Emission control system for internal combustion engine

An emission control system for an internal combustion engine, comprising:

• a catalyst used for purification of exhaust gas discharged from the engine;
• an exhaust-gas sensor provided downstream of the catalyst in a flow direction of the exhaust gas to detect an air-fuel ratio of the exhaust gas or detect whether the exhaust gas is rich or lean, the exhaust-gas sensor including a sensor element that includes a pair of electrodes and a solid electrolyte body located between the pair of electrodes;
• a constant current supply portion configured to change an output characteristic of the exhaust-gas sensor by applying a constant current between the pair of electrodes;
• a catalytic-state determination portion configured to determine whether the catalyst is in a rich state or in a lean state;
• a rich direction control portion configured to perform a rich direction control, in which an air-fuel ratio of the exhaust gas flowing into the catalyst is made to be richer than a normal target air-fuel ratio set based on a normal operation condition, when the catalytic-state determination portion determines that the catalyst is in the lean state, the rich direction control portion being configured to terminate the rich direction control when the catalytic-state determination portion determines that the catalyst is in the rich state after a start of the rich direction control;
• a lean direction control portion configured to perform a lean direction control, in which the air-fuel ratio of the exhaust gas flowing into the catalyst is made to be leaner than the normal target air-fuel ratio set based on the normal operation condition, after the rich direction control portion terminates the rich direction control;
• and
• a characteristic control portion configured to perform a lean responsiveness control, in which the constant current supply portion is controlled to set a flow direction of the constant current so as to increase a detection responsiveness of the exhaust-gas sensor with respect to lean gas, at least during the lean direction control.