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 . A method for reducing NOx in engine exhaust gas flow through an engine exhaust gas after-treatment system by selective catalytic reduction, the method comprising:
 metering reductant into the after-treatment system upstream of a catalyst at engine exhaust gas temperatures both less than and greater than a threshold temperature to render the catalyst effective in reducing NOx at engine exhaust gas temperatures less than and greater than a DEF dosing threshold temperature 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 method as set forth in  claim 1  in which metering reductant into the after-treatment system upstream of a catalyst comprises metering diesel exhaust fluid (DEF) into engine exhaust gas flow through the after-treatment system and operating an electric heater upstream of the catalyst to heat engine exhaust gas when engine exhaust gas has a temperature less than the DEF dosing threshold temperature. 
     
     
         3 . The method as set forth in  claim 2  in which metering DEF into the after-treatment system comprises introducing DEF into engine exhaust gas flow through the after-treatment system at a location upstream of the heater. 
     
     
         4 . The method as set forth in  claim 3  including reducing NOx in engine exhaust gas by selective catalytic reduction by flow of engine exhaust gas across a surface of the heater which contains a catalyst coating for reducing NOx in engine exhaust gas by selective catalytic reduction. 
     
     
         5 . The method as set forth in  claim 4  including reducing NOx in engine exhaust gas by selective catalytic reduction by flow of engine exhaust gas across a surface of the heater which contains a catalyst coating for reducing NOx in engine exhaust gas by selective catalytic reduction and which 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 method as set forth in  claim 2  further comprising introducing DEF into engine exhaust gas flow through the after-treatment system at a location downstream of the heater. 
     
     
         7 . The method as set forth in  claim 1  in which metering reductant into the after-treatment system upstream of a catalyst at engine exhaust gas temperatures both less than and greater than the DEF dosing threshold temperature comprises metering ammonia from an ammonia supply and not DEF from a DEF supply into the after-treatment system at engine exhaust gas temperatures less than the DEF dosing threshold temperature and metering DEF from the DEF supply and not ammonia from the ammonia supply into the after-treatment system at engine exhaust gas temperatures greater than the DEF dosing threshold temperature. 
     
     
         8 . The method as set forth in  claim 7  comprising controlling an ammonia metering valve to meter ammonia into the after-treatment system at engine exhaust gas temperatures less than the DEF dosing threshold temperature and controlling a DEF metering valve to meter DEF into the after-treatment system at engine exhaust gas temperatures greater than the DEF dosing threshold temperature.