Mechanistic insights into evolution of lubricating oil in marine ammonia engines: from molecular interactions to macroscopic performance

Ammonia-powered ships have emerged as a critical direction. However, ammonia being alkaline, can degrade engine oil performance without explicit evolution mechanisms. Hence, the environment of the lubricating oil in marine ammonia engines was simulated, and long-term tests and molecular dynamics were conducted. The dispersion stability, rheological properties, thermogravimetric characteristics, and anti-wear properties of various lubricating oils were analyzed. Additionally, the infrared, nuclear magnetic resonance, and atomic emission spectra of oils were examined. The results indicated that ammonia impaired the performance of the marine lubricating oil. Specifically, ammonia gas underwent mild amidation and complexation with additives. Ammonia solution with high mass fraction reacted via amidation and hydrolysis with additives, while OH- ions formed precipitates with calcium and magnesium. Notably, reactions of OH- ions with metal additives were dominant with a low-mass fraction of ammonia solution. These findings provide data support for the application of ammonia fuel in marine engines.