Development of SCR on Diesel Particulate Filter System for Heavy Duty Applications

Selective Catalytic Reduction (SCR) catalysts have been demonstrated as an effective solution for controlling NOx emissions from diesel engines. Typical 2010 Heavy-Duty systems include a DOC along with a catalyzed soot filter (CSF) in addition to the SCR sub-assembly. There is a strong desire to further increase the NOx conversion capability of such systems, to enable additional fuel economy savings by allowing engines to be calibrated to higher engine-out NOx levels. One potential approach is to replace the CSF with a diesel particulate filter coated with SCR catalysts (SCR-DPF) while keeping the flow-through SCR elements downstream, which essentially increases the SCR volume in the after-treatment assembly without affecting the overall packaging. In this work, a system consisting of SCR-DPF was evaluated in comparison to the DOC + CSF components from a commercial 2010 DOC + CSF + SCR system on an engine with the engine EGR on (standard engine-out NOx) and off (high engine-out NOx). The SCR-DPF system exhibited significantly higher NOx reduction efficiency than the CSF systems under both steady state and heavy-duty FTP transient conditions when the engine operated at the same condition. The soot oxidation activity on these two systems was also evaluated. Net soot oxidation (i.e., more soot removed from the filter than entered it from the engine) was observed over the SCR-DPF system at 400°C presumably because of the high NOx/PM ratio when the engine EGR was turned off. No net soot burn was observed for the 2010 CSF system when operating with standard EGR at the specific conditions chosen for these series of testing. The high passive filter regeneration activity of the SCR-DPF system with the engine operating at high engine-out NOx may result in less frequent active regeneration events and, hence, reduce the fuel penalty associated with active regeneration. The results of this work demonstrated that using SCR-DPF systems not only could meet current NOx reduction regulations but also improve fuel economy for heavy-duty diesel vehicles by allowing them to operate at higher engine-out NOx conditions.