Air-fuel ration control system for multi-cylinder internal combustion engine

An air-fuel ratio control system for a multi-cylinder internal combustion engine having a plurality of cylinders, comprising:

• an exhaust emission control catalyst disposed in an exhaust passage of the internal combustion engine;
• air-fuel ratio control means for individually controlling an air-fuel ratio in each of at least two of the internal combustion engine cylinders, wherein the air-fuel ratio control means controls the air-fuel ratio in at least one of the at least two cylinders to be different from the air-fuel ratio in at least one of the other cylinders of the internal combustion engine;
• and
• catalyst temperature detection means for detecting a temperature of the exhaust emission control catalyst;
• catalyst temperature determination means for determining whether the temperature of the exhaust emissions control catalyst detected by the catalyst temperature detection means has reached a predetermined temperature;
• and
• operation control means for controlling, by means of the air-fuel ratio control means, air-fuel ratios in each of the respective cylinders so as to achieve, until the temperature of the exhaust emission control catalyst detected by the catalyst temperature detection means reaches the first predetermined temperature during a cold start of the internal combustion engine, the air-fuel ratio in each cylinder so as to achieve one of rich-state operation and stoichiometric operation in all the cylinders, and wherein after the temperature of the exhaust emission control catalyst has exceeded the first predetermined temperature, the air-fuel ratio control means controls the air-fuel ratio in each cylinder so as to achieve a lean-state operation in a first portion of the plurality of cylinders and a rich-state operation in a second portion of the plurality of cylinders.

An air-fuel ratio control method for a multi-cylinder internal combustion engine having a plurality of cylinders, comprising the steps of:

• detecting a temperature of an exhaust emission control catalyst disposed in an exhaust passage of the internal combustion engine;
• determining, during a cold start of the internal combustion engine, whether the temperature of the exhaust emission control catalyst has reached a first predetermined temperature;
• and
• performing an individual control of an air-fuel ratio in each of at least two of the internal combustion engine cylinders such that, if the temperature of the exhaust emission control catalyst is lower than the first predetermined temperature, an upper limit of the air-fuel ratio of at least one of the at least two cylinders is reduced relative to an upper limit of an air-fuel ratio in one of the other cylinders and in comparison with an upper limit of the air-fuel ratio of the at least one of the at least two cylinders used if the temperature of the exhaust emission control catalyst is equal to or higher than the first predetermined temperature.