Lubricating oil characteristics for turbine engines include low enough viscosity to facilitate flow between closely fitted, rapidly moving parts, high enough viscosity to prevent metal-to-metal wear, capability of tolerating high temperatures and pressures and a sufficiently low pour point enabling it to flow efficiently under extremely low temperature startup conditions. A high flash point is needed in order to resist burning or vaporization under high temperatures. Minimization of build ups like carbon, varnish, or gum is also necessary. Newer engines have higher compression ratios, engine speeds and operating temperatures. They also have closer tolerances between parts. As a result, specifications for lubricating oils are increasingly more stringent (NavyBMR).
Turbo lubricants generally used today are more often than not synthetic oils which are synthesized from chemically modified petroleum components. Synthetic oils are more effective than mineral oils because they tolerate higher temperatures. They are also better at maintaining good viscosity and stability. Synthetics perform better at low temperatures too. This is because they have a superior capacity to remain fluid. Turbo Oils are composed of Basestock and additives. Ester purity of Basestock is between 94 and 97 percent. It results from a chemical reaction between synthetic alcohols and organic fatty acids as well as synthetic alcohols from chemical processing (Eastman).
Turbo lubricants perform numerous essential functions as they circulate throughout moving parts. These include lubrication, cooling and debris removal. They also serve as a diagnostic medium for engine performance monitoring. There are multiple performance requirements that must be met. Lubricants are required to function in a wide temperature range of 40-310ºC. Low deposit formation, thermal stability and good hydrolytic stability are performance essentials.
Typical gas turbine engines require highly efficient lubrication. All aircraft gas turbine engines have a closed loop lubrication system. Components include a pressurized supply line, a return line, a high-efficiency oil pump and a small oil tank. A fixed amount of oil continuously circulates in the system when the engine is in operation.
Synthetic oils commonly used by turbine engines must conform to equipment specifications. Synthetic oils can be identified by military specification numbers. For example, O-148MIL-PRF-7808 is the specification number for three centistoke turbine engine synthetic lubricating oil. O-156MIL-PRF-23699 is the specification number for five centistoke turbine engine and gearbox oil. MIL-L-23699 is recommended oil for all new and overhauled turbine engines. Turbine engine oil MIL-L-7808 is suitable for use in extremely low temperature ranges. MIL-L-7808 and MIL-L23699 oils are required to be compatible with each other. However, the addition of MIL-L-7808 oil will lower the concentration of MIL-L-23699. Improper mixing of these oils may result in pressure and flow limits differing from those specified for each type of oil (Army).
Synthetic lubricants deteriorate over time. This is why there is a shelf life for all synthetic oils. Use of over-aged oil can cause expensive problems to occur. Most synthetic oil expires after 6 months.
Synthetic oils can degrade rubber materials commonly used in the manufacture of gaskets, O-rings and seals of lubrication systems designed for use with mineral-based oil. Such improper use causes rubber to soften and swell. It eventually loses the ability to seal properly. Synthetic oil will cloud or form other contaminants if stored too long.
Contamination of lubricating oil can destroy engines as can poor maintenance and equipment failure. Carbon contamination can occur when oil evaporates. This can happen in the bearing compartments near hot turbines. When carbon breaks off, it is likely to circulate throughout an engine’s lubrication system. Results can be disastrous. Large enough pieces of carbon can cause pump failure. Smaller pieces can clog nozzles and filters. Use of improperly mixed lubricants can also be highly destructive (NavyBMR).
- Army. Oil Systems. Retrieved from http://aviationmiscmanuals.tpub.com/TM-1-1500-204-23-3/css/TM-1-1500-204-23-3_78.htm
- Eastman Aviation Solutions. Mineral vs. Synthetic Esters (Oils) Turbo Oil and Its Applications.
- NavyBMR. Jet Aircraft Lubricating Systems. Retrieved from http://navybmr.com/study%20material/14008a/14008A_ch5.pdf