Imagine, then, that were talking not about the engine oil in a passenger car, but the lubricant for a turbine engine on a jet airliner. If few of us have time for the lubricant in our cars, its a safe wager that even fewer have sat in the window seat of an Airbus, glanced out at the huge pods slung beneath its wings and thought, Hmmm, I wonder what oil that uses? But if specifying an oil for a car engine is serious business, then surely oils for aviation turbine applications represent a whole different dimension.

Though tailored for their specific environments, automotive engine oils and turbine oils have much in common in terms of the operating concerns they are formulated to address – namely, the reduction of friction and wear, prevention of deposit formations and corrosion, as well as resistance to thermally induced oxidation and cooling of components. Properties such as the need for low volatility, a high flash point, resistance to foaming and excellent elastomer compatibility are further examples of performance criteria of common interest between automotive engine oils and turbine oils.

So-called heavy-duty gas turbines, the massive machines used for industrial power generation, are often lubricated by mineral oils. But light-weight gas turbines of the type used for aviation and smaller industrial applications require lubricants formulated with synthetic ester base stocks in order to cope with their high operating temperatures. Viscosities of formulated oils are typically in the region of 5 centistokes at 100 degrees C. The extreme range of thermal conditions to which these oils are subjected, from cold start to maximum operating temperature, means they need a high viscosity index to guard against an excessive drop in viscosity as temperature levels increase.

The area of elastomer compatibility comes in for particular scrutiny by gas turbine operators, especially when switching from one oil brand or formulation to another. Some lubricant formulations can cause an oil seal to swell, whilst others might cause shrinkage. After a period of use, a seal can take on a permanent set. These tendencies can be assessed and controlled by testing with virgin elastomer materials. But it is extremely difficult to predict how a seal, preconditioned by one formulation, will react to the chemistry of another; either oil may be used new without issue, but the combination can prove problematic. This therefore becomes an important consideration when an operator trials a new product.

A recent press release from Shell makes this very point, as it describes the first six months of in-flight use of AeroShell Ascender by Brazilian carrier Tam Airlines. Shell explained that the trial, initiated in December 2010, involved International Aero Engines (IAE) V2500 gas turbines, fitted to the Number 2 position (right wing only) of two TAM A319 Airbuses. In the press release, Tam Powerplant Engineering Manager Paulo Meneghel said, Following more than six months of flight using AeroShell Ascender, we are satisfied in its compatibility with most elastomer materials, and that it has presented no leakage or performance degradation from the engines. The changeover process has also been smooth thus far.

No doubt the other performance aspects will attract appropriate scrutiny throughout Tams evaluation. But the fact that Meneghel makes elastomer compatibility the central theme of this account underlines its importance. Also of interest is the implication that any compatibility issues would be evident within six months of testing.

The next target for us is in our CFM56-5b engines, as soon as approval is obtained from SNECMA [Brazils National Association for the Study and Construction of Aviation Engines] for flight evaluation, which we anticipate will happen this year, Meneghel said.

So engine oil approvals are even more of a big deal in the world of turbine engines than they are for automobiles, where some manufacturers grant brand-specific approvals and others dont bother. In the turbine world, a new oil doesnt get anywhere near service in an engine without the manufacturers say so, and this is most especially true for aviation use. The approvals process is more protracted, as is the overall product development timeframe. Shell noted that AeroShell Ascender was over ten years in development.

From a product development standpoint then, it is enlightening to compare and contrast the similarities and differences between these two fields of lubrication. It is similarly fascinating to reflect just how much further aviation turbine oil folks have to go to get the job done.