Air-fuel ratio control apparatus and air-fuel ratio control method for internal combustion engine mounted on hybrid vehicle

An air-fuel ratio control apparatus configured to control an air-fuel ratio of air-fuel mixture that is supplied to an internal combustion engine, the internal combustion engine being mounted on a hybrid vehicle as a driving source together with an electric motor, the internal combustion engine including a catalyst in an exhaust passage, the internal combustion engine intermittently repeating a stop and start of its operation in response to an operating state of the hybrid vehicle, comprising:

• an upstream air-fuel ratio sensor configured to generate an output corresponding to an air-fuel ratio of exhaust gas flowing into the catalyst;
• a downstream air-fuel ratio sensor configured to generate an output corresponding to an air-fuel ratio of exhaust gas flowing out from the catalyst;
• a feedback control unit configured to execute air-fuel ratio feedback control of the internal combustion engine on the basis of the output of the upstream air-fuel ratio sensor and the output of the downstream air-fuel ratio sensor such that the air-fuel ratio of air-fuel mixture supplied to the internal combustion engine coincides with a target air-fuel ratio;
• and
• an air-fuel ratio changing unit configured to acquire a catalyst parameter that indicates a state of the catalyst at the time point at which the internal combustion engine is started after the operation of the internal combustion engine is stopped in response to the operating state of the hybrid vehicle, and to change the air-fuel ratio of exhaust gas flowing into the catalyst by changing the target air-fuel ratio on the basis of an acquired catalyst parameter in a predetermined period after the time point at which the internal combustion engine is started.