Patent ID: 7712459

Claim:
An air-fuel ratio control system for an internal combustion engine, comprising: an air-fuel ratio sensor disposed upstream or downstream of an exhaust purification catalyst provided in an engine exhaust passage to detect an air-fuel ratio of exhaust gas, the system being configured to perform feedback control of a fuel supply amount such that an output value of the air-fuel ratio sensor is controlled to a target air-fuel ratio, wherein the feedback control is performed by calculating a correction amount by summing up a value of a proportional and a value of an integral calculated based on a deviation between the output value of the air-fuel ratio sensor and the target air-fuel ratio, and correcting the fuel supply amount based on the obtained correction amount, and, at cold startup of the internal combustion engine, from the startup of the internal combustion engine until a predetermined period elapses, the value of the integral calculated based on the deviation is set to a value smaller than the value of the integral calculated based on the same deviation during normal operation, even when the air-fuel ratio sensor is activated during a startup fuel amount increase control which is performed at cold startup of the internal combustion engine, in which startup fuel amount increase control the fuel supply amount is increased compared to that during normal operation, the predetermined period being longer than a period from the startup of the internal combustion engine until the air-fuel ratio sensor is activated.