Turbine Oil Review
By Frank Feinburg
When early gas turbine engines came into use in the 1940's, mineral oils were used as lubrication. These mineral oils quickly reached the limits of their capability, which lead to extensive research in the late '40s and early '50s. The result was synthetic oil technology.
Type I Oils
Early research, primarily by the military, lead to the Mil-L-7808 specification and oils known as Type I or 3 centistoke (the viscosity at 210F/99C) jet oils. Type I oils are fully synthetic (ester)-based oils. These Type I oils worked well at first but were stressed beyond their limits by the late '50s and early '60s by the newer more powerful — hotter running jet engines. Engines using Type I oils exhibited heavy oil deposits (coking) which required early maintenance action and required the Type I oils to be on fixed drain intervals.
This lead to the development of the Mil-L-23699 specification in the early '60s and the Type II (5 centistoke viscosity at 210F/99C) oils. These Type II oil were also called "2nd generation" jet oils (Type I being "1st generation") by the industry.
Type II Oils
Type II oils are ester-based synthetics, used today by virtually all turbine powered aircraft worldwide, and have proven to be the most technically and commercially successful and long-lived oils developed for aviation. However, they use improved esters with enhanced additive packages to attain about a 100F (38C) improvement in the high temperature serviceable limit, when compared to Type I oils, which eliminates the need for oil drains in most jet engines.
Esters and the recipe for jet oil
An ester is the reaction product of an alcohol and fatty organic acids; which forms a very stable base stock (base oil) both at low temperature (below -40F) and high temperatures (above 250C/482F). The ester is made in a chemical reactor and the finished oil in a blending tank. The source of the raw materials is mostly non petroleum-based — meaning the fatty acids are obtained from sources such as palm and coconut oils, etc. The additives used are typically antioxidants, metal passivators, antifoamants, anti-wear and possibly load carrying or corrosion inhibitor additives.
All jet oil additives are "ashless" containing no metallic components eliminating the formation of metallic soaps (metal containing sludges) formed from the oil alone.
An often asked question
How do synthetic jet oils differ from synthetic automotive oils? Synthetic jet oils differ significantly from synthetic automotive oils in base stock. Automotive oils use synthetic hydrocarbon base oils with less high temperature capability which are more suited to the automotive engine operating environment. Additive packages also vary greatly. Automotive oils often use specialized additives and organo-metallic additives which should not be used in turbine engines.
Type II oils — a remarkable history
The 30 plus years of success of Type II jet oil is unparalleled in the field of aviation lubrication, especially when one considers the performance demands on jet oils
Jet oils must:
• operate in low and high temperature environments
• be compatible with engine and component materials
• provide sufficient load carrying to sustain bearing and gear required operating lives
• have sufficient oxidative and thermal stability to keep viscosity and acid number within limits (eliminating the need for oil drains)
• produce minimal deposits (that can reduce engine life, increase maintenance costs and increase operational costs),
• be compatible with all other approved Type II jet oils (mixing does occur from time to time)
Given all these tough requirements Type II jet oils have performed remarkably well since the early '60s
But, like the earlier turbine engine technology, some newer engines with increased power requirements and increased oil system operating temperatures and/or increased time-on-wing (many in excess of 15,000 and some in excess of 20,000 hours) have challenged and tested the limits of Type II, 2nd generation jet oils.
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