An Experimental Study on the Effects of Pre-Chamber Geometry on the Combustion Characteristics of Ammonia/Air Mixtures Using Turbulent Jet-Post Spark Ignition Strategy

As a carbon-free fuel, ammonia faces challenges in engine applications due to its low flame propagation speed and high ignition energy. The pre-chamber turbulent jet-post spark ignition strategy (TJ-PSI) has been proven effective in accelerating the combustion of stoichiometric ammonia/air mixtures. This study investigates the effects of orifice configuration on the combustion characteristics of stoichiometric ammonia/air premixed mixtures under TJ-PSI mode. Experiments are performed in a constant-volume combustion vessel filled with stoichiometric ammonia/air mixtures, and the spark plug used to trigger the ignition of the mixture in the main chamber is located downstream of the pre-chamber turbulent jet flow. With pre-chamber volume maintained constant, pre-chambers with different orifice numbers (Φ1.4 mm × 1, Φ1.4 mm × 4, Φ1.4 mm × 6) and orifice diameters (Φ4.0 mm × 1) are tested, along with varying time intervals (TI) between the main chamber and pre-chamber spark timings. Experimental results show that the pre-chamber with single large orifice (Φ4.0 mm × 1) produces jet flames but offers limited combustion enhancement. In contrast, a single small orifice (Φ1.4 mm) generates flameless turbulent jets, which reduce combustion duration by 53% compared to the large-orifice case. This improvement is attributed to the pre-chamber jet enhancing turbulence in the main chamber, whereas larger orifices yield lower turbulence intensity. Although multi-orifice configurations provide less pronounced enhancement compared to the single-orifice case, they effectively prevent flame kernel extinction at short TIs (e.g., 10 ms). Consequently, the total combustion duration from pre-chamber spark to the end of main chamber combustion can be significantly shortened.