Exhaust gas control device for internal combustion engine and control method thereof

A control method for an exhaust gas control device for an internal combustion engine, the exhaust gas control device including a catalyst configured to purify exhaust gas discharged from a plurality of cylinders of the internal combustion engine and an electronic control unit, the control method comprising:

• i) acquiring an operation state of the internal combustion engine by the electronic control unit;
• ii) performing catalyst regeneration control by the electronic control unit, the catalyst regeneration control being control of increasing a temperature of the catalyst to regenerate the catalyst by setting a target air-fuel ratio of at least one cylinder among the plurality of cylinders to a rich air-fuel ratio which is lower than a stoichiometric air-fuel ratio and setting target air-fuel ratios of the other cylinders among the plurality of cylinders to a lean air-fuel ratio which is higher than the stoichiometric air-fuel ratio;
• iii) estimating, by the electronic control unit, the temperature of the catalyst based on the acquired operation state of the internal combustion engine and a difference between the lean air-fuel ratio and the rich air-fuel ratio which are set as the target air-fuel ratios during execution of the catalyst regeneration control;
• iv) determining, by the electronic control unit, whether or not the estimated temperature of the catalyst is higher than a threshold value during execution of the catalyst regeneration control;
• and
• v) prohibiting, by the electronic control unit, the catalyst regeneration control when the electronic control unit determines that the estimated temperature of the catalyst is higher than the threshold value during execution of the catalyst regeneration control.