A Review of the Fundamental Understanding of Hydrogen–Diesel Direct Injection Combustion: Recent Advances and Future Outlook

Hydrogen–diesel dual-fuel direct injection (DI) combustion has emerged as a promising strategy for integrating hydrogen into compression-ignition engines, offering potential benefits in terms of efficiency, emissions reduction, and fuel flexibility. Driven by the potential to decarbonize hard-to-electrify sectors while leveraging existing engine platforms, there is increasing interest in hydrogen utilization in heavy-duty applications. This review examines recent advances in the fundamental understanding of hydrogen–diesel dual-fuel DI combustion under compression-ignition engine relevant conditions. It focuses on key factors influencing ignition, jet interactions, and combustion development, addressing a critical knowledge gap in dual-fuel DI technology with hydrogen, where recent advancements have provided new insights. Studies indicate that injection timing and sequence play a crucial role in determining combustion mode, transitioning between premixed and mixing-controlled regimes depending on hydrogen–diesel interactions. Early hydrogen injection promotes premixed combustion but can induce pressure oscillations, whereas later injection favors a mixing-controlled mode with lower peak heat release. Jet–jet interactions further complicate combustion, with converging injection configurations facilitating flame propagation but extending ignition delay due to increased preignition mixing. Experimental investigations have shown that injection duration influences jet momentum balance, affecting the entrainment of pilot combustion products into the hydrogen jet and, consequently, flame stabilization and heat release characteristics. Ambient conditions also have a significant effect on dual-fuel combustion. Lower ambient temperatures extend the ignition delay and fuel–air mixing time before ignition, leading to higher peak heat release rates. Reduced oxygen concentrations shift flame stabilization downstream and increase lift-off distance variability. Forced laser-induced ignition studies, supported by simplified numerical analysis, suggest that edge-flame deflagration mechanisms explain flame recession and stabilization under low-oxygen and low-temperature conditions. Injection parameters, including the pressure and nozzle diameter, also influence hydrogen jet development. Higher injection pressure enhances jet penetration and mixing but may extend the diffusion flame length, increasing heat transfer losses. Similarly, larger nozzle diameters increase the mass flow rate and heat release but also increase the hydrogen flame length. Overall, hydrogen–diesel dual-fuel DI combustion presents a viable pathway toward cleaner and more efficient engine operation. However, further research is required to optimize combustion processes and fully realize its potential.