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