Investigation of Active Pre-Chamber Ignition on an Optically Accessible Ultra-Lean DI Hydrogen Engine

Hydrogen is a promising alternative to conventional fuels for decarbonizing the commercial vehicle sector due to its carbon-free nature. This study investigates the ignition and flame propagation characteristics of hydrogen in a 2-liter single-cylinder optical research engine representative of the commercial vehicle sector. The main objective was to enable high power density operation while minimizing NOx emissions. For that, ultra-lean combustion was employed to lower in-cylinder temperatures, addressing the challenge of NOx formation. To counteract delayed and unstable combustion under lean conditions, an active pre-chamber ignition system was implemented. It uses a gas-purged pre-chamber with separate hydrogen injection and spark plug ignition. Turbulent hot gas jets from the pre-chamber ignite the fresh mixture in the main combustion chamber, enabling faster and more stable ignition compared to conventional spark plugs. Additionally, the low volumetric energy density of hydrogen, which limits performance in port fuel injection due to air displacement, was addressed through direct hydrogen injection into the combustion chamber to increase the mixture heating value. High-speed imaging techniques, including Schlieren and OH chemiluminescence, were used alongside thermodynamic analysis to study combustion dynamics. Results demonstrate that the active pre-chamber ignition system achieved stable combustion at ultra-lean conditions (λ up to 4) without knocking phenomena. Essentially, NOx-free operation was possible for λ > 2.5. Pre-chamber conditions were found to influence performance trade-offs: richer mixtures enhanced stability and combustion speed, while leaner mixtures minimized NOx emissions. Comparative measurements with a standard spark plug showed that pre-chamber ignition leads to more stable and faster ignition, unlocking additional performance and efficiency potential. These findings demonstrate the viability of hydrogen as a carbon-neutral zero-emission fuel for commercial vehicle engines.