A Numerical Investigation of Mixture Formation and Combustion Characteristics of a Hydrogen-Diesel Dual Direct Injection Engine 2021-01-0526

A hydrogen-diesel dual direct injection (H2DDI) combustion strategy in a compression-ignition engine is investigated numerically, reproducing the configuration of previous experimental investigations. These experiments demonstrated the potential of up to 50% diesel substitution by hydrogen while maintaining high engine efficiency; nevertheless, the emission of NO_x increased compared with diesel operation and was strongly dependent on the hydrogen injection timing. This implies the efficiency and NO_x emission are closely associated with hydrogen charge stratification; however, the underlying mechanisms are not fully understood. Aiming to highlight the hydrogen injection-timing influence on hydrogen/air mixture stratification and engine performance, the present study numerically investigates the mixture formation and combustion process in the H2DDI engine concept using Converge, a three-dimensional fluid dynamics simulation code. Increased hydrogen stratification levels are realised by retarding the hydrogen injection timing from 180 °CA to 40 °CA bTDC at fixed energy substitution ratio of 50%, as in the experiments. The simulations are validated against the measured pressure and apparent heat release rate traces. The simulation results show that early hydrogen injection yields an almost homogeneous mixture with entire hydrogen charge being in fuel-lean conditions, leading to a primarily premixed combustion process of the hydrogen fuel. The intermediate injection timings produce a moderately stratified hydrogen mixture with much of the hydrogen at near-stoichiometric conditions, which achieves the highest engine efficiency but also the highest NO_x emissions. The late injection forms a highly stratified charge with most hydrogen mixtures in fuel-rich conditions, presenting a mixing-controlled combustion process. This combustion mode induces the lowest wall heat loss and the lowest NO_x emissions, but yields a relatively high fraction of unburnt hydrogen and efficiency penalty.