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The current publication considers the benefits of placing a SCR system upstream of the DOC. This will enable rapid light-off of the upstream system. The tradeoff between engine out NOx levels, exhaust temperature, and dosing levels between upstream and downstream SCR will be compared with respect to achieving below 0.05 g/hp-hr NOx over the composite FTP and RMC. NOx conversion efficiency will also be compared with respect to N2O formation and NH3 slip.

Current legislative trends regarding diesel emissions are striving to achieve two seemingly competing goals: simultaneously lowering NOx and greenhouse gas (GHG) emissions. These two goals are considered at odds since lower GHG emissions (e.g. CO2) is achieved via high combustion efficiency that result in higher engine out NOx emissions and lower exhaust gas temperatures [1, 2]. Conversely, NOx reduction technologies such as SCR require temperatures above 200°C for dosing the reductant (DEF) [3, 4, 5] as well as for high conversion efficiencies [1, 2, 6, 7, 8, 9]. Dosing DEF requires injection pressures around 5 bar to ensure proper penetration into the exhaust stream as well as generate the appropriate spray pattern and droplet sizes. Dosing DEF generally requires long mixing and/or high turbulence (high restriction) areas so that the aqueous urea solution can be converted into gaseous NH3 without deposit formation [8, 10, 11, 12, 13, 14, 15].