Experimental study of RCCI engine – Ammonia combustion with diesel pilot injection

Ammonia is seen as one potential carbon-free fuel, especially for maritime applications. Since SI engines require a significant ignition energy for large cylinders, engine manufacturers are targeting the use of ammonia in Compressed Ignition (CI) engines. Because of ammonia’s high auto-ignition temperature, to ensure that the combustion occurs in a CI engine, a pilot injection of a higher reactivity fuel must be used, as in Reactivity Controlled Compression Ignition engines. In the present study, the objective was to provide first unique data about the efficiency and pollutant emissions for a single cylinder compression ignition engine with a diesel energy fraction as minimum as possible (down to less than 2%) at a constant 1000 rpm. Experiments cover the impact of a wide variation of equivalence ratios of NH₃-air mixtures from ultra-lean to slightly rich conditions. CO₂, CO, NH₃, NOX, N₂O, UHC values were measured with a Fourier Transform Infrared (FTIR) spectrometer. Results of CO₂ and N₂O are presented as CO₂-Equivalent (CO₂eq) impact. Combustion stability was achieved for most  conditions but not for the leanest ones. Furthermore, under lean conditions for a similar ammonia content, the minimum CO₂eq is reached with a slightly higher Diesel Energy Fraction than the minimum possible. Finally, both leanest and richest conditions present a higher level of CO₂eq compared to the range of ammonia/air mixtures at stoichiometry or just below.