Abstract: The invention relates to a four stroke internal combustion engine with one or more engine operating condition sensors including an exhaust gas composition sensor in the exhaust conduit. An engine management system is arranged to control the operation of the engine including the fuel injection system and the air/fuel ratio &lgr; for at least one cylinder. The engine management system receives from said sensors respective signals Si and oscillates the air/fuel ratio &lgr; between a relatively rich level and a relatively lean level, depending on the signal Si from the exhaust gas sensor. The engine management system determines the temporal characteristics (142) of the oscillation in the air/fuel ratio and determines whether or not the engine is being fueled on the correct stroke by comparing (136) those temporal characteristics with relevant engine operation data (120) stored in the engine management system.

Abstract: The invention relates to a four stroke internal combustion engine with one or more engine operating condition sensors including an exhaust gas composition sensor in the exhaust conduit. An engine management system is arranged to control the operation of the engine including the fuel injection system and the air/fuel ratio &lgr; for at least one cylinder. The engine management system receives from said sensors respective signals Si and oscillates the air/fuel ratio &lgr; between a relatively rich level and a relatively lean level, depending on the signal Si from the exhaust gas sensor. The engine management system determines the temporal characteristics (142) of the oscillation in the air/fuel ratio and determines whether or not the engine is being fuelled on the correct stroke by comparing (136) those temporal characteristics with relevant engine operation data (120) stored in the engine management system.