Abstract: A method of controlling a rate of introduction of a NOx reducing substance or precursor to a catalyst component. The method comprises obtaining the amount of NOx entering the catalyst component, modeling the amount of NOx reducing substance or precursor in the catalyst component and controlling the rate of introduction of the NOx reducing substance or precursor to reduce NOx.

Abstract: A method of controlling a rate of introduction of a NOx reducing substance or precursor to a catalyst component. The method comprises obtaining the amount of NOx entering the catalyst component, modelling the amount of NOx reducing substance or precursor in the catalyst component and controlling the rate of introduction of the NOx reducing substance or precursor to reduce NOx.

Abstract: The operation of a catalyst can be monitored using a two part test. An engine management system is arranged to perturb the oxygen concentration of the exhaust gas of an internal combustion engine such that an oxygen concentration of the gas entering the catalyst varies between oxygen rich and oxygen deficient. A first test involves counting the number of transitions of the gas leaving the catalyst between two predetermined oxygen concentrations in response to a predetermined number of transitions of the gas entering the catalyst. A second test is performed if the first test indicates the catalyst to be faulty. The second test involves forming an integral of the oxygen concentration of the gas leaving the catalyst and correcting the integral for gas flow through the catalyst.