Numerical analysis of the combustion process in a pre-chamber marine engine fueled with ammonia-hydrogen mixtures

Ammonia and hydrogen, as carbon-neutral fuels, possess the potential to play a crucial role in the decarbonization of the mobility sector. This research examines the optimization of the combustion process in a marine spark-ignition engine through the use of a passive pre-chamber. The study has been carried out using computational fluid dynamics (CFD) models. Considering a hydrogen content in the fuel blend of 15% by volume, at a fixed equivalence ratio equal to 0.8, two different nozzle diameters have been tested, and the optimal spark timings have been identified. Then, the effect of different hydrogen amounts in the fuel mixture on the engine’s performance and emissions has been assessed. An optimal spark timing of 712 CAD has been found for both 3 mm and 5 mm nozzles at the specified operating point. The 5 mm nozzle provides slightly higher IMEPH and gross efficiency, with minimal impact on emissions. Reducing hydrogen in the fuel blend from 15% to 10% lowers IMEPH from 31 to 12 bar and gross efficiency from 46.9% to 18.8%. At 5% hydrogen, combustion cannot start. This decrease in the hydrogen amount raises unburned ammonia, NO2, and N2O emissions, while NO emissions are significantly reduced. Decreasing hydrogen content reduces turbulent kinetic energy in the combustion chamber.