Patent ID: 7275363

Claim:
An exhaust gas purifying apparatus for an internal combustion engine, comprising: an NOx trap catalyst trapping NOx in the exhaust gas when an air-fuel ratio of the exhaust gas flowing thereinto is lean and discharging the trapped NOx when the air-fuel ratio of the exhaust gas flowing thereinto is rich; a rich controlling section that performs a rich control to richen the air-fuel ratio flowing into an NOx trap catalyst; a first time point specifying section that specifies a first time point at which the air-fuel ratio of the exhaust gas at an upstream side of the NOx trap catalyst has reached a first predetermined value indicating a rich air-fuel ratio; a second time point specifying section that specifices a second time point at which the air-fuel ratio of the exhaust gas at a downstream side of the NOx trap catalyst has reached a second predetermined value indicating a rich air-fuel ratio; an air-fuel ratio variation time difference detecting section that detects a time duration from the first time point to the second time point in a form of an air-fuel ratio variation time difference; and a performance deterioration determining section that determines a degree of a performance deterioration of the NOx trap catalyst on the basis of the detected air-fuel ratio variation time difference, the performance deterioration determining section determining the degree of the performance deterioration with at least one correcting factor selected from the group consisting of a quantity of NOx trapped by means of the NOx trap catalyst, a reducing agent supply quantity by which a reducing agent is supplied to the NOx trap catalyst, and catalyst temperature which is a temperature of the NOx trap catalyst, at least one of the correcting factors being reflected on the determination of the degree of the performance deterioration of the NOx trap catalyst, wherein the performance deterioration determining section determines whether the NOx trap catalyst reaches a performance deterioration level by comparing the detected air-fuel ratio variation time difference with a reference time difference having a length in time varied in accordance with a predetermined degree of the performance deterioration of the NOx trap catalyst and the reference time difference is corrected by the correcting factor.