A method and system for controlling an engine

A method and system are described for controlling decontamination of an emission control device (72) fitted to an engine having two groups of cylinders (200a,200b). The temperature of the emission control device (72) is maintained at a desired temperature by operating some cylinders of an engine (10) lean and others rich. These lean and rich mixtures react exothermically in the exhaust gas and in the emission control device (72) to generate heat. Efficient contaminant removal is obtained by oscillating the mixture air-fuel ratio about stoichiometric. This oscillation is provided by adjusting the fuel provided to the rich cylinders, or by adjusting the air provided to the lean cylinders, thereby minimizing any torque disturbance corresponding to the oscillations in exhaust air-fuel ratio.