Method and apparatus for controlling the air-fuel ratio of an internal combustion engine

An object of the present invention is to be able to correct for deviation of an actual air-fuel ratio from a point for satisfactory purification performance of an exhaust gas purification catalytic converter, during an interval from immediately after start up until an air-fuel ratio sensor and an exhaust gas purification catalytic converter attain a stable condition. To achieve this, a control constant in an air-fuel ratio feedback control is set and altered during an interval from immediately after start up until the air-fuel ratio sensor and the exhaust gas purification catalytic converter attain a stable condition. In this way, the actual air-fuel ratio can be shifted, and hence the deviation of the actual air-fuel ratio from the point for satisfactory purification performance of the exhaust gas purification catalytic converter during the interval from immediately after start up until the air-fuel ratio sensor and the exhaust gas purification catalytic converter attain a stable condition, can be corrected. Consequently, the situation immediately after start up where the actual air-fuel ratio deviates from the target air-fuel ratio, attributable to the air-fuel ratio sensor and the exhaust gas purification catalytic converter being in an unstable condition, can be suppressed. Therefore the purification efficiency of the exhaust gas purification catalytic converter can be kept high.