Method for feeding reducing agent and motor vehicle and stationary installation in which the method is performed

1. A method for feeding reducing agent into an exhaust-gas mass flow in an exhaust-gas treatment device, the method comprising the following steps:

providing the exhaust-gas treatment device with a feed point for the reducing agent, a storage catalytic converter for storage of reducing agent, and an SCR catalytic converter for performing a selective catalytic reduction of nitrogen oxide compounds in the exhaust gas;

a) implementing a first dosing strategy in which a loading of the storage catalytic converter with reducing agent is monitored and a first target conversion rate is determined on a basis of a present loading;

b) feeding reducing agent in accordance with the first target conversion rate;

c) determining a presently prevailing conversion rate attained with the SCR catalytic converter;

d) comparing the presently prevailing conversion rate with the first target conversion rate, and registering a deviation of the presently prevailing conversion rate from the first target conversion rate; and

e) switching to a further dosing strategy if the deviation exceeds a first threshold value, and disregarding the loading of the storage catalytic converter in the further dosing strategy.

2. The method according to claim 1 , which further comprises performing step e) only if a deviation is registered in step d) over a time period being longer than a predetermined maximum duration.

3. The method according to claim 2 , which further comprises, after step e), implementing the further dosing strategy for a predetermined time period being longer than the predetermined maximum duration, and subsequently switching back to the first dosing strategy according to step a).

4. The method according to claim 3 , which further comprises using a fill level model being reset after the predetermined time period, to determine the loading of the storage catalytic converter.

5. The method according to claim 1 , which further comprises setting a reduced second target conversion rate in the further dosing strategy.

6. The method according to claim 1 , which further comprises:

implementing the dosing strategy in step a) by using a steady-state proportional-integral controller if a steady operating state of the exhaust-gas treatment device prevails, and using a dynamic proportional-integral controller if a dynamic operating state of the internal combustion engine prevails; and

in step d), performing a comparison between a first control value of the steady-state proportional-integral controller and a second control value of the dynamic proportional-integral controller and taking the comparison into consideration in addition to the deviation.

7. The method according to claim 1 , which further comprises:

comparing the deviation with a series of threshold values; and

selecting the further dosing strategy on a basis of the threshold values, in accordance with the following procedure:

upon a deviation smaller than the first threshold value: remain in the first dosing strategy;

upon a deviation smaller than a second threshold value and greater than the first threshold value: switch to a second dosing strategy; and

upon a deviation smaller than a third threshold value and greater than the second threshold value: switch to a third dosing strategy.

8. The method according to claim 1 , which further comprises repeating the method steps a) to e) at regular intervals in a loop.

9. An internal combustion engine in combination with:

an exhaust-gas treatment device for purification of exhaust gases of said internal combustion engine, said exhaust-gas treatment device having a feed point for reducing agent, a storage catalytic converter for storage of reducing agent, and an SCR catalytic converter for performing a selective catalytic reduction of nitrogen oxide compounds in the exhaust gas; and

a control unit constructed and configured for:

a) implementing a first dosing strategy in which a loading of said storage catalytic converter with reducing agent is monitored and a first target conversion rate is determined on a basis of a present loading;

b) feeding reducing agent in accordance with said first target conversion rate;

c) determining a presently prevailing conversion rate attained with said SCR catalytic converter;

d) comparing said presently prevailing conversion rate with said first target conversion rate, and registering a deviation of said presently prevailing conversion rate from said first target conversion rate; and

e) switching to a further dosing strategy if said deviation exceeds a first threshold value, and disregarding said loading of said storage catalytic converter in said further dosing strategy.

10. The combination according to claim 9 , wherein the internal combustion engine and the exhaust gas treatment device part of a motor vehicle.

11. The combination according to claim 9 , wherein the internal combustion engine and the exhaust gas treatment device part of a stationary installation.