AIR-FUEL RATIO CONTROL SYSTEM FOR INTERNAL COMBUSTION ENGINES

An air-fuel control system for use with an internal combustion engine has a catalytic converter, a first exhaust gas sensor, and a second exhaust gas sensor. A sliding mode controller determines a correction quantity to equalize the concentration of the component downstream of the catalytic converter to a predetermined appropriate value based on the output from the second exhaust gas sensor. A feedback controller determines a correction quantity to converge the concentration of the component downstream of the catalytic converter toward the predetermined appropriate value, and feed-back controls the rate at which fuel is supplied to the engine with the determined correction quantity. The sliding mode controller has a hyperplane setting unit for establishing a hyperplane for the sliding mode control process with a linear function having as variables a plurality of quantities of the exhaust system which include at least the concentration, detected by the second exhaust gas sensor, of the component of the exhaust gas downstream of the catalytic converter and a rate of change of the concentration, and a calculator for determining the correction quantity to correct the air-fuel ratio of the internal combustion engine so as to converge the state quantities onto the established hyperplane and also to converge the state quantities toward a balanced point on the established hyperplane while converging the state quantities onto the hyperplane. The hyperplane setting unit variably establishes the hyperplane depending on the manner in which the state quantities converge onto the hyperplane.