Air-fuel ratio control system for internal combustion engines

摘要

An air-fuel ratio control system for an internal combustion engine has an air-fuel ratio sensor arranged in the exhaust system of the engine, for detecting the air-fuel ratio of exhaust gases emitted from the engine, and various engine operating parameter sensors for detecting operating conditions of the engine. An ECU controls the air-fuel ratio of an air-fuel mixture to be supplied to the engine to a desired air-fuel ratio in a feedback manner responsive to an output from the air-fuel ratio sensor. A first learned value for correcting the desired air-fuel ratio is calculated based on the feedback control when the engine is detected to be in a first predetermined operating condition, and a second learned value for correcting the desired air-fuel ratio when the engine is detected to be in a second predetermined engine operating condition. The air-fuel ratio of the air-fuel mixture is controlled to a value leaner than a stoichiometric value, by using the first learned value. The first learned value is learned again when the difference between the first and the second learned values is larger than a predetermined re-learning-determining threshold value.