Limiting nox emissions using two catalysts

Abstract

A method for controlling an internal combustion engine limits emission of undesirable compounds of nitrogen and oxygen and provides increased transient power.

1 . A method of operating a compression ignition engine to limit NO x  exhaust emissions, comprising:
 operating the engine at a first power output by providing a first, lean air/fuel mixture to a combustion chamber of the engine to generate exhaust gas;   receiving a request for an increased power output from the engine, that is greater than the first power output;   providing a second air/fuel mixture to the combustion chamber of the engine which is at stoichiometric or more rich than stoichiometric in response to the request for an increased power output from the engine; and   treating a flow of exhaust gas with a first catalyst formulated to reduce NO x  emissions during operation of the engine with the lean air/fuel mixture and; and   also treating the flow of exhaust gas with a second catalyst formulated to reduce NO x  emissions during operation of the engine with the second, non-lean air/fuel mixture.   
     
     
         2 . The method of  claim 1  in which the first catalyst is formulated to also reduce NO x  emissions during operation of the engine with the non-lean air/fuel mixture. 
     
     
         3 . The method of  claim 1  in which the first catalyst is a diesel oxidation catalyst. 
     
     
         4 . The method of  claim 1  in which the second catalyst is a three way catalyst. 
     
     
         5 . The method of  claim 1  in which the first catalyst is sized and formulated to reduce NO x  emissions to below a predetermined level during operation of the engine with the lean air/fuel mixture. 
     
     
         6 . The method of  claim 1  in which the second catalyst is sized and formulated to reduce NO x  emissions to below a predetermined level during operation of the engine with the non-lean air/fuel mixture. 
     
     
         7 . The method of  claim 6  in which the second catalyst is sized and formulated to reduce NO x  emissions to below a predetermined level during operation of the engine with the non-lean air/fuel mixture throughout the entire operating range of the engine. 
     
     
         8 . The method of  claim 1  in which the first catalyst is sized and formulated to reduce NO x  emissions to below a predetermined level during operation of the engine with the lean air/fuel mixture, and the second catalyst is sized and formulated to reduce NO x  emissions to below the predetermined level during operation of the engine with the non-lean air/fuel mixture. 
     
     
         9 . The method of  claim 5  in which the predetermined level is 0.2 g NO x /hp-hr. 
     
     
         10 . The method of  claim 6  in which the predetermined level is 0.2 g NO x /hp-hr. 
     
     
         11 . The method of  claim 8  in which the predetermined level is 0.2 g NO x /hp-hr. 
     
     
         12 . The method of  claim 8  in which the predetermined level is 0.1 g NO x /hp-hr. 
     
     
         13 . The method of  claim 8  in which the predetermined level is 0.01 NO x /hp-hr. 
     
     
         14 . The method of  claim 1  in which the first catalyst and the second catalyst are formulated to continue to reduce NO x  in exhaust without NO x  saturation of the first or second catalyst regardless of the period of continuous operation. 
     
     
         15 . The method of  claim 1  in which:
 i) the first and second catalysts reduce NO x  emissions to keep NO x  emissions below a first predetermined level during operation of the engine with a lean air/fuel ratio as well as during operation of the engine with a non-lean air/fuel ratio; and 
 ii) the exhaust gases are also treated with an SCR catalyst in an environment which reduces NO x  emissions to a second predetermined level which is less than the first level.