Patent ID: 8146348

Claim:
An exhaust emission control device for reducing nitrogen oxide included in exhaust air from an internal combustion engine, the device comprising: an exhaust pipe defining a passage for the exhaust air discharged from the engine; a catalyst disposed in the exhaust pipe, the catalyst being capable of promoting reduction reaction of the nitrogen oxide in the exhaust air; a supply unit for supplying an additive agent, which is used for the reduction reaction, to an upstream side of the catalyst in a flow direction of the exhaust air; a NOx detecting unit for detecting the nitrogen oxide included in the exhaust air, which has passed through the catalyst; a control unit having a program for controlling the supply unit, wherein the control unit determines a deterioration state of the catalyst based on a result detected by the NOx detecting unit and reduces an amount of the additive agent, which is supplied by the supply unit, in accordance with the determined deterioration state; a rotational speed detecting unit for detecting a rotational speed of the engine; a supply amount detecting unit for detecting an amount of fuel supplied to the engine; a temperature detecting unit for detecting temperature of the exhaust air discharged from the engine; and a memory unit for storing the result detected by the NOx detecting unit, wherein: the control unit makes the memory unit store the result, which is detected by the NOx detecting unit when at least one of the following conditions is satisfied and accordingly the engine is regarded as in continuous high-load operation: the rotational speed of the engine is equal to or larger than a predetermined rotational speed; the amount of fuel supplied to the engine is equal to or larger than a predetermined amount; and the temperature of the exhaust air is equal to or larger than a predetermined temperature; and the control unit determines the deterioration state of the catalyst by comparing the result, which is detected by the NOx detecting unit when the engine is regarded as in continuous high-load operation, and the result detected by the NOx detecting unit and already stored in the memory unit.