Ammonia Production and Utilization in a Hybrid LNT+SCR System

A hybrid LNT+SCR system is used to control NOx from a light-duty diesel engine with in-cylinder regeneration controls. A diesel oxidation catalyst and diesel particulate filter are upstream of the LNT and SCR catalysts. Ultraviolet (UV) adsorption spectroscopy performed directly in the exhaust path downstream of the LNT and SCR catalysts is used to characterize NH₃ production and utilization in the system. Extractive exhaust samples are analyzed with FTIR and magnetic sector mass spectrometry (H₂) as well. Furthermore, standard gas analyzers are used to complete the characterization of exhaust chemistry. NH₃ formation increases strongly with extended regeneration (or “over regeneration”) of the LNT, but the portion of NOx reduction occurring over the SCR catalyst is limited by the amount of NH₃ produced as well as the amount of NOx available downstream of the LNT. Control of lean-rich cycling parameters enables control of the ratio of NOx reduction between the LNT and SCR catalysts. During lean-rich cycling, fuel penalties are similar for either LNT dominant or LNT with supplemental SCR NOx reduction. However, stored NH₃ after multiple lean-rich cycles can enable continued NOx reduction by the SCR after lean-rich cycling stops; thus, requirements for active regeneration of the LNT+SCR system can be modified during transient operation.