Exhaust emission control device for internal combustion engine

An exhaust emission control device for an internal combustion engine, comprising:

• a selective reduction catalyst disposed in an exhaust passage of the engine, for purifying NOx in exhaust gases flowing through the exhaust passage, in the presence of a reducing agent;
• exhaust gas concentration-detecting means disposed in the exhaust passage at a location downstream of said selective reduction catalyst, for outputting a detection signal indicative of concentrations of predetermined components including NOx in exhaust gases;
• a reducing agent supply device for supplying the reducing agent to said selective reduction catalyst;
• NOx purification ratio parameter-calculating means for calculating a NOx purification ratio parameter having a correlation with a NOx purification ratio of said selective reduction catalyst;
• NOx purification ratio-calculating means for calculating the NOx purification ratio, based on the NOx purification ratio parameter, by using a correlation model defining a correlation between the NOx purification ratio parameter and the NOx purification ratio of said selective reduction catalyst degraded to a predetermined degree of degradation;
• supply amount-determining means for determining an amount of supply of the reducing agent to said selective reduction catalyst by said reducing agent supply device, according to the NOx purification ratio;
• and
• supply amount-correcting means for calculating a supply amount correction value according to a value of the detection signal from said exhaust gas concentration-detecting means, and correcting the amount of supply of the reducing agent to said selective reduction catalyst, by using the supply amount correction value,
• wherein said NOx purification ratio-calculating means comprises:
• model modification value-calculating means for calculating a model modification value with a predetermined algorithm such that an absolute value of the supply amount correction value is reduced;
• and
• purification ratio-calculating means for calculating the NOx purification ratio by using the correlation model modified by the model modification value.