Tribological assessment of lubricants used in hydrogen assisted diesel engines

This study investigates the tribological performance of lubricants used in hydrogen-assisted diesel engines with specific focus on their rheology, additives and the formation of tribo-films. While there are concerns about reduced lubricant viscosity and increased wear due to hydrogen combustion, the impact of hydrogen on lubricant quality and tribology in Internal Combustion Engines (ICEs) is a less-explored topic. As an initial step, this study provides an in-depth analysis of lubricant performance in diesel engines, both with and without hydrogen assistance. The study includes chemical evaluations, and controlled tribometric performance analysis of lubricants from refuse trucks after an operation interval equivalent to 2000 km. The results show marginal changes in the lubricant rheology with the use of hydrogen, but there are differences in frictional behaviour under mixed and boundary lubrication regimes. In addition, X-ray Photoelectron Spectroscopy (XPS) analysis shows variations in anti-wear additive adsorption on surfaces, with a noticeable difference between fresh and used oil samples, aligning with the lubricant analysis data. Despite a noticeable difference in measured friction in mixed and boundary regimes of lubrication, the chemical analysis and XPS results show only marginal differences in the anti-wear additive content and the associated tribofilms. The study demonstrates that changes in the chemical and rheological states of the lubricant affect frictional performance and interaction of additives with surfaces in mixed and boundary regimes of lubrication. Understanding these changes is crucial for the future development of suitable formulations for use in dual-fuel hydrogen-assisted ICEs.