AIR-FUEL RATIO CONTROL FOR INTERNAL COMBUSTION ENGINE

PURPOSE:To prevent the deterioration of a catalytic converter by determining the selection timing from feedback control to opened loop control according to the ratio between the number of times of combustion in the high load state over the standard value of an engine and the number of times of combustion less than a standard value. CONSTITUTION:A control circuit 20 determines the fuel injection timing of an injector 15 on the basis of the throttle opening degree detected by sensors 10-14, 16, absolute intake pressure, cooling water temperature, crank angular speed, O2 concentration, and atmospheric pressure. In this case, feedback control is performed so that a prescribed air-fuel ratio is maintained on the basis of the O2 concentration, and opened loop control for obtaining the rich air-fuel ratio is performed in a prescribed high load state. In this case, a prescribed high load standard value is set, and the selection timing from feedback control to opened loop control is determined according to the ratio between the number of times of combustion related to the temperature of a catalytic converter 9 in the case when an engine 4 is over the above-described standard value and the number of times of combustion less than a standard value. Therefore, the deterioration of the catalytic converter can be prevented, maintaining the high purifying action.