As new generations of aero engines display greater efficiency thanks in large part to higher core temperatures and pressures, oil suppliers are needing to freshen up their lubricants to withstand the harsher operating conditions - and meet demands for longer on-wing service times.
To that end, Shell Aviation aims to gain approval for its "Ascender" oil beyond the International Aero Engines V2500. The lubricant was introduced in 2009 after a decade of engineering and testing, but has thus far only been certified for IAE's medium-thrust engine family.
The Anglo-Dutch giant has partnered with all major engine manufacturers to employ the oil on future powerplants, such as the Rolls-Royce Trent XWB, Pratt & Whitney PW1000G geared turbofan and CFM International Leap. But it also targets large legacy fleets, such as the CFM56.
Higher thermal stability and better rubber seal compatibility are the key objectives in formulating new gas turbine engine oils. Synthetic lubricants can completely oxidise on hot surfaces, especially if the parts are covered only by a thin oil layer which cannot absorb enough thermal energy.
One of the oil's main functions is to cool the engine by circulating through internal components, such as bearings, and extracting thermal energy via fuel-cooled heat exchangers. The kerosene, in turn, needs to be warmed up for efficient combustion as its temperature drops in the wing tanks because of the low outside temperature at altitude. It thus passes through the so-called FCOC [fuel-cooled oil cooler] in the engine's fuel supply system.
But when excessively hot oil decomposes in a process called coking, it leaves behind essentially a carbon slur or varnish which can accumulate and restrict oil flow in pipes.
Thermal stability can be improved with higher-quality polyolester base stocks. Additives can also help, but antioxidants that provide greater thermal stability may have adverse effects on elastomers. "There are thermal additives out there which give excellent low-coking [properties]", says David Dowse, Shell Aviation's global technical manager. "But due to their chemical nature, they tend to deteriorate rubber O-rings in the engine."
The trick is to find the optimum balance between the ester and additives. Dowse claims Shell's Ascender oil has no detrimental effect on seals: "We no longer have to compromise between a high thermal stability and seal compatibility."
Martijn van Noordennen, European sales manager for Shell's aviation lubricant division, adds that this was achieved through a clean-sheet oil design rather than further development of existing products.
Seal compatibility is particularly important for older engines and even auxiliary power units, as airlines want to avoid the need for different lubricants across their fleet of new and old aircraft.
But van Noordrennen says the introduction of new powerplant technologies may eventually force airlines to use a range of oils. The use of novel component alloys and ceramic bearings could, for example, require different lubricants, as could architectural changes such as open-rotor engines or powerplants with geared turbines. And, as manufacturers reduce engine weight, oil quantity and sump size might be reduced at the same time as engines run at higher core temperatures for improved combustive efficiency. The result would be to place even greater demands on the oil, particularly for heat absorption.
"Oils have become an integral part of the design process for new engines," says Dowse. "The manufacturers cannot design a new engine and pick an oil off the shelf and hope it works. Those days are gone." He questions whether the current synthetic esters have reached a limit and whether alternative chemistries will need to be employed over the next 20 years.