Hydrogen-fueled internal combustion engines (H2-ICEs) impose unique challenges on engine lubrication because water is an inevitable combustion product. This review summarizes the current understanding of water-induced degradation mechanisms in engine oils for H2-ICEs, with emphasis on physicochemical property variation, additive depletion, tribofilm evolution, and tribological performance. Water present in dissolved, emulsified, or free states can significantly alter lubricant viscosity, destabilize additive systems, and accelerate oxidative aging. In particular, water promotes the depletion of zinc dialkyldithiophosphate (ZDDP) through tribofilm removal and competitive adsorption at rubbing interfaces, while also inducing additive hydrolysis that transforms long-chain phosphates into shorter-chain species with inferior film-forming capability. These processes inhibit tribofilm growth and reduce the mechanical integrity of protective films, thereby deteriorating anti-wear performance. Although substantial progress has been made in understanding the role of liquid water in lubrication, the tribochemical effects of high-temperature water vapor under realistic H2-ICE operating conditions remain largely unexplored. Future research should therefore focus on water vapor-dominated lubrication environments representative of hydrogen combustion, aiming to elucidate the underlying tribochemical mechanisms and support the development of dedicated lubricants for durable and reliable H2-ICE operation.
Ma et al. (Fri,) studied this question.