Method for monitoring an SCR catalyst

Abstract

Method for monitoring an SCR catalyst (3) in an exhaust stream (2) of an internal combustion engine (1) in which a reducing agent solution is dosed for the reduction of nitrogen oxides, wherein a diagnosis of the SCR catalyst (3) is carried out when release criteria are met, characterized in that a difference (Δλ) between a modeled quantity from a model according to a WPA pattern and a corresponding modeled quantity from a model according to a BPU pattern is a release criterion which is met if the difference (Δλ) is above or corresponds to a release threshold (S).

Description

[0001] The present invention relates to a method for monitoring an SCR catalyst using two model parameters. The invention further relates to a computer program that executes each step of the method when run on a computing device, and to a machine-readable storage medium that stores the computer program. Finally, the invention relates to an electronic control unit configured to execute the method according to the invention. State of the art

[0002] Nowadays, the SCR (Selective Catalytic Reduction) process is used in the aftertreatment of exhaust gases from combustion engines to reduce nitrogen oxides (NOx) in the exhaust gas. DE 103 46 220 A1 describes the basic principle. This involves the use of a 32.5% urea-water solution (HWL), also commercially known as AdBlue. ®The process is known to involve dosing the HWL (hydrocarbon liquid) into the exhaust gas. Typically, a dosing system with a dosing module is used to introduce the HWL into the exhaust stream upstream of an SCR catalyst. Ammonia is released from the HWL and subsequently bound to the reactive surface of the SCR catalyst. There, the ammonia combines with the nitrogen oxides, producing water and nitrogen. The HWL is conveyed from a reducing agent tank to the dosing module via a pressure line using a pump and a conveying module.

[0003] As a key component in emissions reduction, the SCR catalyst's emissions-reducing effect is monitored using on-board diagnostics (OBD), also to comply with legal regulations. This monitoring uses a so-called WPA (worst performance acceptable) and a BPU (best performance unacceptable) pattern to assess its functionality. If the SCR catalyst reaches or exceeds the specified assessment values ​​in the WPA pattern, it can be considered fully intact. If the SCR catalyst falls below the specified assessment values ​​in the BPU pattern, it is considered definitively defective. If the assessment values ​​lie between the WPA and BPU patterns, this indicates damage to the SCR catalyst, although the damage is still within an acceptable range that does not necessarily require servicing. Disclosure of the invention

[0004] A method for monitoring an SCR catalyst in the exhaust system of an internal combustion engine is proposed. To reduce nitrogen oxides (NOx) produced during fuel combustion in the engine, a reducing agent solution is metered into the exhaust system upstream of the SCR catalyst via a metering device. This reducing agent is, for example, a urea-water solution from which ammonia (NH3) is released. This ammonia reacts with the NOx to form nitrogen and water. Monitoring of the SCR catalyst involves a diagnostic check when release criteria are met. The release criterion is a difference between a modeled value from a WPA model and a corresponding modeled value from a BPU model.The release criterion is considered fulfilled if the difference between the modeled quantities is above or equal to a release threshold.

[0005] The modeled parameter can be at least one nitrogen oxide conversion rate of the SCR catalyst – often also referred to as efficiency – and / or a nitrogen oxide concentration downstream of the SCR catalyst and / or a nitrogen oxide mass flow rate downstream of the SCR catalyst and / or an ammonia concentration downstream of the SCR catalyst and / or an ammonia mass flow rate downstream of the SCR catalyst. By evaluating the modeled parameters according to the WPA and BPU patterns, it is possible to easily distinguish between phases in which a robust diagnosis can be performed and those in which regeneration cannot. The more robust diagnosis is made possible primarily because, due to the use of the two models, it is advantageously carried out in the phases in which a clearly measurable difference in catalyst behavior is expected.

[0006] According to one aspect, the models based on the WPA pattern and the BPU pattern are calculated as part of a diagnostic function. This means that the models are either already determined within the diagnostic function or indirectly incorporated into it. The advantage of having separate models within the diagnostic function is a better mapping of tolerances to the side appropriate for the diagnosis. For example, the model based on the BPU pattern may tend to be overestimated, while the model based on the WPA pattern is more likely to be underestimated. If the difference between the two models is large enough, the application of the diagnosis is significantly simplified.

[0007] Advantageously, the model according to the WPA pattern is calculated as part of a dosing strategy, and the model according to the BPU pattern is derived independently and separately from the dosing strategy or dependent on the model according to the WPA pattern.

[0008] According to one aspect, the models are calculated according to the WPA pattern and the BPU pattern as part of a dosing strategy. The models for this purpose already exist.

[0009] According to one aspect, a measured parameter is determined for diagnosis. The measured parameter is preferably a measured nitrogen oxide conversion rate, as described below. The SCR catalyst is diagnosed as functioning correctly if the measured parameter is above or equal to a corresponding threshold. Otherwise, the SCR catalyst is diagnosed as defective if the measured parameter is below this threshold.

[0010] Advantageously, the corresponding threshold lies between the corresponding modeled quantity from the model according to the WPA pattern and the corresponding modeled quantities from the model according to the BPU pattern.

[0011] According to one aspect, diagnosing the SCR catalyst involves determining the difference between a measured value and a modeled value from the BPU model. This difference is then integrated over the measurement period. A ratio is then calculated between this integrated difference and the difference between the modeled value from the WPA pattern and the modeled value from the BPU pattern, both integrated over the measurement period. If this ratio is above or equal to a predefined threshold, the SCR catalyst is diagnosed as functional. If the ratio is below this threshold, the SCR catalyst is diagnosed as defective.

[0012] The term "measured" here means that the quantity is measured directly or determined directly from measured quantities.

[0013] The measured quantity can be a nitrogen oxide concentration downstream of the SCR catalyst and / or a nitrogen oxide concentration upstream of the SCR catalyst and / or a nitrogen oxide mass flow rate downstream of the SCR catalyst and / or a nitrogen oxide mass flow rate upstream of the SCR catalyst and / or an ammonia concentration downstream of the SCR catalyst and / or an ammonia mass flow rate downstream of the SCR catalyst and / or a sensor signal from a nitrogen oxide sensor downstream of the SCR catalyst and / or a sensor signal from a nitrogen oxide sensor upstream of the SCR catalyst, whereby the nitrogen oxide sensors output a combined sensor signal of nitrogen oxide and ammonia due to their cross-sensitivity to ammonia.

[0014] Preferably, the measured quantity is a measured nitrogen oxide conversion rate, which is determined, for example, from the measured nitrogen oxide concentration downstream of the SCR catalyst and the measured or modeled nitrogen oxide concentration upstream of the catalyst. Optionally, the nitrogen oxide conversion rate can be determined from the measured nitrogen oxide mass flow rate downstream of the SCR catalyst and the measured or modeled nitrogen oxide mass flow rate upstream of the SCR catalyst. Furthermore, the sensor signal from the nitrogen oxide sensor downstream of the SCR catalyst and the sensor signal from the nitrogen oxide sensor upstream of the SCR catalyst can be used to determine the nitrogen oxide conversion rate. Alternatively or additionally, an ammonia sensor can be used.In particular, a difference is calculated between a determined nitrogen oxide mass flow rate together with a determined ammonia mass flow rate, which were obtained from the sensor signal downstream of the SCR catalyst, and a determined nitrogen oxide mass flow rate obtained from the sensor signal upstream of the SCR catalyst. Furthermore, the nitrogen oxide conversion rate can be determined from a combination of the two methods described above.

[0015] According to one aspect, the integration of differences is started when the release criterion is met. The release criterion is met when the difference between the modeled quantity from the model according to the WPA pattern and the modeled quantity from the model according to the BPU pattern is above or equal to the release threshold.

[0016] The computer program is configured to execute each step of the procedure, particularly when performed on a computer or control unit. It enables the implementation of the procedure in a conventional electronic control unit without requiring any structural modifications. For this purpose, it is stored on a machine-readable storage medium.

[0017] By uploading the computer program to a conventional electronic control unit, the electronic control unit is preserved, which is set up to monitor an SCR catalyst. List of characters

[0018] Exemplary embodiments of the invention are shown in the drawings and explained in more detail in the following description. Fig. Figure 1 shows a schematic representation of an SCR catalyst in an exhaust stream of an internal combustion engine, which is monitored by means of an embodiment of the method according to the invention. Fig. Figure 2 shows a flowchart of a first embodiment of the method according to the invention in the case where a nitrogen oxide conversion rate is evaluated. Fig. Figure 3 shows diagrams of the modeled nitrogen oxide conversion rate over time (a) and the difference between these nitrogen oxide conversion rates over time (b). Fig. Figure 4 shows a flowchart of a second embodiment of the method according to the invention in the case where the formation of a ratio from the difference between the nitrogen oxide conversion rates is evaluated. Fig. Figure 5 shows diagrams of nitrogen oxide conversion rates for an SCR catalyst following the WPA pattern (a) and for an SCR catalyst following the BPU pattern (b). Fig. Figure 6 shows a flowchart of a third embodiment of the method according to the invention in the case where a nitrogen oxide concentration is evaluated downstream of the SCR catalyst. Exemplary embodiments of the invention

[0019] Fig. Figure 1 shows a schematic representation of an internal combustion engine. 1 , an exhaust system 2 and an exhaust aftertreatment system with an SCR catalyst 3 , which is monitored by means of an embodiment of the method according to the invention. A combustion engine 1 exhaust mass flow Q A is powered by the internal combustion engine 1 via the exhaust system 2 to the exhaust aftertreatment system. The combustion engine 1 is controlled by an electronic control unit 6 controlled. The SCR catalyst is responsible for reducing nitrogen oxides. 3The required urea-water solution is supplied using a conveying and dosing system that is known per se. 7 via a metering valve 8 in the exhaust system 2 upstream of the SCR catalyst 3 injected. The conveying and dosing system 7 and the metering valve 8 are via the electronic control unit 6 controlled. In addition, a first nitrogen oxide sensor is 4 upstream of the SCR catalyst 3 before the metering valve 8 arranged. The first nitrogen oxide sensor 4 measures the nitrogen oxide concentration in the exhaust gas upstream of the SCR catalyst 3 and transmits the measurement result to the electronic control unit 6 further. In addition, a second nitrogen oxide sensor is installed. 5 downstream of the SCR catalyst 3 arranged to reduce the nitrogen oxide concentration in the exhaust gas downstream of the SCR catalyst 3measures and also sends the measurement result to the electronic control unit 6 forwards. The nitrogen oxide sensors 4 , 5 They exhibit cross-sensitivity to ammonia. In further embodiments, ammonia sensors can be used alternatively or additionally. By means of the control unit 6 The conveying and dosing system 7 and the metering valve 8 The required mass of urea-water solution is injected into the exhaust stream based on the measured nitrogen concentrations in the exhaust gas. 2 dosed.

[0020] The following applies: The description of the exemplary implementations is based on the nitrogen oxide conversion rates of the SCR catalyst. 3Further embodiments include a nitrogen concentration and / or a nitrogen mass flow rate and / or an ammonia concentration and / or an ammonia mass flow rate and / or sensor signals from the nitrogen oxide sensors. 4 , 5 and / or use sensor signals from the ammonia sensors.

[0021] Fig. Figure 2 shows a flowchart of a first embodiment of the method according to the invention, in which a nitrogen oxide conversion rate is evaluated for diagnosing the functionality of the SCR catalyst. First, a model 10 for a nitrogen oxide conversion rate η m o d W P A according to a WPA template and model 20 for a nitrogen oxide conversion rate η m o d B P U provided for according to a BPU pattern. These models 10 , 20 can be calculated as part of a diagnostic function, where the model 20according to the BPU pattern also from the model 10 can be derived according to the WPA pattern, or calculated as part of a dosing strategy.

[0022] Regarding nitrogen oxide conversion rates η m o d W P A and η m o d B P U These are modeled nitric oxide conversion rates from a WPA pattern and a BPU pattern. For a modeled nitric oxide conversion rate η m o d W P A According to the WPA pattern, this is a nitrogen oxide conversion rate of an SCR catalyst that is just barely acceptable. With a modeled nitrogen oxide conversion rate η m o d B P U Following the BPU pattern, this is a nitrogen oxide conversion rate of an SCR catalyst, the highest value of which is just no longer acceptable. In a next process step 30 A difference Δη is calculated from the nitrogen oxide conversion rate. η m o d W P A and the nitrogen oxide conversion rate η m o d B P U calculated. In a further process step 40 The difference Δη of the nitrogen oxide conversion rates is compared with a defined release threshold S. If the difference Δη of the nitrogen oxide conversion rates is below the release threshold S, no diagnosis is performed. 50. However, if the difference Δη of the nitrogen oxide conversion rates is above the release threshold S, or if the difference Δη of the nitrogen oxide conversion rates corresponds to the release threshold S, a diagnosis is performed with the subsequent procedure steps.

[0023] As part of the diagnosis, a procedure step is performed 60 A diagnostic window DF is defined as the period during which the difference Δη of the nitrogen oxide conversion rates is above the release threshold S. Furthermore, in a subsequent process step 70the current nitrogen oxide conversion rate η mess via the nitrogen oxide sensors 4 , 5 measured in the exhaust system and these in a further process step 80 with a first threshold η OBD compared. Is the measured nitrogen oxide conversion rate η mess above the first threshold η OBD or if it corresponds to this, the SCR catalyst will be used. 3 Diagnosed as functioning: 90. Is the conversion rate η mess below the threshold η OBD , the SCR catalyst 3 diagnosed as defective 95.

[0024] Fig. Figure 3a shows a diagram of the nitrogen oxide conversion rates η over time t. These are the modeled nitrogen oxide conversion rates. η m o d W P A according to the WPA pattern and the modeled nitric oxide conversion rate η m o d B P U represented according to the BPU pattern. Furthermore, the threshold η OBD, which are between the two nitrogen oxide conversion rates η m o d W P A , η m o d B P U The process is depicted. Fig. 3b is the difference Δη of the nitrogen oxide conversion rates η m o d W P A , η m o d B P U out of Fig. Figure 3a is shown in a diagram over time t. In this diagram, the release threshold S is represented as a constant. A diagnostic window DF is defined in the range where the difference Δη of the nitrogen oxide conversion rates exceeds the release threshold S. The measurement 70 the nitrogen oxide conversion rates η mess and the comparison 80 The diagnostic tests (DF) are performed within this diagnostic window. The evaluation time point t is located at the end of the diagnostic window (DF). A for the diagnosis. At this point, the evaluation takes place via the defined diagnostic window DF and the result of the performed diagnosis is output.

[0025] Fig. Figure 4 shows a flowchart of a second embodiment of the method according to the invention. It is first necessary, as already mentioned in connection with Fig. 1 described, a model 110 for a nitrogen oxide conversion rate η m o d W P A according to a WPA template and model 120 for a nitrogen oxide conversion rate η m o d B P U as provided for in a BPU template. In a next procedural step 130 A difference Δη of the nitrogen oxide conversion rates is derived from the modeled nitrogen oxide conversion rate. η m o d W P A according to the WPA pattern and the modeled nitric oxide conversion rate η m o d B P U calculated according to the BPU template. In a further process step 140The difference Δη of the nitrogen oxide conversion rates is compared with a defined release threshold S. If the difference Δη of the nitrogen oxide conversion rates is below the release threshold S, no diagnosis is performed. However, if the difference Δη of the nitrogen oxide conversion rates is above the release threshold S or if the difference Δη equals the release threshold S, the diagnosis is performed. In this case, a further process step is carried out. 160 the current nitrogen oxide conversion rate η mess using the nitrogen oxide sensors 4 , 5 determined. In the next procedural step 170 A ratio r is calculated from the integral of the difference between the current nitrogen oxide conversion rate η. mess and the modeled nitric oxide conversion rate η m o d W P A according to the WPA pattern and the integral of the difference between the modeled nitrogen oxide conversion rate η m o d W P A according to the WPA pattern and modeled nitric oxide conversion rate η m o d B P U formed according to the BPU pattern, as shown below in Formula 1: r = ∫ ( η M e s s − η m o d B P U ) d t ∫ ( η m o d W P A − η m o d B P U ) d t

[0026] In the next procedural step 180 The calculated value of r is then multiplied by a second threshold r OBD compared. The SCR catalyst 3 is diagnosed as functional 190 if the ratio r is above the second threshold r OBD lies or corresponds to this. If the ratio r lies below the second threshold r OBD , the SCR catalyst 3 diagnosed as defective 195.

[0027] Fig. Figure 5a shows the modeled nitrogen oxide conversion rate in an upper diagram of the nitrogen oxide conversion rate η over time t. η m o d W P A according to the WPA pattern and the modeled nitric oxide conversion rate η m o d B P U <?page 6=""?> according to the BPU pattern. It is an exemplary measured nitric oxide conversion rate η. mess shown here, which has a similar course to the modeled nitrogen oxide conversion rate. η m o d W P A according to the WPA pattern. In addition, a threshold η r shown, which lies between the two modeled nitrogen oxide conversion rates and a visualization of the second threshold r OBD of the ratio r, which in the second embodiment (see Fig. 4) described, in comparison to the nitrogen oxide conversion rates. The location of the threshold η r The value midway between the two nitrogen oxide conversion rates corresponds to the value of the second threshold r. OBD of 0.5. In a medium diagram, the difference in the current nitrogen oxide conversion rate η is shown.Mess and the modeled nitric oxide conversion rate η m o d B P U The curve and the integral of this difference over the measurement time are shown as the hatched area below the curve. Furthermore, a lower diagram shows the difference between the modeled nitrogen oxide conversion rate. η m o d W P A according to the WPA pattern and the modeled nitric oxide conversion rate η m o d B P U The BPU pattern is represented as a curve, and the integral of this difference over the measurement time is shown as a hatched area below the curve. The curves and integrals from the middle and lower diagrams have similar shapes and areas. If the ratio r of the two integrals is calculated as a quotient according to Formula 1, this ratio is approximately 1 for the case shown here and is therefore above the second threshold of 0.5. Consequently, the SCR catalyst is diagnosed as conforming to the WPA model and thus as functional (see also the description of...). Fig. 4).

[0028] Fig. Figure 5b shows the modeled nitrogen oxide conversion rate η in an upper diagram of the nitrogen oxide conversion rate η over time t. η m o d W P A according to the WPA pattern and the modeled nitric oxide conversion rate η m o d B P U according to the BPU pattern. It is an exemplary measured nitric oxide conversion rate η. Mess shown, which in this case has a similar course to the modeled nitrogen oxide conversion rate η m o d B P U according to the BPU pattern. In addition, the threshold η corresponding to the second threshold is also present here. r The relationship between the two modeled nitrogen oxide conversion rates is shown. The second threshold r OBD Here too, it is 0.5. In a medium diagram, the difference between the current nitrogen oxide conversion rate η is shown. Mess and the modeled nitric oxide conversion rate η m o d B P U The BPU pattern is represented as a curve, and the integral of this difference over the measurement time is shown as the hatched area below the curve. A lower diagram shows the difference between the modeled nitrogen oxide conversion rate. η m o d W P A according to the WPA pattern and the modeled nitric oxide conversion rate η m o d B P U The BPU pattern is represented as a curve, and the integral of this difference over the measurement time is shown as the hatched area below the curve. The curves and integrals from the middle and lower diagrams differ significantly in their shapes and areas. The upper integral is very small compared to the lower integral and can essentially be considered zero. If the ratio r of the two integrals is calculated as a quotient according to Formula 1, this ratio is approximately 0 for the case presented here and is therefore below the second threshold of 0.5. Consequently, the SCR catalyst is diagnosed as defective according to the BPU model (see also the description of...). Fig. 4).

[0029] Fig.Figure 6 shows a flowchart of a third embodiment of the method according to the invention in the case where the nitrogen oxide concentration is downstream of the SCR catalyst. 3 is evaluated. As already described in the second and third embodiments, a model is first created. 210 for a nitrogen oxide concentration N O x m o d W P A according to a WPA template and model 220 for a nitrogen oxide concentration N O x m o d B P U as provided for in a BPU template. In a next procedural step 230 A difference ΔNOx is calculated between the nitrogen oxide concentration and the modeled nitrogen oxide concentration. N O x m o d W P A according to the WPA pattern and the modeled nitrogen oxide concentration N O x m o d B P U calculated according to the BPU template. In a further process step 240The difference ΔNOx between the nitrogen oxide concentration and a defined release threshold S is compared. If the difference ΔNOx between the nitrogen oxide concentration and the release threshold S is below the release threshold S, no diagnosis is performed. However, if the difference ΔNOx between the nitrogen oxide concentration and the release threshold S is above the release threshold S, or if the difference ΔNOx equals the release threshold S, the diagnosis is performed. In this case, a further process step is carried out. 260 the current nitrogen oxide concentration NOx Mess downstream of the SCR catalyst using the nitrogen oxide sensor 5 determined. In the next procedural step 270 A ratio R is calculated from the integral of the difference between the modeled nitrogen oxide concentration. N O x m o d B P U according to the BPU pattern and the current nitrogen oxide concentration NOx Mess over the measurement time and the integral of the modeled nitrogen oxide concentration N O x m o d B P U according to the BPU pattern and the modeled nitric oxide concentration N O x m o d W P A formed according to the WPA pattern over the measurement time, as shown in Formula 2: R = ∫ ( N O x m o d B P U − N O x M e s s ) d t ∫ ( N O x m o d B P U − N O x m o d W P A ) d t

[0030] In the next procedural step 280 The calculated value of R is then multiplied by a third threshold R. OBD compared. The SCR catalyst is diagnosed as functional 290 if the ratio R is above the second threshold R OBD lies or corresponds to it. If the ratio R is below the third threshold R OBD , the SCR catalyst is diagnosed as defective 295. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10346220 A1

[0002]

Claims

[1] Method for monitoring an SCR catalyst (3) in an exhaust stream (2) of an internal combustion engine (1) in which a reducing agent solution is dosed for the reduction of nitrogen oxides, wherein a diagnosis of the SCR catalyst (3) is carried out when release criteria are met, characterized by , that a difference (Δη) between a modeled quantity from a model according to a WPA pattern and a corresponding modeled quantity from a model according to a BPU pattern is a release criterion that is satisfied if the difference (Δη) is above or equal to a release threshold (S). [2] Method according to claim 1, characterized by that the models are calculated according to the WPA pattern and according to the BPU pattern as part of a diagnostic function. [3] Method according to claim 1, characterized by, that the model according to the WPA pattern is calculated as part of a dosing strategy and the model according to the BPU pattern is derived independently and separately from the dosing strategy or dependent on the model according to the WPA pattern. [4] Method according to claim 1, characterized by that the models are calculated according to the WPA pattern and according to the BPU pattern as part of a dosing strategy. [5] Method according to any one of claims 1 to 4, characterized by , that for diagnosis a measured quantity is determined and the SCR catalyst (3) is diagnosed as functioning if the measured quantity is above an associated threshold (η OBD ) is or corresponds to this, and the SCR catalyst (3) is diagnosed as defective if the measured quantity is below this threshold. [6] Method according to claim 5, characterized by , that the associated threshold (η OBD) between the corresponding modeled size from the model according to the WPA pattern and the corresponding modeled sizes from the model according to the BPU pattern. [7] Method according to any one of claims 1 to 4, characterized by , that for diagnosis a difference between a measured quantity and a modeled quantity from the model according to the BPU pattern is calculated and this difference is integrated over the measurement time and a ratio is formed between this difference and the difference integrated over the measurement time between a modeled quantity from the model according to the WPA pattern and the modeled quantity from the model according to the BPU pattern, wherein the SCR catalyst (3) is diagnosed as functioning if the ratio is above or equal to an associated threshold, and the SCR catalyst (3) is diagnosed as defective if the ratio is below this threshold. [8] Method according to any one of the preceding claims, characterized by , that if the release criterion, namely that the difference between the modeled quantity from the model according to the WPA pattern and the modeled quantity from the model according to the BPU pattern is above the release threshold (S), is met, the integration of the differences is started. [9] Method according to any one of the preceding claims, characterized by , that at least one modeled quantity of the models is one or more of the following quantities: - A nitrogen oxide conversion rate ( η m o d B P U , η m o d W P A ) of the SCR catalyst (3); - a nitrogen oxide concentration downstream of the SCR catalyst (3); - a nitrogen oxide mass flow downstream of the SCR catalyst (3) - an ammonia concentration downstream of the SCR catalyst (3); and / or - an ammonia mass flow downstream of the SCR catalyst (3). [10] Method according to any one of claims 5 to 9, characterized by that the measured quantity is one or more of the following quantities, or is directly derived from these quantities: - a nitrogen oxide concentration downstream of the SCR catalyst (3); - a nitrogen oxide mass flow downstream of the SCR catalyst (3); - a nitrogen oxide concentration upstream of the SCR catalyst (3); - a nitrogen oxide mass flow upstream of the SCR catalyst (3); - an ammonia concentration downstream of the SCR catalyst (3); - an ammonia mass flow downstream of the SCR catalyst (3); - a sensor signal from a nitrogen oxide sensor downstream of the SCR catalyst (3); and / or - a sensor signal from a nitrogen oxide sensor upstream of the SCR catalyst (3). [11] Method according to any one of claims 5 to 10, characterized by, that the measured quantity represents a measured nitrogen oxide conversion rate (η mess ) of the SCR catalyst (3) is determined from a measured nitrogen oxide concentration and / or a measured nitrogen oxide mass flow and / or a sensor signal of a nitrogen oxide sensor downstream of the SCR catalyst (3) and a nitrogen oxide concentration and / or a nitrogen oxide mass flow and / or a sensor signal of a nitrogen oxide sensor upstream of the SCR catalyst (3). [12] Computer program which is configured to perform each step of the method according to any one of claims 1 to 11. [13] Machine-readable storage medium on which a computer program according to claim 12 is stored. [14] Electronic control unit (6) which is configured to monitor an SCR catalyst (3) by means of a method according to any one of claims 1 to 11.

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