Air-fuel ratio control apparatus for an internal combustion engine and a control method of the air-fuel ratio control apparatus

An apparatus for controlling an air-fuel ratio in an internal combustion engine comprising:

• an oxygen sensor in an exhaust passage of the internal combustion engine for detecting a concentration of oxygen in exhaust gas from said internal combustion engine;
• a first operational means for calculating an air-fuel ratio feedback correction value based on a value outputted by said oxygen sensor so that an actual air-fuel ratio in said internal combustion engine is equal to a target air-fuel ratio;
• a second operational means for calculating an air-fuel ratio learning value so that the air-fuel ratio feedback correction value is within a predetermined range, the air-fuel ratio learning value being different from the air-fuel ratio feedback correction value;
• a maximum and minimum setting means for setting a maximum limit and a minimum limit of the air-fuel ratio learning value;
• an air-fuel ratio correcting means for correcting a fuel injection time of a fuel injection valve in an intake passage of said internal combustion engine based on the air-fuel ratio feedback correction value calculated by said first operational means and the air-fuel ratio learning value calculated by said second operational means;
• and
• a maximum and minimum magnifying means for increasing the maximum or decreasing the minimum limit of the air-fuel ratio learning value by stepping degrees in response to a deviation of fuel injection amount, injected by the fuel injection valve, from a fuel injection amount requested for maintaining the target air-fuel ratio after the air-fuel ratio learning value reaches the maximum or minimum limit.