Abstract: An engine rotation speed control system which comprises an actuator for prescribing the recovery position of a throttle valve, a recovery position sensor for detecting that the throttle valve is returned to the recovery position, an idle speed control unit for controlling the engine to a predetermined idling speed, and a negative pressure restricting unit for controlling the throttle actuator such that reduction in intake negative pressure to below a predetermined negative pressure is prevented. When the throttle valve returns to the recovery position with the engine speed in excess of an upper limit of the idling speed, the actuator is controlled by the negative pressure restricting unit. When the engine speed falls below the upper limit, the idle speed control unit is actuated. Upon engine starting, the actuator is set such that the opening degree of the throttle valve suitable for the engine starting is obtained before the starter starts operating.

Abstract: Method of controlling a control system for an air-fuel ratio of air-fuel mixture supplied to an internal combustion engine so that the proper air-fuel ratio is obtained over the whole operation range including a normal operation region and an accelerating operation mode independently of a decrease in oxygen contents at places of high altitudes. When the accelerating operation region is sensed, a proper power duty, corresponding to a negative pressure at that time, is read out from a data table stored in a read-only memory and added to a duty pulse signal which is then utilized for controlling a fuel supply actuator.

Abstract: An engine rotation speed control system which comprises an actuator for prescribing the recovery position of a throttle valve, a recovery position sensor for detecting that the throttle valve is returned to the recovery position, an idle speed control unit for controlling the engine to a predetermined idling speed, and a negative pressure restricting unit for controlling the throttle actuator such that reduction in intake negative pressure to below a predetermined negative pressure is prevented. When the throttle valve returns to the recovery position with the engine speed in excess of an upper limit of the idling speed, the actuator is controlled by the negative pressure restricting unit. When the engine speed falls below the upper limit, the idle speed control unit is actuated. Upon engine starting the actuator is set such that the opening degree of the throttle valve suitable for the engine starting is obtained before the starter starts operating.

Abstract: A method of controlling the air-fuel ratio so as to be effectively maintained at a desired air-fuel ratio of .lambda.=1 is composed of a combination of an electronic map control for effecting the air-fuel ratio control on the basis of map data read out from a data map stored in ROM as air-fuel ratio control data in correspondence to various operation parameters of an engine and an O.sub.2 -feedback control for controlling the air-fuel ratio on the basis of oxygen quantity detected from the exhaust gas. Upon the occurrence of changes in the control quantity for a predetermined number of times in the course of the O.sub.2 -control, the latter is changed over to the map control. At that time point, a control quantity for the map control is corrected on the basis of a mean control quantity during the O.sub.2 -feedback control. Unless a significant variation takes place in the engine operating conditions, the map control is continued for a predetermined time and then changed over to the O.sub.2 -control.