AIR-FUEL RATIO CONTROL SYSTEM FOR INTERNAL COMBUSTION ENGINES

An air-fuel ratio control system for an internal combustion has a plurality of cylinders and an exhaust system. The air-fuel ratio of exhaust gases emitted from the cylinders is detected by an air-fuel ratio sensor arranged in the exhaust system. The air-fuel ratio of a mixture supplied to each of the cylinders is estimated based on an output from the air-fuel ratio sensor by using an observer for observing an internal operative state of the exhaust system by means of a model representative of a behavior of the exhaust system. Feedback control of an amount of fuel to be supplied to the each of the cylinders is carried out in response to the output from the air-fuel ratio sensor by using a controller of a recurrence formula type, such that the air-fuel ratio of the mixture supplied to the each of the cylinders is converged to a desired value. The controller of the recurrence formula type adjusts adaptive parameters for use in the feedback control, for the each of the cylinders based on the estimated air-fuel ratio.