On the lubricant oil potential to serve as auto-ignition centre in hydrogen engines: reactivity alteration, chemical ignition, and propagation modes

Abstract

The present study intends to investigate whether a suspended lubricant oil droplet in a H₂/air environment can initiate an undesired local self-ignition of the charge, while also providing preliminary insights into the nature of the combustion event that might result. A relatively simple analytical model was developed to derive plausible distributions of the temperature and oil vapour mass fraction around the droplet. Based on these results, zero-dimensional numerical simulations were conducted within the OpenSMOKE++ framework to quantify alterations in charge reactivity in the vicinity of the oil droplet induced by temperature and composition inhomogeneities. The “HyLube” chemical model, developed specifically for this purpose in a previous recent work, was employed in the simulations to emulate the reactivity properties of the $H_2/oil/air$ mixture. Thermodynamic conditions comparable to those achievable in the cylinder of an internal combustion engine at the end of the compression stroke were considered. The auto-ignition propagation modes potentially arising from the oil-droplet-induced contamination were then assessed through an analysis based on the comparison between the characteristic times of the reaction and the acoustic waves. To this end, the concept of a “hot spot” serving as the auto-ignition centre originally conceived in the related theory developed by Bradley and colleagues, has been extended to that of a “sensitive spot”, including composition variations. Finally, it was verified whether the energy released by the low-temperature chemical activity suffices to ignite the combustible mixture in the vicinity of the droplet and effectively initiate an uncontrolled combustion process. The findings reveal the capability of lubricant oil to alter the charge reactivity and promote early flame development within the combustion chamber of hydrogen-fuelled engines.