The air/fuel ratio control determines the air/fuel ratio of the engine combustion gases upstream of the catalyser (27), for regulating the air/fuel ratio fed to the engine (11) via a microprocessor, until the air/fuel ratio of the combustion gases reaches a required value. The quantity of material absorbed by the catalyser for each air/fuel ratio is determined from a catalyser model, with indication of the unconverted gas component, the required value for the air/fuel ratio ensuring that the unconverted gas component lies within a defined range.
