Catalyst deterioration judging system

Abstract

The deterioration judgment for an absorption reduction type NOx catalyst is performed quickly and correctly. A catalyst deterioration judging system includes a supply unit which supplies a reducing agent, a measuring unit which measures a NOx concentration in an exhaust gas at a position downstream from the catalyst, a control unit which regulates an amount of the reducing agent so that an air-fuel ratio of the exhaust gas is a lean air-fuel ratio when the reducing agent is supplied from the supply unit, and a judging unit which judges deterioration of the catalyst on the basis of the NOx concentration measured by the measuring unit when the amount of the reducing agent is regulated so that the air-fuel ratio of the exhaust gas is the lean air-fuel ratio when NOx is absorbed by the catalyst.

1 . A catalyst deterioration judging system for judging deterioration of an absorption reduction type NOx catalyst which is provided at an exhaust gas passage of an internal combustion engine to absorb NOx and which reduces the absorbed NOx in accordance with supply of a reducing agent, the catalyst deterioration judging system comprising:
 a supply unit which supplies the reducing agent to the absorption reduction type NOx catalyst and which thereby changes an air-fuel ratio of an exhaust gas allowed to pass through the absorption reduction type NOx catalyst;   a measuring unit which measures a NOx concentration in the exhaust gas at a position downstream from the absorption reduction type NOx catalyst;   a control unit which regulates an amount of the reducing agent so that the air-fuel ratio of the exhaust gas is a lean air-fuel ratio when the reducing agent is supplied from the supply unit; and   a judging unit which judges the deterioration of the absorption reduction type NOx catalyst on the basis of the NOx concentration measured by the measuring unit when the amount of the reducing agent is regulated by the control unit so that the air-fuel ratio of the exhaust gas is the lean air-fuel ratio when NOx is absorbed by the absorption reduction type NOx catalyst.   
     
     
         2 . The catalyst deterioration judging system according to  claim 1 , wherein the judging unit judges that the absorption reduction type NOx catalyst is deteriorated if a maximum value of the NOx concentration measured by the measuring unit is not less than a threshold value. 
     
     
         3 . The catalyst deterioration judging system according to  claim 1 , wherein the judging unit judges that the absorption reduction type NOx catalyst is deteriorated if an added-up value of the NOx concentration measured by the measuring unit is not less than a threshold value. 
     
     
         4 . The catalyst deterioration judging system according to  claim 1 , wherein the control unit regulates the amount of the reducing agent so that the air-fuel ratio of the exhaust gas is a rich air-fuel ratio to reduce NOx before regulating the amount of the reducing agent so that the air-fuel ratio of the exhaust gas is the lean air-fuel ratio. 
     
     
         5 . The catalyst deterioration judging system according to  claim 4 , wherein the control unit regulates the amount of the reducing agent so that the air-fuel ratio of the exhaust gas is the lean air-fuel ratio after elapse of a predetermined time after stopping the supply of the reducing agent from the supply unit after regulating the amount of the reducing agent so that the air-fuel ratio of the exhaust gas is the rich air-fuel ratio to reduce NOx. 
     
     
         6 . The catalyst deterioration judging system according to  claim 1 , wherein the control unit regulates the amount of the reducing agent to provide such an air-fuel ratio that NH 3  is not produced by the absorption reduction type NOx catalyst at the provided air-fuel ratio and NO 2  is produced by the absorption reduction type NOx catalyst at the provided air-fuel ratio, when the control unit regulates the amount of the reducing agent so that the air-fuel ratio of the exhaust gas is the lean air-fuel ratio. 
     
     
         7 . The catalyst deterioration judging system according to  claim 1 , wherein the supply unit supplies the reducing agent to the absorption reduction type NOx catalyst by supplying the reducing agent into the exhaust gas allowed to flow through the exhaust gas passage. 
     
     
         8 . The catalyst deterioration judging system according to  claim 1 , wherein the control unit regulates the amount of the reducing agent so that the air-fuel ratio of the exhaust gas allowed to flow into the absorption reduction type NOx catalyst is lowered and the air-fuel ratio of the exhaust gas allowed to flow into the absorption reduction type NOx catalyst is the lean air-fuel ratio when the deterioration of the absorption reduction type NOx catalyst is judged by the judging unit, as compared with a situation provided before the deterioration of the absorption reduction type NOx catalyst is judged.