Assessment of piston and injector cap designs on the performance of a hydrogen direct-injection spark-ignition engine

Hydrogen is considered a critical solution in the transition to sustainable energy systems. This study provides the first comprehensive evaluation of the combined effects of piston geometry and injector cap design on the performance of a heavy-duty hydrogen direct-injection spark ignition engine using high-fidelity computational fluid dynamics simulations. Four piston geometries: ω-shaped, flat, pent-roof, and a hybrid of flat and pent-roof, were evaluated. Moreover, the hydrogen injector design was analysed by varying the number of cap holes (4-, 5-, and 6-hole) and the jet-included angle (±10˚), alongside two cap orientations (X and + ). The study found that different piston geometries significantly influenced hydrogen jet interaction with the piston wall and overall mixing. The flat piston produced a more homogeneous mixture before ignition, contributing to lower NO_x emissions. Conversely, the bowl-shaped piston resulted in a strongly stratified mixture distribution and faster combustion, yielding the highest thermal efficiency while increasing NO_x emissions. Although the + cap orientation was intended to guide the mixture toward the spark plug, it could not ensure a richer mixture at the spark plug. The 5-hole cap promoted a more uniform mixture and reduced NO_x emissions. Furthermore, adjusting the jet-included angle by 10° led to more stratified mixing, leading to a slower combustion process and negatively impacting engine performance. Considering the best compromise between NO_x emissions and fuel economy, the ω-shaped piston combined with a 5- or 6-hole cap injector exhibited superior performance over the 4-hole configuration, primarily in favor of the significantly reduced NO_x emissions.