Patent ID: 7181331

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 based on at least a stabilization fuel quantity increasing factor that is set 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 stabilization fuel quantity increasing factor 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 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, and to start an air-fuel ratio feedback control upon the air-fuel ratio approximately reaching the stoichiometric value, the target air-fuel ratio revision section being further configured to revise the target air-fuel ratio revising coefficient when the air-fuel ratio feedback control is started by adding an unburned fuel quantity compensating value that is set based on the stabilization fuel quantity increasing factor in effect at that point in time to the target air-fuel ratio revising coefficient while, simultaneously, setting the stabilization fuel quantity increasing factor to zero.