Effect of pre-chamber geometrical parameters and hydrogen enrichment on the combustion and flame characteristics of zero/low carbon fuels

The operation of zero/low-carbon alternative fuels in a spark ignition engine has been widely acknowledged; however, operating it under lean/ultra-lean conditions requires advanced ignition systems or fuel enrichment techniques. Thus, the pre-chamber system and hydrogen enrichment technique were adopted in the current study. Initially, the impact of pre-chamber orifice diameters for fuels with different reactivities was investigated for the first time. Further, due to the limited fundamental studies on combined pre-chamber spark ignition systems and hydrogen enrichment techniques, the current study explored the combined ignition mechanism for the stable combustion of lean/ultra-lean methane-air and ammonia-air mixtures. A Schlieren method with a Phantom high-speed camera was utilized to record the flame development process in an optically accessible constant-volume combustion chamber at a rate of 20,000 frames per second. The results showed that the higher physical diffusivity and chemical reactivity of hydrogen and the physical behavior of turbulent jet from pre-chamber favored the combustion and flame development process for methane-air and ammonia-air mixtures. Thus, an earlier start of combustion and a higher peak heat release rate were observed with hydrogen-enriched methane and ammonia mixtures. Compared to pure methane and ammonia mixtures, hydrogen-enriched methane and ammonia mixtures showed a 28.6 % and 80.7 % reduction in ignition delay and 47.7 % and 78.5 % reduction in combustion duration, respectively. The flame propagation duration was also reduced by 14 ms and 49.4 ms, while the average flame area rise rate was increased by 2.68 times and 3.78 times for the hydrogen-enriched methane and ammonia mixtures, respectively, compared to pure methane and ammonia mixtures.