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.
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.
A critical step towards a healthy engine
By Alan Baker
Effective equipment condition monitoring is vital to efficient and safe aircraft operation. Costly downtime and potentially dangerous in-flight shut-downs are to be avoided. One way of reducing these problems is to develop methods of diagnosing equipment wear or potential failure through an accurate oil analysis program.
Oil analysis provides a reliable means of checking the elemental content within the oil system and involves taking periodic oil samples.
Concentrations of elements present as a result of a units wear or damage can be determined to an accuracy of 0.01parts per million.
Typically, on arrival at the laboratories, samples undergo a variety of tests. These include analysis of viscosity, dilution, oxidation, and acidity.
Checks on an oil's viscosity, oxidation, and acidity are used to determine whether the oil is compliant with the manufacturer's recommendations on oil effectiveness in the given situation.
Viscosity checks are used to determine the grade of oil in use. Contaminants, such as fuel, hydraulic fluid, or another brand of oil, will alter the viscosity of the original oil. The test involves a viscometer [a capillary tube in an oil bath held at a constant temperature] and a known sample volume of the oil to be tested. The process involves placing the oil into the capillary tube and allowing it to pass through. The flow of the oil between two points is timed by the viscometer and the viscosity determined mathematically.
New and used oils may contain acidic constituents as additives or as degradation products formed during service, such as oxidation products. Oxidation of the oil is therefore a measure of the lubricant degradation in service, and is usually indicated by discoloration or by testing for Total Acid Number [TAN]. The TAN is determined by the use of Ptentiometric Titration and checked against condemning limits which have been empirically established.
The detection and monitoring of constituent elements in oil and hydraulic fluid samples requires highly complex technology.
An atomic absorption test consists of heating the diluted oil or fluid sample so that the ions form a gaseous cloud. Light at the appropriate wave length for a particular element is shone through the cloud. The extent of light absorbed by a certain element in the sample can then be measured to reveal the quantity of that element present.
An alternative method of elemental analysis is the inductively coupled plasma (ICP) test. The ICP analysis involves introducing a sample into an argon plasma induced by high frequency where the temperature can reach 10,000¡C. The sample is drawn from a nebulizer in the form of an "aerosol" into the torch area where it undergoes consecutive decomposition, then atomization. The atoms, excited by the plasma, emit energy in the form of light with varying wavelength characteristics of the elements present. The light then passes through optics and a photomultiplier where it is converted into an electrical signal. The intensity of light transmitted is proportional to the concentration of each of the elements present. This is then passed into a data processing system which interprets the data into a usable form.
The ICP equipment is calibrated before each run using samples with known elemental concentrations. The advantage of ICP analysis is that it enables simultaneous analysis of 23 elements in approximately 3 minutes. Results gained from the ICP are trended on an in-house database which has been developed and programmed with alert levels for specific engine types.
Continuous comparison with limits and rates of change
At the time of adding data to the database, a set of limits specific to the subject unit is loaded, complete with a list of the elements which are considered to be critical for the unit.
As the new data is added, the computer checks against the limits, and flags any results that fall outside them. However, checking against pre-set limits is not enough. It is apparent that even when the general wear levels, or limits, are known for a unit type, there will always be odd units which run well below the norm. These units can undergo deterioration without exceeding limits, and consequently there is a continuous monitor for the rate of change from the previous levels for each unit.
Analysis of proportions of elements
A single element out of limits is not necessarily an indication of a serious defect, even when the limit has been greatly exceeded.
Many engineering materials contain a high proportion of a common element [such as iron] combined with much smaller proportions of other elements [molybdenum, chromium, nickel, etc.]. As the ratio of the main element to the secondary element may be in the order of 20:1, a failure which produces for example 4 parts per million [PPM] could be expected to produce only 0.1 or 0.2 PPM of a secondary element.
Modern low level detection therefore allows the examination of the Ôsecondary' elements which can give valuable aid in deciding how serious the problem really is. The programs have been developed to check the relative proportions of the main and secondary elements to determine the likely source material.
A variety of reports are available, each specifically designed to provide quick and easy assimilation of results.
The data is presented both in its numerical and graphical form. The graphical section is selective, reflecting those elements which are considered to be relevant to the particular unit.
The program identifies results which fall outside the limits for that unit or have changed from the previous results by more than a specified amount.
The graphical section makes it possible to recognize simultaneous shifts in the results for several elements and the combinations of elements can give very important clues to the underlying causes. This form of print-out is particularly valuable to operators of large fleets because results for a large number of engines can be scanned in a very short time and any units which deserve more concentrated attention can be identified.
This type of report allows easy assessment of an entire fleet as opposed to a single unit. All the current results for a single element and for all units in the fleet are presented on a single report. This makes it very easy to determine which results are normal for that type of unit and at what stage the results require further investigation.
Particles that are collected on magnetic chip detectors, backflushed from filters or filtered from an oil sample can be analyzed using a scanning electron microscope (SEM).
The SEM provides both a visual assessment and an analysis of the composition of the debris. Visual assessment is made possible by magnifying the particle up to 200,000 times; a far greater resolution than that provided by an optical microscope. From the appearance of the particle it might be possible to determine its origin and whether it has suffered abrasion from other components. A plot is displayed on screen which details the constituent elements of the particle to enable identification of its source. Imaging software now enables operators to receive a printed colour image of the debris that has been analyzed.