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An Eastman Aviation Solutions technician performing SAE ARP5996 (hot-liquid process simulator) testing with a Falex FT2 unit.

Are Fluids and Lubricants Matching Engine Advances?

Lubricants, oils and greases evolve to keep up with engines

Innovation in aviation design usually focuses on solid aircraft parts; for example, new ways of making, testing and repairing composites or the continuing evolution of additive manufacturing of metal or plastic parts. But through these solid structures flow a variety of fluids that also are undergoing continuous improvement. In fact, without these fitter fluids, all those novel solid structures might never make it into the air or last very long without extensive maintenance.  

Better Engine Lubricants

Eastman Aviation Solutions works primarily in two areas. Both of these—lubricants for turbine engines and hydraulic fluids—have seen changes in recent years.

Despite continuing research, new lubricants for engines are not launched frequently. “It takes time to develop formulas, test them and gain approvals,” notes Technology Director Andrew Markson. “All this takes years and is very expensive.”

The new high-performance lubricants are designed to support engines throughout a life cycle of 30 years or more. The design aim is to support higher and wider performance profiles of the newest engines, which are more powerful than their predecessors, generate improved power-to-weight ratios and operate at higher core temperatures. “This demands lubricants that maintain lubricating properties at extremely high temperatures,” Markson explains.

While the primary lubricant challenge is heat management, others are emerging as well. New designs like Pratt & Whitney’s geared turbofan or Rolls-Royce’s UltraFan, another variable-pitch system, represent new ways of assembling engines and impose additional lubricant requirements, especially for load-carrying and anti-wear properties. So new lubricants must provide a high level of protection for critical bearings and gear assemblies.

A first generation of standard lubricants evolved from the 1960s to the 1980s. In the early ’90s, a new high-performance-capability (HPC) class of oils was required for larger, more powerful engines. These early HPC lubricants focused on thermal/oxidative stability, with little attention to load carrying or anti-wear protection.

Standard lubricants are still widely used on older engines. But even carriers with aging powerplants will likely shift toward these new lubricants to standardize their inventories if they need HPC lubricants for their newer engines anyway.

New HPC oils will need to improve load-carrying and anti-wear properties, ensure full compatibility with both older and new engine construction materials and maintain their thermal/oxidative stability.

The aim is to maximize engine reliability and thus reduce maintenance downtime and costs. “We don’t want oil to be the limiting factor to healthy engine operation,” Markson stresses. “We want to assure the engine user that oil will not be the reason for unplanned maintenance.”

New HPC oils are formulated using synthetic base-stocks combined with an advanced, customized additive system to preserve the integrity of the formulation under extreme operating environments. Premature breakdown and degradation of the lubricant can deposit carbon in critical areas of the oil feed or the scavenge portions of the lubrication system. In the worst case, lubricant supply is diminished, and bearings and gears lose lubrication and fail. Keeping things clean is always the goal.

Moving and rotating parts also must be protected against wear. Here, oil is the carrier for additives that are laid down on part surfaces to protect them. “The additives do the work,” Markson says.

Given the lengthy time required to develop and approve new lubricants, it is advantageous to understand the lubrication needs of new engines very early in the development process. Markson says this has been difficult, as OEMs such as Rolls-Royce, Pratt & Whitney and GE only emphasize oil performance in later design stages.

Involving lubricant formulators much earlier would allow discussion of new and alternative lubricant chemistries to address specific lubrication challenges. By addressing such issues earlier and collaboratively, “we have a much better chance of finding more robust and whole solutions,” Markson says. 

For example, gearboxes are much more critical in the new geared turbofan designs, yet they are still lubricated with turbine lubricants rather than special gearbox lubricants. “Lubricating gearboxes effectively with turbine oil can be a challenge,” Markson notes. Turbine oils are primarily designed to lubricate bearings, not gears, and their chemistry may not be ideal for the latter.

The Eastman executive thinks engine makers are now beginning to recognize this need for earlier collaboration, although the level of recognition varies by OEM.

ExxonMobil opened this lubricants blending, packaging and distribution facility in Baton Rouge, Louisiana, in August 2016. The company forecasts that demand for advanced aviation fuels and lubricants such as its Mobil Jet Oil 387 turbine engine oil will increase 55% in 25 years. Credit: ExxonMobil Photos

Another trend Markson sees is increasing willingness—despite proprietary equipment and intellectual property concerns—for engine industry participants to work together to solve common problems such as micro-pitting of bearings and gears. Dealing with this issue may require different chemistries than those used for anti-wear or load-carrying lubricants’ performance.

In hydraulic fluids, the biggest change has been moving from the 3,000-psi fluids in older aircraft to 5,000-psi fluids in newer types like the Boeing 787 and Airbus A350.

Hydraulic fluids are used to manipulate such components as landing gear and control surfaces. Higher-pressure fluids allow for using fewer solid components in the hydraulic system to do the same job, thus saving weight. Since there are usually three redundant hydraulic systems in commercial aircraft to ensure safety in event of failure in any one, this weight saving can be substantial. The weight reduction is most significant in larger aircraft, which is why 787s and A350s use these high-pressure fluids.

Another desirable characteristic of hydraulic fluids is that they be able to withstand higher temperatures for longer periods of time, thus extending the period before these fluids must be drained and replenished.  

HPCs For Generators

It is not just main engines that can benefit from the new lubricants. For example, Mobil Jet Oil 387, a synthetic turbine engine HPC oil, was recently approved for UTC Aerospace Systems’ integrated drive generators (IDG) mounted on accessory gearboxes on many aircraft. IDGs supply constant-frequency alternating current to aircraft systems. UTAS tested 387 in its IDGs in various engines over a two-year period, finding it provided outstanding component protection and oxidative stability, even at high temperatures.

ExxonMobil Sales Manager Vipin Rana argues that airlines can now use 387 for both engine and IDG lubrication, simplifying operations by using a single HPC oil for both duties.

Hotter Greases

Lubricants are hardly the only liquids that have to work at high temperatures in modern engines. Greases may also need to function in fierce heat, at least if some maintenance tasks are to be performed efficiently.  

Tiodize has developed a collection of new greases that can be used on threaded fasteners in jet engines and work at temperatures up to 1,400F. Without such greases, the fastener and the parts it fastens may become cold-welded together in flight, a process known as galling. When galling occurs, parts must be cut or drilled apart, requiring extra time and expense.

Development of the new greases began when the U.S. Air Force sought a lubricant to replace silver on threaded fasteners in very hot engines. Tiodize developed heat-cured coatings that are used by engine manufacturers such as GE, Pratt & Whitney, Rolls-Royce, MTU and Snecma. The company used this experience to develop greases for fasteners during maintenance. These  greases are inorganic and do not contain any metallic or sulfur-producing materials, which could cause fasteners to crack at high temperatures.

Alternative Fuels

Some of the most innovative development is now going on in alternative aviation fuels not based on petroleum or other carbon sources. But the practical impact of this development depends on future fuel economics.

For the last 10 years, Airlines for America (A4A) has been working with the Commercial Aviation Alternative Fuels Initiative (CAAFI) to make these alternatives practical. Five technologies, or pathways, have been certified as capable of producing drop-in jet fuel from biological feedstocks for commercial aircraft, and CAAFI is working on more technologies.

Several alternatives are being used now in small volumes. Whether any of the new fuels becomes viable in substantial volumes depends on markets and environmental policies. The key questions are what alternative fuels will cost when scaled up for commercial production and without public subsidies, and what the cost of petroleum-based fuels will be, including any taxes for carbon emissions.

A4A Environmental Vice President Nancy Young says the economics of alternatives are site-specific. Technologies that tap feedstocks like biomass or municipal waste close to airports will cost less and generate lower emissions. Uncertain, too, are the extra charges for emissions. In October 2016, the U.S. signed on to an International Civil Aviation Organization plan requiring payments for carbon dioxide offsets after 2020 for international air-travel emissions above 2020 levels. She notes that alternative fuels would reduce offset obligations.

The cost of offsets could be anywhere from $100 for a short-haul narrowbody flight to more than $6,000 for a long-haul Airbus A380 flight. The offset burden will be inversely related to the price of conventional jet fuel, at least in the long run, as any pass-through of higher fuel prices would dampen travel demand.       

So far, the A4A-CAAFI efforts have secured wider options for airlines in dealing with major energy and policy uncertainties, a kind of insurance policy whose actual utility will be contingent on unknown events. 

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