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, a detector such as an oxygen sensor for detecting the concentration of oxygen in the exhaust gases, air-fuel mixture supply unit, an on-off electromagnetic valve for correcting the air-fuel ratio of the air-fuel mixture supplied by the air-fuel mixture supply unit and electronic controller. The electronic controller comprises a comparator for comparing the output signal of the detector with a set value, an integrating circuit connected to the comparator, a triangular wave pulse generator, and a driving circuit for producing square wave pulses for driving the on-off electromagnetic valve(s) for output signals of the integrating 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 engine operation detector for detecting a wide open condition of the throttle valve produces a signal during the wide open condition. A switching circuit operatively connects the constant signal generating circuit to the electronic controller for providing a fixed duty ratio of the valve, and renders the electronic controller non-responsive to the output of the exhaust gas detector. The switching circuit is adapted to be operated by the signal of the engine operation detector. A delay circuit is provided to delay the operation of the switching circuit. Thus, the on-off electromagnetic valve is operated at a fixed duty ratio after the wide open condition continues a predetermined period.