Hydrogen represents a promising pathway for decarbonizing heavy-duty transport; however, accurately modeling its injection and combustion behavior in dual-fuel engines remains challenging. This work addresses two critical aspects of hydrogen high-pressure direct injection (HPDI) systems: the characterization of the injection process and the physical mechanisms governing combustion initiation.

A CFD-based methodology was developed to reconstruct injection profiles from apparent heat release rate (aHRR) data and to apply nozzle flow theory for approximating injector behavior, validated through experimental comparison. The findings indicate that classical convergent-nozzle theory fails to capture the observed injection trends. While convergent nozzle theory predicts variable, pressure-dependent mass flow rates, the reconstructed mass flow profiles exhibit nearly constant injection rates when the needle is open, consistent with convergent–divergent nozzle theory predictions. However, the observed variation in maximum injection rates across different cases suggests these deviations may arise not only from geometric constraints but also from aerodynamic phenomena such as boundary layer separation or recirculation within the injector. Such flow features can induce pressure-dependent effects that limit injector performance beyond what nozzle geometry and convergent-nozzle theory alone would predict.

Regarding combustion, the study reveals that hydrogen ignition, triggered by a small diesel pilot, is dominated by localized high-temperature regions produced by the diesel flame. This accelerated autoignition contrasts with alternative hypotheses involving radical transport or direct flame interaction.

Overall, these results advance the understanding of injection and ignition phenomena in hydrogen HPDI engines, providing valuable insights for refining CFD models and supporting the development of efficient hydrogen-powered heavy-duty engines.