Air-fuel ratio control system for internal combustion engines

An air-fuel control system for use with an internal combustion engine, comprising:

• a catalytic converter disposed in an exhaust system of the internal combustion engine, for purifying an exhaust gas emitted from the internal combustion engine;
• a first exhaust gas sensor disposed in said exhaust system for detecting an air-fuel ratio of the exhaust gas upstream of said catalytic converter;
• a second exhaust gas sensor disposed in said exhaust system for detecting the concentration of a component of the exhaust gas which has passed through said catalytic converter, downstream of said catalytic converter;
• sliding mode control means for determining a correction quantity at a first period to correct the air-fuel ratio of the internal combustion engine so as to equalize the concentration of the component of the exhaust gas downstream of said catalytic converter to a predetermined appropriate value, according to a sliding mode control process based on the output from said second exhaust gas sensor;
• feedback control means for determining a correction quantity at a second period for a rate at which fuel is supplied to the internal combustion engine so as to converge the concentration of the component of the exhaust gas downstream of said catalytic converter toward said predetermined appropriate value, based on said correction quantity to correct the air-fuel ratio and the output from said first exhaust gas sensor, and feedback-controlling said rate at which fuel is supplied to the internal combustion engine with the determined correction quantity;
• said first period and said second period being established separately from each other.