Method for monitoring pollutant emissions from a drive unit for a motor vehicle, corresponding drive unit for a motor vehicle and computer program product

Abstract

The invention relates to a method for operating a drive unit for a motor vehicle, which has an exhaust gas-generating drive unit, an exhaust aftertreatment device for treating the exhaust gas, and a lambda sensor arranged downstream of the exhaust aftertreatment device. The exhaust concentration of an exhaust gas component present in the form of nitrogen oxide downstream of the exhaust aftertreatment device is determined by means of an exhaust aftertreatment model. This model is fed an inlet concentration of the exhaust gas component and an inlet air-fuel ratio as input variables, and the inlet air-fuel ratio is determined from a measurement taken by the lambda sensor. The method also takes into account the reaction of oxygen stored in an oxygen reservoir of the exhaust aftertreatment device with ammonia to form nitrogen oxide and water.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 unit for a motor vehicle, which has an exhaust gas-generating drive unit, an exhaust aftertreatment device for treating the exhaust gas, and a lambda sensor arranged downstream of the exhaust aftertreatment device. The exhaust gas concentration of an exhaust gas component present in the form of nitrogen oxide downstream of the exhaust aftertreatment device is determined by means of an exhaust aftertreatment model, to which an inlet concentration of the exhaust gas component and an inlet air-fuel ratio are supplied as input variables. The inlet air-fuel ratio is determined from a measured value of the lambda sensor. The invention further relates to a drive unit for a motor vehicle and a computer program.

[0002] For example, the prior art includes German patent application DE 10 2013 203 495 A1. This describes a method for monitoring a nitrogen oxide storage catalyst in the exhaust gas duct of an internal combustion engine that is operated at least temporarily in a lean-burn condition, wherein, during lean-burn operation of the internal combustion engine, nitrogen oxides from the exhaust gas are stored by the nitrogen oxide storage catalyst, wherein, during a regeneration phase of the nitrogen oxide storage catalyst, the internal combustion engine is operated in a rich-burn condition and the nitrogen oxides stored in the nitrogen oxide storage catalyst are thereby removed, and wherein an exhaust gas component or exhaust gas parameter characteristic of the regeneration process is detected during the regeneration phase by means of an exhaust gas probe.It is intended that changes in a lambda gradient profile after the nitrogen oxide storage catalyst or derived quantities will be evaluated as a feature for monitoring the nitrogen oxide storage catalyst by means of a change in a time-varying lambda curve upstream of the nitrogen oxide storage catalyst in the range λ < 1, and that a diagnosis of the nitrogen oxide storage catalyst's storage capacity will be carried out based on these values.

[0003] The German patent application DE 10 2022 214 379 A1 relates to a method for determining nitrogen oxide emissions and ammonia emissions in the combustion exhaust gas of an engine system using an output-side nitrogen oxide sensor with cross-sensitivity for ammonia, comprising the following steps: acquiring a measurement signal from the nitrogen oxide sensor; determining a partial conversion value for each of several catalyst segments arranged in series in the combustion exhaust gas, depending on local operating conditions, using a provided conversion model that maps a local operating condition to the partial conversion value; determining a conversion value from the several partial conversion values; and determining nitrogen oxide emissions and ammonia emissions depending on the measurement signal and the conversion value.

[0004] Further prior art includes the publications DE 10 2012 209 469 A1 and DE 10 2023 201 660 B3, as well as the non-patent literature NOVA, Isabella et al.: Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts. New York: Springer, March 14, 2014 - ISBN 978-1-4899-8071-7.

[0005] The object of the invention is to propose a method for operating a drive unit for a motor vehicle which has advantages over known methods, in particular reliably monitoring pollutant emissions from the drive unit in order to ensure compliance with limit values.

[0006] According to the invention, this is achieved by a method for operating a drive direction for a motor vehicle with the features of claim 1. It is provided that, when determining the outlet concentration, a reaction of oxygen stored in an oxygen accumulator of the exhaust aftertreatment device with ammonia to form nitrogen oxide and water is taken into account, wherein a reaction rate of the reaction is determined as a function of a fill level of the oxygen accumulator and the measured value of the lambda sensor is corrected with a correction value that is determined from the outlet concentration and the reaction rate.

[0007] Advantageous embodiments with expedient further developments of the invention are specified in the dependent claims. It should be noted that the exemplary embodiments described in the description are not limiting; rather, any variations of the features disclosed in the description, the claims, and the figures are possible.

[0008] The method is designed for operating the drive system. It is preferably implemented by means of a control unit for the drive system. The drive system or its control unit is preferably an integral part of the motor vehicle, but can of course also be separate from it, particularly until the drive system or the control unit is mounted on or in the motor vehicle. The drive system serves to propel the motor vehicle, i.e., to provide a drive torque directed towards propelling the motor vehicle. To provide the drive torque, the drive system includes the drive unit. The drive unit is preferably an internal combustion engine, particularly preferably a gasoline engine.

[0009] During operation, the drive unit is supplied with fuel and fresh gas at least intermittently, with the fresh gas containing fresh air at least intermittently. Additionally, the fresh gas may contain exhaust gas if exhaust gas recirculation is implemented, in which the exhaust gas generated by the drive unit is at least partially returned to the drive unit 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 then reacted within the drive unit.

[0010] During operation of the engine, exhaust gas is produced due to the chemical reaction of fuel and fresh air. This exhaust gas is discharged towards the outside environment of the engine or vehicle. Since the exhaust gas generated by the engine contains pollutants, it is first routed to an exhaust aftertreatment system before being released into the environment. In the exhaust aftertreatment system, the pollutants are at least partially converted into less harmful products. Only after passing through the exhaust aftertreatment system is the exhaust gas discharged into the environment, primarily through an exhaust pipe on the engine.

[0011] The exhaust aftertreatment system is preferably designed as a vehicle catalyst, in particular as a three-way catalyst. The vehicle catalyst may be integrated into a particulate filter, especially a gasoline particulate filter. For this purpose, the particulate filter is, for example, provided with a catalytic coating. The conversion rate, and thus the conversion capacity of the vehicle catalyst, with which the pollutants are converted into less harmful products, depends in particular on the composition of the exhaust gas supplied to the exhaust aftertreatment system or the vehicle catalyst, as well as on the temperature of the exhaust aftertreatment system.

[0012] The components of the exhaust gas produced by the engine are also referred to as raw emissions. Raw emissions describe the composition of the exhaust gas upstream of the exhaust aftertreatment system, or in terms of flow characteristics, between the engine and the exhaust aftertreatment system. As the exhaust gas passes through the aftertreatment system, some of the substances contained within it are transformed, thus changing its composition. The substances present in the exhaust gas downstream of the aftertreatment system, which constitute the exhaust gas, are also referred to as tailpipe emissions, since this is the composition of the exhaust gas released into the environment through the engine's tailpipe.

[0013] As mentioned earlier, the amount of pollutants contained in the tailpipe emissions depends on the raw emissions, but also on the conversion efficiency of the exhaust aftertreatment system or the vehicle's catalytic converter. This efficiency is temperature-dependent. In particular, the conversion efficiency is lower the further the temperature of the exhaust aftertreatment system deviates from its operating temperature; that is, the greater the absolute difference between the temperatures. The temperature of the exhaust aftertreatment system refers, for example, to the temperature of a ceramic honeycomb structure coated with the catalytic material.

[0014] To determine the pollutant emissions of the propulsion system, i.e., the quantity of at least one exhaust component released into the environment, the exhaust aftertreatment model is used. This model simulates the conversion of at least one exhaust component by the aftertreatment system. For this purpose, the inlet concentration of the at least one exhaust component present at an inlet point is fed into the model as a first input. At least nitrogen oxide is used as the exhaust component. Additionally, any number of outlet concentrations of any number of other exhaust components can be determined using the exhaust aftertreatment model.

[0015] The term "inlet point" refers specifically to the point where the exhaust gas enters the exhaust aftertreatment system. Alternatively, it can also refer to the point where the exhaust gas enters a specific section of the exhaust aftertreatment system, particularly one of several sections. Based on the inlet concentration, the exhaust aftertreatment model calculates an outlet concentration present at an outlet point. Analogously, the outlet point refers to the point where the exhaust gas exits the exhaust aftertreatment system or section thereof.

[0016] The inlet concentration can, in principle, be determined in any way. For example, it can be determined as a function of an operating point of the drive unit, where the operating point is characterized, for example, by the drive torque currently provided by the drive unit and / or an instantaneous rotational speed of the drive unit. If the inlet point is the point at which the exhaust gas enters the exhaust aftertreatment system, then the inlet concentration present at the inlet point is, for example, equal to a raw emission from the drive unit, i.e., equal to the quantity of at least one exhaust gas component produced or emitted by the drive unit. If the inlet point is located downstream of this point, the inlet concentration present at the inlet point is preferably determined using the exhaust aftertreatment model.For example, in this case, the inlet concentration for one of the sections corresponds to the outlet concentration for a section of the exhaust aftertreatment system preceding the section.

[0017] As an additional input variable, the inlet air-fuel ratio is fed into the exhaust aftertreatment model. This ratio is determined analogously to the inlet concentration for the inlet point. For example, it is possible to set the inlet air-fuel ratio equal to a combustion air-fuel ratio determined from the measurement of another lambda sensor upstream of the exhaust aftertreatment system. This measurement describes the combustion air-fuel ratio present in the exhaust gas upstream of the exhaust aftertreatment system, or rather, the amount of residual oxygen present in the exhaust gas at that point.

[0018] However, the reading from such an additional lambda sensor may be subject to a comparatively large measurement error, especially if the additional lambda sensor is a wideband sensor. Even an error of less than one percent leads to an intolerable loss of accuracy in the exhaust aftertreatment model. For this reason, it may be necessary to correct the reading from the additional lambda sensor using a trim control based on a reading from the primary lambda sensor. The additional lambda sensor can also be referred to as the first lambda sensor, and the primary lambda sensor as the second lambda sensor. The first lambda sensor is located upstream of the exhaust aftertreatment system, whereas the second lambda sensor is located downstream of the exhaust aftertreatment system.

[0019] The first lambda sensor measures an initial reading, and the second lambda sensor measures a second reading. This second reading describes the air-fuel ratio, or the amount of residual oxygen, downstream of the first lambda sensor, more precisely, downstream of the exhaust aftertreatment system. For example, it is possible to adjust the second reading to a target value as part of a trim control system and then determine an offset applied to the first reading, particularly for lambda control based on the first reading. For instance, it is possible to determine the air-fuel ratio from the reading corrected in this manner and use it as the intake air-fuel ratio.

[0020] However, it could also be provided that the intake air-fuel ratio is determined from the reading of the lambda sensor located downstream of the exhaust aftertreatment system, i.e., the aforementioned second lambda sensor. Specifically, it is provided that the intake air-fuel ratio is determined from the second reading of the second lambda sensor independently of the first reading of the first lambda sensor. This means that the intake air-fuel ratio depends only on the second reading measured downstream of the first lambda sensor, more precisely, downstream of the exhaust aftertreatment system, and not on the first reading.

[0021] In other words, with this approach, the first measurement is not directly considered when determining the inlet air-fuel ratio; only the second measurement is taken into account. This allows the inlet air-fuel ratio to be determined with such high accuracy that the exhaust aftertreatment model can also be successfully implemented, and the outlet concentration accurately reflects the actual concentration of the exhaust component present at the outlet point.

[0022] In summary, the intake air-fuel ratio supplied to the exhaust aftertreatment model can be determined from the measured value of the first lambda sensor and / or the measured value of the second lambda sensor. Preferably, the aforementioned trim control is used here, so that the measured value of the first lambda sensor, or the first measured value in a form corrected based on the measured value of the second lambda sensor, is incorporated into the intake air-fuel ratio.

[0023] The applicant surprisingly discovered that there are operating ranges of the drive unit in which the measured value of the lambda sensor located downstream of the exhaust aftertreatment system deviates from an expected value. This is primarily due to the fact that the exhaust aftertreatment system has an oxygen storage unit, which influences the residual oxygen content, or lambda value, of the exhaust gas downstream of the exhaust aftertreatment system. This can be attributed to the fact that the lambda sensor exhibits cross-sensitivity to nitrogen oxides, and nitrogen oxides are temporarily generated in the exhaust aftertreatment system. Although nitrogen oxides contained in the raw emissions, i.e., those supplied to the exhaust aftertreatment system, are converted within the system, specifically, nitrogen monoxide reacts with molecular hydrogen to form ammonia and water, according to the reaction formula. 2NO + 5H2 → 2NH3 + 2H2O.

[0024] This is particularly the case when the drive unit is operated with a lack of air, i.e., when λ < 1. In this respect, it is assumed that when the drive unit is operated with a rich fuel-fresh gas mixture, no nitrogen oxide is present downstream of the exhaust aftertreatment system.

[0025] However, nitrogen oxides are produced at least temporarily in the exhaust aftertreatment system, particularly when using a rich fuel-air mixture. Since the lambda sensor exhibits cross-sensitivity to nitrogen oxides, but no correction is applied to the measured value due to the rich fuel-air mixture, the air-fuel ratio determined from the lambda sensor reading deviates from the actual air-fuel ratio downstream of the exhaust aftertreatment system. This means, firstly, that the air-fuel ratio measured by the lambda sensor is inaccurate. Secondly, this is also due to the fact that the outlet concentration determined using the exhaust aftertreatment model is flawed and cannot be used to correct the air-fuel ratio.

[0026] For this reason, the reaction of oxygen stored in the oxygen reservoir of the exhaust aftertreatment system with ammonia to form nitrogen oxide and water is taken into account when determining the discharge concentration. This reaction proceeds according to the reaction equation. 4 NH3 + 5 O2 SP → 4 NO + 6 H2O ab, where the superscript SP stands for the oxygen storage, the substance O2 SP This is molecular oxygen present in the oxygen storage unit. Under certain conditions, this oxygen is released from the oxygen storage unit and reacts with the ammonia contained in the exhaust gas to form nitrogen oxide and water.

[0027] The reaction proceeds until the oxygen is depleted from the oxygen reservoir, i.e., until the oxygen reservoir's fill level is sufficiently low. The reaction occurs, in particular, only if oxygen is still present in the oxygen reservoir. It occurs especially after a change in lambda value from a lean fuel-air mixture to a rich fuel-air mixture, i.e., after switching from the lean to the rich mixture. After this switch, the oxygen reservoir is filled with oxygen, so that when the oxygen reservoir is simultaneously emptied, nitrogen oxides are formed, ultimately resulting in a distorted reading from the lambda sensor. This distortion can be counteracted by taking the aforementioned reaction into account, particularly after switching from the lean to the rich mixture.

[0028] The invention provides that the reaction rate is determined as a function of the fill level of the oxygen reservoir. It has already been explained that the reaction can only proceed as long as oxygen is present in the oxygen reservoir. However, the availability of oxygen also depends on the fill level of the oxygen reservoir; thus, oxygen is discharged from the oxygen reservoir more rapidly at a high fill level than at a low fill level.

[0029] In other words, the fill level of the oxygen reservoir not only determines whether the reaction occurs at all, but also, even when the oxygen reservoir is at least partially full, the reaction rate is a function of the fill level. The fill level is used specifically in the form of a relative fill level; that is, it is the quotient between the amount of oxygen stored in the oxygen reservoir and the maximum amount of oxygen that can be stored in the oxygen reservoir. The reaction rate is given by the relationship... k(T)=k(T0)⋅e(E / R⋅(1 / T0−1 / T))⋅Θ, where k(T 0 ) the reaction rate at an initial temperature T 0 , for example at a temperature of T 0= 20 °C. Furthermore, the relationship includes the activation energy E of the reaction, the universal gas constant R, the instantaneous temperature T, and the fill level θ. Ultimately, determining the inlet air-fuel ratio thus determines the reaction rate, which, in addition to the aforementioned constants, depends on the instantaneous temperature of the exhaust aftertreatment system and the fill level of the oxygen storage. High accuracy is achieved with this approach.

[0030] The invention provides that the measured value of the lambda sensor is corrected by a correction value determined from the outlet concentration and the reaction rate. Correcting the measured value can be understood, for example, as first applying the correction value to the measured value and then determining the air-fuel ratio from the corrected measured value. However, it can also be provided that the air-fuel ratio is determined directly from the measured value and the correction value. For example, the correction value is in the form of an offset; the measured value is thus corrected by adding or subtracting the correction value from the measured value. The correction value is a function of the outlet concentration, more precisely the ammonia concentration, and the reaction rate, ultimately a function of the ammonia concentration and the fill level of the oxygen reservoir.This will achieve the aforementioned advantages.

[0031] A further development of the invention provides that the reaction is a first reaction and the outlet concentration is determined taking into account a second reaction, which describes a reaction of the oxygen stored in the oxygen reservoir of the exhaust aftertreatment system with ammonia to form nitrogen and water. The first reaction is the already described reaction of the stored oxygen with the ammonia to form nitrogen oxide and water. The second reaction proceeds according to the reaction equation. 4 NH3 + 3 O2 SP → 2 N2 + 6 H2O This describes the breakdown of the ammonia contained in the exhaust gas. The ammonia, converted into nitrogen and water in this way, is therefore no longer available for the first reaction and thus no longer for the formation of nitrogen oxides. Taking this second reaction into account further increases the accuracy of the determined intake air-fuel ratio.

[0032] A further development of the invention provides that the reaction rate of the second reaction is determined as a function of the fill level of the oxygen reservoir. Ultimately, the reaction rates of both the first and second reactions thus depend on the fill level of the oxygen reservoir. Analogous to the reaction rate of the first reaction, the reaction rate of the second reaction is derived from the relationship k(T)=k(T0)⋅e(E / R⋅(1 / T0−1 / T))⋅Θ with appropriately adjusted reaction rate k(T)0 Reference is therefore made to the corresponding explanations regarding the first reaction and its reaction rate. The reaction rate is taken into account when determining the intake air-to-combustion air ratio, which results in high accuracy.

[0033] A further development of the invention provides that the correction value is determined taking into account the first and second reaction rates. In particular, the second reaction rate is considered in addition to the first reaction rate when determining the correction value. The correction value thus incorporates the discharge concentration or ammonia concentration, the first reaction rate, and the second reaction rate. The aforementioned advantages result accordingly.

[0034] A further development of the invention provides that the outlet concentration is determined taking into account a third reaction, which describes the reaction of nitrogen oxide with hydrogen to form ammonia and water. The availability of ammonia in the exhaust aftertreatment system depends in particular on this third reaction, as it is subsequently converted further according to the first and / or second reactions, namely into nitrogen oxide or molecular nitrogen. The reaction rate of the third reaction depends significantly on the temperature of the exhaust aftertreatment system and is ultimately based on the Arrhenius equation.

[0035] The reaction rate of the third reaction can therefore also be approximated, at least approximately, with k(T)=k(T0)⋅e(E / R⋅(1 / T0−1 / T))⋅Θ This reaction rate is also factored into the correction value. This results in very good accuracy.

[0036] A further development of the invention provides that a lambda value is determined downstream of the exhaust aftertreatment system and compared with the measured value of the lambda sensor. If the deviation lies outside a tolerance range, a fault in the drive unit is detected. For example, the residual oxygen content in the exhaust gas downstream of the exhaust aftertreatment system is calculated using the exhaust aftertreatment model. This calculated residual oxygen content is compared with the measured value of the lambda sensor or with a residual oxygen content determined from the measured value. If the deviation is too large, a fault in the drive unit is detected. The described procedure achieves a reliable plausibility check of the measured value of the lambda sensor and / or the exhaust aftertreatment model.

[0037] For example, it is intended that the lambda value is compared with the measured value during operation of the drive unit with a rich fuel-air mixture, and specifically only during this operation. When using the rich mixture, it is assumed that no nitrogen oxide is present downstream of the exhaust aftertreatment system, as it is broken down within it. However, this is not entirely accurate, since nitrogen oxide is produced in the exhaust aftertreatment system according to the reaction equation provided. By taking the resulting nitrogen oxide into account when correcting the lambda sensor reading, a reliable plausibility check of the measured value is achieved.

[0038] The invention further relates to a drive unit for a motor vehicle, in particular for carrying out the method according to the explanations in this description, wherein the drive unit has an exhaust gas generating drive unit, an exhaust gas aftertreatment device for aftertreatment of the exhaust gas and a lambda sensor arranged downstream of the exhaust gas aftertreatment device, and wherein the drive unit is provided and designed to determine an outlet concentration of an exhaust gas component present in the form of nitrogen oxide downstream of the exhaust gas aftertreatment device by means of an exhaust gas aftertreatment model to which an inlet concentration of the exhaust gas component and an inlet combustion air ratio are supplied as input variables, and the inlet combustion air ratio from a measured value of the lambda sensor.

[0039] The drive unit is also designed and configured to take into account, when determining the exhaust concentration, a reaction of oxygen stored in an oxygen accumulator of the exhaust aftertreatment system with ammonia to form nitrogen oxide and water. The reaction rate is determined as a function of the oxygen accumulator's fill level, and the lambda sensor reading is corrected with a correction value derived from the exhaust concentration and the reaction rate. The advantages of this drive unit design and procedure have already been mentioned. Both the drive unit and the operating method can be further developed as described in this document, and reference is made to these details.

[0040] Furthermore, the invention relates to a computer program product comprising commands that cause the drive device to execute the described method as described herein. For the advantages and possible advantageous embodiments, reference is made to the description in its entirety.

[0041] The features and combinations of features described in the description, in particular those described in the following figure description and / or shown in the figures, can be used not only in the combinations specified, but also in other combinations or individually, without departing from the scope of the invention, in particular the scope of the claims. Thus, embodiments that are not explicitly shown or explained in the description and / or the figures, but which emerge from or can be derived from the explained embodiments, particularly within the scope of the claims, are also to be considered as encompassed by the invention.

[0042] The invention is explained in more detail below with reference to the exemplary embodiments shown in the drawing, without limiting the invention. The drawing shows: Fig. 1. Several diagrams illustrating a method for operating a drive unit for a motor vehicle, as well as Fig. 2 a diagram showing reaction rates of two chemical reactions via a fill level of an oxygen storage device of an exhaust aftertreatment device of the drive unit.

[0043] The Fig. Figure 1 shows several diagrams that illustrate, purely by way of example, a method for operating a drive unit for a motor vehicle. The drive unit has an exhaust gas-generating drive motor and an exhaust aftertreatment system for treating this exhaust gas. A lambda sensor is arranged downstream of the exhaust aftertreatment system, by means of which the air-fuel ratio present in the exhaust gas is determined. The lambda sensor is specifically designed as a switching lambda sensor.

[0044] The diagram above shows curves 1, 2, and 3 plotted against time. Curve 1 describes the air-fuel ratio upstream of the exhaust aftertreatment system, and curve 2 describes the air-fuel ratio actually present in the exhaust gas downstream of the exhaust aftertreatment system. Curve 3 describes the amount of nitrogen oxide present in the exhaust gas downstream of the exhaust aftertreatment system. Nitrogen oxide refers specifically to nitrogen monoxide.

[0045] The lower diagram shows a measurement from the lambda sensor located downstream of the exhaust aftertreatment system, represented by curve 4. The lambda sensor is a switching lambda sensor, so the measurement is given as a voltage. It can be seen that up to a time of approximately t = 3.8 s, the oxygen storage capacity of the exhaust aftertreatment system is full, and accordingly, curve 4 changes from a measurement indicating a rich fuel-air mixture to a measurement indicating a lean fuel-air mixture.

[0046] Subsequently, at approximately t = 4.2 s, the fuel-air mixture used to operate the engine is switched from a lean to a rich mixture. Nevertheless, after the switchover, the lambda sensor reading remains at a low level, corresponding to the lean mixture, specifically for the period of approximately 4.2 s ≤ t ≤ 7 s. During this period, the reading does not change as rapidly as would be expected given the characteristics of the exhaust aftertreatment system.

[0047] This can be explained by the proportion of nitrogen oxide in the exhaust gas downstream of the exhaust aftertreatment system, as shown in diagram 3. Due to the filled oxygen storage tank, ammonia present in the exhaust aftertreatment system is converted into nitrogen oxide, to which the lambda sensor is cross-sensitive. For this reason, the air-fuel ratio determined using the lambda sensor reading should not be calculated directly from the reading. Instead, the formation of nitrogen oxide from the ammonia is taken into account when determining the air-fuel ratio to achieve greater accuracy.

[0048] The Fig.Figure 2 shows a diagram in which curves 5 and 6 depict the reaction rates of two chemical reactions occurring in the exhaust aftertreatment system as they change with the fill level of the oxygen storage tank. Curve 5 shows the reaction rate of a first reaction, which describes the reaction of oxygen stored in the oxygen storage tank with ammonia to form nitrogen oxide and water, whereas curve 6 represents the reaction of a second chemical reaction, which describes the reaction of oxygen stored in the oxygen storage tank with ammonia to form nitrogen and water.

[0049] It is evident that the reaction rates increase with the increasing fill level of the oxygen reservoir. This means that the influence of the two chemical reactions is greater the more oxygen is present in the oxygen reservoir. Accordingly, it is important to consider at least the first reaction, but preferably also the second reaction, when determining the air-fuel ratio from the lambda sensor reading in order to improve its accuracy. REFERENCE MARK LIST: 1. Course 2 Course 3 Course 4 Course 5 Course 6 Course

Claims

[1] Method for operating a drive unit for a motor vehicle, which has an exhaust gas generating drive unit, an exhaust gas aftertreatment device for aftertreatment of the exhaust gas and a lambda sensor arranged downstream of the exhaust gas aftertreatment device, wherein an outlet concentration of an exhaust gas component present in the form of nitrogen oxide downstream of the exhaust gas aftertreatment device is determined by means of an exhaust gas aftertreatment model to which an inlet concentration of the exhaust gas component and an inlet air-fuel ratio are supplied as input variables, and the inlet air-fuel ratio is determined from a measured value of the lambda sensor, characterized by, that when determining the outlet concentration, a reaction of oxygen stored in an oxygen storage unit of the exhaust aftertreatment system with ammonia to form nitrogen oxide and water is taken into account, whereby a reaction rate of the reaction is determined as a function of a fill level of the oxygen storage unit and the measured value of the lambda probe is corrected with a correction value which is determined from the outlet concentration and the reaction rate. [2] Method according to claim 1, characterized by that the reaction is a first reaction and the outlet concentration is determined taking into account a second reaction, which describes a reaction of the oxygen stored in the oxygen storage of the exhaust aftertreatment device with ammonia to nitrogen and water. [3] Method according to claim 2, characterized bythat the reaction rate of the second reaction is determined as a function of the fill level of the oxygen storage. [4] Method according to claim 3, characterized by , that the correction value is determined taking into account the first reaction rate and the second reaction rate. [5] Method according to any one of the preceding claims, characterized by , that the output concentration is determined taking into account a third reaction, which describes a reaction of nitrogen oxide with hydrogen to form ammonia and water. [6] Method according to any one of the preceding claims, characterized by , that a lambda value is determined downstream of the exhaust aftertreatment device and compared with the measured value of the lambda probe, whereby a deviation outside a tolerance range indicates a fault in the drive device. [7] Drive unit for a motor vehicle, in particular for carrying out the method according to one or more of the preceding claims, wherein the drive unit comprises an exhaust gas generating drive unit, an exhaust gas aftertreatment device for aftertreatment of the exhaust gas and a lambda sensor arranged downstream of the exhaust gas aftertreatment device, and wherein the drive unit is provided and configured to determine an outlet concentration of an exhaust gas component present in the form of nitrogen oxide downstream of the exhaust gas aftertreatment device by means of an exhaust gas aftertreatment model to which an inlet concentration of the exhaust gas component and an inlet combustion air ratio are supplied as input variables, and the inlet combustion air ratio from a measured value of the lambda sensor, characterized by, that the drive device is also designed and configured to take into account, when determining the outlet concentration, a reaction of oxygen stored in an oxygen storage unit of the exhaust aftertreatment device with ammonia to form nitrogen oxide and water, wherein a reaction rate of the reaction is determined as a function of a fill level of the oxygen storage unit and the measured value of the lambda probe is corrected with a correction value which is determined from the outlet concentration and the reaction rate. [8] Computer program product comprising instructions that cause the drive device according to claim 7 to execute the method according to one or more of claims 1 to 6.

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