Air-fuel ratio detecting system

An air-fuel ratio detecting system for internal combustion engines comprising:

• oxygen detecting means disposed in an exhaust passage of an internal combustion engine for detecting an absence and presence of oxygen, said oxygen detecting means exhibiting a rich resistance and a lean resistance in response to said absence and presence of oxygen in said exhaust passage, respectively;
• variable resistance means connected in series with said oxygen detecting means;
• power supply means for supplying a series circuit of said oxygen detecting means and said variable resistance means with an electric power so that said series circuit develops a rich and a lean voltages in response to said rich resistance and said lean resistance of said oxygen detecting means, respectively, at a junction between said oxygen detecting means and said variable resistance means;
• reference comparison means for comparing said rich and lean voltages with a reference value so that an air-fuel ratio of mixture supplied to said internal combustion engine is detected;
• sampling means for sampling a peak value of predetermined one of said rich and lean voltages;
• first comparison means for comparing said sampled peak value with a pedetermined first value;
• second comparison means for comparing said sample peak value with a predetermined second value;
• and
• means for varying the resistance value of said variable resistance means in response to the output signals of said first and second comparison means so that one and the other of said rich and lean voltages are kept larger and smaller than said reference value.