Exhaust gas control apparatus for internal combustion engine

Abstract

A SO x trap catalyst, an oxidation catalyst, a particulate filter, an aqueous urea supply valve, and a NO x selective reduction catalyst are arranged in order from upstream to downstream in an engine exhaust passage. It is determined whether a discharge concentration of hydrogen sulfide H 2 S will become equal to or greater than a preset maximum concentration when SO x is released from the SO x trap catalyst. If it is estimated that the discharge concentration of the hydrogen sulfide H 2 S will become equal to or greater than the maximum concentration when SO x is released, an adsorbed ammonia amount adsorbed on the NO x selective reduction catalyst is reduced before SO x is released so that the discharge concentration of the hydrogen sulfide H 2 S is less than the maximum concentration when SO x is released.

1 . An exhaust gas control apparatus for an internal combustion engine, comprising:
 a NO x  selective reduction catalyst which is arranged in an engine exhaust passage and selectively reduces NO x  in exhaust gas using ammonia when an air-fuel ratio of the exhaust gas is lean;   a SO x  trap catalyst which is arranged in the engine exhaust passage upstream of the NO x  selective reduction catalyst and traps SO x  in the exhaust gas; and   a control apparatus that controls the state of the exhaust gas,   wherein the control apparatus i) reduces an adsorbed ammonia amount adsorbed on the NO x  selective reduction catalyst before SO x  is released or ii) reduces the amount of SO x  released from the SO x  trap catalyst when SO x  is released, such that a discharge concentration of hydrogen sulfide is less than a preset maximum concentration when SO x  is released.   
   
   
       2 . The exhaust gas control apparatus according to  claim 1 , further comprising:
 an estimating apparatus that estimates whether the discharge concentration of hydrogen sulfide will be equal to or greater than the maximum concentration when SO x  is released from the SO x  trap catalyst,   wherein when it is estimated that the discharge concentration of hydrogen sulfide will be equal to or greater than the maximum concentration when SO x  is released, the control apparatus i) reduces the adsorbed ammonia amount adsorbed on the NO x  selective reduction catalyst before SO x  is released or ii) reduces the amount of SO x  released from the SO x  trap catalyst when SO x  is released, such that the discharge concentration of hydrogen sulfide is less than the maximum concentration when SO x  is released.   
   
   
       3 . The exhaust gas control apparatus according to  claim 1 , wherein when releasing SO x  from the SO x  trap catalyst, the control apparatus reduces the adsorbed ammonia amount adsorbed on the NO x  selective reduction catalyst before SO x  is released until the adsorbed ammonia amount is less than a preset maximum adsorption amount. 
   
   
       4 . The exhaust gas control apparatus according to  claim 3 , further comprising:
 a determining apparatus that determines whether the adsorbed ammonia amount adsorbed on the NO x  selective reduction catalyst is equal to or greater than the maximum adsorption amount when SO x  is released from the SO x  trap catalyst,   wherein when it is determined that the adsorbed ammonia amount adsorbed on the NO x  selective reduction catalyst is equal to or greater than the maximum adsorption amount, the control apparatus reduces the adsorbed ammonia amount until the adsorbed ammonia amount adsorbed on the NO x  selective reduction catalyst is less than the maximum adsorption amount before releasing SO x .   
   
   
       5 . The exhaust gas control apparatus according to  claim 1 , wherein when releasing SO x  from the SO x  trap catalyst, the control apparatus sets an air-fuel ratio to a first target air-fuel ratio when the adsorbed ammonia amount adsorbed on the NO x  selective reduction catalyst is less than a preset maximum adsorption amount; and the control apparatus sets the air-fuel ratio to a second target air-fuel ratio which is greater than the first target air-fuel ratio when the adsorbed ammonia amount adsorbed on the NO x  selective reduction catalyst is equal to or greater than a maximum adsorption amount and a concentration of released SO x  is larger than a threshold value. 
   
   
       6 . The exhaust gas control apparatus according to  claim 5 , wherein if the adsorbed ammonia amount adsorbed on the NO x  selective reduction catalyst is less than the maximum adsorption amount, the discharge concentration of hydrogen sulfide is less than the maximum concentration regardless of the concentration of released SO x ; and if the concentration of released SO x  is equal to or less than the threshold value, the discharge concentration of hydrogen sulfide is less than the maximum concentration regardless of the adsorbed ammonia amount. 
   
   
       7 . The exhaust gas control apparatus according to  claim 1 , wherein when SO x  is to be released from the SO x  trap catalyst, the control apparatus makes the air-fuel ratio of the exhaust gas flowing into the SO x  trap catalyst rich. 
   
   
       8 . The exhaust gas control apparatus according to  claim 1 , further comprising:
 an aqueous urea supply valve arranged in the engine exhaust passage upstream of the NO x  selective reduction catalyst,   wherein when it is estimated that the discharge concentration of hydrogen sulfide will be equal to or greater than the maximum concentration when SO x  is released from the SO x  trap catalyst, the control apparatus i) reduces the amount of aqueous urea supplied before releasing SO x  or ii) stops the supply of aqueous urea before releasing SO x  before releasing SO x , such that the discharge concentration of hydrogen sulfide is less than the maximum concentration when SO x  is released.   
   
   
       9 . The exhaust gas control apparatus according to  claim 8 , wherein when reducing the amount of aqueous urea supplied or stopping the supply of aqueous urea, the control apparatus increases the amount of NO x  discharged from the engine. 
   
   
       10 . The exhaust gas control apparatus according to  claim 8 , wherein when reducing the amount of aqueous urea supplied or stopping the supply of aqueous urea, the control apparatus raises the temperature of the NO x  selective reduction catalyst. 
   
   
       11 . The exhaust gas control apparatus according to  claim 8 , wherein when it is determined that the adsorbed ammonia amount adsorbed on the NO x  selective reduction catalyst is equal to or greater than the maximum adsorption amount when SO x  is released from the SO x  trap catalyst, the control apparatus reduces the amount of supplied aqueous urea before releasing SO x  or stops the supply of aqueous urea before releasing SO x .