Patent ID: 8209966

Claim:
An exhaust emission control device used for an internal combustion engine and applied to an exhaust gas purifying system including: an NOx catalyst disposed in an exhaust passage of the engine, through which exhaust gas flows, so as to promote selective purification of NOx in the exhaust gas performed by ammonia, which is a reducing agent for reducing NOx; and a reducing agent adding device configured to add the reducing agent to an upstream side of the NOx catalyst in a flow direction of the exhaust gas, the device comprising: a catalyst temperature detector configured to detect a temperature of the NOx catalyst; a reaction ratio calculating unit configured to calculate a reaction ratio, which is a ratio of a reaction amount of ammonia to a reaction amount of NOx in the NOx catalyst, based on the temperature of the NOx catalyst detected by the catalyst temperature detector, using pre-registered control data showing a relationship between the temperature of the NOx catalyst and the reaction ratio; an NO/NO2 calculating unit configured to calculate an NO/NO2 ratio, which is a ratio between NO and NO2 in the exhaust gas, wherein: the reaction ratio calculating unit calculates the reaction ratio based on the temperature of the NOx catalyst detected by the catalyst temperature detector and the NO/NO2 ratio calculated by the NO/NO2 calculating unit; the reaction ratio calculating unit calculates the reaction ratio as a large value when the temperature of the NOx catalyst is high; the reaction ratio calculating unit calculates the reaction ratio as a large value when the NO/NO2 ratio is small; and the reaction ratio is equal to or larger than 1; and an ammonia consumed amount calculating unit configured to calculate a consumed amount of ammonia in the NOx catalyst based on the reaction ratio calculated by the reaction ratio calculating unit.