CN111927607A

CN111927607A - Monitoring the state of a catalytic converter for reducing nitrogen oxides

Info

Publication number: CN111927607A
Application number: CN202010396394.2A
Authority: CN
Inventors: A.巴斯托雷亚勒; T.菲斯特
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2019-05-13
Filing date: 2020-05-12
Publication date: 2020-11-13
Also published as: DE102019206873A1

Abstract

The invention relates to a method for monitoring the state of a catalytic converter (12, 13) for reducing nitrogen oxides, comprising: calculating (31) at least one modeled exhaust gas measured value for a predefined state of the model catalytic converter; detecting (30) at least one corresponding measured exhaust gas measured value after the first catalytic converter (12, 13); and determining whether the catalytic converter (12, 13) is undamaged or damaged based on the at least one modeled exhaust gas measurement and the at least one measured exhaust gas measurement.

Description

Monitoring the state of a catalytic converter for reducing nitrogen oxides

Technical Field

The present invention relates to a method for monitoring the state of a catalytic converter for reducing nitrogen oxides, to a computing unit for carrying out the method, and to a computer program for carrying out the method.

Background

In the field of vehicles, in order to reduce the Nitrogen Oxides (NO) in the exhaust gases_x) In particular, an SCR Catalytic converter (Selective Catalytic reduction) can be usedAnd (selective catalytic reduction)). Herein, nitric oxide NO and nitrogen dioxide NO₂Collectively referred to as nitrogen oxides. The basic principle of the SCR catalytic converter is that: in the presence of ammonia (NH)₃) As a reducing agent, nitrogen oxide molecules are reduced to elemental nitrogen on the catalytic converter surface. Reducing agents typically to release NH₃Is provided in the form of an aqueous urea solution (HWL) which is provided by means of a controlled dosing device upstream of the SCR catalytic converter.

At present, the use of SCR catalytic converters is also based on the requirement of using at least one nitrogen oxide sensor. However, two nitrogen oxide sensors are often used, one before and one after the SCR catalytic converter. In many markets, regulations on vehicle Diagnostics (OBD) require very precise monitoring of the system and in particular of the SCR catalytic converter. If the catalytic converter ages and is therefore no longer able to sufficiently convert nitrogen oxides, the corresponding warning signal lamp must be reliably activated and the catalytic converter replaced before the required limit value is exceeded.

The aging and/or damage of the SCR catalytic converter can be monitored by evaluating the nitrogen oxides by means of suitable sensors upstream and downstream of the catalytic converter. For this purpose, in passive diagnostic methods, the nitrogen oxide concentration, the nitrogen oxide mass flow or the nitrogen oxide conversion are generally measured in a phase in which it is possible to adequately distinguish an intact SCR catalytic converter from a damaged SCR catalytic converter. The diagnostic conditions are usually selected such that the nitrogen oxide conversion of the undamaged SCR catalytic converter is high and the nitrogen oxide conversion of the damaged catalytic converter (boundary part) to be identified is as low as possible. In the case of such an identification, the SCR catalytic converter or its state which has not been damaged is also referred to as WPA (worst performing catalytic converter), while the damaged state is referred to as BPU (best performing unacceptable). The greater the difference between these states under the current conditions, the more robust the diagnostic method based on.

Since conventional NOx sensors are cross-sensitive to ammonia, i.e., exhibit NO_xAnd NH₃So-called ammonia slip behind the SCR catalytic converter can lead to a significant reduction in efficiency, since ammonia also leads to an increased sensor signal which can be misinterpreted as an increase in nitrogen oxides. Thus, on the one hand, the BPU catalytic converter can be more easily identified, since it looks worse than it actually is; on the other hand, WPA components also look worse by mistake, making it difficult to distinguish between the states. Thus, these methods generally have the following objectives: the conditions were chosen such that no ammonia leakage occurred.

If the accuracy of such a passive method is not sufficient, an active method can be applied which determines the ammonia storage capacity of the SCR catalytic converter by means of intervention on the dosage of the urea solution. This ammonia storage capacity is very well associated with thermal or chemical damage to the SCR catalytic converter.

Disclosure of Invention

According to the invention, a method for monitoring the state of a catalytic converter, a computer unit for carrying out the method and a computer program for carrying out the method are proposed, which have the features of the independent patent claims. Advantageous embodiments are the subject matter of the dependent claims and the subsequent description.

The invention herein makes use of the following measures: comparing the measured values detected after (downstream of) the catalytic converter with the measured values of the exhaust gas modeled for the predefined states of the model catalytic converter or respectively derived or filtered values therefrom, for example an integral or a sum; and determining whether the catalytic converter is undamaged or damaged based on the comparison.

By using the model as a comparison, all operating conditions can be physically mapped and thereby taken into account in the diagnosis. Instead of the determination of the efficiency and emissions (Ableitung) of the catalytic converter up to now, this direct determination of the aging of the catalytic converter can hereby be achieved: a better distinction is made between damage to the catalytic converter and damage or malfunction of other components.

In this case, the at least one measured exhaust gas measured value and the at least one modeled exhaust gas measured value can each be, in particular, a nitrogen oxide measured value, an ammonia gas measured value or a combined nitrogen oxide/ammonia gas measured value, and can, for example, each indicate the concentration or mass flow of an exhaust gas constituent (such as the nitrogen oxide or ammonia gas mentioned). For this reason, no structural changes are required with respect to conventional exhaust gas lines, and conventional sensors can be used. In particular, the measured exhaust gas values can be detected by means of a nitrogen oxide sensor, which has a cross-sensitivity to ammonia. Here, this cross-sensitivity is advantageously incorporated into the diagnostic method by taking into account the two aging mechanisms of the catalytic converter — lower storage capacity for ammonia and poorer conversion of nitrogen oxides in terms of quantity.

Active intervention in the dosing, which affects efficiency and emissions, can thereby be avoided. Furthermore, an undamaged catalytic converter can be distinguished more precisely from a damaged catalytic converter, so that the robustness and resolution accuracy of the diagnosis is improved.

Preferably, the determination of whether the catalytic converter is undamaged or damaged based on the at least one modeled exhaust gas measurement and the at least one measured exhaust gas measurement comprises: calculating a difference between the at least one measured exhaust measurement and the at least one modeled exhaust measurement; calculating an integral value of an integral of the plurality of difference values over a predetermined time period; comparing the integrated value with a threshold value; and determining whether the catalytic converter is undamaged or damaged based on the comparison. It is easy to understand that: integration values will be implemented in practice as a sum. In this embodiment, it can be inferred very easily from the sign of the integral value whether the catalytic converter is working better or worse than the model. It is also possible that: the model value and the measured value are first integrated separately and then subtracted.

According to one specific embodiment, each difference value can also be multiplied by a weighting factor before the integration, wherein the weighting factor is determined on the basis of predefined operating conditions of the catalytic converter. In this way, phases in which the accuracy of the modeling and/or measurement is high may affect the diagnosis more, while the impact of values that may not be accurately determined or modeled may be limited.

Such operating conditions for determining the weighting factors may include, for example, one or more of the following: tolerance characteristics of ammonia level of the catalytic converter; temperature of the catalytic converter; temperature gradient of the catalytic converter; the conditions of permission of the sensors used; a tail gas mass flow; mass flow of nitrogen oxides.

Furthermore, according to one embodiment, the admissions conditions can be checked further before the difference is used for integration, and the associated difference can only be used for integration if the admissions conditions are met. Thereby, the entire diagnosis is not forced to be discarded, but it is still ensured that the underevidence values have no influence on the evaluation. In this way, the evaluation frequency and the resolution accuracy are further improved. The admissible conditions may for example include a maximum ammonia slip after the modeling of the catalytic converter and/or a maximum temperature gradient in the catalytic converter, since for example the emission of ammonia at high temperatures may lead to a distortion of the evaluation or a high temperature gradient due to a change in the reaction process may lead to difficulties in the modeling.

The calculation of the at least one modeled exhaust gas measured value can be carried out, for example, on the basis of a reaction dynamics model or a data-based model (i.e., a synthetic characteristic curve model). Known models are also known from the literature, for example "unknown analysis of NO Reduction over Selective Catalyst Reduction-De-NOxMonolith Catalysts", E Tronconi, A. Cavanna, P. Forzatti, Ind. Eng. chem. Res 1998, stage 37, page 2341-2349. These models can be implemented in modern motor vehicle (Kfz) control devices and not only represent the NO of an SCR catalytic converter_xConvert and depict NH₃And (4) leakage.

The computing unit according to the invention, for example, a control device of a motor vehicle, is designed in particular in a program-technical manner for carrying out the method according to the invention.

The implementation of the method according to the invention in the form of a computer program or a computer program product with program code for carrying out all method steps is also advantageous, in particular when the control device which carries out the method is also used for other tasks and is therefore always present, since this results in particularly low costs. Data carriers suitable for providing the computer program are, in particular, magnetic, optical and electronic memories, such as hard disks, flash memories, EEPROMs, DVDs and others. It is also possible to download the program via a computer network (internet, intranet, etc.).

Further advantages and embodiments of the invention emerge from the description and the accompanying drawings.

The invention is schematically illustrated in the drawings and will be described below with reference to the drawings according to embodiments.

Drawings

FIG. 1 schematically illustrates an exemplary catalytic purification system suitable for embodiments of the present invention;

FIG. 2 depicts the principle of catalytic cleaner diagnostics according to an embodiment of the invention;

FIG. 3 illustrates an exemplary graph of integrated difference values for an undamaged (WPA) catalytic converter and a damaged (BPU) catalytic converter in accordance with one embodiment of the present disclosure; while

Fig. 4 shows exemplary method steps according to an embodiment of the invention.

Detailed Description

Fig. 1 illustrates an exemplary system in which embodiments of the present invention may be employed. In this case, one or more catalytic converter elements are arranged in the exhaust gas line, into which the exhaust gas flow 10 is conducted from the internal combustion engine for exhaust gas treatment. In the figure, firstly a diesel oxidation catalytic converter (DOC) 11 is shown, downstream of which two SCR catalytic converter elements 12 and 13 are connected, which may also comprise, for example, a particle filter with an SCR Coating (SCRF) 12. In front of each SCR catalytic converter element 12, 13, a

dosing module

14 and 15 is arranged in each case for the dosed introduction of an aqueous urea solution (HWL) into the system. Each

dosing module

14, 15 is controlled by a control unit 19, wherein preferably all modules of the same unit, e.g. an engine control device, are operated. A plurality of

sensors

16, 17 and 18 are also installed which can measure the exhaust gas values at different locations of the system.

In particular, a nitrogen oxide sensor and/or an ammonia gas sensor can be arranged upstream and/or downstream of each catalytic converter element 11, 12/13, which nitrogen oxide sensor and/or ammonia gas sensor measures the concentration and/or the conversion and/or the mass flow of the respective component in the exhaust gas stream. Here, NO may be involved_xSensor, NO_xSensor also for ammonia (NH)₃) Cross-sensitive, or may involve multiple gas sensors that individually output NO_xAnd NH₃The value of (c). Likewise, it is also possible to arrange for NO at

corresponding positions

16, 17, 18 in the exhaust gas stream_xAnd NH₃A plurality of individual sensors. The measured values of all sensors are forwarded to the control unit 19 for processing. Of course, other sensors not shown here may be used, such as temperature sensors, oxygen sensors, air mass flow meters, and other sensors at different locations of the system.

In order to now identify the state of the SCR catalytic converter 12, 13 in such a system or similar, i.e. to be able to carry out a diagnosis with regard to the deterioration or aging of the catalytic converter, according to one embodiment of the invention the measured exhaust gas values are compared with the modeled exhaust gas values and integrated over a defined period of time.

In this case, a theoretical catalytic converter model can be used, which model is used for the catalytic converter or in a specific predefined state, for example in the still undamaged WPA state (of the catalytic converter) ((ii))Last reforming acceptable) expected values of nitrogen oxides and ammonia in the exhaust stream after the catalytic converter were modeled. The model may be, for example, a reaction kinetics model or a data-based model, which are basically known in the art. As a variable relevant for modeling and measurement, the concentration of the exhaust gas constituent, i.e. in particular NH, can be used₃And NO_xAlternatively, a mass flow of these parameters can also be used. It is also possible that: the exhaust values are modeled using a model based on a comprehensive characteristic curve.

Fig. 2 depicts the principle of catalytic converter diagnostics for a catalytic converter according to an embodiment of the invention. From a suitable model 31 of the SCR catalytic converter, the modeled NO after the relevant catalytic converter is calculated_xConcentration of

And modeled NH₃Concentration of

. For this purpose, the ammonia concentration upstream of the catalytic converter, i.e. upstream of the catalytic converter

As an input quantity influence model 31, the ammonia gas concentration may be determined based on the manipulation of the respective dosing module; and the concentration of nitrogen oxides in front of the catalytic converter

As an input variable influencing model 31, this nitrogen oxide concentration can either be measured by a corresponding sensor upstream of the catalytic converter or can be determined on the basis of a model as a function of the operating state of the engine.

The calculated sum signal 32 of the two modeled variables is then used

NO that may be downstream of catalytic converter 30_xThe signals measured at the sensors are compared.

In this embodiment, the measured sensor signal is NO_xValue of the sensor, NO_xSensor pair NH₃Is cross-sensitive and is arranged downstream after the catalytic converter 30, wherein both components give rise to one signal. I.e. in practice the measured signal indicates NO there_xAnd NH₃Corresponding to a combined value substantially corresponding to the sum of the concentrations present after the catalytic converter 30,

。

then, in order to compare the measured value with the modeled value, a difference 34 between the two summed values may be found,

i.e. for NO after the catalytic converter_xAnd NH₃The modeled summation value 32 for the catalytic converter output is subtracted from the measured summation value.

Difference obtained here

The integration is performed for a predetermined evaluation period, for example, by an integrator 36, and the obtained integration is compared with a threshold value. This comparison is then evaluated by a diagnostic system 38, for example OBD diagnostic software.

If a WPA catalytic converter is used as model 31, this applies in the case of a measured catalytic converter which has not been damaged, namely a WPA catalytic converter

That is to say the difference between the model value and the measured value fluctuates around a zero value and the resulting integral is therefore also close to 0. But also in the case of aged or damaged catalytic converters (BPU)

That is to say the measured signal is above the modelled value, i.e. the integral rises above a threshold. Thereby, it can be provided that: as soon as the threshold value is exceeded in the comparison of the integrated value with the threshold value, the catalytic converter 30 is classified as damaged with respect to the OBD diagnosis 38, whereas as long as the signal obtained is below the threshold value, the catalytic converter 30 is classified as not yet damaged.

The difference of the measured value and the modeled value, which is increased when the catalytic converter is damaged (and thereby also the integral is above the threshold value), can be explained essentially on the basis of two effects: about half involves ammonia (NH)₃) This ammonia can no longer be adequately stored in the damaged catalytic converter compared to the WPA catalytic converter; the other half is from nitrogen oxide NO_xThe nitrogen oxides can no longer be sufficiently converted by the damaged catalytic converter and thus increasingly also occur after the catalytic converter. Whereby two main aging effects, namely reduced NH₃Storage capacity and lower NO_xThe conversion is taken into account in a single monitored variable for diagnosing the catalytic converter.

For exhaust values after a catalytic converter instead of a WPA catalytic converter

And

the so-called intermediate layer catalytic converter between the WPA state and the BPU state can also be considered for the modeling 31 of (1). In this case, integrator 36 catalyzes net at the measured BPUThe quantizer will run in the positive direction in the case of WPA and in the negative direction in the case of WPA.

Fig. 3 shows an exemplary statistical evaluation of a plurality of integrated values, that is to say integrated differences for diagnostic measurements on the BPU component and the WPA component, and a modeling of such an intermediate-layer catalytic converter, wherein these integrated values are plotted against time t [ s ]. Here, the point (more than 0) on the upper side of the graph is the integrated value in the case where the catalytic converter is damaged (BPU), and the value (less than 0) on the lower side indicates the integrated value in the case where the catalytic converter is not damaged (WPA).

E.g. based on NO_xIntegration, a standardized time period can be used as an evaluation time point or length for a single monitoring phase of the diagnosis, so that the diagnosis is NO respectively_xThe amount is evaluated after reaching, for example, 2 grams.

The accuracy of such a diagnosis strongly depends on the accuracy of the modeling used of the catalytic converter. In order that this affects the diagnosis, weighting factors can be applied to the differences between the measured and modeled values, which may vary depending on the operating parameters. These weighting factors are determined separately for each round or for each difference and multiplied with the difference. In this way, phases of high accuracy of the model that are expected can be taken into account to a greater extent, while phases of lower accuracy that are expected only conditionally or not at all influence the diagnosis.

For example, general operating conditions may be used, such as for NO_xAs these weighted conditions are permitted by the sensor and dosing module of the aqueous urea solution. Furthermore, optionally, but for example also separately or in combination, the catalytic converter temperature, the exhaust gas mass flow and the NO upstream of the catalytic converter can be used_xMass flow for restriction.

For all such conditions, it is possible in the limiting case: the integrator is temporarily deactivated and the diagnosis is thus "frozen" so that certain phases of modeling inaccuracy do not affect the diagnosis at all and the diagnostic part is then resumed.

It is also reasonable and possible that: examinationThe temperature gradient of the catalytic converter is taken into account for weighting. In the case of high temperature gradients, in the SCR catalytic converter, in the presence of NO_xReaction with NH₃Competition between absorptions occurs, which makes modeling difficult. To take this into account, the current temperature gradient may be compared to a threshold value. If the threshold is exceeded within a certain time, the integrator can be reset to the previously stored value, which for example corresponds to the value at the first time the threshold of the temperature gradient is exceeded. In this way, imprecise phases are not taken into account in the case where the diagnosis is not completely prevented with each brief rise in the temperature gradient.

For the other of the mentioned admissions conditions, a corresponding interruption of the diagnosis can in principle also be made in a similar manner, wherein the integrator either continues to run after the interruption or is reset to a previously determined value.

If there is NH due to the temperature rise₃Emanating from the catalytic converter, there is a further limitation to the diagnosability of the catalytic converter. This process may result in: the ammonia slip in the case of an undamaged catalytic converter (WPA) is greater than in the case of a damaged catalytic converter (BPU) in which less NH has been stored₃. The combined sensor value measured after the catalytic converter may be greater without a damaged catalytic converter than with a damaged catalytic converter. According to the described diagnostic method, the integral will therefore increase and the measured signal will be erroneously interpreted as a sign of a reduced storage capacity, so that this may lead to a (false) diagnosis of a damaged catalytic converter. To prevent this, the ammonia gas leakage of the catalytic converter may be modeled for a model of the WPA catalytic converter taking into account the temperature. If modeled NH₃If the leak is above the threshold, the diagnosis or integration of the difference can be temporarily stopped again correspondingly. Alternatively, the modeled NH for the WPA state may be₃Signal and modeled NH for BPU states₃The signals are compared. If modeled WPAValues higher than the modeled BPU value, the permission for diagnostics based thereon is not reasonable and can be correspondingly frozen (in the case of using the stored values before the temperature rise) or discarded again.

Furthermore, the accuracy of the diagnosis or the modeling used may also depend on the modeled NH of the catalytic converter₃The accuracy of the stock level. If one would expect NH modeled in one case₃The fill level cannot be determined with sufficient accuracy, so that the diagnosis can be correspondingly throttled or stopped again in these cases. For this purpose, for example, two different additional catalytic converter models can be modeled, wherein the first additional model is based on a tolerance characteristic of the filling level which leads to a maximum NH₃Level, and the second additional model is based on a tolerance characteristic that results in a minimum NH₃And (4) material level. From the absolute or relative difference of these two model values, a confidence factor can be determined, which corresponds to the absolute or relative difference with respect to NH₃Current tolerance characteristics in terms of fill level. The difference between the modeled exhaust value and the measured exhaust value may then be multiplied by the confidence factor to achieve a confidence in NH₃The accuracy aspect of the fill level throttles or further weights the diagnostic method.

Fig. 4 shows a flow chart of the method steps of an exemplary embodiment of the invention. All the steps shown need not be used in the method according to the invention, but other steps or interventions which are not depicted here can likewise be carried out.

After the start of the diagnostic part (step 100), it is checked in step 102: whether a diagnostic permissive condition is met, i.e. whether, as described above, for example a permissive condition for the sensor is present, or whether the temperature gradient is below a certain threshold, as described. If this is not the case, the diagnosis is only continued or carried out when the recheck of the admission conditions is successful.

Next, in step 104, a trust factor is calculated for the current value difference. Also other weighting factors may be determined in this step, which are applied to the corresponding difference values, for example to moderate the potential impact. Here, certain conditions can also be used not only for checking the permission or not but also for weighting, or different conditions and influences can be involved.

In step 106, the integrator that integrates the model with the difference of the measured values and the catalytic converter model used are updated based on the previous steps.

In step 108, it is checked whether a predefined monitoring period has ended. If not, the diagnostic process continues by the loop being traversed again from the beginning (step 100) and the conditions and weighting values being correspondingly checked again and applied to the difference values to be integrated and the integrators being updated again with these values (step 106).

If it is ascertained in step 108 that the monitoring period has ended, i.e. for example a predefined period of time has expired or a predefined criterion for determining the monitoring period is met, the monitoring phase is ended and the obtained integrator value is now compared with a threshold value in step 110. If the integrator value exceeds a predetermined threshold value, it can be assumed that: a diagnosed catalytic converter is damaged (step 112); if the threshold is not exceeded, the reason is that the catalytic converter is not damaged (step 114). The case of this threshold depends on the modeling used.

Subsequently, a further monitoring phase can be introduced, which correspondingly supplies a new integrator value for comparison with the threshold value.

It is also possible that: a number of these integrator values are first evaluated, and it is then concluded whether the catalytic converter is undamaged or damaged, and a corresponding alarm is caused by the diagnostic system (for example OBD); for this purpose, for example, a minimum number of results can be specified which must be successively higher or also discontinuously higher than the threshold value. As already seen in fig. 3, the precise integral values, although possibly differing in value, for a certain catalytic converter state are usually within narrow ranges and above or below defined threshold values, so that a clear and robust distinction of WPA and BPU states can be achieved.

It is easy to understand that: the factors and conditions described with respect to throttling or weighting the diagnostics or with respect to stopping, discarding or freezing the diagnostics may be used alone or in combination with one another. Here, the order in which certain conditions are checked may or may not be the same as described herein; depending on the implementation, certain steps may likewise be omitted.

In another embodiment, it is also possible: substitution of para-NH₃Cross-sensitive NO_xSensors, e.g. using so-called multi-gas sensors, capable of outputting NO_xAnd NH₃Of the individual signals. It is also conceivable that: in the case of the method according to the above exemplary embodiment, sensors with no or negligible cross-sensitivity are used, which can therefore essentially output the mass flow or the concentration of the exhaust gas constituents individually. In these cases, the method steps of the invention can also be applied to NO alone_xAnd/or NH₃The corresponding individual signals, i.e. the modeling of the values, the difference from the measured values to the model values, and the subsequent integration of the obtained difference values and the checking of the threshold values for these integrated values.

It is also possible that: in the case of the described method, only a single catalytic converter is monitored, or else a system of a plurality of catalytic converters is monitored one after the other, as is shown, for example, in fig. 1. In a multi-catalytic converter system, the difference and summation can be carried out individually after each catalytic converter, or a combined model of a plurality of catalytic converters under consideration can be used, so that only the measured exhaust gas values at the sensors after the last catalytic converter of the catalytic converters under consideration are used as measured values. In principle, the invention can be used in all exhaust gas systems with SCR catalytic converters and dosing units.

Preferably, the described method steps and calculations are implemented in one or more control units. In this case, for example, the same control unit, preferably an engine control unit, can be used for all steps, sensor data and control processes.

These steps can be implemented electronically or preferably software-based in a corresponding control unit, such as a processor or microcontroller, so that the control unit can be equipped with the diagnostic system according to the invention in a simple manner as long as there is a corresponding interface with the sensor. Here, the software modules on the control unit can also be combined with other corresponding hardware elements, such as microcontrollers and FPGAs, in order to implement some parts of the embodiment according to the invention. Connections of a plurality of control units are likewise possible.

Claims

1. A method for monitoring the status of a catalytic converter (12, 13) for reducing nitrogen oxides, the method comprising:

calculating (31) at least one modeled exhaust gas measured value for a predefined state of the model catalytic converter;

detecting (30) at least one corresponding measured exhaust gas measured value after the catalytic converter (12, 13); and also

Determining whether the catalytic converter (12, 13) is undamaged or damaged based on the at least one modeled exhaust gas measurement value and the at least one measured exhaust gas measurement value, wherein the determining comprises at least one integration step, at least one difference step and at least one comparison step.

2. The method of claim 1, wherein the at least one measured exhaust measurement and the at least one modeled exhaust measurement are one of: a nitrogen oxide measurement, an ammonia measurement, or a combined nitrogen oxide-ammonia measurement.

3. The method according to claim 1 or 2, wherein the at least one measured exhaust gas measurement and the at least one modeled exhaust gas measurement are indicative of a concentration or a mass flow, respectively, of an exhaust gas constituent.

4. Method according to one of the preceding claims, wherein the measured exhaust gas values are detected by means of a nitrogen oxide sensor (17, 18) having cross-sensitivity to ammonia.

5. The method according to one of the preceding claims, wherein the determination of whether the catalytic converter (12, 13) is undamaged or damaged based on the at least one modeled exhaust gas measurement and the at least one measured exhaust gas measurement comprises:

-taking (34) the difference (ac) of the at least one measured exhaust gas measurement and the at least one modelled exhaust gas measurement;

-finding (36) an integral of the integration of the plurality of difference values (Δ c) over a predetermined time period;

comparing (110) the integrated value to a threshold value; and also

Determining whether the catalytic converter (12, 13) is undamaged or damaged based on the comparison.

6. Method according to claim 5, wherein each difference value (Δ c) is multiplied by a weighting factor before the integration, wherein the weighting factor is determined on the basis of a predefined operating condition of the catalytic converter.

7. The method of claim 6, wherein the predefined operating condition for determining the weighting factor comprises one of the following operating conditions: tolerance characteristics of the ammonia level of the catalytic converter; the temperature of the catalytic converter; a temperature gradient of the catalytic converter; the conditions of permission of the sensors used; a tail gas mass flow; mass flow of nitrogen oxides.

8. The method according to one of claims 5 to 7, further comprising:

checking (102) for a permissive condition before using the difference to integrate; and also

The associated difference is used for the integration only if the admissions condition is met.

9. The method of claim 8, wherein the permission conditions comprise: a modeled maximum ammonia slip after the catalytic converter and/or a maximum temperature gradient in the catalytic converter.

10. The method according to one of the preceding claims, wherein the determination of whether the catalytic converter (12, 13) is undamaged or damaged based on the at least one modeled exhaust gas measurement and the at least one measured exhaust gas measurement comprises:

integrating the exhaust gas measurements over a predetermined time period;

integrating the modeled exhaust measurements over the predetermined time period based on an integration of the plurality of modeled exhaust measurements;

calculating a difference between the integrated exhaust measurement and the integrated modeled exhaust measurement;

comparing the difference to a threshold;

11. Method according to one of the preceding claims, wherein the calculation of the at least one modelled exhaust gas measurement is carried out on the basis of a reaction kinetics model or a data-based model.

12. Method according to one of the preceding claims, wherein the model catalytic converter is a WPA catalytic converter or an intermediate catalytic converter between a WPA catalytic converter and a BPU catalytic converter.

13. A computing unit (19) which is set up to carry out all method steps of a method according to one of the preceding claims.

14. A computer program which, when being executed on a computing unit, causes the computing unit to carry out all the method steps of the method according to one of claims 1 to 12.

15. A machine readable storage medium having stored thereon a computer program according to claim 14.