Ammonia and hydrogen can be produced from water, air and excess electricity using power-to-fuel and are therefore a promising step in the transition from fossil fuel energy to cleaner energy sources. Indeed, produced from excess renewable electricity these two fuels can provide mid- and long-term energy storage. An HCCI engine can be used to convert those two energy vectors to electricity and heat. The purpose of the study was to see the possible range of ammonia concentrations that could be used in a conventional 16:1 compression ratio HCCI engine. Due to the high ignition resistance of ammonia, hydrogen was used to promote and stabilize the combustion. Equivalence ratios of 0.2 to 0.3 were utilized to limit the pressure rise rate. Engine speed was kept constant at 1500 rpm at an intake pressure from 1 to 1.5 bar and with intake temperatures from 428 to 473 K. Stable combustion was achieved with up to 60 molar % of ammonia in hydrogen at an equivalence ratio of 0.2 without ozone by increasing intake pressure and temperature to 1.5 bar and 473 K.