In the late '70s and early '80s, development (by a very few oil companies) of a 3rd generation jet oils began. Third generation oils are designed to enhance oxidative and thermal stability characteristics and reduce oil deposit levels compared to 2nd generation jet oils.
For example, one 3rd generation oil, Mobil Jet Oil 254, exhibits about a 50ûF (10ûC) improvement in bulk oil stability and a very significant reduction in deposit levels (in laboratory testing) compared to typical Type II jet oils. This particular 3rd generation jet oil has worked very well over the last 15 years particularly in hotter running turbine engines in both fixed wing and rotary wing aircraft thus confirming laboratory test results.
Jet oils are among the most tested oils on the market. Laboratory testing is involved in every phase of jet oil development, approval, manufacturing and problem (oil system) solving.
First, new oil requirements are established by field and equipment builder evaluations. Then new oil development begins in the laboratory by testing new formulations. When this phase is completed, the final oil formulation is presented to the U.S. military for mil-spec. approval. This is followed by submitting all test data and military approval to the various equipment builders for their approvals, or to use in a controlled service introduction basis. Successful testing in the laboratory and in flight then leads to manufacturer and FAA approval. Start to finish, this process takes at least 5 years — and often much longer.
Many tests are used to develop, approve, and monitor production of jet oils, but we will discuss only a few of the more significant tests related to oil coking. These tests were designed mostly by testing companies through industry coordination to simulate a type of deposit formation in a particular area of the engine under accelerated and severe conditions. But, before I discuss these tests, let me first briefly discuss the most common type of deposits formed in engines, carbon deposits, or coke.
Coke is the solid residue remaining when oils undergo severe oxidative and thermal breakdown at extreme temperatures. The higher the temperature, the harder, more brittle and blacker the coke becomes.
Deposits are not desirable, but they do form, and when they do we want them to stay put. Coke deposits that break off or shed can migrate in the oil system and plug filters, passageways and oil supply jets.
Coke shedding is influenced by engine thermal cycles and moisture absorbed during extended shut down periods. The coke from jet oils readily absorbs moisture. Also, engine design, operating procedures, and environment can help to reduce or increase coke deposits; but we do not often have control of these influencing factors.
Some of the different types of commonly seen coke and the test used to measure them are:
• Thin film coking which is formed from high surface to oil volume ratio with short residence time and high temperature (the Mobil Thin Film test is used to simulate deposit formation on rotating seals and bearings)
• Vapor/mist coking which forms due to inadequate cooling and wall washing with oil (the EPPI Vapor Phase Coker simulates deposit formation in engine vapor/mist areas and vent lines)
• Dynamic coking which forms inside oil distribution pipework due to high temperatures, inadequate cooling and often in areas of restriction or bends (the Alcor High temperature Deposition test simulates the conditions in oil lines)
These tests have shown very good correlation to field performance of given oils in given engines.
Fourth Generation Jet Oils
It is expected that engines will continue to increase power, time on wing and severity of the oil system environment. That is why oil manufacturers are developing 4th generation jet oils (an example is Mobil Jet Oil 291) designed to improve upon 2nd and 3rd generation jet oils performance in the areas of vapor phase coking, elastomer compatibility, and enhanced load carrying, all while maintaining all the good characteristics of these oils. We believe these 4th generation oils will meet jet engine lubrication demands well into the 21st century.
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