Toward pure ammonia combustion: Experimental study of a hydrogen pre-chamber ammonia engine under different operating conditions

In the context of global carbon reduction efforts, hydrogen jet ignition pre-chamber ammonia engines have attracted growing attention for their carbon-free potential. In this study, bench tests were conducted on a high-compression-ratio (17.5) single-cylinder ammonia engine with a hydrogen-fueled pre-chamber. The hydrogen energy ratio (HER) in the pre-chamber was systematically varied under different engine speeds (1500–3000 rpm) and loads (intake pressures of 72.3–113.1 kPa). The combustion characteristics of both the pre-chamber and the main chamber, along with the corresponding emissions, were analyzed. As HER increases, the hydrogen mixture in the pre-chamber transitions from lean to rich conditions. Both excessively low and high HER values reduce jet energy, impairing main-chamber combustion stability. With increasing engine speed, the allowable HER range for stable combustion narrows. At 1500 rpm (with the widest HER range), the minimum HER required for stable ammonia combustion decreases with increasing intake pressure. At 1500 rpm and 113.1 kPa, stable operation with pure ammonia was achieved using a passive pre-chamber, resulting in a coefficient of variation (COVIMEP) of 1.91% and a brake thermal efficiency (BTE) of 31.76%. The maximum BTE of 42.20% was achieved at 2500 rpm with an HER of 4.61%. NOx and NH3 emissions generally show a trade-off, while N2O decreases with more concentrated heat release in the main chamber. Overall, this study offers experimental evidence and insights into the effects of HER on jet formation and ammonia combustion under various operating conditions, thereby providing guidance for the practical application of hydrogen-assisted ammonia engines.