Tang, W., Youngren, D., SantaMaria, M., and Kumar, S., "On-Engine Investigation of SCR on Filters (SCRoF) for HDD Passive Applications," SAE Int. J. Engines 6(2):862-872, 2013, doi:10.4271/2013-01-1066.
A 4-way Heavy-Duty Diesel (HDD) emissions control aftertreatment system typically consists of diesel oxidation catalyst (DOC), catalyzed soot filter (CSF), urea-based selective catalytic NOx reduction (SCR) and NH₃ slip control catalyst (AMOX). Incorporating the SCR functionality into the soot filter (SCRoF) has great potential to reduce system costs and package volume/weight.In this paper, we discuss some of the recent Cu-Zeolite-based SCR on filter (SCRoF) developments targeting Passive filter regeneration applications. The on-engine investigation of complete DOC+SCRoF+AMOX system focused on three major areas: 1) SCR performance of NOx conversion efficiency and NH₃ slip under both steady state and transient testing conditions; 2) SCRoF system response to sulfur exposure and subsequent sulfur removal for activity recovery; and 3) Characteristics of filter soot load, pressure drop, and passive soot oxidation in SCRoF.Overall, the SCRoF system showed comparable NOx conversion efficiency and sulfur response as the conventional DOC+CSF+SCR+AMOX system. A 92+% cycle cumulative NOx conversion efficiency was demonstrated in a transient emission testing cycle while the peak tailpipe NH₃ slip was well controlled below 5 ppm. With the tested system, soot load does not adversely affect NOx reduction. After sulfur exposure, system performance was fully recovered by desulfation at 500°C - often the threshold temperature for passive systems. It is also important to point out that passive soot oxidation (i.e., C+NO₂) in a SCRoF will be considerably lower than in a catalyzed soot filter (CSF). Passive regeneration capability is inhibited due to SCR reactions competing for NO₂. In this work, intrinsic mechanisms and possible approaches to further enhance passive soot oxidation are reviewed and discussed.