Patent ID: 7980059

Claim:
An exhaust gas purification system for an internal combustion engine comprising: an NOx catalyst provided in an exhaust passage of an internal combustion engine to remove NOx in exhaust gas; an NOx sensor provided in said exhaust passage downstream of said NOx catalyst to detect the NOx concentration in the exhaust gas; NOx concentration estimation unit for estimating the NOx concentration in the exhaust gas in said exhaust passage upstream of said NOx catalyst based on an operation state of said internal combustion engine; deterioration degree estimation unit for estimating the degree of deterioration of said NOx catalyst based on a difference between a detection value of said NOx sensor and an estimated value by said NOx concentration estimation unit at a time when said NOx catalyst is in an active state and the exhaust gas is flowing through said NOx catalyst; and NOx unremoved condition determination unit for determining whether or not an NOx unremoved condition as a condition under which the exhaust gas reaches said NOx sensor with NOx in the exhaust gas not being removed in said NOx catalyst is met, wherein the estimated value of the NOx concentration by said NOx concentration estimation unit in estimating the degree of deterioration of said NOx catalyst by said deterioration degree estimation unit is corrected based on the difference between the detection value of said NOx sensor and the estimated value by said NOx concentration estimation unit at a time when it is determined by said NOx unremoved condition determination unit that the NOx unremoved condition is met.