High-Temperature Ignition Kinetics of Lubricating Oils

Abstract

Ignition of the lubricating fluid in a mechanical system is a highly undesirable and unsafe condition that can arise from the elevated temperatures and pressure to which the lubricant is subjected. It is therefore important to understand the fundamental chemistry behind its ignition to predict and prevent this condition. Lubricating oils, particularly those with a mineral oil base, are very complex mixtures of thousands of hydrocarbons. Additionally, these oils have very low vapor pressures and high viscosities. These present considerable barriers to examining and understanding lubricant ignition chemistry. Therefore, a novel experimental design was devised to create and introduce a lubricant aerosol into a shock-tube facility. In this way, the lubricant can be quasi-homogeneously introduced into the shock tube where it will be vaporized by the incident shock wave, and combustion can be observed behind the reflected shock wave. In this way, the off-the-shelf lubricant Mobil DTE 732, as well as artificially aged and gas turbine field samples of this oil, were tested and compared. In addition, lubricant manufacturers develop highly complex and proprietary additive mixtures to enhance lubricant life, achieve desired physical parameters, and reduce wear. Additionally, a more fundamental look at the base oil ignition chemistry was desired which required a surrogate for lubricant ignition, n-hexadecane (nC16), as well as a pure mineral oil. The ability of nC16 to replicate mineral oil ignition is assessed. Investigation of the effect of traditional lubricant additives on lubricant ignition is also assessed. Four additives were chosen: two antioxidants—one phenol based (cardanol) and one amine based (N, N’-Di-Sec-Butyl-P-Phenylenediamine, PDA); a solvent (ethylene carbonate, EC); and a fire suppressant (dimethyl methylphosphonate, DMMP). The additives were each mixed with nC16 and mineral oil at 10% bv mass for the antioxidants, 5% by mass for EC, and 9% by volume for DMMP. The relative effect on ignition is discussed. Hexadecane was chosen since detailed chemical kinetics models have been previously developed and validated for this fuel. Therefore, these models are compared to the data presented herein. Additionally, kinetics mechanisms exist for DMMP in the literature, and one was chosen to be merged with the nC16 model to investigate its ability to predict the effect of DMMP on oil ignition. Additionally, a rocket propellant binder, hydroxyl terminated polybutadiene, and its mixture with a plasticizer were also tested with the new technique. Conditions tested herein were between 1011 and 1490 K at atmospheric pressure and near stoichiometric mixtures. Good agreement between heated shock tube and injection method results for nC16 is shown and similar results between nC16 and mineral oil is observed. Significant effects on ignition delay time are seen for additive mixtures with EC, PDA and EC when compared to nC16 and mineral oil baselines. Additionally, Mobil DTE 732 is shown to be less reactive than baseline mineral oil, suggesting the additives used in its formulation are suitable for reducing lubricant reactivity.