Air-fuel ratio control system

Claim 1

In an air-fuel ratio control system for an internal combustion engine of the type having an intake passage, an exhaust passage, an air-fuel mixture supply device, an on-off type electromagnetic valve means for correcting the air-fuel ratio of the air-fuel mixture supplied by said air-fuel mixture supply device, O 2 sensor means for detecting the concentration of oxygen in exhaust gases passing through said exhaust passage, the improvement comprising a dither signal generating circuit means for producing a periodical dither signal having an unchanging wave form pattern, a shift control circuit means for shifting the level of a center of said dither signal in dependency on the concentration of oxygen detected by the O 2 sensor means, driving circuit means operatively connected to said shift control circuit means and for producing a driving output for said on-off type electromagnetic valve means, means for detecting a transient state of the engine operation by a variation in a waveform corresponding to the dither signal, said O 2 sensor means for generating said waveform by detecting said concentration of oxygen in the exhaust gases passing through said exhaust passage, a control circuit including an integrating circuit and a square wave generating circuit means for generating square pulses according to the integration output of the integrating circuit for operatively driving said electromagnetic valve means, and switching means for operatively connecting said O 2 sensor means to said driving circuit means via said control circuit and for disconnecting said O 2 sensor means from said driving circuit means via said shift control circuit means when said means for detecting detects the transient state.