Method for checking the plausibility of a NOx sensor in an SCR catalytic converter system

Abstract

In the case of a method for checking the plausibility of a NOx sensor (62; 63) in an SCR catalytic converter system having at least one first SCR catalytic converter device (20) and having at least one second SCR catalytic converter device (30) and having in each case one dosing point (40, 50) for a reducing agent solution for the SCR catalytic converter devices (20, 30) upstream of the respective SCR catalytic converter device, the NOx sensor (62) to be checked for plausibility is situated either between the first SCR catalytic converter device (20) and the second SCR catalytic converter device (30), or the NOx sensor (63) to be checked for plausibility is situated downstream of the second SCR catalytic converter device (30).

Claims

1. A method for checking the plausibility of a NOx sensor (62; 63) in an SCR catalytic converter system having at least one first SCR catalytic converter device (20) and having at least one second SCR catalytic converter device (30) and having in each case one dosing point (40, 50) for a reducing agent solution for the SCR catalytic converter devices (20, 30) upstream of the respective SCR catalytic converter device and having in each case one NOx sensor (62, 63) downstream of the SCR catalytic converter devices (20, 30), with the method comprising: performing substoichiometric dosing of the reducing agent solution for the first SCR catalytic converter device (20) or for the second SCR catalytic converter device (30), and comparing measurable signal values of the NOx sensor (62; 63) respectively to be checked for plausibility with calculable or modellable signal values in the case of NOx conversion that is to be expected in the first SCR catalytic converter device (20) or in the second SCR catalytic converter device (30) respectively, wherein the NOx sensor (62; 63) to be checked for plausibility is situated in each case downstream of the SCR catalytic converter device (20; 30) with the substoichiometric dosing and, in the event of a deviation allowing for predefinable tolerances, determining that the respective NOx sensor (62; 63) is implausible.

2. The method according to claim 1, wherein the NOx sensor (62) to be checked for plausibility is arranged downstream of the first catalytic converter device (20) and at the same time upstream of the second SCR catalytic converter device (30), and in that the defined substoichiometric dosing of the reducing agent solution is performed at the dosing point (40) for the first SCR catalytic converter device (20), and in that the measurable signal values of the NOx sensor (62) are compared with calculable or modellable signal values in the case of NOx conversion that is to be expected in the first SCR catalytic converter device (20).

3. The method according to claim 1, wherein the defined substoichiometric dosing is a dosing quantity ratio , and in that the calculable or modellable signal values (NOx_downstreamofSCR1 or NOx_downstreamofSCR2 respectively) can be derived from a relationship between the dosing quantity ratio and the NOx conversion rate (_SCR1 or _SCR2 respectively) to be expected at the present operating point of the first SCR catalytic converter device (20) or of the second SCR catalytic converter device (30) respectively.

4. The method according to claim 3, wherein the calculable or modellable signal values (NOx_downstreamofSCR1 or NOx_downstreamofSCR2 respectively) can be derived, taking into consideration measurable or modellable NOx data (NOx_upstreamofSCR1) upstream of the first SCR catalytic converter device (20) or measurable or modellable NOx data (NOx_upstreamofSCR2) upstream of the second SCR catalytic converter device (30) respectively, from the following relationship:
NOx_downstreamofSCR1=NOx_upstreamofSCR1.Math.NOx_upstreamofSCR1.Math._SCR1
or
NOx_downstreamofSCR2=NOx_upstreamofSCR2.Math.NOx_upstreamofSCR2.Math._SCR2 respectively.

5. The method according to claim 1, wherein checking of the plausibility of the NOx sensor (62; 63) is performed as part of a monitoring function for the NH.sub.3 storage capacity of the first SCR catalytic converter device (20) or of the second SCR catalytic converter device (30) respectively.

6. The method according to claim 5, wherein the monitoring function for the NH.sub.3 storage capacity is terminated if it can be detected that the NOx sensor (62; 63) is implausible.

7. The method according to claim 5, wherein the monitoring function for the NH.sub.3 storage capacity comprises the following phases: a conditioning phase with substoichiometric dosing for the purposes of setting a defined working point of the first SCR catalytic converter device (20) or of the second SCR catalytic converter device (30) respectively, an overdosing phase until the maximum NH.sub.3 storage of the first SCR catalytic converter device (20) or of the second SCR catalytic converter device (30) respectively is reached, a possible emptying phase with dosing reduced or stopped, and an analysis phase of the NOx conversion rate for the purposes of checking the NH.sub.3 storage capacity, wherein checking of the plausibility of the NOx sensor (62; 63) is performed during the conditioning phase.

8. The method according to claim 1, wherein the first SCR catalytic converter device (20) is a particle filter with SCR coating, and in that the second SCR catalytic converter device (30) is a conventional SCR catalytic converter.

9. The method according to claim 1, wherein, before the plausibility check, it is checked whether operating conditions suitable for a plausibility check are present.

10. The method according to claim 1, wherein the NOx sensor (63) to be checked for plausibility is arranged downstream of the second SCR catalytic converter device (30), and in that the measurable signal values of the NOx sensor (63) are compared with calculable or modellable signal values in the case of NOx conversion that is to be expected in the second SCR catalytic converter device (30), wherein the first SCR catalytic converter device (20) is operated in an operating mode with normal closed-loop control, and in that the defined substoichiometric dosing of the reducing agent solution is performed at the dosing point (50) for the second SCR catalytic converter device (30) by virtue of the dosing being stopped, wherein the calculable or modellable signal values (NOx_downstreamofSCR2) can be derived from the following relationship:
NOx_downstreamofSCR2=NOx_downstreamofSCR1

11. An electronic control unit programmed with instructions that, when executed, carry out the steps of a method according to claim 1.

12. A non-transitory machine-readable storage medium containing computer-readable instructions, which when executed by a computer, cause an SCR catalytic converter system having at least one first SCR catalytic converter device (20) and having at least one second SCR catalytic converter device (30) and having in each case one dosing point (40, 50) for a reducing agent solution for the SCR catalytic converter devices (20, 30) upstream of the respective SCR catalytic converter device and having in each case one NOx sensor (62, 63) downstream of the SCR catalytic converter devices (20, 30), to: perform substoichiometric dosing of the reducing agent solution for the first SCR catalytic converter device (20) or for the second SCR catalytic converter device (30), and compare measurable signal values of the NOx sensor (62; 63) respectively to be checked for plausibility with calculable or modellable signal values in the case of NOx conversion that is to be expected in the first SCR catalytic converter device (20) or in the second SCR catalytic converter device (30) respectively, wherein the NOx sensor (62; 63) to be checked for plausibility is situated in each case downstream of the SCR catalytic converter device (20; 30) with the substoichiometric dosing and, in the event of a deviation allowing for predefinable tolerances, determine that the respective NOx sensor (62; 63) is implausible.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention will emerge from the following description of exemplary embodiments in conjunction with the drawings. Here, the individual features may be realized in each case individually or in combination with one another. In the drawings:

(2) FIG. 1 is a schematic illustration of an exemplary embodiment of an SCR catalytic converter system which is suitable for carrying out the method according to the invention, and

(3) FIG. 2 shows a schematic flow diagram of an exemplary embodiment of the plausibility checking method according to the invention.

DETAILED DESCRIPTION

(4) FIG. 1 schematically shows an exemplary construction of an SCR catalytic converter system which is suitable for carrying out the plausibility checking method according to the invention. The illustration shows the exhaust tract of an internal combustion engine (not shown in any more detail) through which the exhaust gas flows in the direction of the arrow. The exhaust-gas aftertreatment system comprises a diesel oxidation catalytic converter (DOC) 10. The latter is followed by an SCR-coated particle filter (SCRF) 20 as first SCR catalytic converter device. Further downstream, there is situated an SCR catalytic converter (SCR) 30 as second SCR catalytic converter device, downstream of which there may be positioned a further clean-up catalytic converter (CuC), which is not illustrated here. A first dosing point 40 for the required liquid reducing agent solution (e.g. AdBlue) is situated between the DOC 10 and the first SCR catalytic converter device 20. The dosing point 40 is thus situated upstream of the first SCR catalytic converter device 20. A second dosing point 50 for the liquid reducing agent solution is situated downstream of the first SCR catalytic converter device 20 and at the same time upstream of the second SCR catalytic converter device 30. The dosing points 40 and 50 may involve conventional dosing devices, for example dosing valves or injectors. A first NOx sensor 61 is situated upstream of the first SCR catalytic converter device 20, and a second NOx sensor 62 is situated between the first SCR catalytic converter device 20 and the second SCR catalytic converter device 30. A third NOx sensor 63 is situated downstream of the second SCR catalytic converter device 30. Yet further sensors, for example temperature sensors, may additionally be provided. The plausibility checking method according to the invention serves for checking the plausibility of the second NOx sensor 62, which is assigned to the first SCR catalytic converter device 20, or for checking the plausibility of the NOx sensor 63 assigned to the second SCR catalytic converter device 30. The first NOx sensor 61 is not imperatively required for the plausibility checking method according to the invention. Corresponding NOx values may for example also be represented by a calculated model value.

(5) The checking of the plausibility of the NOx sensor 62 will be discussed in more detail below. The checking of the plausibility of said NOx sensor 62 is possible in a particularly advantageous manner because, for the plausibility checking, use can be made of a phase with substoichiometric dosing that is performed in the context of an active diagnosis of the first SCR catalytic converter device 20. In this configuration of the exhaust-gas aftertreatment components, an active diagnosis with substoichiometric dosing does not lead to influences that are relevant with regard to emissions, because the increased NOx concentrations that may arise here downstream of the first SCR catalytic converter device 20 can be compensated by way of the second SCR catalytic converter device 30 with the further reducing agent dosing point 50 arranged upstream thereof. For the checking of the plausibility of the NOx sensor 62, use can thus be made of an underdosing phase during the active diagnosis of the first SCR catalytic converter device 20, wherein influencing of emissions with regard to the system as a whole can be avoided.

(6) During the abovementioned active diagnosis in the context of a monitoring strategy, known per se, for the first SCR catalytic converter device 20, the reducing agent dosing is set so as to be substoichiometric for the conditioning phase. In the conditioning phase, the first SCR catalytic converter device 20 therefore cannot fully convert the nitrogen oxides that are formed. The maximum possible NOx conversion then corresponds to the dosing quantity ratio . The expected NOx concentration downstream of the first SCR catalytic converter device (SCRF) 20 at the position of the NOx sensor 62 that is to be checked for plausibility thus amounts in this case, allowing for possible sensor tolerances, to the following:
NOx_downstreamofSCRF=NOx_upstreamofSCRF.Math.
NOx_upstreamofSCRF.Math._SCRF

(7) Here, _SCRF is the NOx conversion rate of the first SCR catalytic converter device (SCRF) 20 that is expected at the present operating point. If the measured signal of the NOx sensor 62 does not correspond to the model value NOx_downstreamofSCRF calculated in this way, the measurement signal is not plausible, and a further diagnosis of the first SCR catalytic converter device 20 is, under some circumstances, no longer expedient, and may be terminated if necessary.

(8) The plausibility checking method according to the invention may be realized through adaptation of conventional control unit software with an SCR monitoring function by virtue of the checking of the plausibility of the NOx sensor being integrated into a conditioning phase with an underdosing of reducing agent that is performed for the purposes of setting a defined starting point for the diagnosis function. It is then preferably possible for the calculation of the NOx conversion rate (efficiency calculation) of the SCR catalytic converter device to be continued only when, during the conditioning phase, the NOx signal downstream of the SCR catalytic converter device also provides meaningful values.

(9) An exemplary process of an SCR diagnosis function incorporating the plausibility check according to the invention is illustrated in FIG. 2. After the start 100 of the method, it is queried in step 200 whether the SCR catalytic converter device (e.g. SCRF as first SCR catalytic converter device) whose downstream NOx sensor is to be checked for plausibility is in an optimum operating range for efficiency monitoring (SCR catalytic converter diagnosis). If this is not the case, the method returns to the start 100. If the response to the query in step 200 is positive, then the conditioning phase for the SCR catalytic converter diagnosis (active diagnosis) is started in step 300. During the conditioning phase, in which substoichiometric dosing is established, it is checked in step 400 whether the values measured by the NOx sensor that is to be checked for plausibility correspond to a model value. Here, it is in particular checked whether the relationship
NOx_downstreamofSCRFNOx_upstreamofSCRF.Math.

(10) is satisfied. If this is the case, it is to be assumed that a plausible sensor signal is present, and the active diagnosis can be continued in a manner known per se in step 500. If the check in step 400 yields that the measured NOx signal does not correspond to the modelled or calculated NOx signal, it can be inferred that the NOx sensor signal is not plausible, and the further diagnosis of the SCR catalytic converter device can be terminated (step 600). Alternatively, it is possible for the method to return to the start 100 for the purposes of repeating the plausibility check.