Air-fuel ratio control apparatus for internal combustion engine

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

• an upstream sensor provided on an upstream side of an exhaust emission control catalyst in an exhaust system of a multi-cylinder internal combustion engine and configured to detect an air-fuel ratio state based on an exhaust component,
• a downstream sensor provided on a downstream side of the exhaust emission control catalyst in the exhaust system and configured to detect the air-fuel ratio state based on the exhaust component;
• and
• a controller configured to control the internal combustion engine, the controller being programmed to perform:
• main feedback control controlling a fuel supply amount so as to make an exhaust air-fuel ratio equal to a target air-fuel ratio based on an output value from the upstream sensor;
• sub feedback control allowing correction of the fuel supply amount so as to make an exhaust air-fuel ratio equal to the target air-fuel ratio using a correction amount set based on an output value from the downstream sensor;
• correction amount guard control allowing adjustment of the correction amount by setting a limit on the correction amount when an appearance frequency of a state where the output value from the downstream sensor is leaner than a predetermined value is equal to or higher than a predetermined value;
• learning control allowing calculation of a learning value corresponding to a constant deviation between the output value from the upstream sensor and an actual exhaust air-fuel ratio in such a manner that the learning value incorporates at least a part of the correction amount and allowing correction of the fuel supply amount based on the calculated learning value;
• sensor abnormality detection control allowing detection of abnormality in the downstream sensor based on the learning value calculated by the learning control and the output value from the downstream sensor;
• and
• imbalance determination control allowing determination of air-fuel ratio imbalance among cylinders based on the output values from the upstream sensor and the downstream sensor,
• wherein the controller is further programmed to:
• set an incorporation speed at which, during the learning control, the correction amount is incorporated into the learning value to a first speed when a state where the output value from the downstream sensor is leaner than the predetermined value lasts for a duration shorter than a predetermined time, and
• set the incorporation speed to a second speed higher than the first speed and suppress performance of the correction amount guard control until the learning control is completed, when the duration is equal to or longer than the predetermined time.