The future of internal combustion (IC) engine relies on carbon free fuels to mitigate climate change. Ammonia (NH3) is a promising carbon-free fuel which can be used as an energy carrier for hydrogen (H2) and directly as a combustible fuel inside the engines. However, burning pure ammonia fuel is difficult due to its low flammability, burning velocity, and consequent large cycle-to-cycle variation. This study used a multiple-spark-plug approach to burn pure ammonia gas with reduced combustion duration and higher engine power output. The natural flame luminosity (NFL) imaging method was used to capture the multiple flames initiated by various ignition sites. In order to perform the experiment a customized liner having four spark plugs installed at equal spacing, and to compare the results with conventional SI conditions, one spark plug was mounted at the center of the cylinder head. The results show that firing the single central spark plug generated lower in-cylinder pressure and heat release rate (HRR) along with higher combustion duration due to the low flame speed. However, adding more spark plugs increased the cylinder pressure generation and HRR along with creating shorter combustion duration for the same operating conditions. In addition, multiple flames produced by multiple plugs increased the engine power output and reduced the cyclic variation significantly due to higher pressure generation. Additionally, multiple ignition sources allow ammonia fuel to be burnt at lean conditions even at the low compression ratio. Furthermore, firing multiple spark plugs produced higher NOx emission than the single spark plug case due to the higher in-cylinder temperatures generated by multiple flame kernels. At relative air-fuel ratio (lambda) 1.2, higher NOx emission were obtained, because greater amounts of oxygen in the air-fuel mixture permit the oxidation of nitrogen (present in the fuel) into NOx. Further moving to lambda = 1.4, reduces the NOx emission due to the lower in-cylinder temperatures generated by leaner mixtures.
