Identification of N2O formation in an ammonia/n-heptane dual-fueled compression ignition engine using numerical simulations

Abstract

Ammonia, as an alternative fuel to reduce harmful gas emissions from internal combustion engines, has gained increased interest during the last decade. However, the combustion of ammonia could lead to increased formation of nitrous oxide (N 2 O), a potent greenhouse gas that can offset the climate benefits of using a carbon-free fuel. N 2 O is an intermediate species that ideally gets reduced by thermal decomposition or reacting with atomic hydrogen in the later stage of the combustion, leaving low post-flame concentrations. This study investigates events leading to N 2 O formation and accumulation in a dual-fuel direct injection compression ignition engine operated with ammonia and diesel. The injection timing of diesel is fixed, while four different injection times of ammonia are investigated. The results reveal that N 2 O accumulation within the engine cylinder can be traced to phenomena that prevent complete combustion. The findings in this article show accumulation of N 2 O close to the cylinder walls, in regions cooled by ammonia evaporation featuring elevated ammonia concentrations, and in areas with slow oxidation of ammonia during the expansion stroke. A lower consumption of N 2 O in post-flame reactions is observed for early injection timings, caused by misdirected ammonia spray causing low temperatures outside the piston bowl. This increases N 2 O concentrations during the expansion stroke of up to 50%, whereas simultaneous injections minimize ammonia dispersion into the squish area, reducing N 2 O concentrations by 37%. Ensuring proper ammonia injection direction and utilizing the pilot fuel to interact with the ammonia spray reduces the occurrence of phenomena resulting in N 2 O accumulation. The results of this study contribute to the understanding of N 2 O formation, consumption, and accumulation, facilitating the optimization of direct ammonia injection strategies. • Numerical simulations of a high-pressure dual-fuel engine using ammonia and diesel. • N 2 O behavior is analyzed for four different injection timings of ammonia. • Three distinct processes resulting in N 2 O in the exhaust are identified and highlighted. • Ammonia evaporation forms cold, ammonia-rich zones, hindering N 2 O decomposition. • Simultaneous injections favor ammonia burnout and N 2 O decomposition.

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