Idling speed feedback control method having fail-safe function for

Abstract

An idling speed control method which is adapted to detect by detecting means predetermined rotational angle positions of an internal combustion engine having supplementary air supply control means that regulates the quantity of supplementary air being supplied to the engine, and to control the ratio of operating duty of the supplementary air supply control means to an interval of time at which pulses of a signal indicative of the predetermined rotational angle positions are generated, in a feedback manner responsive to the difference between actual idling engine speed and desired idling speed. When an abnormality is detected in the detecting means, which includes a sensor for sensing the predetermined crank angle positions, a maximum quantity of supplementary air is supplied to the engine by operating the supplementary air quantity control means to its operating limit. The supplementary air supply control means may comprise either (1) a solenoid valve disposed in a supplementary air passage, one end of which communicates with an intake passage of the engine at a location downstream of a throttle valve arranged in the intake passage and the other end communicates with the atmosphere, or (2) a negative pressure-operated means disposed for operating the throttle valve to open or close in response to negative pressure in the intake passage at a zone downstream of the throttle valve, and a negative pressure control means for controlling the magnitude of negative pressure in the intake passage acting upon the negative pressure-operated means.