Exhaust purification apparatus for internal combustion engine

Abstract

An exhaust purification apparatus for an internal combustion engine comprises a first oxidation catalyst arranged in an exhaust passage of an engine, a mixer arranged on the downstream side of the first oxidation catalyst, and a second oxidation catalyst arranged on the downstream side of the mixer. A repetitive control is carried out in such a way that a period in which an air-fuel ratio of exhaust gas discharged from an engine body is rich and a period in which the air-fuel ratio is lean are alternately repeated when the temperature of the second oxidation catalyst is to be increased, so that a part of the unburnt fuel contained in a rich exhaust gas discharged from the engine body and a lean exhaust gas discharged from the engine body are mixed in the mixer after passing through the first oxidation catalyst and are subject to oxidization in the second oxidation catalyst.

1 . An exhaust purification apparatus for an internal combustion engine, comprising:
 a first oxidation catalyst arranged in an exhaust passage of an engine;   a mixer arranged in the exhaust passage on the downstream side of the first oxidation catalyst; and   a second oxidation catalyst arranged in the exhaust passage on the downstream side of the mixer;   wherein a repetitive control is carried out in such a way that a period in which an air-fuel ratio of exhaust gas discharged from an engine body is rich and a period in which the air-fuel ratio of the exhaust gas discharged from the engine body is lean are alternately repeated when the temperature of the second oxidation catalyst is to be increased, so that a part of the unburnt fuel contained in exhaust gas which is discharged from the engine body and whose air-fuel ratio is rich and exhaust gas which is discharged from the engine body and whose air-fuel ratio is lean are mixed in the mixer after passing through the first oxidation catalyst, and the mixed exhaust gases are subject to oxidization in the second oxidation catalyst.   
     
     
         2 . An exhaust purification apparatus for an internal combustion engine, according to  claim 1 , wherein said mixer comprises a turbine of a super charger. 
     
     
         3 . An exhaust purification apparatus for an internal combustion engine, according to  claim 1 , wherein the repetitive control is carried out to control the duration in which the air-fuel ratio of the exhaust gas discharged from the engine body is rich and the degree of the air-fuel ratio of the exhaust gas as well as the duration in which the air-fuel ratio of the exhaust gas discharged from the engine body is lean and the degree of the air-fuel ratio of the exhaust gas, so that the air-fuel ratio of the exhaust gas discharged from the mixer is lean. 
     
     
         4 . An exhaust purification apparatus for an internal combustion engine, according to  claim 1 , further comprising an exhaust treatment device arranged in the exhaust passage on the downstream side of the second oxidation catalyst, said exhaust treatment device comprising at least one of a particulate filter which can trap particulate substances and whose temperature must be increased to burn the accumulated particulate substances and a NOx storage and reduction catalyst which can hold NOx through at least one of storage and adsorption and whose temperature must be increased to emit SOx held with in the NOx through at least one of storage and adsorption. 
     
     
         5 . An exhaust purification apparatus for an internal combustion engine, comprising:
 a first oxidation catalyst arranged in an exhaust passage of an engine; and   a second oxidation catalyst arranged in the exhaust passage on the downstream side of the first oxidation catalyst and capable of storing an oxygen;   wherein a iterative control is carried out in such a way that a period in which an air-fuel ratio of exhaust gas discharged from an engine body is rich and a period in which the air-fuel ratio of the exhaust gas discharged from the engine body is lean are alternately repeated when the temperature of the second oxidation catalyst is to be increased, so that a part of the unburnt fuel contained in exhaust gas which is discharged from the engine body and whose air-fuel ratio is rich is subject to oxidization with an oxygen stored in the second oxidation catalyst after passing through the first oxidation catalyst.   
     
     
         6 . An exhaust purification apparatus for an internal combustion engine, according to  claim 5 , wherein said iterative control is carried out to control the duration in which the air-fuel ratio of the exhaust gas discharged from the engine body is rich and the degree of the air-fuel ratio of the exhaust gas as well as the duration in which the air-fuel ratio of the exhaust gas discharged from the engine body is lean and the degree of the air-fuel ratio of the exhaust gas, so that the second oxidation catalyst stores an oxygen when the air-fuel ratio of the exhaust gas is lean and oxidizes the almost all of the unburnt fuel entering in the second oxidation catalyst with the stored oxygen when the air-fuel ratio of the exhaust gas is rich. 
     
     
         7 . An exhaust purification apparatus for an internal combustion engine, according to  claim 5 , further comprising an exhaust treatment device arranged in the exhaust passage on the downstream side of the second oxidation catalyst, said exhaust treatment device comprising at least one of a particulate filter which can trap particulate substances and whose temperature must be increased to burn the accumulated particulate substances and a NOx storage and reduction catalyst which can hold NOx through at least one of storage and adsorption and whose temperature must be increased to emit SOx held with in the NOx through at least one of storage and adsorption.