Patent ID: 7836686

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; at least one oxygen concentration sensor that measures an oxygen concentration in exhaust gas in the exhaust system, wherein at least one of the at least one oxygen concentration sensor is provided on a downstream side of the NOx catalytic converter; a rich purge control means for performing and controlling a rich purge in the internal combustion engine, wherein: the rich purge control means controls the rich purge by temporarily changing an air/fuel ratio of the exhaust gas into a rich range to deoxidize and remove absorbed NOx, which is absorbed by the NOx storage and reduction catalyst; the rich purge control means changes a degree of richness of the air/fuel ratio at a time of performing the rich purge based on a state of the NOx catalytic converter; and the rich purge control means uses an amount of the absorbed NOx of the NOx catalytic converter as a parameter that indicates the state of the NOx catalytic converter; and a purifying performance determining means for determining a purifying performance of the NOx catalytic converter based on a required deoxidization time period, which is required to deoxidize the absorbed NOx and is computed based on a measurement of the at least one oxygen concentration sensor at a time of performing the rich purge, wherein the rich purge control means sets the degree of richness of the air/fuel ratio in such a manner that the degree of richness of the air/fuel ratio gets smaller when the amount of the absorbed NOx becomes smaller, to increase the required deoxidization time period within a permissible range, at which a response time lag of the at least one oxygen concentration sensor does not cause an excess influence on the determination of the purifying performance of the NOx catalytic converter executed by the purifying performance determining means.