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Ammonia-fired piston engines have emerged as a promising technology for ship engines at medium scale (1-10 MW). The use of ammonia cracking, where a catalyst is used to crack ammonia to hydrogen and nitrogen, to produce small quantities of hydrogen that can be used as a combustion promoter instead of fossil fuels, enables this technology to provide a purely carbon-free propulsion and power-generation technology. In this context, a hydrogen-fired prechamber turbulent jet ignition approach has been proposed and it is envisioned to offer significant advantages by accelerating the ammonia ignition and combustion process, increasing its reliability and completeness and achieving low NOx, N2O and NH3 emissions if combined with a non-premixed ammonia flame configuration. This study exploits an OpenFOAM-based Large Eddy Simulation (LES) model to investigate the combustion behaviour of an ammonia main charge ignited by a hydrogen-fired prechamber. Firstly, a conventional configuration where the ammonia is premixed with air before ignition is considered and the hydrogen-fired prechamber is found to provide a strong ignition source for ammonia-air mixtures. The effect of the main-charge equivalence ratio and of the wall temperature on combustion efficiency and emissions formation is evaluated. Secondly, the same hydrogen-fired prechamber configuration is shown to be able to successfully ignite a liquid ammonia main charge directly injected as a spray and modelled using Lagrangian Point Particles (LPP).