Abstract: A method is described for reducing the nitrogen oxide levels in diesel engine exhaust gases with the aid of an exhaust gas treatment system, which includes a device for feeding ammonia and/or a compound decomposing to form ammonia, and two SCR units. An ammonia sensor is situated between the first and second SCR unit. A nitrogen oxide sensor is located in the exhaust pipe downstream from the second SCR unit. The signals of the ammonia sensor and the NOx sensor are combined in a defined manner to form the actual value of a virtual NOx/NH3 sensor, which is used as the input variable for a PI or PID controller, which regulates the quantity of reducing agent to be fed into the exhaust gas as needed.

Abstract: A method is described for reducing the nitrogen oxide levels in diesel engine exhaust gases with the aid of an exhaust gas treatment system, which includes a device for feeding ammonia and/or a compound decomposing to form ammonia, and two SCR units. An ammonia sensor is situated between the first and second SCR unit. A nitrogen oxide sensor is located in the exhaust pipe downstream from the second SCR unit. The signals of the ammonia sensor and the NOx sensor are combined in a defined manner to form the actual value of a virtual NOx/NH3 sensor, which is used as the input variable for a PI or PID controller, which regulates the quantity of reducing agent to be fed into the exhaust gas as needed.