Method and control assembly for operating an exhaust gas system

1. A method for operating an exhaust gas system of a motor vehicle, comprising the steps of:

evaluating measuring values of a sensor which indicate a content of nitrogen oxides in an exhaust gas downstream of a catalytic device adapted to diminish the content of nitrogen oxides in the exhaust gas produced by an engine of the motor vehicle;

assessing a quality of a reducing agent supplied to the catalytic device based on the measuring values;

determining whether reducing agent has been filled into a storage tank by a storage tank level sensor;

capturing a plurality of the measuring values during a predetermined period of time and taking into account a magnitude and a frequency of the plurality of measuring values in the assessing of the quality of the reducing agent; and

modifying an amount of the reducing agent which is supplied to the exhaust gas in response to the step of assessing determining that the quality of the reducing agent is improper;

wherein a magnitude of a measuring value captured within the predetermined period of time is related to an average or to a median of the magnitudes of the plurality of measuring values captured during the predetermined period of time and the related magnitude is utilized in assessing the quality of the reducing agent;

wherein a difference between the magnitude of the measuring value to be related and the average or median is calculated in assessing the quality of the reducing agent.

2. The method according to claim 1 , wherein the reducing agent is assessed to have an improper quality if a value which is based on a plurality of modifications of the amount of the reducing agent is greater than a threshold value.

3. The method according to claim 2 , wherein a sum of differences between a plurality of correction factors is utilized as the value which is based on the plurality of modifications.

4. The method according to claim 1 , wherein in utilizing the related magnitude in assessing the quality of the reducing agent a total of absolute values of differences is created.

5. The method according to claim 4 , wherein the total of absolute values of differences is created over a sampling period which comprises a plurality of predetermined periods of time.

6. The method according to claim 5 , wherein the reducing agent is assessed to have an improper quality if a sum of differences between a plurality of totals is greater than a threshold value.

7. A control assembly for operating an exhaust gas system of a motor vehicle, comprising:

an evaluation unit configured to evaluate measuring values of a sensor which indicate a content of nitrogen oxides in an exhaust gas downstream of a catalytic device, wherein the catalytic device is adapted to diminish the content of nitrogen oxides in the exhaust gas produced by an engine of the motor vehicle;

a detection unit configured to assess a quality of a reducing agent supplied to the catalytic device based on the measuring values, to determine whether reducing agent has been filled into a storage tank, and to take into account a magnitude and a frequency of a plurality of measuring values captured during a predetermined period of time in order to assess the quality of the reducing agent; and

a controller of a dosing unit, wherein the controller modifies an amount of the reducing agent which is supplied to the exhaust gas by the dosing unit if the detection unit assesses that the quality of the reducing agent is improper;

wherein a magnitude of a measuring value captured within the predetermined period of time is related to an average or to a median of the magnitudes of the plurality of measuring values captured during the predetermined period of time and the related magnitude is utilized in assessing the quality of the reducing agent;

wherein a difference between the magnitude of the measuring value to be related and the average or median is calculated in assessing the quality of the reducing agent.