EXHAUST EMISSION PURIFICATION SYSTEM FOR INTERNAL COMBUSTION ENGINE

The invention provides an exhaust purification system for an internal combustion engine having both of an NSR catalyst and an SCR. The system prevents the deterioration of emission performance due to unadsorbed NOx. An exhaust purification system for an internal combustion engine 10 capable of lean-burn driving comprises: an NSR catalyst 16 disposed in an exhaust passageway 12; an SCR 18 disposed downstream of the NSR catalyst 16; a NOx sensor 26, disposed downstream of the SCR 18, for producing output in response to NOx concentration; and rich-spike means for causing a rich-spike. When the NOx sensor 26 has produced output indicative of a NOx concentration higher than a given NOx concentration, the system increases the amount of the NOx contained in the exhaust gas discharged during the rich-spike. Preferably, under a given high-load condition, the air-fuel ratio is made stoichiometric during the rich-spike caused at a particular timing. Under a given low-load condition, in contrast, the amount of exhaust gas is increased during the rich-spike.