Hydrogen–diesel dual-fuel combustion in marine medium-speed engines: Knocking suppression by direct water injection at high hydrogen substitution ratios

Hydrogen is among the most promising alternative fuels for achieving zero-carbon emissions in the maritime sector. However, its application in marine engines faces the bottleneck of substitution ratio limitation. The primary reason is the risk of knocking resulting from hydrogen's rapid heat release and high flame speed. The aim of this study is to explore in-cylinder direct water injection as a knocking-suppression strategy to enable higher hydrogen fractions in the engine. Two knocking-free operating modes are investigated: low hydrogen substitution ratios (0–40 %) without water injection and high hydrogen substitution ratios (90 and 95 %) with water injection. The results show that without water injection, the maximum hydrogen substitution ratio that can maintain stable combustion is 30 %, resulting in a 2.96 % improvement in indicated thermal efficiency and a 3.11 g/kWh rise in NOx emission. With direct water injection, knocking-free combustion can be achieved at high hydrogen substitution (90 and 95 %) under specific injection timing. At the suggested control point of 90 and 95 % HSR (1.5 water-to-fuel ratio, and −100 °CA water injection timing), a 1.43 % and 1.39 % improvement in indicated thermal efficiency and a 6.01 g/kWh and 6.45 g/kWh reduction in NOx emissions can be achieved compared to pure diesel mode. These findings indicate that in-cylinder direct water injection effectively increases hydrogen substitution ratios while enabling high thermal efficiency and low emissions, making it a promising approach for hydrogen–diesel dual-fuel marine engines. • Without DWI, the HSR is limited to 30 %. • 30 % HSR provides a 2.96 % ITE improvement but a 3.11 g/kWh NOx rise. • By adopting DWI, knocking-free combustion can be achieved at high HSRs (90 % and 95 %). • DWI operating envelop for 90 % and 95 % HSRs is identified. • At the suggested DWI control, ITE improvement and NOx reduction can be achieved for both 90 % and 95 % HSR conditions.