Patent ID: 7992378

Claim:
An exhaust gas purifying device for an internal combustion engine, comprising: an exhaust purifying catalyst provided in an exhaust passage of an internal combustion engine for purifying an exhaust gas; a NOx purifying catalyst disposed downstream of the exhaust purifying catalyst for trapping NOx in the exhaust gas in an oxidizing atmosphere and reducing/purifying the trapped NOx in a reducing atmosphere; an upstream air fuel ratio detecting sensor disposed on an upstream side of the exhaust purifying catalyst to detect an air fuel ratio of the exhaust gas flowing into the exhaust purifying catalyst; a downstream air fuel ratio detecting sensor disposed between the exhaust purifying catalyst and the NOx purifying catalyst to detect an air fuel ratio of the exhaust gas flowing out of the exhaust purifying catalyst; a first reducing agent calculating means for computing a first amount of a reducing agent in the exhaust gas supplied to the exhaust purifying catalyst from the detected upstream air fuel ratio; a second reducing agent calculating means for computing a second amount of a reducing agent in the exhaust gas supplied to the NOx purifying catalyst from the detected downstream air fuel ratio; a reduction control terminating means for terminating a reduction control when the amount of a reducing agent supplied to the NOx purifying catalyst exceeds a prescribed reduction termination threshold value; a deterioration degree detecting means for detecting a degree of deterioration of the exhaust purifying catalyst from a ratio of the second amount of the reducing agent to the first amount of the reducing agent; a threshold value correcting means for correcting the reduction termination threshold value according to the degree of deterioration of the exhaust purifying catalyst detected by the deterioration degree detecting means; and a correction prohibiting means for prohibiting the correction of the reduction termination threshold value by the threshold value correcting means when it is estimated that an amount of H 2 and/or CH 4 in the exhaust gas supplied to the exhaust purifying catalyst and/or the exhaust gas supplied to the NOx purifying catalyst exceeds a prescribed correction prohibiting judgment value from a prescribed operating condition of the engine.