Air fuel ratio controller for internal combustion engine for stopping calculation of model parameters when engine is in lean operation

An air fuel ratio controller for an internal combustion engine comprises an exhaust gas sensor, an identifier and a control unit. The exhaust gas sensor detects oxygen concentration of exhaust gas. The identifier calculates model parameters for a model of a controlled object based on the output of the exhaust gas sensor. The controlled object includes an exhaust system of the engine. The control unit is configured to use the model parameters to control the air-fuel ratio so that the output of the exhaust gas sensor converges to a desired value, and to stop the identifier from calculating the model parameters during and immediately after the engine operation with a lean air-fuel ratio. The calculation of the model parameters may be also stopped during and immediately after fuel-cut operation that stops fuel supply to the engine. Such a stop of the calculation of the model parameters reduces the emission of undesired substances contained in exhaust gas when the engine shifts from lean operation to stoichiometric/rich operation.