Device for reducing the NOx content in the exhaust gas of an internal combustion engine

A device for reducing a NOx content in exhaust gas of a vehicle internal combustion engine, wherein a control unit is provided and the engine is operated with excess air, the device comprising:

• an exhaust system conducting an exhaust gas flow in an exhaust gas flow direction;
• an SCR catalytic converter disposed in said exhaust system;
• a reducing agent metering system including:
• a source of reducing agent and a reducing agent injection valve communicating with said source of reducing agent, said injection valve being disposed to inject reducing agent into said exhaust system upstream of said SCR catalytic converter in the exhaust gas flow direction;
• a temperature sensor, an NOx measuring sensor, and an exhaust gas flow sensor connected to said exhaust system;
• a reducing agent control unit directly connected to said temperature sensor, to said NOx measuring sensor, and to said exhaust gas flow sensor;
• said reducing agent control unit determining an amount of reducing agent to be injected, from measurements of said temperature sensor, said NOx measuring sensor, and said exhaust gas flow sensor without recourse to data available in the control unit of the internal combustion engine, and driving said reducing agent metering system and metering said reducing agent based on the measurement.