Exhaust gas control apparatus for internal combustion engine and exhaust gas control method for the same

An exhaust gas control apparatus for an internal combustion engine, the exhaust gas control apparatus comprising:

• a catalyst disposed in an exhaust passage of the internal combustion engine and configured to store oxygen;
• a downstream air-fuel ratio sensor configured to detect an air-fuel ratio of outgoing exhaust gas flowing out of the catalyst;
• an air-fuel ratio control unit configured to control an air-fuel ratio of incoming exhaust gas flowing into the catalyst;
• and
• a catalyst state estimation unit that is configured to calculate a cumulative desorption amount by accumulating an amount of HC desorbed from the catalyst per unit time and configured to estimate activity of the catalyst based on the cumulative desorption amount, wherein
• the air-fuel ratio control unit is configured to control the air-fuel ratio of the incoming exhaust gas such that the air-fuel ratio of the outgoing exhaust gas detected by the downstream air-fuel ratio sensor is maintained at a target air-fuel ratio, and
• the air-fuel ratio control unit is configured to set the target air-fuel ratio to a richer value when the cumulative desorption amount is equal to or larger than a predetermined value than when the cumulative desorption amount is less than the predetermined value.