Exhaust emission control system and method

摘要

An exhaust emission control system for an internal combustion engine is provided which includes a NOx storage/reduction catalyst disposed in an exhaust passage of the engine, and a NOx sensor positioned in the exhaust passage downstream of the catalyst. A controller of the system performs a rich-spike operation to temporarily operate the engine at a rich air/fuel ratio each time a NOx storage state of the catalyst satisfies a predetermined rich-spike condition during a lean-burn operation. When the degree of degradation of the catalyst is determined by comparing the output of the NOx sensor with a predetermined evaluation value, the rich-spike condition is changed so that the amount of NOx stored in the catalyst at the time of a start of the rich-spike operation during a degradation determination period is made larger than that reached during a period other than the determination period.