Air-fuel ratio control method and device for internal combustion engine

A device for controlling an air-fuel ratio of an internal combustion engine, comprising:

• an exhaust purifying catalyst having an oxygen storage capacity which is provided in an exhaust passage of the internal combustion engine;
• an upstream-side air-fuel sensor provided on an upstream side of the exhaust purifying catalyst;
• a downstream-side air-fuel sensor provided on an downstream side of the exhaust purifying catalyst;
• and
• a controller configured to control the air-fuel ratio of the internal combustion engine,
• wherein the controller is configured to:
• estimate an oxygen storage amount of the exhaust purifying catalyst based on an air-fuel ratio detected by the upstream-side air-fuel ratio sensor, and control the air fuel ratio of the internal combustion engine such that the estimated oxygen storage amount matches a target oxygen storage amount;
• and
• reset the estimated oxygen storage amount to a predetermined first oxygen storage amount which is smaller than the target oxygen storage amount when it is detected that the oxygen storage amount of the exhaust purifying catalyst is equal to or smaller than the first oxygen storage amount based on an air-fuel ratio detected by the downstream-side air-fuel ratio sensor, and reset the estimated oxygen storage amount to a predetermined second oxygen storage amount which is larger than the target oxygen storage amount when it is detected that the oxygen storage amount of the exhaust purifying catalyst is equal to or larger than the second oxygen storage amount based on the air-fuel ratio detected by the downstream-side air-fuel ratio sensor,
• wherein the target oxygen storage amount is set within a range where the oxygen storage amount is smaller than a median of the first oxygen storage amount and the second oxygen storage amount.