Influence of passive prechamber orifice diameter on hydrogen combustion dynamics in a spark-ignition optical engine

• Optical limitation: Hydrogen’s low UV luminosity challenges high-speed imaging. • Research gap: PC orifice size impact on H2 combustion was poorly understood. • Smaller orifices (1.0 mm) yield highest jet velocity despite delayed ejection. • Two-stage MC combustion: initial jet-driven, then turbulence-driven propagation. • Study provides unique optical data for 3D-CFD model validation under H2 combustion. Given its potential to extend the lean limit and improve efficiency in spark-ignition engines, prechamber ignition is a key technology for hydrogen combustion. While previous research has confirmed its benefits, the impact of critical geometric prechamber’s parameters like orifice diameter on the in-cylinder flame development dynamics remains insufficiently visualized and quantified in optical engines. This lack of detailed experimental data hinders the optimization of designs and the validation of accurate computational models. This study addresses this gap by employing high-speed natural flame luminosity imaging to directly assess the influence of orifice diameter and air–fuel ratio on jet ejection and main chamber combustion in a single-cylinder optical engine with small-displacement (250 cc). This engine platform is representative of the size used for hybrid powertrains and range extenders, making the findings highly relevant for future hydrogen applications. Despite the challenges posed by hydrogen’s low luminosity, the optical analysis reveals that the prechamber system yields a higher equivalent flame speed than a conventional spark plug. Among the tested orifice diameters, the 1.5 mm configuration exhibited the shortest delay to 50 % flame coverage (C50), advancing ignition by approximately 3 CAD compared to the 1.0 mm case. Conversely, the 1.0 mm orifice, despite a longer initial delay, achieved the shortest overall combustion duration in the main chamber (D50-90), reducing it by up to 25 % compared to the 1.5 mm configuration at λ = 1.9. Peak jet penetration velocities were highest for the 1.0 mm orifice, reaching 175 m/s under rich conditions (λ = 1.55). The findings provide novel, spatially-resolved insights into hydrogen combustion dynamics and deliver a valuable experimental dataset for CFD model validation.