Exhaust emission control system for internal combustion engine (as amended)

Abstract

An object of the present invention is to increase the NO x purification rate of an unactivated SCR catalyst in an exhaust emission control system for an internal combustion engine that has an oxidation catalyst and the SCR catalyst disposed downstream of the oxidation catalyst. In order to achieve this object, the present invention provides an exhaust emission control system for an internal combustion engine, including an oxidation catalyst disposed in an exhaust passage of the internal combustion engine, an SCR catalyst that is disposed downstream of the oxidation catalyst in the exhaust passage, and a supply device for supplying unburnt fuel to the oxidation catalyst when the SCR catalyst is not yet activated, wherein the amount of NO 2 that flows into the SCR catalyst is increased by reducing the amount of unburnt fuel supplied, when a low-load operation of the internal combustion engine is performed.

1 . An exhaust emission control system for an internal combustion engine, comprising:
 an oxidation catalyst disposed in an exhaust passage of the internal combustion engine;   a selective reduction catalyst that is disposed downstream of the oxidation catalyst in the exhaust passage;   temperature increasing unit for increasing a temperature of exhaust flowing out of the oxidation catalyst by supplying unburnt fuel to the oxidation catalyst, when the oxidation catalyst is activated but the selective reduction catalyst is not activated; and   control unit for reducing the amount of unburnt fuel to be supplied to the oxidation catalyst by the temperature increasing unit, when a low-load operation of the internal combustion engine is performed during a period in which the unburnt fuel is supplied to the oxidation catalyst by the temperature increasing unit.   
     
     
         2 . The exhaust emission control system for an internal combustion engine according to  claim 1 , wherein when a temperature of the selective reduction catalyst becomes equal to or higher than a predetermined temperature, the control unit executes either an increasing process for increasing an intake air amount of the internal combustion engine or a reducing process for reducing the intake air amount, whichever process that increases a NO x  purification rate of the selective reduction catalyst higher. 
     
     
         3 . The exhaust emission control system for an internal combustion engine according to  claim 2 , wherein the predetermined temperature is a lowest temperature at which the NO x  purification rate of the selective reduction catalyst increases as a result of increasing the amount of nitrogen dioxide contained in the exhaust. 
     
     
         4 . The exhaust emission control system for an internal combustion engine according to  claim 2 , wherein when the temperature of the selective reduction catalyst drops from the predetermined temperature or higher to less than the predetermined temperature, the control unit reduces the amount of unburnt fuel to be supplied to the oxidation catalyst by the temperature increasing unit. 
     
     
         5 . The exhaust emission control system for an internal combustion engine according to  claim 3 , wherein when the temperature of the selective reduction catalyst drops from the predetermined temperature or higher to less than the predetermined temperature, the control unit reduces the amount of unburnt fuel to be supplied to the oxidation catalyst by the temperature increasing unit.