Achieving Ultra-Low NOx Tailpipe Emissions with a High Efficiency Engine 2020-01-1403
Future regulatory targets for diesel engines pose unique challenges to achieve lower greenhouse gas (GHG) emissions concomitant with lower tailpipe (TP) NOx emissions. Generally, lower GHG emissions are achieved via combustion that generates higher engine out (EO) NOx and lower exhaust gas temperatures. To achieve the lower TP NOx targets with higher EO NOx and lower exhaust gas temperatures will require new aftertreatment architectures. North American on-highway heavy duty diesel (HDD) engines have traditionally used a DOC and DPF since the introduction of the US 2007 emission limits. For these systems, NOx control was achieved using engine calibration parameters. Starting in 2010, this system was augmented with a downstream SCR system. This allowed engine manufacturers to maintain the 2007 EO emission levels while relying on the SCR system to provide an additional 90% NOx reduction to meet the US 2010 limits of 0.2 g/hp.hr NOx. In subsequent years, emission regulations have introduced more stringent GHG limits which have been achieved by increasing EO NOx and relying on higher NOx conversion over the SCR.
Unfortunately there is a limit to the capabilities of the current HDD aftertreatment architecture. In order to achieve ultra-low NOx (below 0.07 g/hp.hr NOx) as well as improved fuel economy, it will be necessary to consider changes to the current design. Preventing NOx slip during cold start or cool exhaust operation will be a challenge since the current architecture has the SCR downstream of the DOC and DPF. High NOx conversion efficiency may be delayed for several minutes before the SCR reaches light-off temperatures. 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 0.2 g/hp.hr NOx over the composite FTP and RMC. Further calibration efforts have been utilized to determine lowest TP NOx possible based on aftertreatment hardware and control algorithm. NOx conversion efficiency will also be compared with respect to N2O formation and NH3 slip.
Brad Adelman, Navtej Singh, Paul Charintranond, Josh Manis