Engine air-fuel ratio control system

An engine air-fuel ratio control system comprising:

• means for setting a air-fuel ratio for an engine based on at least one engine operating condition;
• air-fuel ratio sensor detection means for determining a status of an air-fuel ratio sensor;
• target air-fuel ratio revision means for setting 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
• air-fuel ratio feedback control means for setting 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 means further holding 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.

A method of controlling an engine air-fuel ratio comprising:

• setting the air-fuel ratio for an engine based on at least one engine operating condition;
• determining a status of an air-fuel ratio sensor;
• setting 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;
• setting 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;
• and
• holding 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 starting the air-fuel ratio feedback control after the prescribed amount of time has elapsed.