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

Abstract

The invention relates to a method for operating a drive device (1) for a motor vehicle, which drive device has a drive unit (2) generating exhaust gas, an exhaust gas aftertreatment device (4) comprising a vehicle catalytic converter (5) for aftertreating the exhaust gas, a first lambda probe (8) arranged upstream of the vehicle catalytic converter (5) for determining a first combustion air ratio in the exhaust gas, and a second lambda probe (9) arranged downstream of the first lambda probe (8) for determining a second combustion ratio in the exhaust gas. Provision is made for the exhaust gas aftertreatment device (4) to have a secondary vehicle catalytic converter (7) arranged downstream of the vehicle catalytic converter (5), a diagnostic combustion air ratio present in the exhaust gas downstream of the secondary vehicle catalytic converter (7) being determined and used to determine a catalytic converter state of the secondary vehicle catalytic converter (7).The invention further relates to a drive device (1) for a motor vehicle and a computer program product.

Description

The invention relates to a method for operating a drive device for a motor vehicle, which has an exhaust gas-generating drive unit, an exhaust gas aftertreatment device having a vehicle catalytic converter for aftertreatment of the exhaust gas, a first lambda probe, which is arranged upstream of the vehicle catalytic converter, for ascertaining a first combustion air ratio in the exhaust gas, and a second lambda probe, which is arranged downstream of the first lambda probe, for ascertaining a second combustion air ratio in the exhaust gas, wherein the exhaust gas aftertreatment device has a secondary vehicle catalytic converter, which is arranged downstream of the vehicle catalytic converter, wherein a diagnostic combustion air ratio, which is present downstream of the secondary vehicle catalytic converter in the exhaust gas, is determined and used for ascertaining a catalytic converter state of the secondary vehicle catalytic converter. The invention further relates to a drive device for a motor vehicle and to a computer program product.The prior art discloses, for example, the publication DE 10 2008 027 575 A1. This document describes a diagnostic method for a catalytically active exhaust gas purification element to which exhaust gas of an internal combustion engine of a motor vehicle is supplied, wherein a step response of the exhaust gas purification element occurring in response to a lean-to-rich change and / or rich-to-lean change carried out in a step-like manner during engine operation is evaluated in the form of a signal from a downstream exhaust gas sensor arranged downstream of the exhaust gas purification element with respect to aging of the exhaust gas purification element. It is provided that a nitrogen oxide sensor is used as the downstream exhaust gas sensor.Furthermore, the document DE 10 2014 205 434 A1 discloses a method for detecting an ammonia slip in an exhaust gas aftertreatment system of an internal combustion engine, which has an SCR catalyst and in which, for nitrogen oxide reduction, an ammonia-releasing reducing agent solution is metered into the exhaust gas stream by means of a metering unit in the flow direction of the exhaust gas upstream of the SCR catalyst, wherein a cross sensitivity to nitrogen oxides and ammonia is used by at least one exhaust gas sensor designed as an NO x- sensor, which is arranged downstream of the SCR catalyst in the exhaust gas channel in the flow direction of the exhaust gas, in order to detect an ammonia breakthrough.DE 600 04 132 T2 discloses a method for purifying exhaust gas in the exhaust system of an internal combustion engine, the exhaust system comprising a nitrogen oxide purifying agent for absorbing nitrogen oxide in exhaust gases in a lean exhaust state; a three-way catalyst provided upstream of the nitrogen oxide purifying agent; a first oxygen sensor provided between the nitrogen oxide purifying agent and the three-way catalyst for detecting the oxygen density in the exhaust gas; and a second oxygen sensor provided downstream of the nitrogen oxide purifying agent for detecting the oxygen density in the exhaust gas; wherein the degeneration of the three-way catalyst and the nitrogen oxide purifying agent is evaluated, and wherein the degeneration of the nitrogen oxide purifying agent is evaluated by measuring a first determination period after a lean air-fuel ratio of the exhaust gas is shifted toward a rich air-fuel ratio by a decrease in the air-fuel ratio of a mixture supplied to the internal combustion engine from a time when the value output from the first oxygen sensor changes toward a value indicating a rich air-fuel ratio to the time when the value output from the second oxygen sensor changes toward a value indicating a rich air-fuel ratio. Here, it is arranged that the measured value of the first determination period is corrected according to the degradation state of the three-way catalyst, and that the degradation of the nitrogen oxide purifying agent is determined by comparing the corrected value of the first determination period with a reference value of the determination of the degradation.Furthermore, the publications JP 2003-301 717 A, JP 2004-019 628 A and JP 2009-068 341 A are known from the prior art.In this case, it is provided that a difference signal is formed for plausibility checking an ammonia slip by subtracting a value for the NO x- concentration upstream of the SCR catalyst from the output signal of the exhaust gas sensor downstream of the SCR catalyst as a measure of the sum of a nitrogen oxide and an ammonia concentration, and the temporal profile of the difference signal or the profile of variables derived therefrom is evaluated and compared with applicable limit values.In addition, the document DE 100 23 080 A1 discloses a method for monitoring the storage capacity of an NO x- storage catalytic converter arranged in an exhaust gas duct of a lean-running internal combustion engine, wherein at least one NO x- concentration in the exhaust gas is measured at at least one point in time during a lean operating phase of the internal combustion engine, where λ>1, by means of a measuring device, in particular a NO x- sensor, which is arranged downstream of the NO x- storage catalytic converter, the characteristic value characterizing the storage capacity of the NO x- storage catalytic converter is determined from the measured NO x- concentration and / or a variable derived therefrom and, in the event of termination or interruption of the lean operating phase of the internal combustion engine before the at least one point in time, the at least one characteristic value of the NO x- storage catalytic converter is calculated up to the at least one point in time on the assumption of the presence of lean operating conditions.It is the object of the invention to propose a method for operating a drive device for a motor vehicle, which method has advantages over known methods, in particular enables a reduction of tail pipe emissions of the drive device of the motor vehicle and a reliable monitoring of the tail pipe emissions.This is achieved according to the invention by a method for operating a drive device for a motor vehicle having the features of claim 1. In this case, it is provided that a lambda probe integrated into the vehicle catalytic converter is used as the second lambda probe, and the combustion air ratio integral ascertained from the diagnostic combustion air ratio is adapted on the basis of a state variable describing an aging state of the vehicle catalytic converter.In principle, it is provided that the exhaust gas aftertreatment device has a secondary vehicle catalytic converter arranged downstream of the vehicle catalytic converter, wherein a diagnostic combustion air ratio present in the exhaust gas downstream of the secondary vehicle catalytic converter is determined and used for ascertaining a catalytic converter state of the secondary vehicle catalytic converter.Advantageous embodiments with expedient developments of the invention are specified in the dependent claims. It is pointed out that the exemplary embodiments explained in the description are not restrictive; rather, any variations of the features disclosed in the description, the claims and the figures can be realized.The method is provided for operating the drive device. The drive device serves for driving the motor vehicle, in this respect thus for providing a drive torque directed to driving the motor vehicle. In order to provide the drive torque, the drive direction has the drive unit. The drive unit is preferably present as an internal combustion engine, in particular as an Otto internal combustion engine or as 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 contains fresh air at least temporarily. In addition, the fresh gas can have exhaust gas if exhaust gas recirculation is realized, in which the exhaust gas generated by the drive unit is at least partially recirculated to the drive unit, namely as a constituent of the fresh gas. The fuel and fresh gas supplied to the prime mover form a fuel-fresh gas mixture having a particular composition that is reacted in the prime mover.During operation of the drive unit, exhaust gas arises due to the chemical reaction of fuel and fresh gas with one another, which exhaust gas is discharged in the direction of an external environment of the drive direction or of the motor vehicle. Since pollutants are contained in the exhaust gas generated by the drive assembly, the exhaust gas is first supplied to the exhaust gas aftertreatment device before being discharged into the external environment. In the exhaust gas aftertreatment device, the pollutants are at least partially converted into more harmless products. Only after passing through the exhaust gas aftertreatment device is the exhaust gas discharged into the external environment, in particular through an end tube of the drive device.The exhaust gas aftertreatment device has the vehicle catalytic converter, which is preferably designed as a three-way catalytic converter, oxidation catalytic converter, NO x- storage catalytic converter or SCR catalytic converter. A conversion rate and thus the conversion performance of the vehicle catalytic converter, with which the pollutants are converted into the more harmless products, depend in particular on the composition of the exhaust gas which is fed to the exhaust gas aftertreatment device or to the vehicle catalytic converter and on the temperature of the vehicle catalytic converter.The components of the exhaust gas generated by the drive unit are also referred to as raw emissions. The raw emissions describe in this respect the composition of the exhaust gas upstream of the exhaust gas aftertreatment device or fluidically between the drive unit and the exhaust gas aftertreatment device. The substances contained in the exhaust gas are partially converted by the exhaust gas when the exhaust gas post-treatment device is passed through, so that the composition of the exhaust gas changes. The substances present in the exhaust gas downstream of the exhaust gas aftertreatment device, which form the exhaust gas, are also referred to as final pipe emissions, since the exhaust gas having this composition is discharged into the external environment through the final pipe of the drive device.As already mentioned, the quantity of pollutants contained in the tail pipe emissions depends on the raw emissions, but additionally also on the conversion performance of the exhaust gas aftertreatment device or of the vehicle catalytic converter. This is temperature-dependent. In particular, the conversion performance is lower the further a temperature of the exhaust gas aftertreatment device or of the vehicle catalytic converter is spaced apart from an operating temperature of the exhaust gas aftertreatment device or of the vehicle catalytic converter, that is to say the greater an absolute value of the difference between the temperatures. The temperature of the exhaust gas aftertreatment device or of the vehicle catalytic converter is to be understood in particular as a temperature of a ceramic honeycomb body which is provided with the catalytic coating.For operating the drive device, the first combustion air ratio and the second combustion air ratio are used, among other things. The first combustion air ratio corresponds to a combustion air ratio in the exhaust gas upstream of the vehicle catalytic converter, i.e. fluidically between the drive unit and the vehicle catalytic converter. The second combustion air ratio is a combustion air ratio present in the exhaust gas downstream of the first lambda probe, in particular in the vehicle catalytic converter or downstream of the vehicle catalytic converter. The first combustion air ratio is determined using the first lambda probe and the second combustion air ratio is determined using the second lambda probe. For this purpose, the first lambda probe is arranged upstream of the vehicle catalytic converter and the second lambda probe is arranged downstream of the first lambda probe; in particular, the second lambda probe is present in the vehicle catalytic converter or downstream of the vehicle catalytic converter.For example, the two combustion air ratios, i.e. the first combustion air ratio and the second combustion air ratio, are used to carry out lambda control and trim control. In this case, in particular the composition of the fresh fuel gas mixture is set on the basis of the first combustion air ratio, whereas the second combustion air ratio is used for correcting the first combustion air ratio or a setpoint value to which the first combustion air ratio is set in the course of the lambda control in the course of the trim control.In principle, it may be provided to adjust the composition of the fresh fuel gas mixture solely on the basis of the first combustion air ratio or solely on the basis of the second combustion air ratio, that is to say without taking into account the respective other combustion air ratio. In each case, the respective combustion air ratio is set to the corresponding setpoint value. For this purpose, it is provided in particular to adapt the composition of the fresh fuel gas mixture in such a way that the respective combustion air ratio changes in the direction of the setpoint value, in particular up to the setpoint value. For example, the respective combustion air ratio is controlled by setting the composition of the fresh fuel gas mixture to the setpoint value.However, it is particularly preferably provided to use both the first combustion air ratio and the second combustion air ratio for adjusting the composition of the fresh fuel gas mixture. In this case, the two combustion air ratios are preferably set to a respective setpoint value. In particular, it is thus provided that the first combustion air ratio is set to a first setpoint value and the second combustion air ratio is set to a second setpoint value, namely by corresponding adaptation of the composition of the fresh fuel gas mixture.In order to further improve the conversion performance of the exhaust gas aftertreatment device, the secondary vehicle catalytic converter is arranged downstream of the vehicle catalytic converter. The vehicle catalytic converter can thus also be referred to as a primary vehicle catalytic converter. The exhaust gas supplied to the exhaust gas aftertreatment device flows first through the vehicle catalytic converter and then through the secondary vehicle catalytic converter. The vehicle catalytic converter is thus arranged fluidically closer to the drive unit than the secondary vehicle catalytic converter. For example, the vehicle catalytic converter is arranged in an engine compartment of the motor vehicle, whereas the secondary vehicle catalytic converter is designed as an underbody catalytic converter or underbody catalytic converter and is therefore present on an underbody of the motor vehicle. Preferably, the secondary vehicle catalyst is in the form of a three-way catalyst.The vehicle catalytic converter is operated in a regulated manner with the aid of the two lambda sensors, whereas the operation of the secondary vehicle catalytic converter can be carried out in an uncontrolled manner. However, it is to be assumed that a sensor present downstream of the secondary catalytic converter must be installed in the future, by means of which a combustion air ratio can also be determined, which is referred to here as a diagnostic combustion air ratio. This sensor, which serves for ascertaining the diagnostic combustion air ratio present in the exhaust gas downstream of the secondary catalytic converter, is, for example, a lambda probe or a nitrogen oxide sensor or NO x- sensor. In the latter case, the NO x- emissions in the exhaust gas downstream of the secondary vehicle catalytic converter are determined at least temporarily on the basis of the sensor. If these exceed a specific threshold value, an error signal is generated, which is preferably displayed to a user of the motor vehicle and prompts him to visit a workshop.A diagnosis of the vehicle catalytic converter can be implemented easily on the basis of the first lambda probe and the second lambda probe. For example, it is customary to set the first combustion air ratio alternately to different setpoint values, in particular to a setpoint value corresponding to a lean fuel-fresh gas mixture and to a setpoint value corresponding to a rich fuel-fresh gas mixture. In particular, it is provided to ascertain in this way how much oxygen can be stored in the vehicle catalytic converter, i.e., to ascertain an oxygen storage capacity of the vehicle catalytic converter. On the basis of this oxygen storage capacity, a catalytic converter state of the vehicle catalytic converter is determined, which in particular describes an aging state of the vehicle catalytic converter.No such diagnostic method is known for the secondary vehicle catalyst, the catalyst state of the secondary vehicle catalyst is not determined. In the event of a defect in the secondary vehicle catalytic converter, this will certainly remain unnoticed over a longer period of time and the conversion line of the exhaust gas aftertreatment device will be reduced. In order to always ensure adequate conversion performance of the exhaust gas aftertreatment device, the catalytic converter state of the secondary vehicle catalytic converter should therefore also be determined. This is done on the basis of the diagnostic combustion air ratio which is present in the exhaust gas downstream of the secondary vehicle catalytic converter.This ensures that not only the vehicle catalytic converter but also the secondary vehicle catalytic converter have sufficient conversion power, so that a total conversion power of the exhaust gas aftertreatment device, which is composed of the conversion power of the vehicle catalytic converter and the conversion power of the secondary vehicle catalytic converter, is sufficiently high to meet all predefined exhaust gas limit values during proper operation of the drive device. This is reliably implemented using the described procedure.A further development of the invention provides that the determination of the catalyst state takes place as a function of the first combustion air ratio and / or the second combustion air ratio. Therefore, in the determination of the catalytic converter state, not only the diagnostic combustion air ratio, but also the first combustion air ratio or the second combustion air ratio is taken into account. On the basis of the first combustion air ratio and / or the second combustion air ratio on the one hand and the diagnostic combustion air ratio on the other hand, for example, the difference between oxygen input into the secondary vehicle catalytic converter and oxygen output from the secondary vehicle catalytic converter is known. From this, the catalyst state can be reliably deduced.A further development of the invention provides that the diagnostic combustion air ratio is used within a diagnostic period for ascertaining the catalytic converter state, which diagnostic combustion air ratio is ascertained on the basis of the first combustion air ratio and / or the second combustion air ratio. The diagnostic combustion air ratio is thus not permanently taken into account in order to determine the catalyst state. Rather, it is used only within the diagnosis period or is taken into account only within the diagnosis period. The diagnostic period is set based on the first combustion air ratio and / or the second combustion air ratio. For example, it is provided that the first combustion air ratio or the second combustion air ratio flows only indirectly into the catalytic converter state of the secondary vehicle catalytic converter, namely by defining the diagnostic period on the basis of the second combustion air ratio. The described procedure enables the determination of the catalytic converter state with high accuracy.A further development of the invention provides that the diagnostic period begins when the first combustion air ratio and / or the second combustion air ratio falls below a first threshold value and ends when the first combustion air ratio and / or the second combustion air ratio exceeds a second threshold value. In other words, the diagnostic period continues for as long as, in particular exclusively as long as, the first combustion air ratio or the second combustion air ratio is within a setpoint value range which is limited by the first threshold value and / or the second threshold value. In particular, the diagnostic period begins when the respective combustion air ratio enters the setpoint value range and ends when the respective combustion air ratio leaves the setpoint value range.If the respective combustion air ratio falls below the first threshold value, the diagnostic period begins. In particular, the diagnostic period only begins if the respective combustion air ratio falls below the first threshold value starting from larger values, i.e. the combustion air ratio is initially larger than the first threshold value. Conversely, the diagnostic period ends as soon as the respective combustion air ratio exceeds the second threshold value, in particular starting from smaller values.Of course, instead of the combustion air ratio, a variable different from this may also be used, which however describes the combustion air ratio. For example, a voltage supplied by the respective lambda probe can be used instead of the combustion air ratio, in particular if this is present as a jump lambda probe. With the aid of the described procedure, the diagnosis period within which the diagnosis of the secondary vehicle catalytic converter takes place is fixed with high accuracy.A further development of the invention provides that the first threshold value and the second threshold value are selected to be equal to a common threshold value or different from one another. The two threshold values can thus be identical, i.e. correspond to the same value. However, they may also have different values. This enables the diagnosis to be accurately matched to the respective drive device.A further development of the invention provides that a combustion air ratio integral is determined for determining the catalyst state by integrating the diagnostic combustion air ratio or a variable determined from it over the diagnostic period. In other words, the combustion air ratio integral corresponds to the integral of the diagnostic combustion air ratio during the diagnostic period, in particular over the entire diagnostic period. Before the diagnosis period or at the beginning of the diagnosis period, the combustion air ratio integral is reset, namely to an initial value, for example to 0.Starting from the initial value, the diagnostic combustion air ratio or the variable is periodically added to the combustion air ratio integral, i.e. the diagnostic combustion air ratio or the variable is added to the combustion air ratio integral. The combustion air ratio integral is preferably determined by integrating a deviation of the diagnostic combustion air ratio from a stoichiometric combustion air ratio over the diagnostic period, i.e., in particular the extent of a deviation of the diagnostic combustion air ratio from a lambda value of λ=1. The aforementioned variable is therefore, for example, the difference between the stoichiometric combustion air ratio and the diagnostic combustion air ratio, in particular is corrected for sign. From the combustion air ratio integral, the catalyst state of the secondary vehicle catalyst can be deduced with high accuracy in the described manner.A further development of the invention provides that at or after the end of the diagnostic period the catalytic converter state is determined from the combustion air ratio integral. Specifically, the smaller the combustion air ratio integral, the better the catalyst state is assumed. Conversely, the greater the combustion air ratio integral at the end or after the end of the diagnostic period, the poorer the catalyst state is deduced. For example, if an error threshold value is exceeded by the combustion air ratio integral, an error is detected and an error signal is generated.Alternatively, it is provided, for example, that a plurality of catalytic converter states and associated threshold values are stored. On the basis of the combustion air ratio integral, from the deposited catalyst states, that catalyst state is selected whose threshold value is closest to the combustion air ratio integral over all deposited catalyst states. Alternatively, the one can be selected from the catalyst states which has the largest threshold value of all catalyst states, which is exceeded by the combustion air ratio integral. The selected catalyst state is then used as the catalyst state of the secondary vehicle catalyst. This enables accurate diagnosis of the secondary vehicle catalyst.The invention provides that a lambda probe integrated into the vehicle catalytic converter is used as the second lambda probe, and the combustion air ratio integral determined from the diagnostic combustion air ratio is adapted on the basis of a state variable describing an aging state of the vehicle catalytic converter.In a variant not according to the invention, the second lambda probe is arranged downstream of the vehicle catalytic converter, so that the second combustion air ratio directly corresponds to the combustion air ratio measured by means of the second lambda probe. Consequently, the measured value of the second lambda probe can be used uncompensated in order to determine the second combustion air ratio. According to the invention, the second lambda probe is integrated into the vehicle catalytic converter, i.e., it is situated downstream of an beginning of the vehicle catalytic converter and upstream of an end of the vehicle catalytic converter, as seen in terms of flow. Consequently, the combustion air ratio measured using the second lambda sensor does not correspond to the combustion air ratio which is actually present downstream of the vehicle catalytic converter. This influences the diagnostic period determined on the basis of the second combustion air ratio and thus also the combustion air ratio integral.This means that a compensation of the combustion air ratio integral is necessary. This compensation is carried out on the basis of the state variable which describes the state of aging of the vehicle catalytic converter. For example, the combustion air ratio integral is therefore adapted or corrected on the basis of the state variable and only then used for ascertaining the catalytic converter state. The state of aging is in turn determined from the oxygen storage capacity of the vehicle catalytic converter, as has already been explained above. In any case, the described procedure enables the diagnosis of the secondary vehicle catalytic converter with high accuracy.A further development of the invention provides that a broad-band lambda probe is used as the first lambda probe and a jump lambda probe is used as the second lambda probe. The broadband lambda probe enables the detection of the residual oxygen content or the corresponding combustion air ratio over a further measurement range than the jump lambda probe. The wide-band lambda sensor is preferably used for carrying out the already mentioned lambda control and accordingly for adjusting the composition of the fresh fuel gas mixture with which the drive unit is operated.The sudden-action lambda probe has a narrower measurement range than the broadband lambda probe; in particular, it is (only) used for detecting a combustion air ratio of λ=1. However, the measurement accuracy of the surge lambda probe is higher than that of the wide band lambda probe. Deviations and errors of the broadband lambda probe are preferably at least partially compensated for by means of the trim control or by using the jump lambda probe. Thereby, the adjustment of the composition of the fresh fuel gas mixed with high accuracy is realized.The invention further relates to a drive device for a motor vehicle, in particular for carrying out the method according to the statements within the scope of this description, wherein the drive device has an exhaust gas-generating drive unit, an exhaust gas aftertreatment device having a vehicle catalytic converter for aftertreatment of the exhaust gas, a first lambda probe arranged upstream of the vehicle catalytic converter for ascertaining a first combustion air ratio in the exhaust gas, and a second lambda probe arranged downstream of the first lambda probe for ascertaining a second combustion air ratio in the exhaust gas.It is provided here that the exhaust gas aftertreatment device has a secondary vehicle catalytic converter arranged downstream of the vehicle catalytic converter, wherein the drive direction is provided and configured to determine a diagnostic combustion air ratio present in the exhaust gas downstream of the secondary vehicle catalytic converter and to use it for determining a catalytic converter state of the secondary vehicle catalytic converter.Furthermore, it is provided that the second lambda probe is a lambda probe integrated into the vehicle catalytic converter and the drive direction is furthermore provided and configured to adapt the combustion air ratio integral ascertained from the diagnostic combustion air ratio on the basis of a state variable describing an aging state of the vehicle catalytic converter.The advantages of such a configuration of the drive device and such a procedure have already been pointed out. Both the drive device for a motor vehicle and the method for operating it can be further developed according to the explanations within the scope of this description, so that in this respect reference is made to it.The invention also relates to a computer program product comprising instructions which cause the drive direction to execute the explained method according to the statements of this description. With regard to the advantages and possible advantageous developments, reference is made in its entirety to the description.The features and combinations of features described in the description, in particular the features and combinations of features described in the following description of the figures and / or shown in the figures, can be used not only in the respectively specified combination but also in other combinations or alone without departing from the scope of the invention. Embodiments are therefore also to be considered as encompassed by the invention which are not explicitly shown or explained in the description and / or the figures, but which emerge from the explained embodiments or can be derived from them.The invention is explained in more detail below with reference to the exemplary embodiments shown in the drawing, without any restriction of the invention being effected. The following shows: FIG. 1 shows a schematic illustration of a drive device for a motor vehicle having an exhaust-gas-generating drive unit and an exhaust-gas aftertreatment device, and FIG. 2 shows a plurality of diagrams, on the basis of which a method for ascertaining a catalytic converter state of a secondary vehicle catalytic converter is explained.FIG. 1 shows purely schematically a drive device 1 for a motor vehicle, which has a drive unit 2, which is present here in the form of an internal combustion engine, and an exhaust tract 3. In the exhaust tract 3, an exhaust gas aftertreatment device 4 is present, which has a vehicle catalytic converter 5, an optional particulate filter 6 and a secondary vehicle catalytic converter 7. Exhaust gas is supplied to the exhaust gas aftertreatment device 4, which is generated by the drive unit 2. Upstream of the vehicle catalytic converter 5, a first lambda probe 8 is present and downstream of a part of the vehicle catalytic converter 5, a second lambda probe 9.A first combustion air ratio determined by means of the first lambda probe 8 is fed to a lambda controller, for example, and a second combustion air ratio determined by means of the second lambda probe 9 is fed to a trim controller. Both the lambda controller and the trim controller influence a composition of a fresh fuel gas mixture with which the drive unit 2 is operated.Downstream of the secondary vehicle catalyst 7, an NO x- sensor 10 is arranged. This serves at least temporarily to determine a nitrogen oxide concentration in the exhaust gas. However, it is also temporarily used for determining a combustion air ratio downstream of the secondary vehicle catalytic converter 7, wherein this combustion air ratio is also referred to as a diagnostic combustion air ratio. Instead of the NO x- sensor 10, a lambda probe, in particular a broadband lambda probe, or an equivalent sensor can be used. The vehicle catalytic converter 5 and the secondary vehicle catalytic converter 7 are preferably designed as a three-way catalytic converter.FIG. 2 shows a plurality of diagrams, on the basis of which a method for carrying out a diagnosis of the secondary vehicle catalytic converter 7 or for ascertaining a catalytic converter state of the secondary vehicle catalytic converter 7 is described. In a first diagram, a curve 11 shows a setpoint value for the first combustion air ratio and a curve 12 shows the first combustion air ratio, in each case over time. In a second diagram, a probe voltage of second lambda probe 9, describing the second combustion air ratio, is shown in a curve 13 over time.In a third diagram, a curve 14 indicates an amount of oxygen temporarily stored in the vehicle catalytic converter 5 over time. A fourth diagram shows the diagnostic combustion air ratio for different catalytic converter states of the secondary vehicle catalytic converter 7 in curves 15, 16 and 17, In a fifth diagram, the diagnostic combustion air ratio integrated during a diagnostic period, corresponding to a combustion air ratio integral, is plotted over time, namely in curves 18, 19 and 20, again for different catalytic converter states.Curves 15 and 18 correspond to a first catalytic converter state, curves 16 and 19 correspond to a second catalytic converter state, and curves 17 and 20 correspond to a third catalytic converter state. The first catalyst state is present, for example, for a newly valued secondary vehicle catalyst 7, the second catalyst state is present for a used secondary vehicle catalyst 7 and the third catalyst state is present for a defective secondary vehicle catalyst 7.The diagrams show the respective variables during a diagnosis of the vehicle catalytic converter 5. For this purpose, the fuel-fresh gas mixture is first adjusted to a rich mixture in order to bring the vehicle catalytic converter 5 into a defined state in which an oxygen storage of the vehicle catalytic converter 5 is completely or at least almost completely emptied. If the measured value of the second lambda probe 9 reaches a specific value, the system switches from the rich mixture to a lean mixture. On the basis of the measured values of the two lambda sensors 8 and 9, an oxygen balance is established via the vehicle catalytic converter 5 and, on the basis of these, the oxygen storage capacity is deduced.During this procedure, a diagnosis of the secondary vehicle catalytic converter 7 is additionally carried out. For this purpose, a diagnostic period is determined which begins when a threshold value Vs is exceeded by the measured value of the second lambda probe 9 and ends when the measured value of the second lambda probe 9 falls below the threshold value. The former is the case at the time t 1 and the latter is the case at the time t 2. Between these two points in time, the diagnostic combustion air ratio is determined by means of the NO x- sensor 10, see curves 15, 16 and 17.The diagnostic combustion air ratio is integrated during the diagnostic period. This results in the combustion air ratio integral corresponding to curves 18, 19 and 20. The described procedure enables an accurate conclusion to be drawn about the state of the secondary vehicle catalytic converter 7, so that not only the state of the vehicle catalytic converter 5 but also the state of the secondary vehicle catalytic converter 7 is known with high accuracy. It is particularly advantageous in the described procedure that the diagnosis of the secondary vehicle catalytic converter 7 is carried out or at least can be carried out during the diagnosis of the vehicle catalytic converter 5, so that, apart from this diagnosis, no targeted setting of the drive unit 2 has to be carried out for a diagnosis of the secondary vehicle catalytic converter 7.LIST OF REFERENCE CHARACTERS:1 Drive device 2 Drive unit 3 Exhaust tract 4 Exhaust gas aftertreatment device 5 Vehicle catalytic converter 6 Particle filter 7 Secondary vehicle catalytic converter 8 1 Lambda probe 9 2 Lambda probe 10 NOx sensor 11 Course 12 Course 13 Course 14 Course 15 Course 16 Course 17 Course 18 Course 19 Course 20 Course

Claims

Method for operating a drive device (1) for a motor vehicle, which has an exhaust-gas-generating drive unit (2), an exhaust-gas aftertreatment device (4) which has a vehicle catalytic converter (5) for aftertreatment of the exhaust gas, a first lambda probe (8) which is arranged upstream of the vehicle catalytic converter (5) and is intended for ascertaining a first combustion air ratio in the exhaust gas, and a second lambda probe (9) which is arranged downstream of the first lambda probe (8) and is intended for ascertaining a second combustion ratio in the exhaust gas, wherein the exhaust-gas aftertreatment device (4) has a secondary vehicle catalytic converter (7) which is arranged downstream of the vehicle catalytic converter (5), wherein a diagnostic combustion air ratio which is present downstream of the secondary vehicle catalytic converter (7) in the exhaust gas is determined and is used for ascertaining a catalytic converter state of the secondary vehicle catalytic converter (7), characterized in that, using as second lambda probe (9) a lambda probe integrated into the vehicle catalytic converter (5) and adapting the combustion air ratio integral determined from the diagnostic combustion air ratio on the basis of a state variable describing an ageing state of the vehicle catalytic converter (5).Method according to Claim 1, characterized in that the determination of the catalyst state takes place as a function of the first combustion air ratio and / or the second combustion air ratio.Method according to one of the preceding claims, characterized in that, for ascertaining the catalyst state, the diagnostic combustion air ratio is used within a diagnostic period which is ascertained on the basis of the first combustion air ratio and / or on the basis of the second combustion air ratio.Method according to Claim 3, characterized in that the diagnostic period begins when the first combustion air ratio and / or the second combustion air ratio falls below a first threshold value and ends when the first combustion air ratio and / or the second combustion air ratio exceeds a second threshold value.Method according to claim 4, characterised in that the first threshold value and the second threshold value are chosen to be equal to a common threshold value or different from one another.Method according to one of the preceding claims, characterized in that, in order to determine the catalyst state, a combustion air ratio integral is determined by integrating the diagnostic combustion air ratio or a variable determined from it over the diagnostic period.Method according to Claim 6, characterized in that, at or after the end of the diagnostic period, the catalyst state is determined from the combustion air ratio integral.Drive device (1) for a motor vehicle, in particular for carrying out the method according to one or more of the preceding claims, wherein the drive device (1) has an exhaust-gas-generating drive unit (2), an exhaust-gas aftertreatment device (4) having a vehicle catalytic converter (5) for aftertreatment of the exhaust gas, a first lambda probe (8) arranged upstream of the vehicle catalytic converter (5) for ascertaining a first combustion air ratio in the exhaust gas and a second lambda probe (9) arranged downstream of the first lambda probe (8) for ascertaining a second combustion air ratio in the exhaust gas, wherein the exhaust-gas aftertreatment device (4) has a secondary vehicle catalytic converter (7) arranged downstream of the vehicle catalytic converter (5), wherein the drive device (1) is provided and configured for this purpose, a diagnostic combustion air ratio present in the exhaust gas downstream of the secondary vehicle catalytic converter (7) and to be used for ascertaining a catalytic converter state of the secondary vehicle catalytic converter (7), characterized in that the second lambda probe (9) is a lambda probe integrated into the vehicle catalytic converter (5) and the drive device (1) is furthermore provided and configured to adapt the combustion air ratio integral ascertained from the diagnostic combustion air ratio on the basis of a state variable describing an aging state of the vehicle catalytic converter (5).A computer program product comprising instructions for causing the driving device (1) according to claim 8 to carry out the method according to one or more of claims 1 to 7.

Images