Scr after-treatment of engine exhaust gas

Abstract

Reductant is metered into an engine exhaust gas after-treatment system upstream of an SCR catalyst at engine exhaust gas temperatures both less than and greater than a DEF dosing threshold temperature to render the catalyst effective in reducing NOx without creating undesired reaction products which would impair effectiveness of the after-treatment system at engine exhaust gas temperatures less than the DEF dosing threshold temperature.

What is claimed is: 
     
         1 . An internal combustion engine comprising:
 an engine exhaust gas after-treatment system comprising a catalyst for reducing NOx in engine exhaust gas by selective catalytic reduction, a supply of reductant, and a controller for metering reductant into the after-treatment system upstream of the catalyst at engine exhaust gas temperatures both less than and greater than a DEF dosing threshold temperature to render the catalyst effective in reducing NOx without creating a reaction product which would impair effectiveness of the after-treatment system by depositing and collecting on components of the after-treatment system at engine exhaust gas temperatures less than the DEF dosing threshold temperature.   
     
     
         2 . The internal combustion engine as set forth in  claim 1  in which the reductant comprises diesel exhaust fluid (DEF), the after-treatment system comprises an electric heater upstream of the catalyst for heating engine exhaust gas, and the controller controls operation of the electric heater to heat engine exhaust gas which has a temperature less than the DEF dosing threshold temperature. 
     
     
         3 . The internal combustion engine as set forth in  claim 2  in which the after-treatment system comprises an injector through which DEF is introduced into engine exhaust gas flow through the after-treatment system, and the heater is disposed in the engine exhaust gas flow at a location downstream of the injector. 
     
     
         4 . The internal combustion engine as set forth in  claim 3  in which the heater comprises a surface across which engine exhaust gas flows and which contains a catalyst coating for reducing NOx in engine exhaust gas by selective catalytic reduction. 
     
     
         5 . The internal combustion engine as set forth in  claim 4  in which the surface of the heater which contains a catalytic coating for reducing NOx in engine exhaust gas by selective catalytic reduction has a surface area which is less than a surface area of the catalyst across which engine exhaust gas flows after having flowed past the heater. 
     
     
         6 . The internal combustion engine as set forth in  claim 2  in which the after-treatment system comprises an injector through which DEF is introduced into engine exhaust gas flow through the after-treatment system, and the heater is disposed in the engine exhaust gas flow at a location upstream of the injector. 
     
     
         7 . The internal combustion engine as set forth in  claim 1  in which the supply of reductant comprises a supply of DEF and a supply of ammonia, and the controller controls metering of reductant into the after-treatment system to meter ammonia and not DEF into the after-treatment system at engine exhaust gas temperatures less than the DEF dosing threshold temperature and to meter DEF and not ammonia into the after-treatment system at engine exhaust gas temperatures greater than the DEF dosing threshold temperature. 
     
     
         8 . The internal combustion engine as set forth in  claim 7  in which the controller comprises an ammonia metering valve which is controlled to meter ammonia into the after-treatment system at engine exhaust gas temperatures less than the DEF dosing threshold temperature and a DEF metering valve which is controlled to meter DEF into the after-treatment system at engine exhaust gas temperatures greater than the DEF dosing threshold temperature.