DE102016211575A1 - Error detection in an SCR system using an ammonia level - Google Patents

Abstract

The invention relates to a method for fault detection in an SCR system of an internal combustion engine in a motor vehicle, which has two SCR catalysts and two nitrogen oxide sensors. A nitrogen oxide sensor is upstream and a nitrogen oxide sensor is located downstream of the two SCR catalysts. The method comprises the following steps: First, heating (306) of the first SCR catalyst to a temperature at which a maximum ammonia level that can be stored by the first SCR catalyst is approximately zero. This is followed by adjusting (307) a fixed ammonia level in the first SCR catalyst which is above the maximum ammonia level for the second SCR catalyst. The following is a measurement (308) of a nitrogen oxide concentration (NOxVor) upstream of the SCR catalysts and a sum of a nitrogen oxide concentration (NOx after) and an ammonia concentration (NH ₃ Nach) downstream of the SCR catalysts. This results in a comparison (311) of an actual ammonia level (NH ₃ Tat) of the second SCR catalyst experiencing ammonia slip with an expected ammonia level (NH ₃ Erw) of the second SCR catalyst ammonia slip occurs during an evaluation phase. Eventually, a fault will occur in at least one of the two SCR catalysts, depending on the comparison (311) of the actual ammonia level (NH ₃ Tat) and the expected ammonia level (NH ₃ Erw) of the second SCR catalyst on the occurrence of ammonia Slip detected (320; 330).

Description

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

State of the art

A now widely used technology for reducing nitrogen oxides (NOx) in the exhaust of internal combustion engines in motor vehicles, the selective catalytic reduction (SCR). In SCR systems is a urea-water solution, commercially known as AdBlue ^® by an injection module in the exhaust line, upstream of at least one SCR catalyst injected. The separated from the urea-water solution ammonia reacts to the SCR catalysts in the selective catalytic reduction with the nitrogen oxides to elemental nitrogen.

The introduction of stricter emission regulations will use several SCR catalysts that act on the same exhaust gas. In the case of insufficient efficiency of the SCR catalysts to reduce nitrogen oxide emission in the exhaust system, an error detection with an on-board test procedure (usually implemented in an electronic control unit) is prescribed. For this reason, continuous monitoring is performed during normal operation of the vehicle. In the usual test methods, at least one nitrogen sensor is used upstream and at least one nitrogen sensor downstream of the SCR catalyst. For a single SCR catalyst, two nitric oxide sensors are sufficient to calculate the efficiency of the SCR system while monitoring nitric oxide emissions.

An extension of the SCR systems to several SCR catalysts in the same exhaust gas line traditionally requires n + 1 nitrogen oxide sensors in order to determine, by means of a pin-point strategy, at least one non-functioning or malfunctioning SCR catalyst. Accordingly, the non-functioning or malfunctioning SCR catalyst can be selectively repaired or replaced, while the well-functioning SCR catalyst or the SCR system remains unaffected. However, the use of the pin-point strategy is not necessary during normal operation of the vehicle, since it is sufficient here only to monitor the efficiency. In the event of a malfunctioning or malfunctioning SCR catalytic converter, the driver may be alerted, for example via a signal lamp on the dashboard, whereupon the driver may, for example, drive the vehicle. into a workshop. During the workshop activity, the said pin-point strategy is used in the test procedure to determine the malfunctioning SCR catalyst.

Disclosure of the invention

The method relates to an SCR system of an internal combustion engine in a motor vehicle. Here, the SCR system has two successive arranged SCR catalysts in a common exhaust system. The exhaust gas first passes through a first SCR catalyst and is then forwarded to a second SCR catalyst so that both SCR catalysts act on the exhaust gas. Furthermore, the SCR system has two nitrogen oxide sensors, which are also arranged in this exhaust system. A first nitrogen oxide sensor is arranged upstream of the two SCR catalysts and can there measure a nitrogen oxide concentration before an exhaust gas treatment by the SCR catalysts. A second nitrogen oxide sensor is arranged downstream of the two SCR catalysts and can there measure a sum of a nitrogen oxide concentration and an ammonia concentration after the exhaust aftertreatment by the SCR catalysts, wherein the ammonia concentration corresponds to an ammonia slip, This occurs when an ammonia level exceeds a maximum ammonia level.

The method comprises the following steps. First, the first SCR catalyst is heated to a temperature at which a maximum ammonia level that can be stored by the first SCR catalyst is approximately zero. This can be implemented, for example, by representing a temperature dependence of the maximum ammonia level as a characteristic and from this the temperature is determined at which the characteristic falls below a specified limit. In particular, this temperature is in a range of 400 ° C to 600 ° C. It should be noted that the temperature of the second SCR catalyst is preferably kept as low as possible, so that it can store an ammonia filling quantity which corresponds approximately to the normal operation. Preferably, the heating of the first SCR catalyst is carried out via a heat flow upstream of the SCR catalysts, for example by heating the exhaust gas in the internal combustion engine. Due to a resulting higher heat exchange of the first SCR catalyst compared to the second SCR catalyst, only a lesser Heat flow, the desired temperature differences are easily reached.

Then a fixed ammonia level is set in the first SCR catalyst. This fixed ammonia level is above a sum of the maximum ammonia level that can be stored in the second SCR catalyst and an ammonia level that reacts with the nitrogen oxides during the reduction. Accordingly, it can be assumed that an amount of ammonia that was not used to reduce nitrogen oxides in the first SCR catalyst is completely stored in the second SCR catalyst.

Subsequently, a nitrogen oxide concentration upstream of the SCR catalysts and a sum of the nitrogen oxide concentration and an ammonia concentration downstream of the SCR catalysts is measured. Conventional nitrogen oxide sensors have a cross-sensitivity to an ammonia concentration, so that the sum of the nitrogen oxide concentration and the ammonia concentration can be measured in particular by the nitrogen oxide sensor arranged downstream of the SCR catalysts. In particular, a change in the sum signal measured by the sensor relative to an expected nitrogen oxide concentration can result from an ammonia concentration which corresponds to an ammonia slip of the second SCR catalyst. The ammonia slip indicates an amount of ammonia that passes through the SCR catalyst without participating in the reduction. Preferably, an actual ammonia charge of the second SCR catalyst at which ammonia slip occurs is determined from the ammonia concentration downstream of the two SCR catalysts. According to a further aspect, it can be provided that an expected ammonia charge quantity of the second SCR catalytic converter, in which the ammonia slip occurs, is determined from a characteristic curve for the second SCR catalytic converter.

Subsequently, during an evaluation phase, a comparison is made of the actual ammonia charge and the expected ammonia charge of the second SCR catalyst at which ammonia slip occurs. Finally, based on the comparison, an error is detected in at least one of the two SCR catalysts. It is particularly preferred here that a fault in the second SCR catalytic converter is recognized when the actual ammonia charge quantity and the expected ammonia charge quantity of the second SCR catalytic converter, in which the ammonia slip occurs, differ.

Optionally, at the beginning of the process, a measurement of the nitrogen oxide concentration upstream of the SCR catalysts and a measurement of the nitrogen oxide concentration downstream of the two SCR catalysts can be carried out. Both SCR catalysts work in normal operation. An error in the SCR system is detected if the nitrogen oxide concentration downstream of the two SCR catalysts does not correspond to an expected nitric oxide concentration. It is particularly preferred here that a fault in the first SCR catalytic converter is detected when an error in the SCR system, as indicated, is detected and additionally the actual ammonia charge quantity and the expected ammonia charge quantity of the second SCR catalytic converter, in which the ammonia slip occurs, match.

According to a further aspect, it may be provided that after the faulty catalytic converter has been repaired or replaced on the basis of the method, the motor vehicle travels a fixed distance and / or time and the method is then carried out again. This also takes into account the rare case where both SCR catalysts malfunction at the same time. Optionally, when the process is carried out again, the evaluation phase can also be changed so that a larger range of errors can be covered.

The computer program is set up to perform each step of the method, in particular when it is performed on a computing device or controller. It allows the implementation of the method in a conventional electronic control unit without having to make any structural changes. For this purpose it is stored on the machine-readable storage medium.

By loading the computer program on a conventional electronic control unit, the electronic control unit is obtained, which is set up to perform the error detection in the SCR system. The electronic control unit may in this case be both an on-board control unit and an external control unit, for example a diagnostic unit which is connected to the SCR system during fault detection and controls the method.

Brief description of the drawings

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

1 schematically shows an SCR system, which comprises two SCR catalysts and three nitrogen oxide sensors and by means of a conventional Method an error detection can be performed.

2 schematically shows an SCR system, which comprises two SCR catalysts and two nitrogen oxide sensors and by means of the method according to the invention an error detection can be performed.

3 shows a flowchart of an embodiment of the method according to the invention.

4 shows a diagram of a temperature dependence of an ammonia level of an SCR catalyst, which can be used in an embodiment of the method according to the invention.

Embodiment of the invention

In 1 is a common SCR system 100 an internal combustion engine, not shown, in a motor vehicle, with a first SCR catalytic converter 101 and a second SCR catalyst 102 , in which an error can be detected by a conventional method. The two SCR catalysts 101 and 102 are in a row in an exhaust system 120 arranged, wherein the first SCR catalyst 101 closer to an injection module 130 which is a urea-water solution upstream of the two SCR catalysts 101 and 102 in the exhaust system 120 injects. Furthermore, the SCR system includes 100 a first nitric oxide sensor 110 the between the injection module 130 and the first SCR catalyst 101 is arranged there and a nitrogen oxide concentration NOxVor the exhaust gas before exhaust aftertreatment by the SCR catalysts 101 and 102 can measure. It also includes the SCR system 100 a second nitrogen oxide sensor 111 downstream of the second SCR catalyst 102 and there is a nitrogen concentration NOx after the exhaust gas after exhaust aftertreatment by both SCR catalysts 101 and 102 can measure. In addition, the SCR system also includes a third nitric oxide sensor 112 that is between the first SCR catalyst 101 and the second SCR catalyst 102 is arranged there and a nitrogen oxide concentration of the exhaust gas after exhaust aftertreatment by the first SCR catalysts 101 can measure. Said three nitric oxide sensors 110 . 111 and 112 as well as the injection module 130 are with an electronic control unit 140 connected and controlled by this.

In a conventional method of error detection, an actual difference in nitrogen oxide concentrations at the first nitrogen oxide sensor becomes 110 and the third nitric oxide sensor 112 or an actual difference in the nitrogen oxide concentrations at the third nitrogen oxide sensor 112 and the second nitrogen oxide sensor 111 determined and compared with an expected difference. If at least one of the actual differences does not match the corresponding expected difference, a fault will be detected in the intervening SCR catalyst 101 respectively. 102 closed.

2 shows an SCR system 200 an internal combustion engine, not shown, in a motor vehicle, which also has a first SCR catalytic converter 201 and a second SCR catalyst 202 includes, in which an error can be detected by means of an embodiment of the method according to the invention. The two SCR catalysts 201 and 202 are consecutively in an exhaust line 220 arranged, wherein the first SCR catalyst 201 closer to an injection module as previously described 230 is arranged. However, the SCR system shown here includes 200 only a first nitric oxide sensor 210 that is between the injection module 230 and the first SCR catalyst 201 is arranged, and a second nitrogen oxide sensor 211 downstream of the second SCR catalyst 202 is arranged. The two SCR catalysts 210 and 211 as well as the injection module 230 are connected to an electronic control unit and are at least controlled during fault detection of this. In a further embodiment, the electronic control unit may be an external apparatus used during fault detection with the SCR system 200 connected is.

The first nitric oxide sensor 210 measures a nitrogen oxide concentration NOx before the exhaust gas before exhaust aftertreatment by the SCR catalysts 101 and 102 and the second nitric oxide sensor 211 measures, due to its cross sensitivity to an ammonia concentration, a cumulative signal from the nitrogen concentration NOx after and the ammonia concentration NH ₃ After the exhaust gas after the exhaust aftertreatment by both SCR catalysts 101 and 102 , The SCR system 200 out 2 therefore differs from the SCR system 1 only by the absence of the third nitric oxide sensor 112 ,

An embodiment of the method according to the invention for error detection in the previously described SCR system 200 is as a flowchart in 3 shown. In a first step, a measurement 300 the nitrogen oxide concentration NOx before the first nitrogen oxide sensor 210 and the nitrogen oxide concentration NOx after the second nitrogen oxide sensor 211 carried out. After the exhaust aftertreatment by the two SCR catalysts, the nitrogen oxide concentrations subsequently deviate from NOx 201 and 202 from an expected value, which is determined from the nitrogen oxide concentration NOxVor and a conversion rate of the two SCR catalysts, becomes an error in the SCR system 200 recognized 301 , That is, at least one of the two SCR catalysts 201 or 202 , in rare cases both, have a malfunction and / or at least a reduced conversion rate. Will not be an error in the SCR system 200 recognized 301 , the procedure is terminated 302 ,

Otherwise, in a further step, the internal combustion engine of the stationary motor vehicle is switched to idling 303 and then operated at a specified operating point 304 , Thereafter, it is waited until the nitrogen oxide concentration NOxVor on the first nitrogen oxide sensor 210 and the nitrogen oxide concentration NOx after the second nitrogen oxide sensor 211 are identical. Subsequently, the first SCR catalyst 201 heated to a temperature T 306 in which a maximum ammonia level NH ₃ FüllMax of the first SCR catalyst 201 is approximately zero. This is a characteristic curve 400 the temperature dependence of the first SCR catalyst, as in 4 shown used. In this embodiment, this temperature (T) is 500 ° C. Subsequently, the urea-water solution in the exhaust line 220 injected 306 , As a result, the ammonia level NH ₃ filling in the first SCR catalyst increases 201 up to a specified value. This is above a maximum ammonia filling level NH ₃ FüllMax of the second SCR catalyst 202 , and an amount of ammonia in the first SCR catalyst 201 reacts in the reduction of nitrogen oxides. As a result, all ammonia, which is not in the reduction of nitrogen oxides on the first SCR catalyst 201 responded to the second SCR catalyst 202 passed. At the second SCR catalyst 202 occurs due to the ammonia level NH ₃ filling, which is higher than the maximum ammonia level NH ₃ FüllMax, an ammonia slip, in which a non-stored amount of ammonia this SCR catalyst 202 happens without participating in the reduction of nitrogen oxides.

If the specified ammonia level NH ₃ Füll achieved, on the one hand again a measurement 308 the nitrogen oxide concentration NOx before the first nitrogen oxide sensor 210 and the sum signal from the nitrogen oxide concentration NOxNach and the ammonia concentration NH ₃ After the second nitrogen oxide sensor 211 carried out. The ammonia concentration NH ₃ After can be attributed to an occurring ammonia slip on the second SCR catalyst. An actual ammonia level NH ₃ Tat at which the ammonia slip occurs is thus determined as soon as the sum signal is increased by the ammonia slip occurring. On the other hand, an expected ammonia level NH ₃ Erw when ammonia slip occurs from a characteristic 500 is the second SCR catalyst, determined, in particular, the maximum ammonia level NH ₃ FüllMax of the second SCR catalyst serves as a reference.

In a comparison 311 For example, the actual ammonia level will be NH ₃ Tat and the expected ammonia level will be NH ₃ Erw if ammonia slip occurs on the second SCR catalyst 202 during a valuation phase. If the actual ammonia level NH ₃ Act differs from the expected ammonia level NH ₃ Erw, a fault in the second SCR catalytic converter becomes apparent 202 recognized 320 , As a further step 321 becomes the faulty second SCR catalyst 202 repaired or replaced. If the actual ammonia-level NH ₃ fact and the expected ammonia-level NH ₃ Erw agree, then there can be no error on the second SCR catalyst 202 be determined. However, in the measurement 300 an error in the SCR system 200 was detected, it can be assumed that the first SCR catalyst 201 is faulty. Accordingly, in this case, an error in the first SCR catalyst 201 recognized 330 , Again, as a further step 331 the faulty first SCR catalyst 201 repaired or replaced.

To the rare case where both SCR catalysts 201 and 202 are flawed, ruled out, especially if a fault in the second SCR catalyst 202 recognized 320 was, the motor vehicle is driven a fixed time and / or distance 340 , Subsequently, the procedure is repeated from the beginning. In a further embodiment, it is provided, in the repetition of the method, to change the evaluation phase.

4 shows a characteristic 400 of the maximum ammonia level NH ₃ FüllMax depending on the temperature T. In this embodiment, the maximum ammonia level NH ₃ FüllMax of the second SCR catalyst 202 at 500 ° C at the point 401 indistinguishable from zero. It should be noted that the temperature at which the point 401 is dependent on the SCR catalyst used and may be in another embodiment, for example, at 450 ° C.

Claims (11)

Method for error detection in an SCR system ( 200 ) of an internal combustion engine in a motor vehicle, which comprises two SCR catalysts ( 201 . 202 ) and two nitrogen oxide sensors ( 210 ; 211 ), wherein a nitrogen oxide sensor ( 210 ) upstream and a nitrogen oxide sensor ( 211 ) downstream of the two SCR catalysts ( 201 . 202 ), comprising the following steps: - heating ( 306 ) of the first SCR catalyst ( 201 ) to a temperature (T) at which a maximum Ammonia level (NH ₃ FüllMax) obtained from the first SCR catalyst ( 201 ) is approximately zero; - To adjust ( 307 ) of a fixed ammonia level (NH ₃ filling) in the first SCR catalyst ( 201 ), which is above the maximum ammonia level (NH ₃ FüllMax) for the second SCR catalyst ( 202 ) lies; - Measure up ( 308 ) of a nitrogen oxide concentration (NOxVor) upstream of the SCR catalysts ( 201 ; 202 ) and a sum of a nitrogen oxide concentration (NOx after) and an ammonia concentration (NH ₃ Nach) downstream of the SCR catalysts ( 201 ; 202 ); - Comparison ( 311 ) of an actual ammonia level (NH ₃ Tat) of the second SCR catalyst ( 202 ), where ammonia slip occurs, with an expected ammonia level (NH ₃ Erw) of the second SCR catalyst ( 202 ) in which ammonia slip occurs during an evaluation phase; - Detect ( 320 ; 330 ) of an error in at least one of the two SCR catalysts ( 201 ; 202 ), depending on the comparison ( 311 ) of the actual ammonia level (NH ₃ Tat) and the expected ammonia level (NH ₃ Erw) of the second SCR catalyst ( 202 ) when ammonia slip occurs. A method according to claim 1, characterized in that a fault in the second SCR catalyst ( 202 ) ( 320 ), when the actual ammonia level (NH ₃ Tat) and the expected ammonia level (NH ₃ Erw) of the second SCR catalyst ( 202 ) when ammonia slip occurs ( 311 ). A method according to claim 1, characterized in that at the beginning of a measurement ( 300 ) of the nitrogen oxide concentration (NOxVor) upstream of the SCR catalysts ( 202 ; 203 ) and the sum of the nitrogen oxide concentration (NOx after) and the ammonia concentration (NH ₃ Nach) downstream of the two SCR catalysts ( 201 ; 202 ), both SCR catalysts ( 201 ; 202 ) in normal operation, and an error in the SCR system ( 200 ) ( 301 ), if the sum of the nitrogen oxide concentration (NOx after) and the ammonia concentration (NH ₃ Nach) downstream of the two SCR catalysts ( 201 ; 202 ) does not correspond to an expected nitric oxide concentration. A method according to claim 3, characterized in that a fault in the first SCR catalyst ( 201 ) ( 330 ), if an error in the SCR system ( 200 ) ( 301 ) and the actual ammonia level (NH ₃ Tat) and the expected ammonia level (NH ₃ Erw) of the second SCR catalyst ( 202 ) when ammonia slip occurs ( 311 ). Method according to one of the preceding claims, characterized in that after the faulty catalyst ( 201 ; 202 ) was repaired or replaced ( 321 ; 331 ), the motor vehicle travels a fixed distance and / or time ( 340 ) and then the process is repeated. A method according to claim 5, characterized in that when repeating the method, the evaluation phase is changed. Method according to one of the preceding claims, characterized in that the actual ammonia level (NH ₃ act) of the second SCR catalyst ( 202 ) when ammonia slip occurs from an ammonia concentration (NH ₃ Nach) downstream of the two SCR catalysts ( 201 ; 202 ) ( 309 ) becomes. Method according to one of the preceding claims, characterized in that the expected ammonia level (NH ₃ Erw) of the second SCR catalyst ( 202 ) when ammonia slip occurs from a characteristic curve ( 500 ) for the second SCR catalyst ( 202 ) ( 310 ) becomes. A computer program adapted to perform each step of the method of any one of claims 1 to 8. A machine-readable storage medium on which a computer program according to claim 9 is stored. Electronic control unit, which is set up to carry out a fault detection in the SCR system by means of a method according to one of Claims 1 to 8.

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