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To meet the severe PM (Particulate Matter) emission regulation for diesel vehicles, use of a DOC (Diesel Oxidation Catalyst) + CSF (Catalyzed Soot Filter) emission control system has recently been started in Japan. The CSF is used for diesel exhaust soot filtering, and the accumulated soot is periodically combusted to regenerate the CSF. The heat required for the soot regeneration is supplied from the DOC pre-cat under a specific regeneration mode. Additional fuel is supplied to the DOC by post injection and/or fuel dosing into the exhaust pipe upstream of DOC. The resulting exotherm provides the heat for soot combustion on the CSF. Uniform and controlled soot combustion in the CSF is necessary to maintain system durability during the vehicle's life. There are concerns that non-uniform fuel supply to the DOC front face may lead to non-uniform soot combustion in the CSF.