Exhaust gas purification system for an internal combustion engine

Abstract

The present invention is to provide a technique which can detect the level of degradation of a precatalyst in an exhaust gas purification system for an internal combustion engine that includes a NOx catalyst arranged on an exhaust passage of the internal combustion engine, and the precatalyst arranged on the exhaust passage at a location upstream of the NOx catalyst and having an oxidation function. In the present invention, the level of degradation of the precatalyst is detected based on the width of change in temperature of the NOx catalyst at the time when a reducing agent is intermittently supplied from an upstream side of the precatalyst to the precatalyst and the NOx catalyst. At this time, it is determined that the larger the width of change in temperature of the NOx catalyst, the higher the level of degradation of the oxidation catalyst is.

1 . An exhaust gas purification system for an internal combustion engine, comprising:
 an NOx storage-reduction catalyst that is arranged on an exhaust passage of the internal combustion engine;   a precatalyst that is arranged on the exhaust passage at a location upstream of said NOx storage-reduction catalyst and has an oxidation function;   a temperature detecting unit that detects the temperature of said NOx storage-reduction catalyst;   a reducing agent supplying unit that intermittently supplies a reducing agent to said precatalyst and said NOx storage-reduction catalyst from an upstream side of said precatalyst; and   a degradation level detection unit that detects the level of degradation of said precatalyst based on the width of change in temperature of said NOx storage-reduction catalyst at the time when said reducing agent is intermittently supplied by said reducing agent supplying unit.   
   
   
       2 . The exhaust gas purification system for an internal combustion engine as set forth in  claim 1 , wherein:
 said degradation level detection unit determines that the larger the width of change in temperature of said NOx storage-reduction catalyst at the time when the reducing agent is intermittently supplied by said reducing agent supplying unit, the higher the level of degradation of said precatalyst is.   
   
   
       3 . The exhaust gas purification system for an internal combustion engine as set forth in  claim 1 , comprising:
 an NOx reduction control execution unit that executes NOx reduction control for reducing the NOx occluded in said NOx storage-reduction catalyst by intermittently supplying the reducing agent by unit of said reducing agent supplying unit; and   a precatalyst temperature raising unit that, when NOx reduction control is performed by said NOx reduction control execution unit, serves to make the temperature of said precatalyst higher, in case where the width of change in temperature of said NOx storage-reduction catalyst at time when the reducing agent is intermittently supplied by said reducing agent supplying unit is equal to or more than a predetermined change width, than in case where the temperature change width of said NOx storage-reduction catalyst is less than said predetermined change width.   
   
   
       4 . The exhaust gas purification system for an internal combustion engine as set forth in  claim 3 , wherein:
 the larger the width of change in temperature of said NOx storage-reduction catalyst at the time when the reducing agent is intermittently supplied by said reducing agent supplying unit, the higher a target temperature at the time when the temperature of said precatalyst is raised by said precatalyst temperature raising unit is made.   
   
   
       5 . The exhaust gas purification system for an internal combustion engine as set forth in  claim 4 , wherein:
 the execution of the NOx reduction control by said NOx reduction control execution unit is inhibited when said target temperature becomes higher than a predetermined upper limit value.   
   
   
       6 . The exhaust gas purification system for an internal combustion engine as set forth in  claim 2 , comprising:
 an NOx reduction control execution unit that executes NOx reduction control for reducing the NOx occluded in said NOx storage-reduction catalyst by intermittently supplying the reducing agent by unit of said reducing agent supplying unit; and   a precatalyst temperature raising unit that, when NOx reduction control is performed by said NOx reduction control execution unit, serves to make the temperature of said precatalyst higher, in case where the width of change in temperature of said NOx storage-reduction catalyst at time when the reducing agent is intermittently supplied by said reducing agent supplying unit is equal to or more than a predetermined change width, than in case where the temperature change width of said NOx storage-reduction catalyst is less than said predetermined change width.