Abstract: In a water-cooled engine comprising a catalyst for purifying exhaust in an exhaust pipe, a catalyst is rendered active at an early stage by arranging an air-fuel ratio of a fuel mixture supplied to the engine to be lean on startup. A catalyst activation water temperature corresponding to full activation of this catalyst is set, and the air-fuel ratio is arranged to be leaner than a theoretical air-fuel ratio during a period from engine startup to when a cooling water temperature reaches the catalyst activation water temperature. After the cooling water temperature reaches the catalyst activation water temperature, the air-fuel ratio is shifted to the stoichiometric air-fuel ratio. In this way, lean control is optimized to activate the catalyst.

Abstract: The determining precision of catalyst activation, which is the basis for stopping a secondary air supply to accelerate catalyst activation and start air-fuel ratio feedback control, is enhanced. An engine cooling water temperature is detected, and a catalyst activation water temperature is determined from an engine startup water temperature based on predetermined characteristics. The catalyst activation water temperature is set to be lower the lower the startup cooling water temperature, and is set such that a difference between a predetermined temperature and an engine startup cooling water temperature is larger the lower the startup cooling water temperature. In this way, catalyst activation is determined with high precision according to the startup water temperature and subsequent cooling water temperature variation.