2025 Sustainable Energy & Powertrains

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 prechamber ignition system was implemented. It uses a gas-purged prechamber with separate hydrogen injection and spark plug ignition. Turbulent hot gas jets from the prechamber ignite the fresh mixture in the main 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 analyses to study combustion dynamics. Results demonstrated that the active prechamber ignition system achieved stable combustion at highly lean conditions (¿ up to 4) without knocking phenomena. NOx-free operation was possible for ¿ > 2.5. Prechamber conditions were found to influence performance trade-offs: richer mixtures inside the prechamber enhanced stability and combustion speed, while leaner mixtures minimized NOx emissions. Comparative measurements with a standard spark plug showed that prechamber ignition leads to more stable and faster ignition, unlocking additional performance and efficiency potential. These findings highlight the importance of optimized prechamber injection and ignition strategies for practical applications, demonstrating the viability of hydrogen as a carbon-neutral fuel for commercial vehicle engines.