Investigating active pre-chamber jet ignition in ammonia-hydrogen engines to minimize hydrogen requirements

Hydrogen-assisted combustion has emerged as an efficient technique for improving ignition and stability in low-reactivity ammonia engines. However, the high hydrogen demand in existing approaches limits practical deployment. This study experimentally investigates ignition and combustion characteristics in an active pre-chamber engine under ultra-low hydrogen blending ratios. The effects of hydrogen injection timing and hydrogen blending ratio are systematically evaluated using in-cylinder pressure analysis and optical diagnostics. The results demonstrate that stable combustion can be sustained even at very low hydrogen fractions (2.0-3.0%), accompanied by improved combustion phasing and reduced cycle-to-cycle variation. An optimal injection timing is identified at approximately −60°CA ATDC, with an effective operating window between −100°CA and −40°CA ATDC. Emission measurements show that hydrogen-assisted combustion reduces ammonia slip, indicating improved combustion completeness, while NO emissions increase moderately, reflecting the typical trade-off between enhanced combustion efficiency and NOx formation. Optical observations further reveal that pre-chamber jet ignition promotes early flame kernel development and rapid spatial propagation, contributing to improved combustion stability. This enhancement is primarily associated with localized ignition reinforcement and accelerated flame growth, rather than a uniform increase in global mixture reactivity. Overall, this study demonstrates the feasibility of ultra-low hydrogen-assisted combustion and provides quantitative guidance for optimizing injection strategies in pre-chamber engine systems.