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

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.