Clarification of NH₃ Formation Mechanism on a Diesel Engine NOx Storage Reduction Catalyst Under Rich Conditions and Evaluation of the SCR Benefit at WLTC

Abstract

Recent studies have shown that the combination of a NOx storage reduction catalyst (NSR) and a passive selective catalytic reduction catalyst (pSCR) would be a technical solution to meet post EU6 emission legislation for small passenger cars. In this case, NH₃ formed during rich NSR regeneration is used to reduce remaining NOx on a SCR catalyst which is located downstream of the NSR. In this study, first, the NH₃ formation mechanism of NSR under different emission λs, rich durations, and stored NOx values are clarified using a diesel engine operating under steady-state operation. Finally, NOx conversion and the associated fuel consumption penalty are clarified under the Worldwide harmonized light vehicle test cycle (WLTC). It was identified that NH₃ starts to be produced by the NSR after oxygen storage capacity depletion. Next, a NH₃ peak occurs as a result of the converted pre-stored and engine out NOx. If the rich operation is long enough, the pre-stored NOx contribution fades and NH₃ concentration lowers to an equivalent level as the engine out NOx. From this understanding, several WLTC patterns were measured varying the number of rich operations and their duration in order to identify best regeneration strategy with a minimum fuel consumption penalty. The optimal pSCR benefit, in terms of fuel penalty, is achieved when the rich operation duration is (1) longer than the breakthrough time, (2) includes the initial NH₃ peak resulting from the conversion of the pre-stored NOx.