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

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 α, 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 α and the NOx conversion rate (η_SCR1 or η_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:

NO x _downstreamofSCR1=NO x _upstreamofSCR1·α≈NO x _upstreamofSCR1·η_SCR1

or

NO x _downstreamofSCR2=NO x _upstreamofSCR2·α≈NO x _upstreamofSCR2·η_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 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 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 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 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 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:

NO x _downstreamofSCR2=NO x _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.