Exhaust emission control device of internal combustion engine

Abstract

An exhaust emission control device of an internal combustion engine capable of effectively restoring an NSR catalyst from sulfur poisoning while suppressing reduction of fuel efficiency and worsening of emission, in the internal combustion engine including the NSR catalyst and SCR. An exhaust emission control device of an internal combustion engine capable of a lean burn operation includes a TWC that is placed in an exhaust passage of the internal combustion engine and supports a precious metal in an inside thereof, an NSR catalyst that is placed at an exhaust downstream side of the TWC and supports a precious metal and a base in an inside thereof, and an SCR that is placed in the exhaust passage at a downstream side of the NSR catalyst, wherein the NSR catalyst is placed in a region in which a bed temperature at a time of an operation thereof becomes 500° C. to 750° C. The TWC and the NSR catalyst are preferably configured as an SC of a tandem structure in which the TWC and the NSR catalyst are integrated by being serially arranged.

1 . An exhaust emission control device of an internal combustion engine capable of a lean burn operation, comprising:
 a first catalyst that is placed in an exhaust passage of the internal combustion engine, and supports a precious metal in an inside thereof;   a second catalyst that is placed at an exhaust downstream side of the first catalyst, and supports a precious metal and a base in an inside thereof; and   an NOx selective catalytic reduction that is placed in an exhaust passage at a downstream side of the second catalyst,   wherein the second catalyst is placed in a region in which a bed temperature at a time of an operation thereof becomes 500° C. to 750° C.   
     
     
         2 . (canceled) 
     
     
         3 . The exhaust emission control device of an internal combustion engine according to  claim 1 ,
 wherein the first catalyst and the second catalyst are of a tandem structure in which the first catalyst and the second catalyst are integrated by being serially arranged.   
     
     
         4 . The exhaust emission control device of an internal combustion engine according to  claim 1 , further comprising:
 determining means that determines whether or not sulfur poisoning of the second catalyst advances; and   rich gas introducing means that introduces rich gas into the second catalyst, when it is determined that the sulfur poisoning advances.   
     
     
         5 . The exhaust emission control device of an internal combustion engine according to  claim 4 , further comprising:
 NOx concentration acquiring means that acquires an NOx concentration of exhaust gas at a downstream side of the NOx selective catalytic reduction,   wherein the determining means determines that sulfur poisoning of the second catalyst advances, when the NOx concentration is higher than a predetermined concentration.   
     
     
         6 . The exhaust emission control device of an internal combustion engine according to  claim 4 ,
 wherein the determining means determines that sulfur poisoning of the second catalyst advances, when a traveling distance after previous execution of the rich gas introducing means is longer than a predetermined distance.   
     
     
         7 . The exhaust emission control device of an internal combustion engine according to  claim 4 ,
 wherein the internal combustion engine is an internal combustion engine having a plurality of cylinders,   the rich gas introducing means includes air-fuel ratio control means that, out of a first and a second cylinder groups that are a result of dividing the plurality of cylinders, controls an exhaust air-fuel ratio of the first cylinder group to a predetermined rich air-fuel ratio, and controls an exhaust air-fuel ratio of the second cylinder group to a predetermined rich or a theoretical air-fuel ratio.   
     
     
         8 . The exhaust emission control device of an internal combustion engine according to  claim 4 ,
 wherein the rich gas introducing means introduces slight rich exhaust gas to the second catalyst.   
     
     
         9 . The exhaust emission control device of an internal combustion engine according to  claim 4 , further comprising:
 second determining means that determines whether or not it is timing for performing malfunction diagnosis of the first catalyst; and   malfunction diagnosing means that executes active air-fuel ratio control that forcefully switches an air-fuel ratio of the internal combustion engine to a rich side and a lean side alternately with a theoretical air-fuel ratio set as a control center, and executes malfunction diagnosis of the first catalyst, based on an oxygen storage capacity measured thereby, when it is timing for performing the malfunction diagnosis of the first catalyst,   wherein the rich gas introducing means   shifts the control center in the active air-fuel ratio control to a rich direction by a predetermined amount, when it is timing for performing the malfunction diagnosis of the first catalyst, and it is determined that sulfur poisoning of the second catalyst advances.