Exhaust emission control system for internal combustion engine

摘要

An exhaust emission control system for a nitrogen oxide removing device, the control system having a first oxygen concentration sensor provided upstream of the nitrogen oxide removing device, a second oxygen concentration sensor provided downstream of the nitrogen oxide removing device. A first response delay time period from the time when the output value from the first oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio to the time when the output value from the second oxygen concentration sensor becomes a value indicative of a rich air-fuel ratio is measured. A second response delay time period from the time when the output value from the first oxygen concentration sensor has changed to a value indicative of a lean air-fuel ratio, to the time when the output value from the second oxygen concentration sensor becomes a value indicative of a lean air-fuel ratio is measured. Deterioration of the nitrogen oxide removing device is determined according to the first and second response delay time periods.