Patent ID: 7127344

Claim:
An engine air-fuel ratio control system comprising: an air-fuel ratio setting section configured to set an air-fuel ratio for an engine based on at least one engine operating condition; an air-fuel ratio sensor detection section configured determine a status of an air-fuel ratio sensor; a target air-fuel ratio revision section configured to set a target air-fuel ratio revising coefficient to richen the air-fuel ratio immediately after the engine is started and afterwards to gradually decrease the air-fuel ratio over time to gradually converge towards a stoichiometric value, with the target air-fuel ratio revising coefficient decreasing at a higher rate upon determining the air-fuel ratio sensor to be active than a prior decreasing rate before determining the air-fuel ratio sensor to be active; and an air-fuel ratio feedback control section configured to set an air-fuel ratio feedback revising coefficient to perform an air-fuel ratio feedback control that converges the air-fuel ratio towards the stoichiometric value based on a signal from the air-fuel ratio sensor when an air-fuel ratio feedback control condition is satisfied, the air-fuel ratio feedback control section being further configured to hold the air-fuel ratio feedback revising coefficient at a reference value for a prescribed amount of time after the air-fuel ratio feedback control condition is satisfied and start the air-fuel ratio feedback control after the prescribed amount of time has elapsed.