Patent ID: 7997070

Claim:
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; a reducing agent supply device for supplying the reducing agent to said selective reduction catalyst; 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 a concentration of NOx and a concentration of the reducing agent in the exhaust gases, said exhaust gas concentration-detecting means having a characteristic that a direction of a change in a value of the detection signal responsive to an increase/decrease of the concentration of NOx is the same as a direction of a change in the value of the detection signal responsive to an increase/decrease of the concentration of the reducing agent; variation value-calculating means for calculating a variation value an amplitude of which changes periodically at a predetermined repetition period; correlation parameter-calculating means for calculating a correlation parameter indicative of a correlation between the value of the detection signal and the variation value by using the value of the detection signal and the variation value; control input value-calculating means for calculating a value of a control input for controlling the value of the detection signal to an extremal value, based on the correlation parameter; and 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 such that the supply amount of the reducing agent includes the value of the control input and the variation value.