a diagram in which measured values ​​of two lambda sensors are plotted over time.

[0044] TheFigure 1shows two diagrams in which two curves 1 and 2 are plotted against time t. In an upper diagram, curve 1 shows a first combustion air ratio measured by a first lambda probe. In a lower diagram, curve 2 shows a measured value of a second lambda probe, namely a voltage of a Nernst cell of the second lambda probe, wherein the voltage corresponds to a second combustion air ratio or at least describes it. The first lambda probe is arranged upstream of an exhaust gas aftertreatment device of a drive device designed as a vehicle catalytic converter, and the second lambda probe is arranged downstream of the exhaust gas aftertreatment device.This means that the first lambda probe describes the combustion air ratio in exhaust gas generated by a drive unit of the drive device between the drive unit and the exhaust gas aftertreatment device and the second combustion air ratio downstream of the exhaust gas aftertreatment device, in particular in terms of flow between the exhaust gas aftertreatment device and a tailpipe of the drive device.

[0045] The drive unit of the drive device is intended to operate with a first composition of a fuel-fresh gas mixture up to a time t1. For example, the first composition is used from time t0 up to time t1. This composition corresponds to a rich composition, i.e., there is an excess of fuel. The first combustion air ratio measured upstream of the exhaust gas aftertreatment device reacts comparatively quickly to the first composition, whereas the measured value of the second lambda sensor reacts more slowly. It changes towards a value of U = 0.8 V, given here only as an example, which corresponds to a specific combustion air ratio.

[0046] The specified value or the corresponding combustion air ratio is reached at time t1. At this time, a gradient of the measured value or the combustion air ratio is determined. From this, further curves 3 and 4 of the measured value or the combustion air ratio can be projected, which would be present if the first composition were to continue to be used. Curve 3 occurs for a better condition of the exhaust gas aftertreatment system, while curve 4 occurs for a worse condition. The better condition exists, for example, with a shorter operating time than the worse condition.

[0047] Since operating the drive unit using the first composition beyond time t 1 is undesirable because it leads to high pollutant emissions and high fuel consumption, the drive unit is operated from time t 1 with a second composition that differs from the first composition. In particular, the second composition is a lean composition, meaning there is an excess of oxygen. The second composition is used to operate the drive unit until time t 2 is reached. From time t 2 onwards, the composition is selected such that a combustion air ratio of λ = 1 is present or established.

[0048] A diagnosis of the exhaust aftertreatment system is performed between times t 0 and t 2. In particular, an oxygen reservoir of the exhaust aftertreatment system is completely emptied by time t 1 and then completely filled by time t 2.

[0049] The oxygen capacity of the exhaust gas aftertreatment system can be determined from the time interval between times t 1 and t 2 and the composition used during this time. This is preferably incorporated into a state variable that describes the state of the exhaust gas aftertreatment system.

[0050] In any case, however, the state variable is determined based on the gradient of the combustion air ratio that exists at time t1, i.e., immediately or precisely when the specified combustion air ratio is reached by the combustion air ratio measured by the second lambda probe or the corresponding measured value. The described procedure achieves a particularly precise and reliable determination of the state variable and thus the condition of the exhaust gas aftertreatment system. LIST OF REFERENCE SYMBOLS:

[0051] 1History 2History 3History 4History

Claims

1. A method for operating a drive device for a motor vehicle, which has a drive unit generating exhaust gas, an exhaust gas aftertreatment device designed as a vehicle catalyst for aftertreating the exhaust gas, and a lambda probe arranged downstream of the exhaust gas aftertreatment device for determining a combustion air ratio in the exhaust gas, characterized in that When a specific combustion air ratio is detected by means of the lambda probe, a gradient of the combustion ratio is recorded and used to determine a state variable describing a state of the exhaust gas aftertreatment device.

2. Method according to claim 1, characterized in thatthe drive unit is operated with a first composition of the fuel-fresh gas mixture corresponding to a rich fuel-fresh gas mixture until the specific combustion air ratio is detected by means of the lambda probe, and / or that after the specific combustion air ratio has been detected by means of the lambda probe, the drive unit is operated with a second composition of the fuel-fresh gas mixture different from the first composition.

3. Method according to one of the preceding claims, characterized in that the drive unit is operated with a third composition of the fuel-fresh gas mixture before operation with the first composition, wherein the third composition is selected such that a combustion air ratio measured by means of the lambda probe changes away from the determined combustion air ratio.

4. Method according to one of the preceding claims, characterized in thatthe drive unit is operated with the first composition of the fuel-fresh gas mixture as part of a diagnosis of the exhaust gas aftertreatment system and / or after an overrun phase.

5. Method according to one of the preceding claims, characterized in that the specific combustion air ratio is determined as a function of a temperature of the lambda probe and / or the temperature of the lambda probe is used to correct the gradient.

6. Method according to one of the preceding claims, characterized in that the temperature is determined from an electrical resistance of the lambda sensor.

7. Method according to one of the preceding claims, characterized in thatupstream of the exhaust gas aftertreatment device there is a first lambda probe for measuring a first combustion air ratio in the exhaust gas and the lambda probe is used as a second lambda probe for measuring the combustion air ratio present as a second combustion air ratio, wherein a lambda control of the drive unit is carried out on the basis of the first combustion air ratio and the second combustion air ratio.

8. Method according to one of the preceding claims, characterized in that If the state variable leaves a setpoint range, a fault in the exhaust gas aftertreatment system is detected.

9. Drive device for a motor vehicle, in particular for carrying out the method according to one or more of the preceding claims, wherein the drive device has a drive unit generating exhaust gas, an exhaust gas aftertreatment device designed as a vehicle catalyst for aftertreating the exhaust gas and a lambda probe arranged downstream of the exhaust gas aftertreatment device for determining a combustion air ratio in the exhaust gas, characterized in that the drive device is provided and designed to detect a gradient of the combustion air ratio when a specific combustion air ratio is detected by means of the lambda probe and to use it to determine a state variable describing a state of the exhaust gas aftertreatment device.

10. A computer program product comprising instructions causing the drive device according to claim 9 to execute the method according to one or more of claims 1 to 8.

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