Future Directions in Diesel SCR Systems

Christine  Lambert, Ford Motor Co.

Urea SCR systems were introduced on diesel trucks in the U.S. in 2010. An important diesel segment is the Class 2b and 3 medium-duty trucks in which >70% of customers choose the diesel powertrain. These weight classes were certified as a complete chassis on the light-duty Federal Test Procedure for LEVII emissions. This required the addition of lean NOx aftertreatment in addition to the diesel oxidation catalyst (DOC) and diesel particulate filter (DPF) that were already part of the aftertreatment system. It took over ten years of research and development to reach the required NOx efficiencies of >80% at 120,000 miles of durability, and most manufacturers chose Urea SCR over alternative lean NOx trap technology for its enhanced performance at lower cost and lower fuel penalty. While the initial systems met the standards, further improvements in performance and cost are always beneficial. SCR systems are now undergoing materials cost reductions through reduced use of precious metals and more use of lower cost Pd than Pt. Aftertreatment efficiency will improve through enhanced catalyst washcoat durability, catalyst and filter structure, and heating strategies. As we prepare for LEVIII, aftertreatment modeling can help sort through potential designs and eliminate those systems early on that have less chance to meet future performance and cost targets.

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