Exhaust Gas Purification Device For Internal Combustion Engine

Abstract

A NOx absorbent is arranged in an exhaust passage of an internal combustion engine, and a fuel supply valve ( 28 ) is arranged in the exhaust passage upstream of the NOx absorbent. If the temperature of the NOx absorbent is lower than a predetermined temperature when the NOx must be released from the NOx absorbent, the air-fuel ratio of exhaust gas flowing through the NOx absorbent is first switched from a basic lean air-fuel ratio to and maintained at a lean air-fuel ratio with a lower leanness for a predetermined lean time, and is then switched to a rich air-fuel ratio. If the temperature of the NOx absorbent is higher than the predetermined temperature when the NOx must be released from the NOx absorbent, the air-fuel ratio of the exhaust gas flowing through the NOx absorbent is switched to the rich air-fuel ratio without being switched to the lean air-fuel ratio with a lower leanness.

1 . An exhaust gas purification device for an internal combustion engine having an exhaust passage, combustion being carried out under a basic lean air-fuel ratio, comprising:
 a NOx absorbent arranged in the exhaust passage, the NOx absorbent absorbing NOx contained in exhaust gas therein when the air-fuel ratio of exhaust gas is lean and releasing absorbed NOx therefrom when the air-fuel ratio of exhaust gas is switched to rich; and   control means for controlling the air-fuel ratio of exhaust gas flowing through the NOx absorbent,   
     wherein, when NOx must be released from the NOx absorbent, the air-fuel ratio of exhaust gas flowing through the NOx absorbent is first switched from the basic lean air-fuel ratio to and maintained at a lean air-fuel ratio with a lower leanness for a predetermined lean time, and is then switched to a rich air-fuel ratio. 
   
   
       2 . An exhaust gas purification device for an internal combustion engine according to  claim 1 , wherein the air-fuel ratio of the exhaust gas flowing through the NOx absorbent is returned to and maintained at the basic lean air-fuel ratio until the NOx must be released from the NOx absorbent once more. 
   
   
       3 . An exhaust gas purification device for an internal combustion engine according to  claim 1 , further comprising means for obtaining a temperature of the NOx absorbent, wherein, if the temperature of the NOx absorbent is lower than a predetermined temperature when the NOx must be released from the NOx absorbent, the air-fuel ratio of exhaust gas flowing through the NOx absorbent is first switched from the basic lean air-fuel ratio to and maintained at the lean air-fuel ratio with a lower leanness for the predetermined lean time, and is then switched to the rich air-fuel ratio, if the temperature of the NOx absorbent is higher than the predetermined temperature when the NOx must be released from the NOx absorbent, the air-fuel ratio of exhaust gas flowing through the NOx absorbent is switched to the rich air-fuel ratio without being switched to the lean air-fuel ratio with a lower leanness. 
   
   
       4 . An exhaust gas purification device for an internal combustion engine according to  claim 3 , further comprising means for obtaining a degree of deterioration of the NOx absorbent, wherein the predetermined temperature when the degree of deterioration of the NOx absorbent is high is set higher than that when the degree of deterioration is low. 
   
   
       5 . An exhaust gas purification device for an internal combustion engine according to  claim 1 , further comprising means for obtaining a temperature of the NOx absorbent, wherein the lean time is set in accordance with the obtained temperature of the NOx absorbent. 
   
   
       6 . An exhaust gas purification device for an internal combustion engine according to  claim 1 , further comprising means for obtaining an amount of the exhaust gas flowing through the NOx absorbent, wherein the lean time is set in accordance with the amount obtained in the exhaust gas. 
   
   
       7 . An exhaust gas purification device for an internal combustion engine according to  claim 1 , further comprising means for obtaining a degree of deterioration of the NOx absorbent, wherein the lean time is set in accordance with the obtained degree of deterioration. 
   
   
       8 . An exhaust gas purification device for an internal combustion engine according to  claim 1 , wherein the lean time is set to make the temperature of the NOx absorbent or the increment thereof, obtained by maintaining the air-fuel ratio of the exhaust gas inflowing the NOx absorbent at the lean air-fuel ratio with a lower leanness for the lean time, equal to a target value.