Air-fuel ratio control system for internal combustion engine having improved air-fuel ratio-shift correction method

Claim 1

An air-fuel ratio control system for an internal combustion engine, wherein the engine includes an evaporative fuel purge system including a canister for temporarily storing fuel vapor, the air-fuel ratio control system comprising: air to fuel ratio detecting means for detecting an air-fuel ratio in exhaust gas discharged from the engine; air-fuel ratio control means for controlling the air-fuel ratio in exhaust gas from the engine by varying a fuel quantity supplied to the engine so that the air-fuel ratio approaches a predetermined target air-fuel ratio; wherein the evaporative fuel purge system includes purging means for purging evaporated fuel from the canister to the engine, the purging being executed at a purging rate determined by the purging means; air-fuel ratio-shift correction means for optimizing the air-fuel ratio control by deriving a first component of a detected air-fuel ratio-shift amount, which first component is an amount by which the air-fuel ratio has shifted from the target air-fuel ratio due to a cause independent of the purging executed by the purging means, wherein the first component is relatively constant over time in comparison to a second component of the detected air-fuel ratio shift amount occurring as a result of the purging operation executed by said purging means; and wherein the air-fuel ratio-shift correction means derives the first component based on a first detected air-fuel ratio-shift amount and a second detected air-fuel ratio-shift amount, wherein the first detected air-fuel ratio-shift amount is detected by the air to fuel ratio detecting means when the purging means is purging at a first purging rate and wherein the second air-fuel ratio-shift amount is detected by the air to fuel ratio detecting means when the purging means is purging at a second purging rate different from the first purging rate.