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) designed as a vehicle catalyst for aftertreating the exhaust gas, a first lambda probe (6) arranged upstream of the exhaust gas aftertreatment device (4) for determining a first combustion air ratio in the exhaust gas, and a second lambda probe (7) arranged downstream of the exhaust gas aftertreatment device (4) for determining a second combustion air ratio in the exhaust gas, wherein a composition of a fuel-fresh gas mixture used to operate the drive unit (2) is adjusted based on the first combustion air ratio and / or the second combustion ratio.It is provided that a target value to which the first combustion air ratio or the second combustion air ratio is set is selected from several stored preset target values ​​depending on an operating state of the drive unit (2). The invention further relates to a drive device (1) 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 a drive unit generating exhaust gas, an exhaust gas aftertreatment device configured as a vehicle catalyst for aftertreating the exhaust gas, a first lambda probe arranged upstream of the exhaust gas aftertreatment device for determining a first combustion air ratio in the exhaust gas, and a second lambda probe arranged downstream of the exhaust gas aftertreatment device for determining a second combustion ratio in the exhaust gas, wherein a composition of a fuel-fresh gas mixture used to operate the drive unit is adjusted based on the first combustion air ratio and / or the second combustion air ratio. The invention further relates to a drive device for a motor vehicle and a computer program product.

[0002] For example, the document US 7,197,866 B2 is known from the prior art.This describes a method of controlling fuel injection into an engine having an exhaust system with an emissions control device arranged therein, the method comprising: reading information from a sensor located downstream of the emissions control device, the information including a substantially linear indication of an air-fuel ratio in exhaust gas, the linear indication being substantially linear over a full range of air-fuel ratios of at least 12:1 to 18:1, the information also including a substantially non-linear indication of stoichiometry; setting a setpoint for an upstream sensor based on this information; and setting fuel injection into the engine based on the set setpoint and a signal from the upstream sensor.

[0003] 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 enables a more precise adjustment of the composition of the fuel-fresh gas mixture.

[0004] This is achieved according to the invention with a method for operating a drive device for a motor vehicle with the features of claim 1. It is provided that a target value to which the first combustion air ratio or the second combustion air ratio is set is selected from a plurality of stored default target values ​​depending on an operating state of the drive unit, in particular independently of the first combustion air ratio and / or the second combustion air ratio.

[0005] 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.

[0006] 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 the fuel-fresh gas mixture with a specific composition, which is reacted in the drive unit.

[0007] 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.

[0008] 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 and the temperature of the exhaust gas aftertreatment device.

[0009] 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 direction.

[0010] The amount 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 refers, in particular, to the temperature of a ceramic honeycomb body provided with the catalytic coating.

[0011] The first combustion air ratio and the second combustion air ratio, among other things, are used to operate the drive system. The first combustion air 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 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 exhaust aftertreatment device, and the second lambda probe is arranged downstream of the exhaust aftertreatment device.

[0012] For example, the two combustion air ratios, i.e., the first combustion air ratio and the second combustion air ratio, are used to implement lambda control and trim 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 or the target value to which the first combustion air ratio is adjusted within the framework of trim control.

[0013] In principle, the composition of the fuel-fresh gas mixture can be adjusted based solely on the first combustion air ratio or solely on the second combustion air ratio, i.e., without taking the other combustion air ratio into account. In either case, the respective combustion air ratio is adjusted to the target value. For this purpose, it is particularly provided that the composition of the fuel-fresh gas mixture is adjusted such that the respective combustion air ratio changes toward the target value, in particular up to the target value. For example, the respective combustion air ratio is regulated by adjusting the composition of the fuel-fresh gas mixture to the target value.

[0014] However, it is particularly preferred to use both the first combustion air ratio and the second combustion air ratio to adjust the composition of the fuel-fresh gas mixture. In this case, the two combustion air ratios are preferably set to a respective target value. In particular, it is therefore provided to set the first combustion air ratio to a first target value and the second combustion air ratio to a second target value, namely by appropriately adjusting the composition of the fuel-fresh gas mixture.

[0015] For stability reasons, the second combustion air ratio is adjusted to the second setpoint with a longer time constant than the first combustion air ratio to the first setpoint. This is due to the arrangement of the two lambda sensors on opposite sides of the exhaust aftertreatment system; the exhaust gas first flows over the first lambda sensor, then passes through the exhaust aftertreatment system, and only then reaches the second lambda sensor. Therefore, the exhaust gas takes significantly longer to travel from the drive unit to the second lambda sensor than it does from the drive unit to the first lambda sensor.

[0016] For example, when designing the drive device or drive unit, it is provided to determine a target value for the second combustion air ratio at which the exhaust gas aftertreatment system has the best conversion performance for the pollutants contained in the exhaust gas. Pollutants in this case include, in particular, hydrocarbons, carbon oxide, in particular carbon monoxide, and nitrogen oxide. If the second combustion air ratio corresponds to the second target value, a specific value is set for the first combustion air ratio, which is subsequently used as the first target value. This procedure is repeated, for example, for different operating points until a corresponding first target value is available for several operating points.These first setpoints are subsequently used depending on the operating point set on the drive unit to adjust the composition of the fuel-fresh gas mixture or at least to pre-control this adjustment.

[0017] However, the first combustion air ratio and the second combustion air ratio are not identical for different raw emission compositions that have the same combustion air ratio. The combustion air ratio follows the relationship 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

[0018] This means that, for example, 1 ppm CO 2 can be replaced by 1 ppm H 2 without changing the combustion air ratio, since 1 / 2 O 2 is required for the oxidation of both exhaust gas components. The relationship can be calculated using the diffusion rate v x ¯ = 8 k B T πm x as 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 This means that the diffusion rate of the exhaust gas components into the lambda sensor depends on their respective molar mass. Accordingly, the square root of the reciprocal of the molar mass must be taken into account for the combustion air ratio measured using the respective lambda sensor.

[0019] Tests conducted by the applicant have surprisingly shown that this can be achieved by selecting the target value to which the first combustion air ratio or the second combustion air ratio is set from several stored predefined target values ​​depending on the operating state of the drive unit. Different predefined target values ​​are stored for different operating states, in particular in a control unit used to operate the drive device, in particular to control the drive unit.

[0020] Based on the current operating state of the drive unit, a preset target value is to be selected from the stored preset target values ​​and subsequently used as the target value. This selection is preferably carried out independently of the first combustion air ratio, independently of the second combustion air ratio, or independently of both the first combustion air ratio and the second combustion air ratio. The target value is therefore selected from the preset target values ​​without taking into account the first combustion air ratio and / or the second combustion air ratio. The described procedure, in particular, precontrols the composition of the fuel / fresh gas mixture.

[0021] Of course, however, it is possible to adjust the setpoint after selecting from the multiple preset setpoints, for example, using the respective other combustion air ratio. Thus, if the first combustion air ratio is set to the setpoint, the setpoint is adjusted based on the second combustion air ratio, particularly within the framework of the trim control described above. However, if the second combustion air ratio is set to the setpoint, the first combustion air ratio is used to adjust the setpoint.

[0022] The described procedure allows the different operating states of the drive unit to be effectively accounted for. This occurs in particular during a warm-up operation of the drive system, i.e., while a temperature of the drive unit and / or the exhaust gas aftertreatment system is lower than the respective operating temperature. For example, the warm-up operation is preferably performed only while the temperature of the drive unit is lower than an operating temperature of the drive unit and / or while a temperature of the exhaust gas aftertreatment system is lower than an operating temperature of the exhaust gas aftertreatment system.

[0023] Once the temperature reaches the respective operating temperature, warm-up operation is terminated. Outside of warm-up operation, especially after warm-up operation, the setpoint should no longer be selected. Instead, the setpoint is set to a specific default value and adjusted, for example, based on the first combustion air ratio and / or the second combustion air ratio. This achieves overall low-emission operation of the drive system.

[0024] A further development of the invention provides that when the operating state changes from a first operating state to a second operating state, the setpoint is adjusted from a first specified setpoint selected for the first operating state towards a second specified setpoint selected for the second operating state, in particular continuously and / or steadily, preferably over a specific period of time. The selected setpoint should therefore not change abruptly, but rather a continuous or steady change is provided. The setpoint is changed, for example, in such a way that it is changed over the specific period of time starting from the first setpoint to the second setpoint. It therefore only reaches the second setpoint upon expiration or end of the specific period of time.This avoids discontinuities in the setpoint and in turn ensures particularly low-emission operation of the drive system.

[0025] A further development of the invention provides that the setpoint is determined based on an operating point set on the drive unit from a characteristic map containing several predefined setpoints, which are selected from several stored characteristic maps depending on the operating state of the drive unit. Thus, it is not intended to select a single predefined setpoint directly; instead, the setpoint is selected by selecting the characteristic map from the several characteristic maps.

[0026] Each of the multiple characteristic maps contains multiple default setpoints, in particular for different operating points of the drive unit. Preferably, the stored characteristic maps each contain default setpoints for the same operating points, so that a default setpoint can be determined for each of the operating points from each of the characteristic maps.

[0027] Overall, the proposed engine map is selected from the multiple engine maps depending on the operating state, preferably independently of the first combustion air ratio and / or the second combustion air ratio. The setpoint is then determined from the selected engine map based on the operating point set on the drive unit, for example, by selecting the preset setpoint assigned to the operating point from the selected engine map or by interpolating or extrapolating the preset setpoints stored in the selected engine map. Using the engine maps enables particularly targeted setting of the setpoint.

[0028] A further development of the invention provides that the operating state is described by a temperature of the drive unit and the target value and / or the characteristic map is selected depending on the temperature. The operating state is therefore dependent on the temperature of the drive unit, for example on a temperature of a combustion chamber of the drive unit. At low temperatures in the combustion chamber, the fuel introduced into it cannot evaporate optimally, so that the lambda sensor, in particular the first lambda sensor, is exposed to a higher proportion of unburned hydrocarbons. These have a lower diffusion rate into the lambda sensor, so that their influence on the lambda value determined by the lambda sensor is too small and the lambda sensor indicates an exhaust gas that is too lean. This influence decreases as the temperature of the combustion chamber increases.

[0029] Accordingly, the described advantages can be achieved by storing the specified setpoints and / or maps for different temperatures and selecting the setpoint or map based on the temperature from the specified setpoints or maps. The resulting advantages have already been discussed.

[0030] A further development of the invention provides that, for the setpoint, a first preset value is selected that results in a leaner composition of the fuel / fresh gas mixture at a lower first temperature, and a second preset value is selected that results in a richer composition of the fuel / fresh gas mixture at a higher second temperature. The background to this procedure has already been mentioned. If the temperature is lower, the lambda sensor indicates a mixture that is too lean, whereas a richer mixture is actually present. Accordingly, the setpoint must be shifted further toward the lean composition, the lower the temperature. This achieves the advantages already mentioned.

[0031] A further development of the invention provides that the operating state is described by a throughput of secondary air and the setpoint and / or the characteristic map is selected depending on the throughput of secondary air. Secondary air is understood to be air which is chemically unchanged, in particular unburned, in the exhaust gas downstream of the drive unit. The secondary air can, for example, be introduced into the exhaust gas downstream of the drive unit or can be expelled by the drive unit itself together with the exhaust gas or as a component of the exhaust gas. Throughput is understood to be a quantity of secondary air per unit of time, for example a mass flow or a volume flow.

[0032] The secondary air is present, for example, while the exhaust gas aftertreatment system is actively heated, namely by operating the drive unit with a rich fuel-fresh gas mixture and additionally introducing secondary air into the exhaust gas, so that a post-oxidation of the combusted fuel still contained in the exhaust gas occurs downstream of the drive unit and upstream or in the exhaust gas aftertreatment system. In the combustion chamber of the drive unit, a rich mixture is present at correspondingly high temperatures. Sufficient secondary air is introduced so that the initial combustion air ratio equals the target value.

[0033] Although the first combustion air ratio is equal to the target value, the exhaust gas composition is different than without the exhaust aftertreatment system being heated up. In particular, the raw emissions contain a comparatively high amount of molecular hydrogen and a comparatively low amount of hydrocarbons, so that the first lambda sensor indicates that the mixture is too rich. Accordingly, the target value must be selected such that the mixture is enriched. For this reason, the target value or the characteristic map is adjusted depending on the secondary air throughput. The specified target values ​​from which the target value is selected and the stored characteristic maps from which the characteristic map is selected are selected for this type of enrichment. Once again, this procedure serves to achieve particularly low emissions from the drive system by precisely adjusting the desired mixture composition.

[0034] A further development of the invention provides that, for the setpoint value, a first preset value is selected that results in a leaner composition of the fuel / fresh gas mixture at a lower first flow rate, and a second preset value is selected that results in a richer composition of the fuel / fresh gas mixture at a higher second flow rate. The background and advantages of this have already been discussed. This effectively counteracts the shift in the mixture composition toward a lean mixture caused by the secondary air, and low exhaust emissions are achieved.

[0035] A further development of the invention provides that the secondary air is admixed to the exhaust gas in terms of flow between the drive unit and the exhaust gas aftertreatment device or is conveyed by the drive unit by operating the drive unit with a valve overlap. The admixture of the secondary air into the exhaust gas preferably takes place with the aid of a secondary air source, for example a pump or a pressure vessel. For example, ambient air is used as secondary air, which is taken from the exterior environment of the motor vehicle and subsequently introduced into the exhaust gas, namely downstream of the drive unit and upstream of the exhaust gas aftertreatment device. As a result, the secondary air reacts with unburned fuel still contained in the exhaust gas, so that the temperature of the exhaust gas and thus the temperature of the exhaust gas aftertreatment device are increased.

[0036] However, secondary air can also be introduced into the exhaust gas by operating the drive unit with valve overlap. In this case, the drive unit is operated in a scavenging mode, in which a rich fuel-fresh gas mixture is generated in the combustion chamber, and the resulting rich exhaust gas is mixed with purge air or secondary air that enters the combustion chamber through the valve overlap. In any case, the described procedure effectively adapts the setpoint to the presence of secondary air or the secondary air flow rate.

[0037] A further development of the invention provides that the setpoint is adjusted taking the second combustion air ratio into account within the framework of a trim control. The selected setpoint, or the setpoint determined from the selected characteristic map, initially serves to pre-control the lambda control. It is then corrected using the trim control by adjusting the second combustion air ratio to the corresponding setpoint. In particular, the first setpoint, to which the first combustion air ratio is adjusted, is adjusted by setting the second combustion air ratio to the second setpoint. This achieves the advantages already mentioned.

[0038] A further development of the invention provides for a broadband lambda sensor to be used as the first lambda sensor and a step-type lambda sensor to be used as the second 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.

[0039] 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.

[0040] The invention further relates to a drive device for a motor vehicle, in particular for carrying out the method according to the statements in the context of this description, 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, a first lambda probe arranged upstream of the exhaust gas aftertreatment device for determining a first combustion air ratio in the exhaust gas and a second lambda probe arranged downstream of the exhaust gas aftertreatment device for determining a second combustion air ratio in the exhaust gas, wherein the drive device is provided and designed to adjust a composition of a fuel-fresh gas mixture used to operate the drive unit based on the first combustion air ratio and / or the second combustion air ratio.

[0041] The drive device is further provided and designed to select a setpoint value to which the first combustion air ratio or the second combustion air ratio is set, depending on an operating state of the drive unit, from a plurality of stored preset setpoint values, in particular independently of the first combustion air ratio and / or the second combustion air ratio.

[0042] The advantages of such a design of the drive direction 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.

[0043] Furthermore, the invention 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.

[0044] 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.

[0045] The invention is explained in more detail below with reference to exemplary embodiments illustrated in the drawings, without limiting the invention. In the drawings: Figure 1 shows a schematic representation of a drive device for a motor vehicle with an exhaust gas generating drive unit and an exhaust gas aftertreatment device. The Figure 1 shows, purely schematically, a drive device 1, which has a drive unit 2, which here is in the form of an internal combustion engine, and an exhaust tract 3. In the exhaust tract 3, there is an exhaust gas aftertreatment device 4, which here is designed as a vehicle catalytic converter. Exhaust gas generated by the drive unit 2 is fed to the exhaust gas aftertreatment device 4 according to the arrow 5. Upstream of the exhaust gas aftertreatment device 4 there is a first lambda probe 6, and downstream of the exhaust gas aftertreatment device 4 there is a second lambda probe 7. A first combustion air ratio determined by means of the first lambda probe 6 is fed to a lambda controller 8 and a second combustion air ratio determined by means of the second lambda probe 7 is fed to a trim controller 9.Both the lambda controller 8 and the trim controller 9 influence a setpoint 10 for a composition of a fuel-fresh gas mixture 11 with which the drive unit 2 is operated.

[0046] It is provided that the setpoint 10 is selected from a plurality of stored predefined setpoints depending on an operating state of the drive unit 2, preferably independently of the first combustion air ratio and / or the second combustion air ratio. Particularly preferably, a characteristic map is selected from a plurality of stored characteristic maps based on the operating state, and the setpoint is read from this characteristic map using the operating point set on the drive unit 2. The operating point is defined in particular by a torque to be generated by the drive unit 2 and / or a rotational speed of the drive unit 2. With the aid of the described procedure, the lambda controller 8 is pre-controlled in particular, so that deviations such as those that can occur in certain operating states are reliably avoided. This achieves particularly low emissions from the drive device. LIST OF REFERENCE SYMBOLS:

[0047] 1Drive system 2Drive unit 3Exhaust tract 4Exhaust aftertreatment system 5Arrow 61. Lambda probe 72. Lambda probe 8Lambda controller 9Trim controller 10Setpoint 11Fuel-fresh gas mixture

Claims

1. A method for operating a drive device (1) for a motor vehicle, which has a drive unit (2) generating exhaust gas, an exhaust gas aftertreatment device (4) designed as a vehicle catalyst for aftertreating the exhaust gas, a first lambda probe (6) arranged upstream of the exhaust gas aftertreatment device (4) for determining a first combustion air ratio in the exhaust gas, and a second lambda probe (7) arranged downstream of the exhaust gas aftertreatment device (4) for determining a second combustion air ratio in the exhaust gas, wherein a composition of a fuel-fresh gas mixture used to operate the drive unit (2) is adjusted based on the first combustion air ratio and / or the second combustion ratio, characterized in thata setpoint to which the first combustion air ratio or the second combustion air ratio is set is selected from several stored preset setpoints depending on an operating state of the drive unit (2).

2. Method according to claim 1, characterized in that the setpoint is determined on the basis of an operating point set on the drive unit (2) from a characteristic map containing several preset setpoints, which is selected from several stored characteristic maps depending on the operating state of the drive unit (2).

3. Method according to one of the preceding claims, characterized in that the operating state is described by a temperature of the drive unit (2) and the setpoint and / or the characteristic map is selected depending on the temperature.

4. Method according to one of the preceding claims, characterized in thatfor the setpoint, a first preset setpoint is selected which results in a leaner composition of the fuel-fresh gas mixture at a lower first temperature and a second preset setpoint is selected which results in a richer composition of the fuel-fresh gas mixture at a higher second temperature.

5. Method according to one of the preceding claims, characterized in that the operating state is described by a flow rate of secondary air and the setpoint and / or the characteristic map is selected depending on the flow rate of secondary air.

6. Method according to one of the preceding claims, characterized in that for the setpoint, a first preset setpoint is selected which results in a leaner composition of the fuel-fresh gas mixture at a lower first throughput and a second preset setpoint is selected which results in a richer composition of the fuel-fresh gas mixture at a higher second throughput.

7. Method according to one of the preceding claims, characterized in that the secondary air is fluidically mixed with the exhaust gas between the drive unit (2) and the exhaust gas aftertreatment device (4) or is conveyed by the drive unit (2) by operating the drive unit (2) with a valve overlap.

8. Method according to one of the preceding claims, characterized in that the setpoint is adjusted taking into account the second combustion air ratio as part of a trim control.

9. 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 a drive unit (2) generating exhaust gas, an exhaust gas aftertreatment device (4) designed as a vehicle catalyst for aftertreating the exhaust gas, a first lambda probe (6) arranged upstream of the exhaust gas aftertreatment device (4) for determining a first combustion air ratio in the exhaust gas, and a second lambda probe arranged downstream of the exhaust gas aftertreatment device (4) for determining a second combustion air ratio in the exhaust gas, wherein the drive device (1) is provided and designed to adjust a composition of a fuel-fresh gas mixture used to operate the drive unit (2) based on the first combustion air ratio and / or the second combustion air ratio, characterized in thatthe drive device (1) is further provided and designed to select a setpoint value to which the first combustion air ratio or the second combustion air ratio is set from a plurality of stored preset setpoint values ​​depending on an operating state of the drive unit (2).

10. Computer program product comprising instructions which cause the drive device (1) according to claim 9 to carry out the method according to one or more of claims 1 to 8.

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