Patent ID: 8899014

Claim:
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 that changes an output characteristic of the exhaust-gas sensor by applying a constant current between the pair of electrodes; a rich direction control portion which performs 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 target air-fuel ratio set based on a normal operation condition, after a termination of a fuelling-stop control in which fuel injection of the engine is stopped; and a characteristic control portion which performs a rich responsiveness control, in which the constant current supply portion is controlled to increase a detection responsiveness of the exhaust-gas sensor with respect to rich gas, during the rich direction control, wherein one of the pair of electrodes is exposed to the exhaust gas, the other of the pair of electrodes is exposed to air introduced from an atmosphere, and the constant current supply portion applies the constant current flowing from the other of the pair of electrodes to the one of the pair of electrodes to increase the detection responsiveness of the exhaust-gas sensor with respect to the rich gas in the rich responsiveness control.