Impact of Second NH ₃ Storage Site on SCR NO _x Conversion in an Ultra-Low NO _x Aftertreatment System

Typical two-site storage-based SCR plant models in literature consider NH₃ stored in the first site to participate in NH₃ storage, NO_x conversion and second site to only participate in NH₃ storage passively. This paper focuses on quantifying the impact of stored NH₃ in the second site on the overall NO_x conversion for an ultra-low NO_x system due to intra site NH₃ mass transfer. Accounting for this intra site mass transfer leads to better prediction of SCR out NH₃ thus ensuring compliance with NH₃ coverage targets and improved dosing characteristics of the controller that is critical to achieving ultra-low NO_x standard. The stored NH₃ in the second site undergoes mass transfer to the first site during temperature ramps encountered in a transient cycle that leads to increased NO_x conversion in conditions where the dosing is switched off. The resultant NH₃ coverage fraction prediction is critical in dosing control of SCR. This phenomenon is evaluated and quantified with different aging conditions, where the increased second site storage and reduced standard SCR activity due to hydrothermal aging leads to further increase in the reported phenomena. Although this phenomenon was observed for both light-off SCR (Lo-SCR) and downstream SCR based on analysis of the data, the impact on Lo-SCR performance was found to be higher compared to the downstream system due to the transient thermal conditions and higher temperatures experienced by the Lo-SCR system. This mass transfer mechanism also plays a role in determining NH₃ slip characteristics of Lo-SCR for real world conditions where the gradual transfer of NH3 in the axial direction leads to NH3 slip. This phenomenon is demonstrated using experimental data collected on a production engine for a set of HFTP, CFTP, RMC and LLC cycles.