Method for metering a reducing agent into an exhaust-gas treatment device, exhaust-gas treatment device and motor vehicle

1. A method for metering a reducing agent into an exhaust-gas treatment device having a feed point and an SCR catalytic converter for the conversion of nitrogen oxide compounds in exhaust gas from a connected internal combustion engine, the method comprising the following steps:

a) calculating the following target conversion rates indicating what fraction of the nitrogen oxide compounds present in the exhaust gas can be converted by the SCR catalytic converter:

a.1) a first target conversion rate determined from a power output of the connected internal combustion engine;

a.2) a second target conversion rate determined from a mass flow of nitrogen oxide compounds in purified exhaust gases; and

a.3) a third target conversion rate determined from a ratio between a quantity of nitrogen oxide compounds upstream of the SCR catalytic converter and a quantity of nitrogen oxide compounds downstream of the SCR catalytic converter;

b) selecting the lowest target conversion rate;

c) determining a dosing quantity of reducing agent using the selected target conversion rate; and

d) dosing the determined dosing quantity into the exhaust-gas treatment device.

2. The method according to claim 1 , which further comprises calculating the target conversion rates in steps a.1) to a.3) by using at least the following parameters:

at least one temperature of the SCR catalytic converter;

at least one nitrogen oxide concentration in the exhaust gas;

a rotational speed of the internal combustion engine; and

a torque of the internal combustion engine.

3. The method according to claim 1 , which further comprises determining the dosing quantity in step c) by using a control unit having an integral control component and a proportional control component, comparing a presently prevailing rate of conversion by the SCR catalytic converter with the selected target conversion rate, and determining an error being used for the determination of the dosing quantity.

4. The method according to claim 3 , which further comprises:

calculating the presently prevailing conversion rate from a difference between a first signal of a first nitrogen oxide sensor upstream of the SCR catalytic converter in an exhaust-gas flow direction and a second signal of a second nitrogen oxide sensor downstream of the SCR catalytic converter in the exhaust-gas flow direction;

determining the first signal at a first time interval before the second signal; and

setting the first time interval to correspond to a flow time of the exhaust gas from the first nitrogen oxide sensor to the second nitrogen oxide sensor.

5. The method according to claim 3 , which further comprises:

selectively operating the control unit in a steady-state mode and in a dynamic mode;

calculating a gradient of a nitrogen oxide quantity in the exhaust gas upstream of the SCR catalytic converter for step c);

operating the control unit in the dynamic mode if the gradient is greater than a threshold value, and operating the control unit in the steady-state mode if the gradient is less than or equal to the threshold value;

using the target conversion rate selected in step b) only in the dynamic mode; and

aiming for a complete conversion of the nitrogen oxide compounds in the exhaust gas in the steady-state mode, for the calculation of the dosing quantity in step c).

6. The method according to claim 3 , which further comprises resetting the integral control component to zero if the internal combustion engine connected to the exhaust-gas treatment device undergoes a load alteration.

7. The method according to claim 1 , which further comprises providing the SCR catalytic converter with a coating which, under normal operating conditions of the SCR catalytic converter, has no storage capacity for reducing agent, and providing no additional storage catalytic converter for reducing agent in the exhaust-gas treatment device.

8. An exhaust-gas treatment device for the purification of exhaust gases of an internal combustion engine, the exhaust-gas treatment device comprising:

an SCR catalytic converter configured to perform a selective catalytic reduction of nitrogen oxide compounds in the exhaust gas;

a feed point configured to feed reducing agent, said feed point disposed upstream of said SCR catalytic converter in an exhaust-gas flow direction;

a second nitrogen oxide sensor downstream of said SCR catalytic converter in said exhaust-gas flow direction; and

a control unit configured to control metering of reducing agent at said feed point, said control unit programmed for:

a) calculating the following target conversion rates indicating what fraction of the nitrogen oxide compounds present in the exhaust gas can be converted by said SCR catalytic converter:

a.1) a first target conversion rate determined from a power output of the internal combustion engine;

a.2) a second target conversion rate determined from a mass flow of nitrogen oxide compounds in purified exhaust gases; and

a.3) a third target conversion rate determined from a ratio between a quantity of nitrogen oxide compounds calculated upstream of the SCR catalytic converter and a quantity of nitrogen oxide compounds downstream of the SCR catalytic converter;

b) selecting the lowest target conversion rate;

c) determining a dosing quantity of reducing agent using the selected target conversion rate; and

d) dosing the determined dosing quantity into said feed point.

9. A motor vehicle, comprising:

an internal combustion engine; and

an exhaust-gas treatment device according to claim 8 for the purification of exhaust gases of said internal combustion engine.