ENGINE AIR-FUEL RATIO CONTROL

A fuel injector (5) injects fuel to generate an air-fuel mixture that is burnt in the engine (1), and a sensor (16) detects an air-fuel ratio of the air-fuel mixture from an exhaust gas composition of the engine (1). A controller (11) calculates a dead time representing a lag between the air-fuel ratio variation of the air-fuel mixture and the air-fuel ratio variation detected by the sensor (16), and calculates an estimated state quantity by applying a dead time compensation according to Smith method and a disturbance compensation to the air-fuel ratio detected by the sensor (16). By determining an air-fuel ratio feedback correction amount based on the estimated state quantity applying a sliding mode control process, the response and stability of air-fuel ratio control is improved.