Ammonia is seen as one potential carbon-free fuel, especially for maritime applications. Since SI engines require important ignition energy for large cylinders, engine manufacturers are targeting the use of ammonia in 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 engine. 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 constant 1000 rpm. Experiments cover the impact of a wide variation of the equivalence ratio of NH3-air mixture, 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 impact. The combustion stability was achieved for most of conditions but not 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. Last, both leanest and richest conditions present a higher level of CO2eq compared to the range of ammonia/air mixture at stoichiometry or just below.