A Computational Study of Hydrogen Direct Injection Using a Pre-Chamber in an Opposed-Piston Engine

Combustion characteristics of a hydrogen (H₂) direct-injected (DI) pre-chamber (PC)-assisted opposed piston two-stroke (OP2S) engine are investigated by 3D computational fluid dynamics (CFD) simulations. The architecture of the OP2S engine has potential features for reducing wall heat losses, as the DI H₂ jets are not directed towards the piston face. To overcome the high resistance to autoignition of H₂, a PC technology was implemented in order to enhance the ignition of the mixture by the multiple hot reactive jets. To further investigate the interaction between the H₂ plume and the chamber walls, three different piston bowl designs were evaluated and ranked based on a merit function. For the cases under study, the flat piston design was found to be most favorable (compared to the narrow and wide pistons) due to its reduced surface area for lower wall heat losses. The results also showcase that a co-optimization approach considering various parameters is an effective strategy to minimize the flame-wall interaction. The analysis showed that the PC jet must guarantee ignition and also a high-momentum exchange to support mixing-controlled and late combustion stages, while keeping safety limits from being exceeded. Finally, the results highlight that DI-PC H₂ combustion exhibits Diesel-like behavior, which can be exploited to achieve high efficiency and low emissions. Similar to conventional Diesel combustion (CDC), DI-PC H₂ combustion can provide the control of combustion phasing by adjusting the timing of the hot jet injection. While more work is needed to achieve the same level of efficiency as CDC, the present study demonstrated additional benefits of DI-PC concept as a robust carbon-free engine operation option. Finally, the analysis with respect to the fuel energy distribution and the DI-PC H₂ combustion phases shows that it is possible to further optimize combustion, especially in mixing-controlled and late stages.