Long breathing lean NO_x trap regeneration with supplemental n-butanol

Abstract

This paper evaluates a long breathing strategy of lean NO x trap for achieving ultra-low nitrogen oxide (NO x) emissions, with an aim to reduce the associated fuel penalty. The fuel impacts on the long breathing strategy of lean NO x trap operation are examined on a heated flow bench with diesel and n-butanol as the reductants. Engine tests are performed to identify suitable working regions for the lean NO x trap strategies. For a very low engine-out NO x emission level of ~30 ppm, the long breathing adsorption of the lean NO x trap shows a significant improvement in NO x storage efficiency compared to a conventional lean NO x trap operational strategy for a moderate level of NO x emissions. The use of n-butanol fuel in diesel engines produces much lower NO x and particulate matter emissions, which is deemed advantageous for operating the long breathing lean NO x trap strategy. As a reductant for lean NO x trap regeneration, n-butanol is found to be more effective in terms of regeneration effectiveness, NO x conversion efficiency, and potential hydrogen (H2) yield compared to using diesel fuel in the after-treatment. A fuel penalty analysis is conducted for the selected cases with combinations of different combustion modes and lean NO x trap strategies. Given a low level of NO x emissions from n-butanol combustion, the long breathing lean NO x trap strategy can potentially achieve ultra-low NO x emissions with a minimum fuel penalty.

Catalytic NO_x Aftertreatment—Towards Ultra-Low NO_x Mobility

“…The use of onboard fuel for LNT regeneration as reductant induces a fuel penalty because of periodic fuel injection events in the aftertreatment system. This fuel penalty (additional fuel energy required for LNT regeneration) can be as high as 2.5% for conventional LNT operation [65]. The conventional hightemperature combustion in IC engines results in significant NO x generation that can quickly saturate the LNT catalyst, requiring frequent regeneration events that increase the fuel penalty.…”

“…Lowering the combustion temperature by using a moderate level of EGR can significantly curtail the engine-out NO x emissions without a considerable impact on the engine performance, thus prolonging the lean NO x storage period and reducing the frequency of required regeneration cycles. This combination of in-cylinder and tailpipe NO x mitigation, termed as "long breathing" LNT strategy, has achieved low tailpipe NO x emissions with diminished fuel penalty as low as 0.3% [65][66][67]. The dispersion and spatial distribution of alkaline metal compounds and PGMs have a significant effect on the storage and reduction reactions on LNT [62].…”