Exhaust gas purification system of internal combustion engine

Abstract

A catalyst suitable for reducing NOx in exhaust gas by ammonia in the presence of excess oxygen is arranged in an engine exhaust passage. The catalyst is fed an aqueous urea solution from an addition control valve, part of the urea fed to the catalyst is stored in the catalyst, and the ammonia produced from the urea stored in the catalyst is used to reduce the NOx in the exhaust gas. It is judged whether the storage capacity of the catalyst is larger than a preset allowable upper limit capacity. When it is judged that the storage capacity of the catalyst is larger than the allowable upper limit capacity, the feed of the aqueous urea solution is prohibited.

1 . An exhaust purification system of an internal combustion engine arranging a catalyst suitable for reducing NOx in the exhaust gas by ammonia under an excess of oxygen in an engine exhaust passage, the system comprising:
 a feeding means for feeding an ammonia generating compound to the catalyst; and   a feed controlling means for controlling the amount of feed of the ammonia generating compound,   the catalyst having a function of storing at least part of the ammonia generating compound fed to the catalyst in the catalyst and generating ammonia from the ammonia generating compound stored in the catalyst and using the generated ammonia to reduce the NOx in the exhaust gas,   the system further comprising a judging means for judging if a storage capacity of the catalyst is larger than a preset allowable upper limit capacity,   wherein the feed controlling means prohibits the feed of the ammonia generating compound when the storage capacity of the catalyst is larger than the allowable upper limit capacity.   
     
     
         2 . An exhaust purification system of an internal combustion engine as set forth in  claim 1 , wherein the allowable upper limit capacity is set so that the amount of a substance derived from the ammonia generating compound discharged from the catalyst later does not exceed the allowable upper limit amount. 
     
     
         3 . An exhaust purification system of an internal combustion engine as set forth in  claim 2 , wherein the substance derived from the ammonia generating compound is at least one substance selected from ammonia, hydrogen cyanide, and isocyanic acid. 
     
     
         4 . An exhaust purification system of an internal combustion engine as set forth in  claim 2 , wherein the allowable upper limit capacity is set so that the ammonia, hydrogen cyanide, and isocyanic acid discharged later from the catalyst all do not exceed the respective allowable upper limit amounts. 
     
     
         5 . An exhaust purification system of an internal combustion engine as set forth in  claim 1 , wherein the storage capacity of the catalyst becomes larger when the temperature of the catalyst is low compared with when the temperature of the catalyst is high, and wherein the judging means judges that the storage capacity of the catalyst is larger than the allowable upper limit capacity when the temperature of the catalyst is lower than a preset temperature set to a temperature where the storage capacity of the catalyst becomes the allowable upper limit capacity. 
     
     
         6 . An exhaust purification system of an internal combustion engine as set forth in  claim 5 , wherein an NOx reduction rate of the catalyst becomes higher when the temperature of the catalyst is high compared with when the temperature of the catalyst is low, and wherein the preset temperature is set higher than the temperature where the NOx purification rate of the catalyst becomes a preset allowable lower limit rate. 
     
     
         7 . An exhaust purification system of an internal combustion engine as set forth in  claim 1 , wherein the feed controlling means allows the feed of the ammonia generating compound when the storage capacity of the catalyst is smaller than the allowable upper limit capacity.