Experimental and numerical analysis of unburned ammonia and nitrous oxide emission characteristics in ammonia/diesel dual-fuel engine

Abstract

Ammonia (NH₃) combustion has been investigated as a carbon-free fuel for internal combustion engines. The marine sector attempts to use NH₃ as fuel for diesel engines. However, NH₃ combustion in diesel engines can emit unburned NH₃ and nitrous oxide (N₂O) via the greenhouse effect, and the mechanisms of emission production remain unclear. In this study, the combustion of a premixed NH₃–diesel dual-fuel initiated by a pilot fuel is investigated experimentally and numerically. The experiments reveal a change in combustion phasing and emission characteristics for up to 80% of the energy fraction of NH₃. As the energy fraction of NH₃ increases, the onset of combustion is delayed, the center of combustion is slightly advanced and then retracted, and the end of combustion is slightly advanced. NO and unburned NH₃ emissions increased, whereas CO emission decreased as the energy fraction of NH₃ increased; by contrast, N₂O emissions increased. However, the increase in N₂O diminished when the energy fraction of NH₃ increased by 40% or more. Computation fluid dynamics simulations based on n-heptane and ammonia reaction kinetics qualitatively reproduced the experimental results. The numerical analysis facilitates the understanding of the underlying phenomena of emission via ammonia–diesel dual-fuel combustion. N₂O formation can be categorized into two stages: a steep formation of N₂O with main heat release and CO₂ production, followed by a relatively low formation rate of N₂O with a decreasing rate of NH₃ decomposition and reduction in CO₂ production. The early injection of pilot fuel allowed the pilot fuel to distribute to the wide area of the combustion chamber and NH₃ to rapidly combust without remaining N₂O. The rapid combustion eliminates the second stage of relatively slow combustion, which generated N₂O. The low NH₃ and N₂O emissions observed experimentally with the early pilot fuel injection may be due to this mechanism