Patent ID: 7430854

Claim:
A method for controlling an air-fuel ratio of an internal combustion engine, comprising the steps of: receiving an output of an exhaust gas sensor that detects oxygen concentration of exhaust gas; calculating model parameters for a model of an object controlled by the air-fuel ratio control based on the output of the exhaust gas sensor, the controlled object being a system including a catalyst and the exhaust gas sensor in an exhaust manifold of the engine; determining a desired air-fuel ratio with use of the model parameters so that the output of the exhaust gas sensor converges to a desired value; controlling the air-fuel ratio based on the desired air-fuel ratio; stopping the calculation of the model parameters when the engine is operating with a lean air-fuel ratio and during a predetermined period after the engine stops operating with a lean air-fuel ratio; and during the stopping of the calculation, continuing the determination of the desired air-fuel ratio, with use of the model parameters that were last calculated by the identifier before the engine started the operation with a lean air-fuel ratio, wherein both the continuing the determination of the desired air-fuel ratio and the stopping of the identifier are performed in the same way for all cylinders in the engine.