Coke Formation from Aircraft Turbine Engine Oils: Part I — Deposit Analysis and Development of Laboratory Oil Coking Test

Abstract

The authors report here the results of a study undertaken to determine how aircraft engine oil degrades to form coke in static films on engine components and the impacts of additives and surface materials upon the coke production processes. The research is presented in two parts. This first paper deals with the initial research to develop a simple laboratory oil-coking test capable of monitoring antioxidant depletion, oil degradation, and coke formation of stressed aircraft turbine engine oils. Details of the analytical chemistry experiments performed on the stressed samples to quantitate the percentage of oil, polymer dissolved in the oil, and insoluble coke are also presented. The second paper (Part 2) (I) deals with more quantitative measurements of the degradation of thin layers of oil on heated surfaces. Effects of different oils and surfaces are explored.

The results presented in Part I show that simple open-vial laboratory tests involving thin films of oil produce polymer and coke similar to the deposits seen on failed face seals taken from operating aircraft engines and isolated from used engine oils. Coking reactions require oxygen to deplete the antioxidant package and to polymerize/decompose the ester basestock of the oil. After the antioxidant package is depleted, the ester basestock polymerizes to form small oil-soluble polymers. As the oil spends additional time at elevated temperature, these polymers increase in molecular weight and change in chemical composition becoming insoluble in the oil, producing deposits. Although the antioxidants inhibit the polymer/coke reactions, the antioxidants are incorporated in the formed deposits increasing the deposit amount. Consequently, these results indicate the proper concentration and choice of antioxidants will allow elimination, not just reduction, of deposits on engine components coated with thin layers of static oil films.

Presented at the 52th Annual Meeting Kansas City, Missouri May 18–22, 1997

KEY WORDS:

• Lubricant Degradation/Stability
• Gas Turbine Oils
• Antioxidant Analyses

Notes

Presented at the 52th Annual Meeting Kansas City, Missouri May 18–22, 1997