Study on lean combustion of ammonia-hydrogen mixtures in a pre-chamber engine

Abstract

Ammonia holds promise for achieving zero carbon emissions from internal combustion engines. The pre-chamber turbulent jet ignition, characterized by multipoint distributed ignition and rapid combustion, could be applied to ammonia combustion to extend combustion limits and stability. This study investigates the impacts of hydrogen energy fraction (Hfrac), equivalence ratio, and ignition timing on the combustion and emission characteristics of ammonia-hydrogen mixture at lean combustion conditions in a pre-chamber (PC) marine engine by CFD Simulation. In H2-fuel active PC mode, increasing the hydrogen content or achieving stoichiometric combustion within the PC will shorten the cold jet phase and accelerate the formation of a more intense hot jet. As the Hfrac and the equivalence ratio in the main-chamber increase, the combustion pressure, temperature, and heat release rate (HRR) rise, while the combustion duration shortens due to enhanced reactivity, potentially leading to end gas auto-ignition and knock due to lower auto-ignition energy of the mixture. NO primarily forms near the flame front, and both NO concentration and the generation area increase with an increase in hydrogen content and equivalence ratio. Fuel-NO is likely to be the dominant factor in NO generation with lean combustion conditions. N2O, as a product of NH3 low-temperature combustion, is mainly concentrated near the flame front. An increase in unburned NH3 emissions corresponds to an increase in N2O emissions. Delaying the ignition timing to reduce NOx emissions is primarily achieved by controlling NO emissions, even though there is a slight increase in N2O.