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Ammonia and hydrogen can be produced from water, air and excess renewable electricity (Power-to-fuel) and are therefore a promising alternative in the transition from fossil fuel energy to cleaner energy sources. An Homogeneous-Charge Compression-Ignition (HCCI) engine is therefore being studied to use both fuels under a variable blending ratio for Combined Heat and Power (CHP) production. Due to the high auto-ignition resistance of ammonia, hydrogen is required to promote and stabilize the HCCI combustion. Therefore the research objective is to investigate the HCCI combustion of varying hydrogen-ammonia blending ratios in a 16:1 compression ratio HCCI engine, with a specific focus maximizing the ammonia proportion as well as on the NOx emissions that could arise from the nitrogen contained in the ammonia. A single-cylinder, constant speed, HCCI engine has been used with an intake pressure varied from 1 to 1.5 bar and with intake temperatures ranging from 428 to 473 K. Stable combustion was achieved with up to 70 %vol. ammonia proportion by increasing intake pressure to 1.5 bar, intake temperature to 473 K, and equivalence ratio to 0.28. From pure hydrogen to 60 %vol. ammonia proportion, the combustion efficiency only lost 0.6 points. Pure hydrogen Indicated Mean Effective Pressure (IMEP) was limited to 2.7 bar to avoid ringing (i.e. too high pressure rise rate) but blended with ammonia the IMEP safely reached 3.1 bar. For pure hydrogen, NOx emissions were below 6 ppm. For hydrogen-ammonia blends, NOx were between 1000 and 3500 ppm. Exhaust Gas Recirculation (EGR) operations significantly reduced NOx emissions through a reduced oxygen availability but with a noticeable negative effect on combustion efficiency due to lower in-cylinder temperatures. The performed simulations showed the production of significant N2O quantities under 1400 K. Ammonia showed to be an effective fuel for HCCI conditions and EGR revealed itself as a promising NOx reducing technique through a decreased oxygen availability. Still the combustion temperature must be kept above 1400 K

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