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

An air-fuel ratio control method of an internal combustion engine, comprising the steps of:

• detecting the concentration of a predetermined component in the exhaust gas in the engine;
• detecting the operating condition of the engine to discriminate whether or not the engine is in a predetermined first operating condition;
• calculating an air-fuel ratio correction coefficient depending upon said detected concentration, when the engine is in the first operating condition;
• holding an air-fuel ratio correction coefficient to a value, when the engine is not in the first operating condition;
• detecting the operating condition of the engine to discriminate whether or not the engine is in a predetermined second operating condition which is included within said first operating condition;
• calculating an average value of said calculated air-fuel ratio correction coefficient only when the engine is in the second operating condition;
• and
• correcting the amount of fuel supplied to the engine in accordance with said air-fuel ratio correction coefficient, said correcting being performed by a closed-loop control when the engine is in the first operating condition and performed by an open-loop control when the engine is not in the first operating condition, an initial value of the air-fuel ratio correction coefficient when said fuel correction changes from open-loop control to closed-loop control being determined according to said calculated average value.