Patent ID: 8406980

Claim:
An air-fuel ratio control method for an internal combustion engine that includes: an upstream air-fuel ratio sensor that is arranged on an exhaust upstream side of an exhaust purification catalyst provided within an engine exhaust passage and detects an air-fuel ratio of exhaust gas; and a downstream air-fuel ratio sensor that is arranged on an exhaust downstream side of the exhaust purification catalyst and detects an air-fuel ratio of exhaust gas, the air-fuel ratio control method comprising: executing a main feedback control that controls a fuel supply amount on the basis of an output value of the upstream air-fuel ratio sensor so that an exhaust air-fuel ratio becomes a target air-fuel ratio; and executing a sub-feedback control that compensates for an error between the output value of the upstream air-fuel ratio sensor and an actual exhaust air-fuel ratio by correcting the fuel supply amount on the basis of an output value of the downstream air-fuel ratio sensor so that the exhaust air-fuel ratio becomes the target air-fuel ratio, wherein when executing the sub-feedback control, a correction amount for the fuel supply amount is calculated on the basis of a value of an integral term that integrates a deviation between the output value of the downstream air-fuel ratio sensor and the target air-fuel ratio; and when a fuel increase or decrease control that increases or decreases the fuel supply amount irrespective of the target air-fuel ratio is executed, updating of the value of the integral term in the sub-feedback control is stopped for a predetermined period after completion of the fuel increase or decrease control; the method further comprising: calculating a learned value, which corresponds to a steady-state error between the output value of the upstream air-fuel ratio sensor and the actual exhaust air-fuel ratio, based on the integral term, and correcting the fuel supply amount based on the calculated learned value; and calculating the learned valued even during the predetermined period after completion of the fuel increase or decrease control.