System and method to minimize the amount of NOx emissions by optimizing the amount of supplied reductant

Abstract

A method is described for controlling lean and rich operation of an internal combustion engine coupled to a lean NOx trap. In one example, the termination of the lean air-fuel mixture is based on an oxygen storage capacity of the NOx trap. In another example, the level and duration of rich air-fuel ratio purging operation is also controlled based on the oxygen storage capacity of the NOx trap.

We claim:  
     
         1 . A method for controlling an engine having an exhaust with an emission control device capable of storing NOx during lean operating conditions, and converting at least a portion of said NOx during stoichiometric or rich operating conditions, the method comprising: 
 operating the engine to produce a lean exhaust gas mixture fed to the emission control device;    after said lean operation, operating the engine to produce a rich exhaust gas mixture fed to the emission control device, said rich air-fuel ratio determined as a function of at least the oxygen storage capacity of the device.    
     
     
         2 . The method of  claim 1  wherein said rich air-fuel ratio is further based on temperature of the device.  
     
     
         3 . The method of  claim 1  wherein as oxygen storage capacity of the device decreases, the rich air-fuel ratio becomes less rich.  
     
     
         4 . The method of  claim 1  wherein the rich air-fuel ratio is selected to provide a select amount of CO and H2.  
     
     
         5 . The method of  claim 1  wherein the oxygen storage capacity of the device is determined based on device degradation.  
     
     
         6 . The method of  claim 5  wherein device degradation is based on at least one of an amount of sulfur contaminating the device and thermal degradation of the device.  
     
     
         7 . The method of  claim 1  wherein the oxygen storage capacity of the device is determined from rich to lean transition time.  
     
     
         8 . A method for controlling an engine having an 
 exhaust with an emission control device capable of storing NOx during lean operating conditions, and converting at least a portion said NOx during stoichiometric or rich operating conditions, the method comprising: 
 operating the engine to produce a lean exhaust gas mixture fed to the emission control device;  
 estimating the amount of NOx release based on the oxygen storage capacity of the device;  
 after said lean operation, operating the engine to produce a rich exhaust gas mixture fed to the emission control device, said rich air-fuel ratio is determined based at least on the amount of NOx released.  
   
     
     
         9 . The method of  claim 8  wherein said rich air-fuel ratio is further based on temperature of the device.  
     
     
         10 . The method of  claim 8  wherein said amount of NOx release is further based on operating conditions.  
     
     
         11 . The method of  claim 8  wherein as oxygen storage capacity of the device decreases the rich air-fuel ratio becomes less rich.  
     
     
         12 . The method of  claim 8  wherein the rich air-fuel ratio is selected to provide a selected amount of CO and H2 to the device.  
     
     
         13 . The method of  claim 8  wherein the oxygen storage capacity of the device is determined based on device degradation.  
     
     
         14 . The method of  claim 13  wherein device degradation is based on an amount of sulfur contaminating the device.  
     
     
         15 . The method of  claim 8  wherein the oxygen storage capacity of the device is determined from rich to lean transition time.  
     
     
         16 . A computer storage medium having instructions encoded therein for controlling an engine having an 
 exhaust with an emission control device capable of storing NOx during lean operating conditions, and converting at least a portion of said NOx during stoichiometric or rich operating conditions, said medium comprising: 
 code for operating the engine to produce a lean exhaust gas mixture fed to the emission control device;  
 after said lean operation, code for operating the engine to produce a rich exhaust gas mixture fed to the emission control device, said rich air-fuel ratio determined as a function of at least the oxygen storage capacity of the device.