CN110821622A

CN110821622A - Method for monitoring an SCR catalyst

Info

Publication number: CN110821622A
Application number: CN201910729492.0A
Authority: CN
Inventors: A.弗朗茨; T.菲斯特
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2018-08-09
Filing date: 2019-08-08
Publication date: 2020-02-21
Also published as: DE102018213380A1

Abstract

The invention relates to a method for monitoring an SCR catalyst in an exhaust gas line of an internal combustion engine. Wherein a reducing agent solution is metered in for reducing nitrogen oxides. When the start-up criterion is fulfilled, a diagnosis of the SCR catalyst (3) is carried out. The starting criterion is characterized by a modeling value from a model according to WPA mode (

) With corresponding modeled values from the model according to BPU mode: (

) Difference between (A) and (B)) Is a start-up criterion, which is fulfilled when said difference is higher than or equal to a start-up threshold (S).

Description

Method for monitoring an SCR catalyst

Technical Field

The invention relates to a method for monitoring an SCR catalyst by means of two model values. Furthermore, the invention relates to a computer program which, when run on a computing device, implements each step of the method, and to a machine-readable storage medium storing the computer program. Finally, the invention relates to an electronic control device, which is provided for implementing the method according to the invention.

Background

At present, the SCR method (selective catalytic reduction) is used in the aftertreatment of the exhaust gases of internal combustion enginesSelectiveCatalyticRReduction) to reduce nitrogen oxides (NOx) in the exhaust. DE 10346220 a1 describes the basic principle. Here, a 32.5% aqueous urea solution (HWL) (also known commercially as AdBlue) is added^®) Metered into the offgas. Typically, the metering system is provided for this purpose with a metering module to meter the HWL into the exhaust gas stream upstream of the SCR catalyst. Ammonia is cracked from the HWL, which then binds to the reactive surface of the SCR catalyst. There, ammonia combines with nitrogen oxides, thereby producing water and nitrogen. The HWL is conveyed by the delivery module from the reducing agent tank via a pressure line to the metering module by means of a delivery pump.

As an important component for reducing pollutants, the SCR catalytic converter is monitored by on-board diagnostics (OBD) with regard to its pollutant reduction effect, also in order to comply with legal regulations. Using the so-called WPA mode (worst acceptable performance) in the monitoringworstperformanceacceptable) and so-called BPU mode (unacceptable best performance)bestperformanceunacceptable) to evaluate functional capacity. If the SCR catalyst reaches or exceeds the evaluation set in WPA mode, it may be evaluated as intact. If the SCR catalyst is below the evaluation set in BPU mode, it should be evaluated as definitely defective. If the evaluation value is between the WPA mode and the BPU mode, this indicates an SCR catalyst damage, wherein the damage is still within an acceptable range, wherein no servicing measures are mandatory.

Disclosure of Invention

The invention provides a method for monitoringMethod of an SCR catalyst in an exhaust line of an internal combustion engine. In order to reduce the nitrogen oxides produced during the combustion of fuel in an internal combustion engine by means of an SCR catalyst, a reducing agent solution is metered into the exhaust gas line upstream of the SCR catalyst by means of a metering device. The reducing agent is, for example, an aqueous urea solution from which ammonia (NH) is cleaved₃) The ammonia reacts with the nitrogen oxides to form nitrogen and water. When the start-up criterion is fulfilled, a diagnosis of the SCR catalytic converter, which is carried out in the monitoring range, is carried out in this case. Used as the starting criterion is the difference between the modeled value from the model according to the WPA mode and the corresponding modeled value from the model according to the BPU mode. When the difference in the modeled values is above or equal to a startup threshold, the startup criteria are considered to have been met.

The at least one modeled value of the model may be a nitrogen oxide conversion (alternatively often also referred to as efficiency) of the SCR catalyst and/or a nitrogen oxide concentration downstream of the SCR catalyst and/or a nitrogen oxide mass flow downstream of the SCR catalyst and/or an ammonia concentration downstream of the SCR catalyst and/or an ammonia mass flow downstream of the SCR catalyst. By evaluating the modeled values according to the WPA-and BPU-modes, phases are easily defined in which the diagnosis can be robustly implemented and the regeneration cannot be robustly implemented. More robust diagnosis is achieved in particular by: it is advantageously carried out in a phase in which a clearly measurable difference in the catalyst performance is expected, thanks to the use of these two models.

According to one aspect, models according to the WPA mode and according to the BPU mode are computed as part of a diagnostic function. This means that the model is always determined within the diagnostic function or is indirectly incorporated into the diagnostic function. The advantage of an independent model within the diagnostic function is that tolerances are better reflected in terms of suitability for diagnosis. For example, models according to the BPU mode tend to be overestimated, while models according to the WPA mode are more likely to be underestimated. If the difference between the two models is large enough, the application of the diagnosis is significantly simplified.

Advantageously, the model according to the WPA mode is calculated as part of the metrology strategy and the model according to the BPU mode is independent and independent of the metrology strategy or derived therefrom in dependence on the model according to the WPA mode.

According to one aspect, models according to WPA mode and BPU mode are computed as part of a metering strategy. These models already exist for this purpose.

According to one aspect, for diagnosis, a measurement value is determined. The measured value is preferably a measured nitrogen oxide conversion as described below. When the measured value is higher than or equal to the associated threshold value, the SCR catalytic converter is diagnosed as functional. Otherwise, the SCR catalyst is diagnosed as defective when the measured value is below the threshold value.

Advantageously, said threshold value is located between a respective modeled value from the model according to the WPA mode and a respective modeled value from the model according to the BPU mode.

According to one aspect, to diagnose the SCR catalyst, a difference between the measured value and a modeled value from the BPU model is determined. The difference between the measured value and the modeled value from the model according to the BPU mode is integrated over the measurement time in the next step. Then, a ratio between the difference and the difference between the modeled value from the model according to the WPA mode and the modeled value from the BPU mode integrated across the measurement time is formed. When the ratio is above or equal to the associated threshold value, the SCR catalyst is diagnosed as functional. When the ratio is below the threshold, the SCR catalyst is diagnosed as defective.

The term "measuring" herein means that these values are measured directly or determined directly from the measured values.

The measured values can be the nitrogen oxide concentration downstream of the SCR catalyst and/or the nitrogen oxide concentration upstream of the SCR catalyst and/or the nitrogen oxide mass flow downstream of the SCR catalyst and/or the nitrogen oxide mass flow upstream of the SCR catalyst and/or the ammonia concentration downstream of the SCR catalyst and/or the ammonia mass flow downstream of the SCR catalyst and/or a sensor signal of a nitrogen oxide sensor upstream of the SCR catalyst, the nitrogen oxide sensors giving a combined sensor signal of nitrogen oxide and ammonia due to their cross-sensitivity to ammonia.

Preferably, the measured value is a measured nitrogen oxide conversion, which is determined, for example, from a measured nitrogen oxide concentration downstream of the SCR catalyst and a measured or modeled nitrogen oxide concentration upstream of the catalyst. Optionally, the nitrogen oxide conversion may be determined from a measured nitrogen oxide mass flow downstream of the SCR catalyst and a measured or modeled nitrogen oxide mass flow upstream of the SCR catalyst. In addition, for determining the nitrogen oxide conversion, a sensor signal of a nitrogen oxide sensor downstream of the SCR catalyst and a sensor signal of a nitrogen oxide sensor upstream of the SCR catalyst can be used. Alternatively or additionally, an ammonia sensor may be used. In particular, a difference between the determined nitrogen oxide mass flow (which is determined together with the determined ammonia mass flow) determined from the sensor signal downstream of the SCR catalyst and the determined nitrogen oxide mass flow determined from the sensor signal upstream of the SCR catalyst is calculated. Furthermore, the nitrogen oxide conversion can be determined by a combination of the two possibilities mentioned above.

According to one aspect, integration of the difference is initiated when the start-up criteria are met. The startup criterion is here fulfilled when the difference between the modeled value from the model according to the WPA mode and the modeled value from the model according to the BPU mode is higher than or equal to the startup threshold.

The computer program is provided for carrying out each step of the method, in particular when the computer program is carried out on a computing or control device. This enables the method to be carried out in a conventional electronic control device without having to make constructional changes thereto. Further, it is stored on a machine-readable storage medium.

By running the computer program on a conventional electronic control unit, an electronic control unit is obtained which is provided for carrying out the monitoring of the SCR catalyst.

Drawings

Embodiments of the invention are illustrated in the drawings and are set forth in more detail in the description that follows.

Fig. 1 shows a schematic representation of an SCR catalyst in an exhaust line of an internal combustion engine, which SCR catalyst is monitored by an embodiment of the method according to the invention.

Fig. 2 shows a flow chart of a first embodiment of the method according to the invention for evaluating the nitrogen oxide conversion.

FIG. 3 shows a modeled graph of NOx conversion over time (a) and a graph of the difference between NOx conversion over time (b).

Fig. 4 shows a flow chart of a second embodiment of the method according to the invention for evaluating the formation of a ratio from the difference between the nitrogen oxide conversions.

Fig. 5 shows a graph of the nitrogen oxide conversion in the case of an SCR catalyst according to the WPA mode (a) and in the case of an SCR catalyst according to the BPU mode (b).

Fig. 6 shows a flow chart of a third exemplary embodiment of the method according to the invention for evaluating the nox concentration downstream of the SCR catalyst.

Detailed Description

Fig. 1 shows in a schematic way an internal combustion engine 1, an exhaust gas line 2 and an exhaust gas aftertreatment system with an SCR-catalyst 3, which is monitored by an embodiment of the method according to the invention. Exhaust gas mass flow Q from internal combustion engine 1_AIs conveyed from the combustion engine 1 via an exhaust line 2 to an exhaust aftertreatment system. The internal combustion engine 1 is controlled by an electronic control apparatus 6. The aqueous urea solution required for reducing nitrogen oxides in the SCR catalyst 3 is injected into the exhaust gas line 2 upstream of the SCR catalyst 3 via a metering valve 8 by a delivery and metering system 7 known per se. The delivery and metering system 7 and the metering valve 8 are controlled by the electronic control device 6. In addition, a first nitrogen oxide sensor 4 upstream of the SCR catalyst 3 is arranged upstream of the metering valve 8. 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. Furthermore, a second nitrogen oxide sensor 5 is arranged downstream of the SCR catalyst 3, which measures the nitrogen oxide concentration in the exhaust gas downstream of the SCR catalyst 3 and transmits the measurement result likewise to the electronsA control device 6. The nitrogen oxide sensors 4, 5 have a cross-sensitivity to ammonia. In other embodiments, an ammonia sensor may alternatively or additionally be used. The delivery and metering system 7 and the metering valve 8 are controlled by the control device 6 and the desired mass amount of aqueous urea solution is metered into the exhaust gas line 2 in accordance with the determined nitrogen concentration in the exhaust gas.

The following applies: the description of the embodiments is based on the nitrogen oxide conversion of the SCR catalyst 3. Other embodiments provide for using the nitrogen concentration and/or the nitrogen mass flow and/or the ammonia concentration and/or the ammonia mass flow and/or the sensor signal of the nitrogen oxide sensor 4, 5 and/or the sensor signal of the ammonia sensor.

Fig. 2 shows a flow chart of a first embodiment of the method according to the invention, in which the nitrogen oxide conversion is evaluated for diagnosing the functional capability of the SCR catalyst. First, model 10 was set for nitrogen oxide conversion according to the WPA modelAnd setting the model 20 for NOx conversion according to the BPU mode

. These

models

10, 20 may be calculated as part of a diagnostic function, where the model 20 according to the BPU mode may also be derived from the model 10 according to the WPA mode, or may be calculated as part of a metrology strategy.

Conversion of nitrogen oxides

And

is the modeled nitrogen oxide conversion from the WPA mode and the BPU mode. Modeled nitrogen oxide conversion according to WPA mode

Is just still acceptable nox conversion of the SCR catalyst. Root of herbaceous plantModeled NOx conversion according to BPU mode

Is the nox conversion of the SCR catalyst whose highest value is just no longer acceptable. In a next method step 30, the nitrogen oxide conversion is calculated

With nitrogen oxide conversion

Difference of (2)

. In a further method step 40, the difference in the nitrogen oxide conversion is determined

Compared to a defined activation threshold S. If difference in the conversion of nitrogen oxides

Below the activation threshold S, no diagnosis 50 is performed. Conversely, if there is a difference in the conversion of nitrogen oxides

Above the start-up threshold S or the difference in the NOx conversion

Equal to the start threshold S, the diagnosis is carried out by the subsequent method steps.

In the context of diagnostics, the diagnostic window DF is defined as the difference in the nox conversion in method step 60

A period of time above the start-up threshold S. Furthermore, in a subsequent method step 70, the current nitrogen oxides are measured by the nitrogen oxide sensors 4, 5 in the exhaust gas lineConversion rate

And this is compared with the first threshold value in a further method step 80

And (6) comparing. If the measured nitrogen oxide conversion rate

Above a first threshold value

Or equal to the first threshold value

Then the SCR catalyst 3 is diagnosed as functional 90. If the conversion rate is high

Below threshold

The SCR catalyst 3 is diagnosed as defective 95.

FIG. 3a shows the nitrogen oxide conversion

Graph over time t. Shows the modeled nitrogen oxide conversion according to WPA model

And modeled NOx conversion according to BPU mode. In addition, two nitrogen oxide conversions are shown

Threshold value extending therebetween. In FIG. 3b, the nitrogen oxide conversion from FIG. 3a is shown in a graph over time t

Difference of (2). In this graph, the activation threshold S is shown as a constant. Difference in conversion of nitrogen oxides

Above the activation threshold S, a diagnostic window DF is defined. Conversion of nitrogen oxides

The measurement 70 and the comparison 80 are carried out during the time period of the diagnostic window DF. Evaluation time T for diagnosis_AAt the end of the diagnostic window DF. At this point in time, a defined diagnostic window DF is evaluated and the results of the performed diagnosis are given.

Fig. 4 shows a flow chart of a second embodiment of the method according to the invention. First, as already described in connection with fig. 1, the model 110 is set up for nitrogen oxide conversion according to the WPA mode

And setting the model 120 for NOx conversion according to the BPU mode

. In a next method step 130, the nitrogen oxide conversion from the modeling according to the WPA model is calculated

NOx conversion with modeling according to BPU modeDifference in nitrogen oxide conversion. In a further method step 140, the difference in the nitrogen oxide conversion is determined

Compared to a defined activation threshold S. If there is a difference between the nitrogen oxide conversions

Below the activation threshold S, no diagnosis is performed 150. Conversely, if there is a difference in the conversion of nitrogen oxides

Above a starting threshold S or a difference

Equal to the activation threshold S, the diagnosis is implemented. In this case, in a further method step 160, the current nox conversion is determined by means of the nox sensors 4, 5

. In a next method step 170, the conversion from the current nitrogen oxides is carried out

Nitrogen oxide conversion with modeling according to WPA model

Integral of difference between and nitrogen oxide conversion according to modeling of WPA mode

NOx conversion with modeling according to BPU mode

Integral forming ratio of difference between

As shown below in equation 1:

(formula 1)

In a next method step 180, the

Calculated value of (d) and second threshold value r_OBDAnd (6) comparing. When ratio of

Above a second threshold value r_OBDOr equal to a second threshold value r_OBDThe SCR catalyst 3 is diagnosed as being functional 190. If ratio

Below a second threshold value r_OBDThe SCR catalyst 3 is diagnosed as defective 195.

FIG. 5a above nitrogen oxide conversionModeled NOx conversion according to the WPA model is shown in the graph over time t

And modeled NOx conversion according to BPU mode

. Shows an exemplary measured nitrogen oxide conversion

Here with nitrogen oxide conversion in accordance with the modeling of the WPA modelSimilar trend. In addition, threshold values are displayed

Between the two modeled NOx conversions and visually displaying a second threshold value r of the ratio r compared to the NOx conversion_OBDWhich is described in the second embodiment (see fig. 4). Intermediate threshold between two nitrogen oxide conversions

Corresponds to the second threshold value r_OBDThe value of (A) is 0.5. In the middle diagram, the current nitrogen oxide conversion is plotted graphically

Nitrogen oxide conversion rate with modeling

And the integral of this difference over the measurement time is depicted as the shaded area under the curve. Furthermore, in the following graph, modeled nitrogen oxide conversion according to the WPA mode is plotted as a curve

NOx conversion with modeling according to BPU mode

The difference between, and the integral of this difference over the measurement time is depicted as the shaded area under the curve. Curves and integrals from the middle and lower graphs have similar shape and area values. If the ratio r of the two integrals is formed by a quotient according to equation 1, it is approximately 1 for the case shown here and is therefore above the second threshold value 0.5. Thus, the SCR catalyst is diagnosed as corresponding to the WPA model and thus functionally capable (see also the description of fig. 4).

FIG. 5b above nitrogen oxide conversion

The modeled NOx conversion according to the WPA model is likewise shown in the graph over time tAnd modeled NOx conversion according to BPU mode. Shows an exemplary measured nitrogen oxide conversion

In this case, the nitrogen oxide conversion rate is similar to the modeled nitrogen oxide conversion rate according to the BPU modeSimilar trend. In addition, here too, a threshold value between the two modeled NOx conversion rates is shown, which corresponds to the second threshold value

. Second threshold value r_OBDHere also 0.5. In the middle diagram, the current nitrogen oxide conversion is plotted graphically

NOx conversion with modeling according to BPU mode

And the integral of this difference over the measurement time is depicted as the shaded area under the curve. In the following chart, modeled nitrogen oxide conversion according to the WPA model is plotted as a curve

NOx conversion with modeling according to BPU modeThe difference between, and the integral of this difference over the measurement time is depicted as the shaded area under the curve. The curves and integrals from the middle and lower graphs have significant deviations from each other in their shape and their area values. The above integral is very small compared to the below integral and can be considered substantially zero. If the ratio r of the two integrals is formed by a quotient according to equation 1, it is approximately 0 for the case shown here and is therefore below the second threshold value of 0.5. Thus, the SCR catalyst is diagnosed as corresponding to the BPU model and is therefore defective (see also the description of fig. 4).

Fig. 6 shows a flow chart of a third embodiment of the method according to the invention for evaluating the nox concentration downstream of the SCR catalyst 3. As already described in the second and third embodiments, the model 210 is first set for the nox concentration according to the WPA mode

And setting the model 220 for NOx concentration according to the BPU mode

. In a next method step 230, the nitrogen oxide concentration from the modeling according to the WPA model is calculated

With modeled NOx concentration according to BPU modeOf the nitrogen oxide concentration of

. In a further method step 240, the difference between the nitrogen oxide concentrations is determined

With defined activation thresholdAnd S is compared. If the difference between the concentrations of nitrogen oxides

Below the activation threshold S, no diagnosis 250 is performed. Conversely, if the difference in NOx concentration

Above a starting threshold S or a difference

Equal to the activation threshold S, the diagnosis is implemented. In this case, in a further method step 260, the current nitrogen oxide concentration downstream of the SCR catalyst is determined by means of the nitrogen oxide sensor 5

. In a next method step 270, the modeled NOx concentration is determined from the BPU modelWith the current concentration of NOxIntegral of the difference between over the measurement time and modeled NOx concentration according to BPU mode

NOx concentration modeling according to WPA mode

The integration of the difference between over the measurement time forms the ratio R, as shown below in equation 2:

(formula 2)

In a next method step 280, the calculated value of R is compared with a third threshold value R_OBDAnd (6) comparing.When the ratio R is higher than the third threshold R_OBDOr equal to a third threshold value R_OBDThe SCR catalyst is diagnosed as functional 290. When the ratio R is lower than a third threshold value R_OBDThe SCR catalyst is diagnosed as defective 295.

Claims

1. Method for monitoring an SCR catalyst (3) in an exhaust gas line (2) of an internal combustion engine (1), wherein a reducing agent solution for reducing nitrogen oxides is metered in, wherein a diagnosis of the SCR catalyst (3) is carried out when a start-up criterion is fulfilled,

characterized by the difference between a modeled value from the model according to the WPA schema and a corresponding modeled value from the model according to the BPU schema (

) Is a starting criterion when the difference (

) Above or equal to the start-up threshold (S), the start-up criterion is fulfilled.

2. The method of claim 1 wherein the model based on the WPA mode and based on the BPU mode is calculated as part of a diagnostic function.

3. The method of claim 1, wherein the model according to the WPA schema is computed as part of a metering strategy and is derived independently and independently of the metering strategy or dependent on the model according to the WPA schema.

4. The method of claim 1, wherein the model according to the WPA mode and according to the BPU mode are calculated as part of a metering strategy.

5. The method of any one of claims 1 to 4Characterized in that for diagnosis, a measured value is determined and when the measured value is above the threshold value(s) (() Or equal to said threshold value () The SCR catalyst (3) is diagnosed as functional and the SCR catalyst (3) is diagnosed as defective when the measured value is below the threshold value.

6. The method of claim 5, wherein the threshold value(s) ((m))) Between the corresponding modeled values from the model according to the WPA schema and the corresponding modeled values from the model according to the BPU schema.

7. Method according to any one of claims 1 to 4, characterized in that for the diagnosis a difference between the measured value and the modeled value from the model according to the BPU mode is calculated and integrated over the measurement time and a ratio between the difference and the difference between the modeled value from the model according to the WPA mode and the modeled value from the model according to the BPU mode integrated over the measurement time is formed, wherein the SCR catalyst (3) is diagnosed as functional when the ratio is above or equal to an attributed threshold value and the SCR catalyst (3) is diagnosed as defective when the ratio is below the threshold value.

8. Method according to any of the preceding claims, characterized in that the integration of the difference is started when a start criterion is fulfilled that the difference between the modelled value from the model according to the WPA mode and the modelled value from the model according to the BPU mode is higher than a start threshold (S).

9. The method according to any of the preceding claims, characterized in that at least one modeled value of the model is one or more of the following values:

-nitrogen oxide conversion of the SCR catalyst (3) ((

、

）；

-a nitrogen oxide concentration downstream of the SCR catalyst (3);

-mass flow of nitrogen oxides downstream of the SCR catalyst (3);

-ammonia concentration downstream of the SCR catalyst (3); and/or

-mass flow of ammonia downstream of the SCR catalyst (3).

10. A method according to any one of claims 5 to 9, characterized in that the measured values are one or more of the following values, or are determined directly from these values:

-a nitrogen oxide concentration downstream of the SCR catalyst (3);

-mass flow of nitrogen oxides downstream of the SCR catalyst (3);

-a nitrogen oxide concentration upstream of the SCR catalyst (3);

-mass flow of nitrogen oxides upstream of the SCR catalyst (3);

-ammonia concentration downstream of the SCR catalyst (3);

-ammonia mass flow downstream of the SCR catalyst (3);

-a sensor signal of a nitrogen oxide sensor downstream of the SCR catalyst (3); and/or

-a sensor signal of a nitrogen oxide sensor upstream of the SCR catalyst (3).

11. A method according to any one of claims 5 to 10, wherein the method is carried out in a batch plantThe measured value is the measured nitrogen oxide conversion of the SCR catalyst (3) ((

) Which is determined from the measured nitrogen oxide concentration and/or the measured nitrogen oxide mass flow and/or the sensor signal of the nitrogen oxide sensor downstream of the SCR catalyst (3) and from the nitrogen oxide concentration and/or the nitrogen oxide mass flow and/or the sensor signal of the nitrogen oxide sensor upstream of the SCR catalyst (3).

12. A computer program arranged to implement each step of the method according to any one of claims 1 to 11.

13. A machine-readable storage medium having stored thereon a computer program according to claim 12.

14. Electronic control device (6) which is provided for carrying out a monitoring of the SCR catalyst (3) by a method according to one of claims 1 to 11.