Emission control system for internal combustion engine

An emission control system for an internal combustion engine,

• a catalyst used for purification of exhaust gas discharged from the engine;
• an upstream exhaust-gas sensor provided upstream of the catalyst in a flow direction of the exhaust gas to detect an upstream air-fuel ratio of the exhaust gas flowing upstream of the catalyst or to detect whether the exhaust gas is rich or lean;
• a main feedback control portion which corrects a fuel injection amount of the engine based on an output value of the upstream exhaust-gas sensor in a main feedback control so that the upstream air-fuel ratio becomes equal to an upstream target air-fuel ratio;
• a downstream exhaust-gas sensor provided downstream of the catalyst in the flow direction of the exhaust gas to detect a downstream air-fuel ratio of the exhaust gas flowing downstream of the catalyst or to detect whether the exhaust gas is rich or lean, wherein the downstream exhaust-gas sensor includes a sensor element that includes a pair of electrodes and a solid electrolyte body located between the pair of electrodes;
• a sub feedback control portion which corrects the upstream target air-fuel ratio or the fuel injection amount based on an output value of the downstream exhaust-gas sensor in a sub feedback control so that the downstream air-fuel ratio becomes equal to a downstream target air-fuel ratio;
• a constant current supply portion which changes an output characteristic of the downstream exhaust-gas sensor by applying a constant current on the pair of electrodes and setting a flow direction of the constant current between the pair of electrodes;
• and
• a characteristic control portion which controls the constant current supply portion, wherein
• the characteristic control portion controls the constant current supply portion to apply the constant current on the pair of electrodes so as to advance a timing of a lean detection of the downstream exhaust-gas sensor, when the downstream air-fuel ratio is richer than the downstream target air-fuel ratio, and when the sub feedback control portion corrects the downstream air-fuel ratio such that the downstream air-fuel ratio becomes leaner in the sub feedback control, and
• the characteristic control portion controls the constant current supply portion to apply the constant current on the pair of electrodes so as to advance a timing of a rich detection the downstream exhaust-gas sensor, when the downstream air-fuel ratio is leaner than the downstream target air-fuel ratio, and when the sub feedback control portion corrects the downstream air-fuel ratio such that the downstream air-fuel ratio becomes richer in the sub feedback control.