Air-fuel ratio control system

Abstract

A system for controlling air-fuel ratio for a carburetor for an internal combustion engine having an induction passage, a throttle valve, an exhaust passage, an exhaust gas detector such as oxygen sensor for detecting the concentration of oxygen in the exhaust gases, air-fuel mixture supply means, an on-off electromagnetic valve for correcting the air-fuel ratio of the air-fuel mixture supplied by the air-fuel mixture supply means and an electronic controller. The electronic controller comprises a comparing circuit for comparing the output signal of the detector with a stoichiometric reference, an integration circuit connected to the comparing circuit, a triangular wave pulse generator, and a driving circuit for producing square wave pulses for driving the on-off electromagnetic valve means from the output signal of the integration circuit and of the triangular wave pulse generator for controlling the air-fuel ratio to a value approximately equal to the stoichiometric air-fuel ratio. A constant signal generating circuit is selectively connected operatively to the electronic controller. An idling operation detector produces a signal during idling operation. A switching unit operatively connects the contant signal generating circuit to the electronic controller and renders the electronic control means non-responsive to the output of the exhaust gas detector. The switching unit is adapted to be operated only after a predetermined time delay when the duty ratio of the square wave pulses reaches the maximum or minimum value during the idling operation.