Patent ID: 8117829

Claim:
An exhaust emission control device for an internal combustion engine, for purifying exhaust gases discharged from the engine into an exhaust system, comprising: an upstream catalyst that is disposed in the exhaust system and has an oxidation function for purifying exhaust gases; a NOx catalyst that is disposed in the exhaust system at a location downstream of the upstream catalyst, for trapping NOx contained in exhaust gases in an oxidizing atmosphere, and reducing the trapped NOx in a reducing atmosphere to thereby purify the trapped NOx; NOx reduction control means for executing reduction control in which the exhaust gases flowing into said NOx catalyst are controlled to the reducing atmosphere, by supplying reducing agent to an upstream side of said upstream catalyst so as to cause said NOx catalyst to carry out an operation for reducing NOx; degradation degree-estimating means for estimating a degree of degradation of said upstream catalyst; and correction means for correcting a time period over which the reduction control should be executed by said NOx reduction control means, according to the estimated degree of degradation of said upstream catalyst, wherein said degradation degree-estimating means includes: upstream temperature-detecting means for detecting a temperature at a location upstream of said upstream catalyst; downstream temperature-detecting means for detecting a temperature at a location downstream of said upstream catalyst; catalyst temperature-detecting means for detecting a temperature of said upstream catalyst; rise degree-calculating means for calculating a degree of rise in the temperature at the location downstream of said upstream catalyst with respect to the temperature at the location upstream of said upstream catalyst, the temperature being detected during execution of the reduction control; temperature correction value-calculating means for calculating a temperature correction value such that as the detected temperature of said upstream catalyst is higher, the temperature correction value becomes larger; and calculating degree of rise in the detected downstream temperature of said upstream catalyst by subtracting the temperature correction value from a provisional degree of rise, and estimates the degree of degradation of said upstream catalyst such that as a cumulative degree of rise in the temperature is smaller, the degree of degradation becomes higher.