Air-fuel ratio control apparatus for internal combustion engine and air-fuel ratio control method therefor

An air-fuel ratio control apparatus configured to control an air-fuel ratio of exhaust gas flowing into an exhaust gas control catalyst provided in an exhaust passage of an internal combustion engine and having an oxygen storage capacity, the air-fuel ratio control apparatus comprising a processor configured to control the air-fuel ratio of the exhaust gas flowing into the exhaust gas control catalyst, wherein:

• the processor is configured to alternately repeat a rich process for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust gas control catalyst to reach a rich air-fuel ratio that is richer than a stoichiometric air-fuel ratio and a lean process for controlling the air-fuel ratio of the exhaust gas flowing into the exhaust gas control catalyst to reach a lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio;
• and
• the processor is configured to, during the rich process, execute periodically, in a predetermined cycle, a lean pulse process for controlling, over a period shorter than a period of one execution of the lean process, the air-fuel ratio of the exhaust gas flowing into the exhaust gas control catalyst to reach a lean air-fuel ratio having a higher lean level than the air-fuel ratio during the lean process.