Air-fuel ratio control system

In an air-fuel ratio control system for an internal combustion engine having an intake passage, an exhaust passage, an air-fuel mixture supply means, an on-off type electromagnetic valve for correcting the air-fuel ratio of the air-fuel mixture supplied by said air-fuel mixture supply means, dither signal generating circuit means for producing a periodical dither signal, a shift control circuit means for shifting the level of the center of said dither signal, driving circuit means, operatively connected to said shift control circuit means, for producing a driving output for said on-off type electromagnetic valve, and an O.sub.2 sensor means for detecting the concentration of oxygen in exhaust gases passing through said exhaust passage and producing an output waveform of output voltage, the improvement comprising

• first circuit means for detecting a middle value between the maximum value and the minimum value of the output voltage of the O.sub.2 sensor means in each cycle;
• second circuit means for detecting a middle value of the output waveform of said O.sub.2 sensor means between upper and lower portions thereof with respect to a predetermined reference value;
• and
• shift signal generating circuit means for comparing outputs of said first and second circuit means for generating a difference of said outputs as a shift signal;
• said shift control circuit means for controlling the air-fuel ratio of the mixture in such a direction that said difference is decreased.