Hydrogen internal combustion engines are emerging as a promising technology for decarbonizing the transportation sector. However, due to hydrogen's low density, achieving sufficient fuel delivery often requires a high flow rate. Despite its excellent diffusivity, optimizing the hydrogen-air mixing in the context of an engine combustion chamber remains a significant challenge. This mixing process critically influences key phenomena such as ignition, flame propagation, knocking, and oxides of nitrogen formation, all of which are shaped by injector design, injection parameters, and in-cylinder flow characteristics. The Injector caps have been found to be useful for providing more control over injection from hollow-cone injectors, which deliver high flow rates, offering the advantage of not needing to modify the entire injector system. Therefore, modeling and understanding the impact of injector cap designs on jet evolution is essential to improving efficiency in heavy-duty engines.
In this study, high-fidelity computational fluid dynamics simulations were performed using the Reynolds-Averaged Navier-Stokes (RANS) turbulence model within a constant volume chamber to investigate hydrogen jet behavior with various cap designs mounted on a hollow-cone injector (Bosch HDEV4). The study investigates several critical cap design parameters, including the number of cap holes (single, four, six, and ten holes), the hole axis angle (40°, 45°, 50°, and 55°), the cap inner volume (71.98 and 257.17 mm³) and inner geometry (simple, spherical, and jet-directed configuration), and the corresponding cap throat area to injector throat area ratio (0.5, 1, 2, 4, and 8). The objective is to evaluate how these parameters influence overall jet behavior, jet penetration length, and mixture distribution. By capturing the effects of these design variations, this study offers insights into optimizing injector cap designs to enhance hydrogen jet performance in engines.
The results of this study reveal that the implementation of a cap demonstrated a clear enhancement in mixture formation, generating a progressively higher proportion of lean mixture over time. Key parameters such as the cap's inner volume and the ratio of throat areas were found to directly influence jet penetration and mixing quality, with a larger throat area ratio promoting deeper penetration and richer mixtures. Shock waves were observed both at the injector and cap exits, with their intensity varying based on the cap's internal volume and throat dimensions. The internal shape of the cap had minimal effect on jet dynamics, provided the internal volume remained constant. In multi-hole caps, the jet angle played a crucial role in controlling penetration length, without considering plume-to-plume interactions. Furthermore, it was found that increasing the number of holes reduced jet penetration length but had little effect on the overall mixture formation. Ultimately, the optimal cap design among the sample space was determined to feature four holes, a minimal internal volume, and a carefully calibrated throat area and jet angle, tailored to specific engine requirements for improved hydrogen jet performance.