Exhaust purification system for internal combustion engine

An exhaust purification system for an internal combustion engine capable of lean-burn driving, the system comprising:

• a NOx storage reduction (hereinafter referred to as an NSR catalyst) catalyst disposed in an exhaust passageway of the internal combustion engine;
• a NOx selective catalytic reduction (hereinafter referred to as an SCR) disposed downstream of the NSR catalyst;
• an exhaust sensor, disposed downstream of the SCR, for producing output in response to NOx concentration;
• rich-spike device for causing a rich-spike at a particular timing during the lean-burn driving;
• and
• NOx-quantity augmenting device for increasing the amount of NOx contained in exhaust gas discharged during the rich-spike, in the event that the exhaust sensor has produced output indicative of a NOx concentration higher than a given NOx concentration,
• wherein the NOx-quantity augmenting device includes gas-quantity augmenting device for increasing the amount of exhaust gas during the rich-spike, in the event that the internal combustion engine is being operated under a given low-load condition.