Patent attributes
An object is to estimate the degree of deterioration of an NOx catalyst with improved accuracy in an exhaust gas purification system having the NOx catalyst provided in an exhaust passage. According to the present invention, an NOx sensor is provided downstream of an NOx catalyst, and the difference between an estimated value of the NOx concentration in the exhaust gas in the exhaust passage upstream of the NOx catalyst and a detection value of the NOx sensor at a time when NOx in the exhaust gas is not removed in the NOx catalyst is calculated. In addition, in estimating the degree of deterioration of the NOx catalyst, the estimated value of the NOx concentration in the exhaust gas in the exhaust passage upstream of the NOx catalyst is corrected based on this difference, and the degree of deterioration of the NOx catalyst is estimated based on the difference between this corrected value and the detection value of the NOx sensor.
