Method for controlling exhaust emission control device

Abstract

When urea water U is injected by an addition nozzle 7 upstream of a NO x reduction catalyst 6 incorporated in the passage 5 so as to reduce and purify NO x in exhaust gas G, a target adsorption amount curve is set by shifting to lower temperature side a saturated adsorption amount curve of ammonia to the catalyst. A target adsorption amount of the reducing agent corresponding to the catalyst temperature is calculated and an actual adsorption amount of the reducing agent to the catalyst is determined. The amount of the urea water U to be added upstream of the catalyst is reduced when the actual adsorption amount reaches the target adsorption amount, and is increased when the actual adsorption amount is lower than the target adsorption amount. Thus, the adsorption amount of ammonia is secured while preventing ammonia from adsorbing so as to save the urea water U.

1 . A method for controlling an exhaust gas emission control device wherein a reducing agent is added upstream of NO x  reduction catalyst incorporated in an engine exhaust passage so as to reduce and purify NO x  in exhaust, characterized in that a target adsorption amount curve is set by shifting to lower temperature side a saturated adsorption amount curve which represents relationship between catalyst temperature and saturated adsorption amount of the reducing agent to the catalyst, that a target adsorption amount of the reducing agent corresponding to the catalyst temperature is calculated and an actual adsorption amount of the reducing agent to the catalyst is determined and that the amount of the reducing agent to be added upstream of the catalyst is reduced when the actual adsorption amount reaches the target adsorption amount, and is increased when the actual adsorption amount is lower than the target adsorption amount.  
   
   
       2 . A method for controlling an exhaust gas emission control device according to  claim 1 , wherein predictive catalyst temperatures after elapse of predetermined time periods are determined on the basis of catalyst inlet and outlet temperatures, ambient temperature, sucked air amount and heat capacity of the catalyst, the predictive catalyst temperatures being used to correct a shift amount for the target adsorption amount curve.