Split injection strategies for a high-pressure hydrogen direct injection in a small-bore dual-fuel diesel engine

Hydrogen -diesel dual direct -injection (H2DDI) engines present a promising pathway towards cleaner and more efficient transportation. In this study, hydrogen split injection strategies were explored in an automotive -size single -cylinder compression ignition (CI) engine, with a focus on varying the injection timings and energy fractions. The engine was operated at an intermediate load with fixed combustion phasing through adjustments of pilot diesel injection timing. An energy substitution principle guided the variation in energy fraction between the two hydrogen injections and then diesel injection while keeping the total energy input constant. The findings demonstrate that early first hydrogen injection timings lead to characteristics indicative of premixed combustion, reflecting a high homogeneity of the hydrogen -air mixture. In contrast, hydrogen stratification levels were predominantly influenced by later second injection timings, with mixing -controlled combustion behaviour apparent for very late injections near top dead centre or when the second hydrogen injection held high energy fractions, which led to decreased nitrogen oxides (NOx: NO and NO2) emissions. The carbon dioxide (CO2) emissions did not show high sensitivity to the hydrogen split injection strategies, exhibiting about 77 % reduction compared to the diesel baseline due primarily to increased hydrogen energy fraction of up to 90 %.