Abstract: A propulsion system, for example for a motor vehicle, that includes an internal combustion engine, a catalyst for reducing nitrogen oxides NOx contained in the exhaust faces of the internal combustion engine, a fuel injector arranged upstream of the catalyst, and a mechanism that determines quantity of the nitrogen oxides NOx received from the internal combustion engine. The propulsion system also includes a mechanism that determines a fuel quantity required for attaining a proportion lower or substantially equal to a stoichiometric proportion with respect to the nitrogen oxide NOx quantity, and a mechanism that triggers injection of the required quantity when the ratio therebetween and the nitrogen oxide NOx quantity is less than the triggering threshold.