EXHAUST EMISSION CONTROL SYSTEM FOR ENGINE

摘要

PROBLEM TO BE SOLVED: To completely release NOx stored in a NOx storage catalyst and prevent unpurified HC and CO from being emitted when an air-fuel ratio in exhaust gas is made temporarily rich for reducing and purifying NOx. SOLUTION: In this exhaust emission control system of an engine 1, a NOx storage catalyst 3 is provided for storing NOx in exhaust gas when an exhaust gas air-fuel ratio is lean and releasing the stored NOx for reduction and purification when the exhaust gas air-fuel ratio is stoichiometric or rich. An engine control unit 9 judges lowering of NOx storage ability of the NOx storage catalyst 3 caused by an increase of the stored NOx amount. When it is judged that the NOx storage ability is lowered, the stored NOx is released and reduced by increasing an air-fuel ratio intermittently for several times. Therefore, NOx stored in the NOx storage catalyst can be released completely. An air-fuel ratio in the catalyst is maintained at a substantially stoichiometric ratio while an air-fuel ratio in the exhaust gas is made temporarily rich for reducing and purifying NOx, and unpurified HC and CO can be prevented from being emitted.