In-cylinder NO_x active reduction via ammonia post-injection in ammonia direct-injection engines

Abstract

High NO_x emissions pose a critical challenge for ammonia engines. This study proposes ammonia post-injection as a strategy to achieve in-cylinder NO_x active reduction in ammonia direct-injection engines, offering an innovative approach for NO_x emission control. Computational fluid dynamics (CFD) simulation results elucidate the characteristics of ammonia combustion and NO_x evolution under ammonia post-injection conditions. The post-injected ammonia can efficiently reduce the in-cylinder NO_x it encounters, leading to a significant decrease in NO_x concentration. Chemical kinetics analysis was conducted to reveal the underlying mechanisms and reaction pathways of the SNCR effect on NO_x. The reduction of NO_x primarily proceeds through the reactions between NO/NO₂ and NH/NH₂. NO is reduced via three pathways, yielding NNH (by NH₂), N₂O (by NH), and N₂ (by NH and NH₂), respectively. In contrast, NO₂ is reduced via a single pathway that yields N₂O under the action of NH and NH₂. NH₂ plays the overwhelmingly dominant role in reducing both NO and NO₂. The effectiveness and feasibility of ammonia post-injection in reducing NO_x emissions were evaluated through engine experiments. The experimental results demonstrate that the ammonia post-injection strategy enables significant NO_x reduction for ammonia direct-injection engines. In the current work, a 14.4% NO_x reduction was achieved without compromising combustion efficiency by appropriately delaying the ammonia post-injection timing, confirming the great potential of ammonia post-injection in NO_x emission control.

Graphical abstract

Keywords

NO_x reduction / ammonia post-injection / selective non-catalytic reduction / ammonia combustion / ammonia direct-injection engines

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