Control of NO/NO2 Ratio to Improve SCR Efficiency for Treating Engine Exhaust Using Bypass Oxidation Catalyst

Methods and systems for treating NOx-containing exhaust from an internal combustion engine. An exhaust aftertreatment system has at least a primary oxidation catalyst, a particulate filter, and a selective reduction catalyst (SCR). A bypass line diverts a portion of the exhaust from the exhaust line from a point downstream the particulate filter to a point upstream the SCR. A secondary oxidation catalyst on the bypass line is used to generate NO or NO2 to be returned to the exhaust line upstream the SCR.