CN107542563B

CN107542563B - Fault detection in an SCR system by means of ammonia fill level

Info

Publication number: CN107542563B
Application number: CN201710500625.8A
Authority: CN
Inventors: F.米科利斯; A.沃洛诺; M.通多
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2016-06-28
Filing date: 2017-06-27
Publication date: 2021-04-16
Anticipated expiration: 2037-06-27
Also published as: DE102016211575A1; CN107542563A; KR20180002057A

Abstract

The invention relates to a method for fault detection in an SCR system of an internal combustion engine in a motor vehicle, comprising two SCR catalysts and two nitrogen oxide sensors. The method comprises the following steps: heating the first SCR catalyst to a temperature for which a maximum ammonia fill level that can be stored by the first SCR catalyst is approximately zero; setting a prescribed ammonia fill level in the first SCR catalyst; measuring the concentration of nitrogen oxides upstream of the SCR catalyst and the sum of the concentration of nitrogen oxides and the concentration of ammonia downstream of the SCR catalyst; deriving a comparison of an actual ammonia fill level of the second SCR catalyst to an expected ammonia fill level of the second SCR catalyst when ammonia slip occurs; a fault in at least one of the two SCR catalysts is identified based on a comparison of an actual ammonia fill level of the second SCR catalyst to an expected ammonia fill level when ammonia slip occurs.

Description

Fault detection in an SCR system by means of ammonia fill level

Technical Field

The invention relates to a method for fault detection in an SCR system having two SCR catalysts by means of an ammonia fill level when ammonia slip occurs. Furthermore, the invention relates to a computer program which executes each step of the method when the computer program runs on a computer, and to a machine-readable storage medium which stores the computer program. Finally, the invention relates to an electronic control unit which is set up to carry out the method.

Background

Selective Catalytic Reduction (SCR) is a widely spread technology for reducing nitrogen oxides (NOx) in the exhaust gases of combustion motors in motor vehicles. In an SCR system, it will be commercially also referred to as AdBlue^®The known urea-water solution is injected into the exhaust system by an injection module upstream of the at least one SCR catalytic converterIn (1). The ammonia separated from the urea-water-solution reacts with the nitrogen oxides at the SCR catalyst for selective catalytic reduction to form elemental nitrogen.

Due to the introduction of more stringent emission regulations, a plurality of SCR catalysts are used, which act on the same exhaust gas. In the event of an insufficient efficiency of the SCR catalytic converter for reducing the nitrogen oxide emissions in the exhaust system, provision is made for a fault detection to be carried out by the vehicle's own detection method (which is usually implemented in an electronic control unit). For this reason, continuous monitoring is carried out during normal operation of the vehicle. For a common detection method, at least one nitrogen sensor arranged upstream of the SCR catalyst and at least one nitrogen sensor arranged downstream of the SCR catalyst are used. For a single SCR catalyst, two nox sensors are sufficient for calculating the efficiency of the SCR system and for monitoring the nox emissions at the same time.

The extension of the SCR system to a plurality of (n) SCR catalysts in the same exhaust system is conventionally based on an n +1 nox sensor for detecting at least one SCR catalyst which is not or poorly functioning by means of a pin-point strategy. Accordingly, a non-or poorly functioning SCR catalyst can be repaired or replaced in a targeted manner, while a better functioning SCR catalyst or the SCR system remains unaffected. However, the use of the needle-point strategy is not necessary during normal operation of the vehicle, since it is sufficient here to monitor the efficiency only. In the case of a non-or poorly functioning SCR catalyst, the driver can be alerted, for example, by a signal light on the dashboard, whereupon he drives the vehicle, for example, to a service point. During the repair-point repair, the needle-point-type strategy mentioned is used in the detection method for detecting the non-or poorly functioning SCR catalyst.

Disclosure of Invention

The method relates to an SCR system of a combustion motor in a motor vehicle. In this case, the SCR system has two SCR catalytic converters arranged one behind the other in a common exhaust system. The exhaust gas first passes through the first SCR catalyst and is subsequently conducted on to the second SCR catalyst, so that both SCR catalysts act on the exhaust gas. Furthermore, the SCR system has two nox sensors, which are likewise arranged in this exhaust system. A first nitrogen oxide sensor is arranged upstream of the two SCR catalysts and is able to measure there the nitrogen oxide concentration before the exhaust gas treatment by the SCR catalysts. A second nitrogen oxide sensor is arranged downstream of the two SCR catalysts and can measure there the sum of the nitrogen oxide concentration and the ammonia concentration after exhaust gas aftertreatment by the SCR catalysts, wherein the ammonia concentration corresponds to an ammonia slip (Ammoniak-Schlupf) occurring when the ammonia fill level exceeds a maximum ammonia fill level.

The method comprises the following steps. The first SCR catalyst is first heated to a temperature for which the maximum ammonia fill level that can be stored by the first SCR catalyst is almost zero. This can be achieved, for example, by: the temperature dependence of the maximum ammonia filling level is shown as a characteristic curve and the temperature for which the characteristic curve is below a defined limit value is determined therefrom. In particular, such temperatures are in the range of 400 ℃ to 600 ℃. It is noted that the temperature of the second SCR catalyst is preferably kept as low as possible, so that it can store an ammonia charge which corresponds more or less to normal operation. The first SCR catalyst is preferably heated by a heat flow upstream of the SCR catalyst, for example by heating the exhaust gas in the internal combustion engine. The desired temperature difference is easily reached due to the resulting higher heat exchange of the first SCR catalyst compared to a second SCR catalyst which also only encounters a smaller heat flow.

Subsequently, a prescribed ammonia filling level in the first SCR catalyst is set. The specified ammonia fill level (Ammoniak-Silstand) is above the sum of the maximum ammonia fill level that can be stored in the second SCR catalyst and the ammonia fill level that reacts with the nitrogen oxides during reduction. It can accordingly be assumed that the amount of ammonia which is not used in the first SCR catalyst for reducing nitrogen oxides is completely stored in the second SCR catalyst.

The nitrogen oxide concentration upstream of the SCR catalyst and the sum of the nitrogen oxide concentration and the ammonia concentration downstream of the SCR catalyst are subsequently measured. Conventional nox sensors have a lateral sensitivity with respect to the ammonia concentration, so that the sum of the nox concentration and the ammonia concentration can be measured, in particular, by a nox sensor arranged downstream of the SCR catalyst. In particular, a change in the sum signal measured by the sensor relative to the expected nitrogen oxide concentration may result from an ammonia concentration that corresponds to the ammonia slip of the second SCR catalyst. The ammonia slip indicates the amount of ammonia that passes through the SCR catalyst without participating in the reduction. The actual ammonia filling of the second SCR catalyst at the occurrence of the ammonia slip is preferably derived from the ammonia concentration downstream of the two SCR catalysts. According to a further aspect, it can be provided that the expected ammonia filling level (Ammoniak-Sillmenge) of the second SCR catalyst when the ammonia slip occurs is determined from a characteristic curve for the second SCR catalyst.

In a further process, during an analysis phase, a comparison of an actual ammonia fill level of the second SCR catalyst with an expected ammonia fill level at the time of the occurrence of the ammonia slip is performed. Finally, a fault in at least one of the two SCR catalysts is identified on the basis of the comparison. It is particularly preferred here to identify a fault in the second SCR catalyst if the actual ammonia filling quantity of the second SCR catalyst differs from the expected ammonia filling quantity when the ammonia slip occurs.

Alternatively, the measurement of the nitrogen oxide concentration upstream of the SCR catalyst and the measurement of the nitrogen oxide concentration downstream of the two SCR catalysts can be carried out at the beginning of the method. In this case, the two SCR catalytic converters are operated in normal operation. A malfunction in the SCR system is identified if the NOx concentration downstream of the two SCR catalysts does not match the expected NOx concentration. It is particularly preferred here to identify a fault in the first SCR catalyst if a fault in the SCR system is identified as described and additionally the actual ammonia filling quantity of the second SCR catalyst corresponds to the expected ammonia filling quantity when the ammonia slip occurs.

According to a further aspect, it can be provided that, after the defective catalytic converter has been repaired or replaced on the basis of the method, the motor vehicle travels the specified distance and/or time and then the method is carried out again. This also takes into account the rare case that both SCR catalysts have a failure at the same time. Optionally, the analysis phase can also be changed when the method is re-implemented, so that a larger fault spectrum (Fehlerspektrum) can be covered.

The computer program is set up for: in particular, each step of the method is carried out when it is implemented on a computer or a controller. The computer program enables the method to be implemented in a conventional electronic controller without structural modifications thereto. To this end, the computer program is stored on the machine-readable storage medium.

The electronic control unit is obtained by loading the computer program onto a conventional electronic control unit, which is set up to carry out fault detection in the SCR system. In this case, the electronic control unit can be both the control unit of the vehicle itself and an external control unit, such as a diagnostic unit, which is connected to the SCR system during fault detection and controls the method.

Drawings

Embodiments of the invention are illustrated in the drawings and are explained in detail in the following description. Wherein:

fig. 1 schematically shows an SCR system which comprises two SCR catalysts and three nitrogen oxide sensors and which enables fault detection by means of conventional methods;

fig. 2 shows schematically an SCR system which comprises two SCR catalysts and two nitrogen oxide sensors and which can carry out fault detection by means of the method according to the invention;

FIG. 3 shows a flow chart of an exemplary embodiment of the method according to the present invention; and is

Fig. 4 shows a graph of the temperature dependence of the ammonia filling level of the SCR catalyst, which can be used in one exemplary embodiment of the method according to the invention.

Detailed Description

Fig. 1 shows a generic SCR system 100 of a combustion motor, not shown, in a motor vehicle, having a first SCR catalyst 101 and a second SCR catalyst 102, for which a fault can be identified in a conventional manner. The two

SCR catalysts

101 and 102 are arranged one behind the other in the exhaust system 120, wherein the first SCR catalyst 101 is arranged closer to an injection module 130, which injects a urea/water solution into the exhaust system 120 upstream of the two

SCR catalysts

101 and 102. Furthermore, the SCR system 100 comprises a first nitrogen oxide sensor 110, which is arranged between the injection module 130 and the first SCR catalyst 101 and is able to measure there the nitrogen oxide concentration NOx _ of the exhaust gas before the_{Before treatment}. Furthermore, the SCR system 100 comprises a second NOx sensor 111, which is arranged downstream of the second SCR catalyst 102 and is able to measure there the nitrogen concentration NOx \ of the exhaust gas after the exhaust gas aftertreatment by the two

SCR catalysts

101 and 102_{After treatment}. The SCR system additionally comprises a third nitrogen oxide sensor 112, which is arranged between the first SCR catalyst 101 and the second SCR catalyst 102 and is able to measure there the nitrogen oxide concentration of the exhaust gas after the exhaust gas aftertreatment by the first SCR catalyst 101. Three nitrogen oxides mentionedThe object

sensors

110, 111 and 112 and the injection module 130 are connected to and controlled by an electronic control unit 140.

In conventional methods for fault detection, the actual difference between the nox concentration at the first nox sensor 110 and at the third nox sensor 112 or the actual difference between the nox concentration at the third nox sensor 112 and at the second nox sensor 111 is detected and compared with the expected difference. If at least one of the actual differences does not correspond to the respective expected difference, a fault in the

SCR catalyst

101 or 102 lying between them is concluded.

Fig. 2 shows an SCR system 200 of a combustion motor, not shown, in a motor vehicle, which likewise comprises a first SCR catalyst 201 and a second SCR catalyst 202, for which a fault can be identified by means of an embodiment of the method according to the invention. The two

SCR catalysts

201 and 202 are respectively arranged one behind the other in the exhaust system 220, wherein the first SCR catalyst 201 is arranged closer to the injection module 230 as described above. However, the SCR system 200 shown here only comprises a first nitrogen oxide sensor 210 arranged between the injection module 230 and the first SCR catalyst 201 and a second nitrogen oxide sensor 211 arranged downstream of the second SCR catalyst 202. The two SCR

catalytic converters

210 and 211 and the injection module 230 are connected to an electronic control unit and are controlled by the latter at least during the fault detection. In another embodiment, the electronic control unit can be an external device which is connected to the SCR system 200 during the fault detection.

The first nitrogen oxide sensor 210 measures the nitrogen oxide concentration NOx _ of the exhaust gas before the exhaust gas aftertreatment by the

SCR catalysts

101 and 102_{Before treatment}And the second nox sensor 211 measures the ammonia passing through the two

SCR catalysts

101 and 102 according to its lateral sensitivity with respect to ammonia concentrationNitrogen concentration NOx _ of exhaust gas after exhaust gas aftertreatment_{After treatment}And ammonia concentration NH_{After 3_ treatment}The resulting sum signal. The SCR system 200 of fig. 2 therefore differs from the SCR system of fig. 1 only in the absence of the third nitrogen oxide sensor 112.

An exemplary embodiment of a method according to the present invention for fault detection in the SCR system 200 described above is illustrated in fig. 3 as a flow chart. In a first step, the nitrogen oxide concentration NOx _ at the first nitrogen oxide sensor 210 is determined_{Before treatment}And the nitrogen oxide concentration NOx _ at the second nitrogen oxide sensor 211_{After treatment} A measurement 300 is performed. If the nitrogen oxide concentration NOx after exhaust gas aftertreatment by means of the two

SCR catalysts

201 and 202 is present_{After treatment}With the concentration of nitrogen oxides NOx __{Before treatment}And the expected values obtained in the conversion of the two SCR catalysts, a fault in the SCR system 200 is identified 301. That is to say that at least one of the two

SCR catalysts

201 or 202, and in rare cases also both SCR catalysts, has a malfunction and/or at least one reduced conversion rate. If no fault in the SCR system 200 is identified 301, the method is ended 302.

Otherwise, in a further step, the combustion motor of the stopped motor vehicle is switched 303 to idle and then operated 304 at the specified operating point. Then, the process waits until the nitrogen oxide concentration NOx _ at the first nitrogen oxide sensor 210_{Before treatment}With the nitrogen oxide concentration NOx _ at the second nitrogen oxide sensor 211_{After treatment}The same is true. Subsequently, the first SCR catalyst 201 is heated 306 to a temperature T for which the maximum ammonia filling level NH of the first SCR catalyst 201_{3_ maximum fill level}Almost zero. For this purpose, a temperature-dependent characteristic curve 400 of the first SCR catalyst is used, as shown in fig. 4. In this embodiment, the temperature (T) is about 500 ℃. The urea-water solution is then injected 306 into the exhaust train 220.

Thus, the ammonia filling level NH in the first SCR catalyst 201_{3_ fill level}Until it increases to the value specified. This prescribed value is at the maximum ammonia fill level NH of the second SCR catalyst 202_{3_ maximum fill level}And above the amount of ammonia that reacts in the first SCR catalyst 201 upon reduction of the nitrogen oxides. As a result, all ammonia that has not reacted at the first SCR catalyst 201 during the reduction of the nitrogen oxides is diverted to the second SCR catalyst 202. Due to a higher than maximum ammonia filling level NH of the second SCR catalyst_{3_ maximum fill level}Ammonia fill level of (NH)_{3_ fill level}At the second SCR catalyst 202, an ammonia slip occurs, for which the ammonia amount that is not stored passes this SCR catalyst 202 without participating in the reduction of the nitrogen oxides.

NH if the specified ammonia fill level is reached_{3_ fill level}On the one hand, the NOx concentration NOx _ at the first NOx sensor 210 is then renewed_{Before treatment}And from the nitrogen oxide concentration NOx _ at the second nitrogen oxide sensor 211_{After treatment}And ammonia concentration NH_{After 3_ treatment}The resulting summed signal is measured 308. The ammonia concentration NH_{After 3_ treatment}Can be attributed to ammonia slip occurring at the second SCR catalyst. Thus, once the sum signal has been increased due to the ammonia slip that occurs, the actual ammonia fill level NH at the time of the ammonia slip is determined_{3_ actual}. On the other hand, the expected ammonia filling level NH at the occurrence of the ammonia slip is determined from the characteristic curve 500 of the second SCR catalyst_{3_ expectation}Wherein first the maximum ammonia filling level NH of the second SCR catalyst_{3_ maximum fill level}For reference.

In comparison 311, during the analysis phase, the actual ammonia fill level NH at which the ammonia slip occurs at the second SCR catalyst 202_{3_ actual}And expected ammonia fill level NH_{3_ expectation}Controls were performed. If the actual ammonia fill level NH_{3_ actual}And expected ammonia fill level NH_{3_ expectation}Otherwise, a fault in the second SCR catalyst 202 is identified 320. As a further step 321, the faulty second SCR catalyst 202 is repaired or replaced. If the actual ammonia fill level NH_{3_ actual}And expected ammonia fill level NH_{3_ expectation}In agreement, a fault at the second SCR catalyst 202 cannot be determined. However, since a fault in the SCR system 200 is identified in the measurement 300, it can be assumed that the first SCR catalyst 201 is faulty. Accordingly, a fault in the first SCR catalyst 201 is identified 330 in this case. Here, as a further step 331, the faulty first SCR catalyst 201 is repaired or replaced.

In order to rule out the rare case that both

SCR catalysts

201 and 202 are defective, the motor vehicle is first driven 340 for a defined time and/or distance when a defect in the second SCR catalyst 202 is detected 320. The method is then repeated from the beginning. In a further embodiment, it is provided that the analysis phase is changed during the repetition of the method.

FIG. 4 shows the maximum ammonia fill level NH_{3_ maximum fill level}Is dependent on the temperature T, characteristic curve 400. In such an embodiment, the maximum ammonia fill level NH of the second SCR catalyst 202 at point 401 at a temperature of 500 deg.C_{3_ maximum fill level}There is no difference from zero. It is noted that the temperature at which the point 401 is located depends on the SCR catalyst used and may in another embodiment be, for example, approximately 450 ℃.

Claims

1. Method for fault detection in an SCR system (200) of an internal combustion engine in a motor vehicle, having two SCR catalysts (201; 202) and two nitrogen oxide sensors (210; 211), wherein one nitrogen oxide sensor (210) is arranged upstream of the two SCR catalysts (201; 202) and one nitrogen oxide sensor (211) is arranged downstream of the two SCR catalysts, comprising the following steps:

-activating a first SCR catalyst (201)) Heating to a temperature (T) for which a maximum ammonia filling level (NH) that can be stored by the first SCR catalyst (201)_{3_ maximum fill level}) About zero;

-setting an ammonia fill level (NH) specified in the first SCR catalyst (201)_{3_ fill level}) The specified ammonia fill level is at a maximum ammonia fill level (NH) for the second SCR catalyst (202)_{3_ maximum fill level}) Above;

-measuring the nitrogen oxide concentration (NOx) upstream of the SCR catalyst (201; 202)_{Before treatment}) And by the SCR catalyst (201; 202) downstream of (1) nitrogen oxide concentration (NOx __{After treatment}) And ammonia concentration (NH)_{After 3_ treatment}) The sum of the constituents;

-during an analysis phase, an actual ammonia fill level (NH) of the second SCR catalyst (202) at occurrence of ammonia slip_{3_ actual}) And an expected ammonia fill level (NH) of the second SCR catalyst (202) when ammonia slip occurs_{3_ expectation}) Comparing;

-depending on the actual ammonia filling level (NH) of the second SCR catalyst (202) when ammonia slip occurs_{3_ actual}) And expected ammonia fill level (NH)_{3_ expectation}) To identify the two SCR-catalysts (201; 202) of the SCR catalyst.

2. The method of claim 1, wherein the actual ammonia fill level (NH) of the second SCR catalyst (202) is determined if ammonia slip occurs_{3_ actual}) And expected ammonia fill level (NH)_{3_ expectation}) Otherwise, a fault in the second SCR catalyst (202) is identified.

3. Method according to claim 1, characterized in that the nitrogen oxide concentration (NOx) is initially upstream of the SCR catalyst (201; 202)_{Before treatment}) And by the two SCR catalysts (201; 202) is/are as followsDownstream nitrogen oxide concentration (NOx _ @)_{After treatment}) And ammonia concentration (NH)_{After 3_ treatment}) The sum of the compositions is measured, wherein two SCR catalysts (201; 202) is operated in normal operation and, if the two SCR catalysts (201; 202) downstream of (1) nitrogen oxide concentration (NOx __{After treatment}) And ammonia concentration (NH)_{After 3_ treatment}) If the sum of the values does not match the expected NOx concentration, a fault in the SCR system (200) is identified.

4. Method according to claim 3, characterized in that the actual ammonia fill level (NH) of the second SCR catalyst (202) is reached if a malfunction in the SCR system (200) is identified and when ammonia slip occurs_{3_ actual}) And expected ammonia fill level (NH)_{3_ expectation}) In agreement, a fault in the first SCR catalyst (201) is identified.

5. Method according to any of the preceding claims, characterized in that after repair or replacement of a defective SCR catalyst (201; 202), the motor vehicle is driven for a defined distance and/or time and the method is subsequently repeated.

6. The method of claim 5, wherein said analysis phase is changed while said method is repeated.

7. Method according to any of claims 1 to 4, characterized in that the actual ammonia fill level (NH) of the second SCR catalyst (202) at the occurrence of ammonia slip_{3_ actual}) From the two SCR-catalysts (201; 202) downstream ammonia concentration (NH)_{After 3_ treatment}) And (4) obtaining.

8. The method as claimed in any of claims 1 to 4, characterized in that the expected ammonia filling level (NH) of the second SCR catalyst (202) in the event of ammonia slip_{3_ expectation}) Is used for theA characteristic curve (500) of the second SCR catalyst (202) is obtained.

9. A machine-readable storage medium, on which a computer program is stored, which computer program is set up for: carrying out each step of the method according to any one of claims 1 to 8.

10. An electronic controller configured to: fault detection is carried out in the SCR system by means of the method according to any one of claims 1 to 8.