CN111911271A

CN111911271A - Method for zero calibration of a NOx sensor

Info

Publication number: CN111911271A
Application number: CN202010382389.6A
Authority: CN
Inventors: J.雷德; S.克莱因; T.杭德勒
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2019-05-09
Filing date: 2020-05-08
Publication date: 2020-11-10
Also published as: DE102019206680A1

Abstract

The invention relates to a method for zero-point calibration (83) of a nitrogen oxide sensor that is laterally sensitive to ammonia and that is arranged downstream of an SCR catalytic converter in an exhaust gas system of an internal combustion engine, wherein at least one measured value of the nitrogen oxide sensor is used as an adaptation value in an operating state of the internal combustion engine in which the exhaust gas system does not contain nitrogen oxides. The zero calibration (83) is released (82) only if the exhaust gas train does not contain ammonia at the location of the nitrogen oxide sensor.

Description

Method for zero calibration of a NOx sensor

Technical Field

The invention relates to a method for zero calibration of a nitrogen oxide sensor that is laterally sensitive to ammonia. Furthermore, the invention relates to a computer program for carrying out each step of the method and to a machine-readable storage medium, which stores the computer program. Finally, the invention relates to an electronic control unit which is designed to carry out the method.

Background

For example, exhaust aftertreatment systems for reducing nitrogen oxide emissions use technologies such as catalysts for storing nitrogen oxides (NSCs) or Selective Catalytic Reduction (SCR) of nitrogen oxides. In both technologies, the output of ammonia emissions may occur during operation.

Under the action of the noble metal catalyst, the NSC catalyst stores nitrogen oxides of the oxygen-rich exhaust gas in the form of nitrates. By enriching the fuel-air mixture in the internal combustion engine, nitrogen oxide molecules are discharged into the exhaust gas flow and are ideally reduced to nitrogen by incompletely combusted, reducing hydrocarbons and/or carbon monoxide. However, in part, it is also reduced further to ammonia.

When using an SCR catalyst, provision is made for ammonia to be generated in the exhaust gas system by introducing a reducing agent solution, which is a urea solution containing water (urea aqueous solution, HWL), and for the ammonia to react with the nitrogen oxides present in the exhaust gas in the SCR catalyst to form nitrogen gas and water. In this case, the unreacted ammonia leaves the SCR catalyst again, however, as so-called ammonia slip. Furthermore, the metering of the amount of reducing agent and the diagnostic function for detecting a faulty SCR catalyst are carried out on the basis of the measured values of the nitrogen oxide sensor in the exhaust system. These nitrogen oxide sensors react laterally sensitively to ammonia.

The zero point output by the nox sensor is deviated from the actual value. The zero point offset may be positive or negative. Therefore, zero calibration is typically employed. Such a zero point calibration is carried out in an operating state in which the exhaust gas contains no nitrogen oxides. However, due to the lateral sensitivity of the nox sensor to ammonia, the ammonia concentration of the exhaust gas must be modeled in order to perform a zero point calibration.

Disclosure of Invention

In a known manner, a method for zero-point calibration of a nitrogen oxide sensor that is laterally sensitive to ammonia is provided in an exhaust gas system of an internal combustion engine downstream of an NSC catalyst or an SCR catalyst, in an operating state of the internal combustion engine in which the exhaust gas system does not contain nitrogen oxide at least one measured value of the nitrogen oxide sensor being used as an adaptation value for the zero-point calibration. However, the release of the zero calibration only occurs when the exhaust gas train does not contain ammonia at the location of the nitrogen oxide sensor. If the ammonia concentration, the position of the nox sensor, has to be modeled in a conventional manner within the scope of the zero calibration, systematic errors that are not taken into account in the model behavior can lead to inaccuracies in the zero calibration. These inaccuracies can be avoided by using the described release conditions.

Preferably, it is checked by means of the zero-point-calibrated ammonia signal of the ammonia sensor whether the exhaust gas train does not contain ammonia at the location of the nitrogen oxide sensor. The zero calibration of the ammonia signal is advantageous because the ammonia signal is subject to similar interference factors and has similar tolerances as the signal of the nitrogen oxide sensor.

In principle, an ammonia sensor may refer to an ammonia sensor which is installed in the exhaust system in the vicinity of a nitrogen oxide sensor. However, in order to be able to dispense with additional components and to achieve a maximum spatial proximity between the ammonia measurement and the nitrogen oxide measurement, it is preferred that the ammonia sensor is embodied as a circuit of a sensor element of the nitrogen oxide sensor.

Operating states of the internal combustion engine in which the exhaust gas system does not contain nitrogen oxides, for example, in coasting mode, can be achieved by flushing combustion residues out of the exhaust gas system with fresh air, and in such operating states it cannot be ensured that the exhaust gas system does not contain ammonia at the location of the nitrogen oxide sensor. Therefore, ammonia desorption of the SCR catalyst may also occur during engine-type coasting. The zero calibration of the ammonia signal is therefore preferably carried out in an operating state in which it is concluded from the temperature of the SCR catalyst and from the quantity of reducing agent dosing in the SCR catalyst that the exhaust system does not contain ammonia at the location of the ammonia sensor. Depending on the catalyst technology used, the zero calibration of the ammonia signal can only be carried out at temperatures below the light-off temperature of the SCR catalyst. In order to ensure a sufficient time share for the zero calibration of the ammonia signal, it may therefore be necessary to heat the ammonia sensor to below its dew point as well.

Even if the coasting of the internal combustion engine is insufficient to exclude the presence of ammonia at the position of the ammonia sensor, it is preferred to perform a zero calibration of the ammonia signal during the coasting of the internal combustion engine. Thereby, the accuracy of the calibration is improved in case of possible lateral sensitivity of the ammonia sensor to nitrogen oxides or other exhaust gas components.

Furthermore, in the zero calibration of the ammonia sensor, the measured values of the ammonia sensor are averaged over a predeterminable period of time and attenuated using a transfer function. In this way, the influence of signal noise can be reduced, and the accuracy of zero calibration of the ammonia sensor can be improved.

A computer program is provided for carrying out each step of the method, in particular when the computer program is run on a computing device or an electronic controller. This makes it possible to implement different embodiments of the method according to the invention on an electronic controller without having to make structural changes thereto. To this end, the computer program is stored on a machine-readable storage medium.

By running a computer program on a conventional electronic controller, an electronic controller is obtained which is set up to carry out a zero calibration of the nitrogen oxide sensor which is laterally sensitive to ammonia by means of the method.

Drawings

Embodiments of the invention are illustrated in the drawings and are further described in the following description.

Fig. 1 shows an exhaust gas aftertreatment system of an internal combustion engine, in which a nitrogen oxide sensor, which is laterally sensitive to ammonia, can be zero-calibrated by means of a method according to an embodiment of the invention.

FIG. 2 shows a flow diagram of a method for zero calibration of an ammonia laterally sensitive NOx sensor according to an embodiment of the present invention.

Detailed Description

The internal combustion engine 10 shown in fig. 1 has an exhaust gas system 11. In the exhaust gas system 11, an NSC catalyst 21, a first SCR catalyst 22 arranged on a diesel particulate filter, and a second SCR catalyst 23 combined with a CUC catalyst (cleaning catalyst) are arranged one after the other. The HWL is stored in a reducing agent tank 30, on the bottom of which a delivery module 31 is arranged. The first metering module 32 is arranged upstream of the first SCR catalyst in the exhaust gas system 11. The second metering module 33 is arranged upstream of the second SCR catalyst 23 in the exhaust gas system 11. The two metering modules 32, 33 are supplied with HWL by means of the delivery module 31, which meter HWL into the exhaust system 11. Ammonia is released from the HWL and reacts with nitrogen oxides in a selective catalytic reduction reaction in the

SCR catalysts

22, 23. A plurality of ammonia sensors 41 to 43 and nitrogen oxide sensors 51 to 53, which are laterally sensitive to ammonia, are arranged in the exhaust gas system 11. The first ammonia sensor 41 and the first nox sensor 51 are arranged between the NSC catalyst 21 and the first dosing module 32. The second ammonia sensor 42 and the second nitrogen oxide sensor 52 are arranged between the first SCR catalyst 22 and the second dosing module 33. Third ammonia sensor 43 and third nitrogen oxide sensor 53 are arranged downstream of second SCR catalyst 23. Furthermore, each of the

SCR catalysts

22, 23 has a

temperature sensor

61, 62, respectively. The measured values of all sensors 41-43, 51-53, 61, 62 are fed to an electronic control unit 70. The electronic controller controls the internal combustion engine 10 and the two dosing modules 32, 33.

In one exemplary embodiment of the method according to the present invention for zero-point calibration of nox sensors 51 to 53, a plurality of variables detected in electronic controller 70 are used as the release conditions. These release conditions include the idle time t of the internal combustion engine 10, the fuel mass flow q in the internal combustion engine 10_m(Kra) intake air mass flow q in internal combustion engine 10_m(Luf), operating mode B of internal combustion engine 10, temperature T of first SCR catalyst 22 or second SCR catalyst 23, amount of ammonia m stored in first SCR catalyst 22 or second SCR catalyst 23 (NH 3), and reductant mass flow q through first dosing module 32 or through second dosing module 33_m(Red). Before the zero-point calibration of one of the nox sensors 51 to 53 is carried out, the method provides that first a zero-point calibration of the ammonia sensors 41 to 43 arranged in the vicinity thereof is carried out. For this purpose, a check 80 is first made as to whether all the release conditions have values that are appropriate for the zero calibration of the respective ammonia sensors 41 to 43. If the release condition is fulfilled, the ammonia signal NH3 is measured by means of the respective ammonia sensor 41 to 43_rohAnd zero point calibration 81 is performed with the aid of the original values. In this case, the measurement is repeated a plurality of times and the measured values are averaged and attenuated using the transfer function. By zero calibration, a calibrated ammonia signal NH3 is now provided_kalFor use.

For the zero-point calibration of nox sensors 51 to 53, a check 82 is now made as to whether the variable which has been checked for the zero-point calibration of the ammonia signal now has assumed a value which satisfies the release conditions for the zero-point calibration of nox sensors 51 to 53. On the other hand, it is also checked here whether a calibrated ammonia signal NH3 is present_kal. If these conditions are met, the NOx signal NOx of the NOx sensors 51-53 is used_rohThe zero calibration 83 of the nox sensors 51 to 53 is carried out with the original values of (b). In this way, a calibrated nitrogen oxide signal NOx may be obtained_kal。

In a not illustrated embodiment of the invention, it may alternatively also be provided that the first ammonia sensor 41 and the first nox sensor 51 or the second ammonia sensor 42 and the second nox sensor 52 or the third ammonia sensor 43 and the third nox sensor 53 are each implemented in a single sensor component, in which the ammonia sensors are implemented as a circuit of a sensor element of the respective nox sensor.

Claims

1. Method for zero-point calibration (83) of a nitrogen oxide sensor (51-53) that is laterally sensitive to ammonia, which is arranged in an exhaust gas system (11) of an internal combustion engine (10) downstream of an NSC catalyst (21) and an SCR catalyst (22, 23), wherein at least one measured value of the nitrogen oxide sensor (51-53) is used as an adaptation value in an operating state of the internal combustion engine (10) in which the exhaust gas system (11) does not contain nitrogen oxide, characterized in that the zero-point calibration (83) is released (82) only when the exhaust gas system (11) does not contain ammonia at the location of the nitrogen oxide sensor (51-53).

2. Method according to claim 1, characterized in that the zero-point-calibrated ammonia signal (NH 3) of the ammonia sensor (41-43) is used as a reference_kal) To check whether the exhaust line (11) does not contain ammonia at the location of the nitrogen oxide sensors (51-53).

3. Method according to claim 2, characterized in that the ammonia sensor is implemented as an electric circuit of a sensor element of the nitrogen oxide sensor (51-53).

4. Method according to claim 2 or 3, characterized in that the ammonia signal (NH 3) is carried out in one of the following operating states_roh) Is determined, and a zero point calibration (81) of (A) is carried out, in which operating state it is concluded from the temperature (T) of the SCR catalyst (22, 23) and the quantity (m (NH 3)) of reductant dosing in the SCR catalyst (22, 23) that the exhaust gas system (11) does not contain ammonia at the location of the ammonia sensor (41-43).

5. The method according to claim 4, characterized in that the ammonia signal (NH 3) is carried out during coasting operation of the internal combustion engine (10)_roh) Zero point calibration (81).

6. Method according to one of claims 2 to 5, characterized in that in the zero point calibration (81) of the ammonia sensor (41-43), the measured values of the ammonia sensor (41-43) are averaged over a predeterminable period of time and attenuated with a transfer function.

7. A computer program arranged to perform each step of the method according to any one of claims 1 to 6.

8. A machine-readable storage medium on which a computer program according to claim 7 is stored.

9. An electronic controller (70) which is provided for carrying out a zero calibration (83) of a nitrogen oxide sensor (51-53) which is laterally sensitive to ammonia by means of a method according to one of claims 1 to 6.