Experimental and numerical analysis of unburned ammonia and nitrous oxide emission characteristics in ammonia/diesel dual-fuel engine
Yoichi Niki
Abstract
Ammonia (NH 3 ) combustion has been investigated as a carbon-free fuel for internal combustion engines. The marine sector attempts to use NH 3 as fuel for diesel engines. However, NH 3 combustion in diesel engines can emit unburned NH 3 and nitrous oxide (N 2 O) via the greenhouse effect, and the mechanisms of emission production remain unclear. In this study, the combustion of a premixed NH 3 –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 NH 3 . As the energy fraction of NH 3 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 NH 3 emissions increased, whereas CO emission decreased as the energy fraction of NH 3 increased; by contrast, N 2 O emissions increased. However, the increase in N 2 O diminished when the energy fraction of NH 3 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. N 2 O formation can be categorized into two stages: a steep formation of N 2 O with main heat release and CO 2 production, followed by a relatively low formation rate of N 2 O with a decreasing rate of NH 3 decomposition and reduction in CO 2 production. The early injection of pilot fuel allowed the pilot fuel to distribute to the wide area of the combustion chamber and NH 3 to rapidly combust without remaining N 2 O. The rapid combustion eliminates the second stage of relatively slow combustion, which generated N 2 O. The low NH 3 and N 2 O emissions observed experimentally with the early pilot fuel injection may be due to this mechanism