Experimental investigation of a novel hydrogen nanobubble water injection in a hydrogen direct-injection spark-ignition engine

Hydrogen offers a credible route to zero-carbon combustion in internal combustion engines and can deliver high thermal efficiency, but its physical and combustion properties create persistent challenges in fuel delivery, storage, and NOx control. This work reports experiments on a boosted single-cylinder spark-ignition engine running on hydrogen direct injection (DI) as the primary fuel, with intake-port water injection used as a NOx mitigation strategy and further enriched with hydrogen-charged nanobubbles. The objectives are to quantify how far water injection can suppress NOx and to test whether the additional hydrogen carried by nanobubbles can recover, or even improve, the power and efficiency lost through water dilution. Bulk nanobubbles (50–500 nm) remain suspended in water for weeks because their negligible buoyancy and strongly negative zeta potential suppress rise and coalescence; despite established uses in agriculture, water treatment, and biomedicine, their application as a hydrogen carrier in a working engine has received very limited experimental attention. A bespoke generator was used to charge water with hydrogen nanobubbles, and their size, concentration, and zeta potential were measured by dynamic light scattering (Malvern Zetasizer Ultra); by optimising the generator, a maximum concentration of 5.12 × 10 11 nanobubbles per millilitre was achieved. Hydrogen nanobubble water injection was then compared directly with conventional water injection. Relative to conventional water, nanobubble water raised indicated thermal efficiency by about 4% and torque by about 5%. NOx fell substantially in both cases, with a slightly greater reduction for nanobubble water; this difference is attributed principally to the lower measured lambda of that campaign rather than to a distinct NOx-suppression mechanism. Three independent diagnostics — residual exhaust oxygen, wideband lambda and exhaust-gas temperature — shift consistently in a direction that is compatible with additional in-cylinder hydrogen release from the nanobubble suspension. It must be emphasised, however, that such a release was not measured directly. The present dataset establishes the performance and emissions outcome; the underlying mechanism remains a hypothesis that requires dedicated optical and pressure-decay diagnostics before it can be regarded as demonstrated.