Patent ID: 7797925

Claim:
An exhaust gas purifying system for an internal combustion engine, comprising: an NOx catalytic converter that includes an NOx storage and reduction catalyst and is provided in an exhaust system of the internal combustion engine; an NOx deoxidization control means for supplying and controlling an NOx deoxidizing agent to the NOx catalytic converter at a time of deoxidizing and removing absorbed NOx, which is absorbed by the NOx storage and reduction catalyst; a purifying performance determining means for determining a purifying performance of the NOx catalytic converter based on one of: an amount of the supplied NOx deoxidizing agent, which is required to deoxidize and remove the absorbed NOx at a time of operating the NOx deoxidization control means; and a parameter that correlates with the amount of the supplied NOx deoxidizing agent; an unburnt fuel state determining means for determining whether supplied unburnt fuel, which is supplied to the NOx catalytic converter, is in an excessive state at the time of operating the NOx deoxidization control means; and a prohibiting means for prohibiting the purifying performance determining means from determining the purifying performance of the NOx catalytic converter when the unburnt fuel state determining means determines that the supplied unburnt fuel is in the excessive state; wherein the NOx deoxidization control means includes: a rich purge means for temporarily shifting an air/fuel ratio of the internal combustion engine into a rich range; and a fuel adding means for adding and supplying the unburnt fuel to the NOx catalytic converter; the NOx deoxidization control means selects and uses one of the rich purge means and the fuel adding means to supply the NOx deoxidizing agent to the NOx catalytic converter; and the unburnt fuel state determining means determines that the unburnt fuel is in the excessive state whenever the fuel adding means is operated to add and supply the unburnt fuel to the NOx catalytic converter.