Patent ID: 6941744

Claim:
An exhaust emission control system for an internal combustion engine, comprising: a NOx storage/reduction catalyst disposed in an exhaust passage of the internal combustion engine, the NOx storage/reduction catalyst selectively storing NOx contained in exhaust gas flowing into the catalyst, through adsorption, absorption, or both of adsorption and absorption, when an air/fuel ratio of the exhaust gas is lean, and reducing and removing the stored NOx by using reducing components in the exhaust gas when the air/fuel ratio of the exhaust gas becomes equal to a stoichiometric air/fuel ratio or a rich air/fuel ratio; a NOx sensor positioned in the exhaust passage downstream of the NOx storage/reduction catalyst, the NOx sensor being operable to detect a NOx concentration in the exhaust gas; and a controller that: performs a rich-spike operation to temporarily operate the engine at a rich air/fuel ratio each time a NOx storage state of the NOx storage/reduction catalyst satisfies a predetermined rich-spike condition during an operation of the engine at a lean air/fuel ratio, so as to supply exhaust gas having a rich air/fuel ratio to the NOx storage/reduction catalyst for reduction and removal of the NOx stored in the NOx storage/reduction catalyst; determines a degree of degradation of the NOx storage/reduction catalyst by comparing an output value of the NOx sensor obtained in a predetermined determination period during the operation of the engine at a lean air/fuel ratio, with a predetermined evaluation value; and changes the rich-spike condition so that an amount of NOx stored in the NOx storage/reduction catalyst at the time of a start of the rich-spike operation during the determination period is made larger than the amount of NOx stored in the catalyst at the time of the start of the rich-spike operation during a period other than the determination period.