Air-fuel ratio control system for internal combustion engines

An air-fuel ratio control system for an internal combustion engine having an exhaust system, comprising:

• air-fuel ratio-detecting means arranged in said exhaust system, for detecting an air-fuel ratio of exhaust gases emitted from said engine;
• operating condition-detecting means for detecting operating conditions of said engine;
• feedback control means for controlling an air-fuel ratio of an air-fuel mixture to be supplied to said engine to a desired air-fuel ratio in a feedback manner responsive to an output from said air-fuel ratio-detecting means;
• first learned value-calculating means for calculating a first learned value for correcting said desired air-fuel ratio, based on an output from said feedback control means, when said engine is detected to be in a first predetermined operating condition by said operating condition-detecting means;
• second learned value-calculating means for calculating a second learned value for correcting said desired air-fuel ratio, based on said output from said feedback control means, when said engine is detected to be in a second predetermined engine operating condition by said operating condition-detecting means;
• leaning control means for controlling said air-fuel ratio of said air-fuel mixture to a value leaner than a stoichiometric value, by using said first learned value;
• and
• re-learning means for re-learning said first learned value when a difference between said first learned value and said second learned value is larger than a predetermined re-learning-determining threshold value.