Computational comparison of the conventional diesel and hydrogen direct-injection compression-ignition combustion engines

Most research and development on hydrogen (H-2) internal combustion engines focus on premixed-charge spark ignition (SI) or diesel-hydrogen dual-fuel technologies. Premixed charge limits the engine efficiency, power density, and safety, while diesel injections give rise to CO2 and particulate emissions. This paper demonstrates a non-premixed compression-ignition (CI) neat H-2 engine concept that uses H-2 pilots for ignition. It compares the CI H-2 engine to an equivalent diesel engine to draw fundamental insights about the mixing and combustion processes. The Converge computational fluid dynamics solver is used for all simulations. The results show that the brake thermal efficiency of the CI H-2 engine is comparable or higher than diesel, and the molar expansion with H-2 injections at TDC constitutes 5-10 % of the total useful work. Fuel-air mixing in the free-jet phase of combustion is substantially higher with H-2 due to hydrogen's gaseous state, low density, high injection velocity, and transient vortices, which contribute to the 3 times higher air entrainment into the quasi-steady-state jet regions. However, the H-2 jet momentum is up to 4 times lower than for diesel, which leads to not only ineffective momentum-driven global mixing but also reduced heat transfer losses with H-2. The short H-2 flame quenching distance may also be inconsequential for heat transfer in CI engines. Finally, this research enables future improvements in CI H-2 engine efficiency by hypothesizing a new optimization path, which maximizes the free-jet phase of combustion, hence is totally different from that for conventional diesel engines.