DE102020211729A1 - Process for checking the plausibility of nitrogen oxide sensors - Google Patents

Abstract

The invention relates to a method for checking the plausibility of nitrogen oxide sensors (31 - 34) in an SCR catalytic converter system (20) with two SCR catalytic converters (21, 22), which has at least one first nitrogen oxide sensor (31, 32) which is located upstream of the SCR catalytic converters (21, 22) is arranged in an exhaust system (10), has at least one second nitrogen oxide sensor (33) which is arranged between the SCR catalytic converters (21, 22) and has at least a third nitrogen oxide sensor (34) which is arranged downstream of the SCR -Catalysts (21, 22) is arranged. During regeneration of a particle filter (12) arranged in the exhaust system (10), signals from the first nitrogen oxide sensor (31, 32) and the second nitrogen oxide sensor (33) are checked for plausibility (54, 55). Outside of the regeneration of the particulate filter (12), signals from the second nitrogen oxide sensor (33) and the third nitrogen oxide sensor (34) are checked for plausibility with one another.

Description

The present invention relates to a method for checking the plausibility of nitrogen oxide sensors of an SCR catalytic converter system. Furthermore, the present invention relates to a computer program that executes each step of the method and a machine-readable storage medium that stores the computer program. Finally, the invention relates to an electronic control unit that is set up to carry out the method.

State of the art

SCR catalytic converters (selective catalytic reduction) can be used to treat exhaust gases from internal combustion engines. Upstream of such an SCR catalytic converter, a urea water solution (HWL) is metered into the exhaust line and reacts there to form ammonia. In the SCR catalytic converter, ammonia reacts with nitrogen oxides to form water and nitrogen. Some of the ammonia can also be stored in the SCR catalytic converter.

Exhaust aftertreatment systems with two SCR catalysts arranged one behind the other have at least one metering valve upstream of the first SCR catalyst and optionally another metering valve between the SCR catalysts in order to meter HWL into the exhaust system. In order to monitor the SCR reaction and to model an ammonia filling level of the SCR catalytic converters, nitrogen oxide sensors are usually arranged in front of, between and behind the SCR catalytic converters in the exhaust line. Furthermore, these can also be used to record and store nitrogen oxide emissions from the internal combustion engine and make them available for evaluations outside the motor vehicle (tailpipe monitoring). Furthermore, with the help of the nitrogen oxide sensors, manipulation of the exhaust aftertreatment can be detected (anti-tempering) and the effectiveness of the entire exhaust aftertreatment system can be monitored (on board monitoring; OBM) instead of just identifying individual defective components.

Disclosure of Invention

The method is used to check the plausibility of nitrogen oxide sensors in an SCR catalytic converter system with two SCR catalytic converters. This has at least one first nitrogen oxide sensor which is arranged upstream of the SCR catalytic converters in an exhaust system, in particular of a motor vehicle. Furthermore, it has at least one second nitrogen oxide sensor, which is arranged between the SCR catalytic converters, and at least one third nitrogen oxide sensor, which is arranged downstream of the SCR catalytic converters. In many countries, these nitrogen oxide sensors must be monitored by on-board diagnostics (OBD) due to legal requirements. This applies to both their suitability for reducing emissions in connection with the SCR catalytic converters and their suitability for diagnosing other components of the exhaust aftertreatment.

A particle filter is also arranged in the exhaust line. This can be provided as a separate component independently of the SCR catalytic converters, or it can also be provided that an SCR catalytic converter is arranged on the particle filter (SCR on filter; SCRoF). In particular, the SCR catalytic converter arranged further upstream from the two SCR catalytic converters is arranged on the particle filter.

Irrespective of whether the particulate filter is combined with an SCR catalytic converter or is a separate component, the method provides that signals from the first nitrogen oxide sensor and the second nitrogen oxide sensor are checked for plausibility during regeneration of the particulate filter and signals from the second outside of the regeneration of the particulate filter Nitrogen oxide sensor and the third nitrogen oxide sensor are checked for plausibility with each other. During the operation of an internal combustion engine whose exhaust gas is to be post-treated by the SCR catalytic converter system, the temperature of the second SCR catalytic converter in the direction of flow is generally lower than that of the first SCR catalytic converter. Depending on the operating point, one or the other SCR catalytic converter is better suited to converting nitrogen oxides. In particular, the temperature plays a major role here. Too low a temperature hinders the evaporation of the HWL and the formation of ammonia upstream of the SCR catalyst and the SCR reaction. Too high a temperature also causes a reduction in conversion performance, since a large proportion of the HWL that is dosed reacts with oxygen instead of reducing nitrogen oxides. Therefore, when the exhaust gas temperature is low, the first SCR catalytic converter is used to a greater extent for reducing nitrogen oxides, and when the exhaust gas temperature is high, the second SCR catalytic converter is used. For this purpose, the distribution of the HWL metered in between a metering point upstream of the first SCR catalytic converter and a metering point between the two SCR catalytic converters can be varied, which can go as far as dispensing with metering at one metering point.

During the regeneration of the particle filter, the temperature of the particle filter, but also that of the entire exhaust system, increases significantly, which has the effects on the efficiency of the SCR catalytic converters described above. During regeneration, therefore, the dosage is usually the first dosing device greatly reduced, since the temperature of the first SCR catalytic converter is too high without achieving efficient nitrogen oxide reduction. The plausibility check during the regeneration is therefore preferably carried out at a point in time at which metering of a reducing agent solution into the exhaust system upstream of the SCR catalytic converters is reduced by a predefinable value compared to metering outside of the regeneration. In this case, the reduction particularly preferably takes place to a dosage of zero.

Outside of the regeneration of the particle filter, there are operating points at which the metering quantity of the second metering device is greatly reduced because the temperature of the second SCR catalytic converter is too low to achieve efficient nitrogen oxide reduction. The plausibility check outside of the regeneration therefore preferably takes place at a point in time at which metering of a reducing agent solution into the exhaust line between the SCR catalytic converters is reduced by a predefinable value compared to metering during the regeneration. The dosage is particularly preferably reduced to zero.

In both cases, the metering of the reducing agent solution into one of the two SCR catalytic converters is greatly reduced or even deactivated. For some time after the sharp reduction or deactivation of the reducing agent metering, a reaction of nitrogen oxides with previously stored ammonia takes place in the SCR catalytic converter behind it. An electronic control unit to which the nitrogen oxide sensors are connected can model the ammonia storage level of the SCR catalytic converters. After the ammonia storage of an SCR catalytic converter has been emptied, there is little or no change in the nitrogen oxide concentration via this SCR catalytic converter. The plausibility check is therefore preferably carried out at a point in time when an ammonia storage level of an SCR catalytic converter which is arranged between the nitrogen oxide sensors to be checked for plausibility is below a threshold value. The electronic control unit can then compare the nitrogen oxide signals from the nitrogen oxide sensors in the section of the exhaust system in which the SCR reaction is no longer taking place and mutually check their plausibility.

If the SCR catalytic converter system has a plurality of first nitrogen oxide sensors, their signals can also be checked for plausibility with one another during regeneration of the particle filter.

If, during the plausibility check, the nitrogen oxide signals of the nitrogen oxide sensors that have been checked for plausibility differ from one another by at least a predeterminable threshold value, it can also be provided in particular that the OBD system detects an error, stores an error code and, in particular, only lights up a fault lamp (MIL) after repeated error detection. activated. Additional measures can also be initiated as part of the monitoring of additional functionalities of the nitrogen oxide sensors. The further measures can in particular involve the generation of an error message that is only visible to a workshop, a hidden error message or a reduction in the power of the internal combustion engine (inducement). Since the particle filter is rarely regenerated and the emissions during this regeneration are converted to all journeys using a factor depending on the frequency of regeneration, higher emissions are also permitted during the rare regeneration of the particle filter, which is caused by the lower level of journeys can be compensated for without regenerating the particle filter. This can also include the plausibility check that is carried out in the course of the regeneration of the particle filter.

The computer program is set up to carry out each step of the method when it runs on a computing device or on an electronic control unit. It enables different embodiments of the method to be implemented on an electronic control unit without having to make structural changes thereto. For this purpose, it is stored on the machine-readable storage medium. By loading the computer program onto a conventional electronic control unit, the electronic control unit is obtained, which is set up to check nitrogen oxide sensors for plausibility using the method.

character list

• 1 zeigt einen schematischen Ausschnitt eines Abgasstrangs, in dem ein SCR-Katalysatorsystem angeordnet ist, dessen Stickoxidsensoren mittels eines Ausführungsbeispiels des erfindungsgemäßen Verfahrens plausibilisiert werden können.
• 2 zeigt ein Ablaufdiagramm eines Ausführungsbeispiels des erfindungsgemäßen Verfahrens.
• 3 zeigt schematisch die Plausibilisierung von Stickoxidsensoren während einer Regeneration eines Partikelfilters in einem Ausführungsbeispiel der Erfindung.
• 4 zeigt schematisch die Plausibilisierung von Stickoxidsensoren außerhalb der Regeneration eines Partikelfilters in einem Ausführungsbeispiel des erfindungsgemäßen Verfahrens.

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

• 1 shows a schematic section of an exhaust line in which an SCR catalytic converter system is arranged, the nitrogen oxide sensors of which can be checked for plausibility by means of an exemplary embodiment of the method according to the invention.
• 2 shows a flow chart of an embodiment of the method according to the invention.
• 3 shows schematically the plausibility check of nitrogen oxide sensors during a regeneration of a particle filter in an embodiment of the invention.
• 4 shows schematically the plausibility of nitrogen oxide sensors outside the regeneration of a particle filter in an embodiment of the method according to the invention.

Embodiments of the invention

1 shows the exhaust system 10 of an internal combustion engine, not shown, of a motor vehicle. An SCR catalytic converter system 20 for exhaust gas aftertreatment is arranged in this. The exhaust system 10 has a diesel oxidation catalytic converter (DOC) 11 and a particle filter 12 in series. A first SCR catalytic converter 21 of the SCR catalytic converter system 20 is arranged on the particle filter 12 . A second SCR catalytic converter 22 follows downstream of this first SCR catalytic converter 21, which is designed as a SCRoF . A second metering valve 24 for metering HWL into the exhaust line 10 is arranged between the two SCR catalytic converters 21, 22 in the exhaust line 10.

The SCR catalytic converter system 20 has four nitrogen oxide sensors 31 to 34 which send signals to an electronic control unit 40 . A nitrogen oxide sensor 31 is arranged upstream of the DOC 11 in the exhaust line 10 . Another nitrogen oxide sensor 32 is arranged between the DOC 11 and the first metering valve 23 . Another nitrogen oxide sensor 33 is arranged between the first SCR catalytic converter 21 and the second metering valve 24 . The last nitrogen oxide sensor 34 is arranged downstream of the second SCR catalytic converter 22 .

As in 2 is shown, after a start 50 of an exemplary embodiment of the method according to the invention, a check 51 first takes place as to whether regeneration of the particle filter 12 is taking place. If such a regeneration is active, a further check 52 takes place as to whether a modeled ammonia storage level of the first SCR catalytic converter 21 indicates that the ammonia storage of the first SCR catalytic converter 21 has been completely emptied. As soon as this condition is met, several plausibility checks 53 to 55 are carried out. These are in 3 shown. In the first plausibility check 53, the two nitrogen oxide sensors 31, 32 immediately upstream and downstream of the DOC 11 are checked for plausibility. In a second plausibility check 54, the two nitrogen oxide sensors 32, 33 immediately upstream and downstream of the first SCR catalytic converter 21 are checked for plausibility with one another. In a third plausibility check 55, the nitrogen oxide sensor 31 upstream of the DOC is checked for plausibility with the nitrogen oxide sensor 33 between the two SCR catalytic converters 21, 22.

If, on the other hand, test 51 shows that the regeneration of particle filter 12 is not active, a test 56 is carried out to determine whether a modeled ammonia storage level in first SCR catalytic converter 21 indicates that the ammonia storage in first SCR catalytic converter 21 has been completely emptied. As soon as this condition is met, a plausibility check 57 of the two nitrogen oxide sensors 33, 34, which are each arranged directly upstream and downstream of the second SCR catalytic converter 22, takes place. this is in 4 shown.

When the plausibility checks 53 to 55 or the plausibility check 57 have been completed, the method can be restarted 50 .

Claims (10)

Method for checking the plausibility of nitrogen oxide sensors (31 - 34) of an SCR catalytic converter system (20) with two SCR catalytic converters (21, 22), which has at least one first nitrogen oxide sensor (31, 32) which is installed upstream of the SCR catalytic converters (21, 22 ) is arranged in an exhaust line (10), has at least one second nitrogen oxide sensor (33) which is arranged between the SCR catalytic converters (21, 22) and has at least one third nitrogen oxide sensor (34) which is arranged downstream of the SCR catalytic converters (21 , 22) is arranged, characterized in that - during a regeneration of a particle filter (12) which is arranged in the exhaust line (10), signals of the first nitrogen oxide sensor (31, 32) and the second nitrogen oxide sensor (33) are checked together for plausibility ( 54, 55), and - outside the regeneration of the particle filter (12), signals from the second nitrogen oxide sensor (33) and the third nitrogen oxide sensor (34) are checked for plausibility with one another (57). procedure after claim 1 , characterized in that during regeneration of the particulate filter, signals from a plurality of first nitrogen oxide sensors (31, 32) are checked for plausibility with one another (53). procedure after claim 1 or 2 , characterized in that the plausibility check (53 - 55) takes place during the regeneration at a point in time at which metering of a reducing agent solution into the exhaust system (10) upstream of the SCR catalytic converters (21, 22) by a predeterminable value compared to metering outside of regeneration is reduced. Procedure according to one of Claims 1 until 3 , characterized in that the plausibility check (57) takes place outside of the regeneration at a time at which a dosing of a Reducing agent solution in the exhaust line (10) between the SCR catalytic converters (21, 22) is reduced by a predeterminable value compared to dosing during regeneration. procedure after claim 3 or 4 , characterized in that the reduction is to zero dosing. Procedure according to one of claims 3 until 5 , characterized in that the plausibility check (53 - 55, 57) takes place at a point in time at which an ammonia storage level of an SCR catalytic converter (21, 22) which is arranged between nitrogen oxide sensors (31 - 34) to be checked for plausibility is below a threshold value. Procedure according to one of Claims 1 until 6 , characterized in that an SCR catalytic converter (21) is arranged on the particle filter (12). Computer program which is set up to carry out each step of the method according to one of Claims 1 until 7 to perform. Machine-readable storage medium on which a computer program claim 8 is saved. Electronic control unit (40), which is set up to use a method according to one of Claims 1 until 7 To check nitrogen oxide sensors (31 - 34) for plausibility.

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