3D‐Printed Adiabatic Jet Ignition Pre‐Chambers for High‐Efficiency, Low‐Emission Hydrogen Engines: A Narrative Review of Progress, Performance Benchmarks, and Additive Manufacturing Pathways

Pre‐chamber jet ignition (JI) is a transformative technology enabling stable ultra‐lean combustion and enhanced thermal efficiency in hydrogen‐fueled internal combustion engines. This narrative review synthesizes decades of research on JI systems, from foundational hydrogen‐assisted JI concepts to modern applications synergized with direct injection (DI) and turbocharging. We critically analyze key geometric parameters governing pre‐chamber design, including volume ratios and nozzle configurations, and their impact on jet dynamics, heat loss, and combustion stability. The integration of JI with DI is shown to mitigate hydrogen's combustion challenges (e.g., knock, NOx) while enabling throttle‐less load control. Furthermore, electric turbocharging eliminates lag and enhances power density, while additive manufacturing facilitates near‐adiabatic ceramic pre‐chambers to minimize thermal losses. This review further explores the comparative advantages of active versus passive JI strategies and examines synergistic technologies such as electric turbocharging. A dedicated case study on rotary valve engines contextualizes critical design trade‐offs. Finally, we evaluate the transformative potential of additive manufacturing in fabricating near‐adiabatic pre‐chambers, which promise significant reductions in heat loss and gains in thermal efficiency. This review underscores JI's pivotal role in unlocking hydrogen's potential as a zero‐carbon fuel, highlighting pathways toward 45%–50% brake efficiency and near‐zero emissions.