In-cylinder hydrogen concentration effects on lubricant-surface interactions and tribofilm formation in hydrogen internal combustion engines

This study aims to quantify the effects of different in-cylinder hydrogen concentrations on lubricant performance and to elucidate the underlying mechanisms. A specialized friction testing system, capable of precisely controlling hydrogen concentration, was employed to evaluate the tribological behavior of piston ring-cylinder liner pairs. Post-test analysis of the worn surfaces involved scanning electron microscopy (SEM) for morphological examination and X-ray photoelectron spectroscopy (XPS) for chemical state analysis. Experimental results show that hydrogen concentrations in the range of 10-30% markedly improve lubrication at the piston ring-cylinder liner interface and substantially reduce wear. With increasing hydrogen concentration, the surface concentrations of zinc, sulfur, and phosphorus on the rubbing surfaces increase significantly, indicating that the hydrogen atmosphere promotes boundary-film formation. However, at excessively high hydrogen concentration (100%) the beneficial lubricating effect is attenuated. It should be noted that the short-term beneficial effects observed, which may stem from the formation of transient surface films, can change substantially under prolonged hydrogen exposure due to potential risks such as additive degradation and hydrogen embrittlement. Therefore, the results presented here primarily reflect initial performance, and long-term durability remains to be established. The results also indicate that, under hydrogen-enriched conditions, blends containing 40% Group III+60% Group IV and 85% Group III+15% Group IV exhibited markedly greater improvements in lubrication performance in this study. This study provides an important basis for the establishment of evaluation standards for lubricants used in hydrogen internal combustion engines.