KEY CONCEPTS
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Aviation lubricants have become very specialized to meet the demands of high-performance jet turbine engines.
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Exacting certification requirements constrain changes in aviation lubricant formulations, but safety is paramount, and today’s lubricants work very well.
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Future electric, hybrid electric and hydrogen-fueled aircraft present a need and an opportunity for significant changes in lubrication and cooling fluids.
Over the past several years, the word “sustainability” has appeared more and more frequently in corporate initiative statements. Driven by consumer and investor demands, regulatory requirements and supply chain disruptions, companies across multiple industry sectors are devoting time and money toward setting and achieving sustainability goals.
Sustainability means different things to different industries, but several factors stand out. Sustainable feedstocks are those that come from environmentally benign sources: extraction methods that minimize pollution or habitat destruction, with safe working conditions and fair compensation for workers and profits that do not contribute to abusive or corrupt entities or efforts. Sustainable production emphasizes the efficient use of these feedstocks through maximizing the yield of the desired products using processes with the fewest steps, the fewest toxic or polluting starting materials or byproducts and least energy consumption economically achievable. Sustainable usage entails reducing product waste, increasing a product’s performance and service life and reducing energy usage and pollution during operations. Sustainable disposal includes reducing the amount of product to be disposed of, reclaiming and recycling as much as is practical and converting toxic or polluting components into something more benign before disposing of the product.
How does aviation factor into the drive toward greater sustainability? In the U.S., demand for jet fuel was about 13% as large as the demand for light-duty automotive gasoline in 2019. In 2020, this figure shrank to slightly less than 10% as pandemic lockdowns grounded many commercial flights.
1 Even though aviation represents a small sector of the fuel and lubricant market, it is a very visible sector. With thousands of passenger jets carrying millions of people over heavily populated areas, commercial aviation is an often-cited reminder of modern society’s demand for fossil fuels
(see Greener, More Efficient Aviation Fuels).
Greener, more efficient aviation fuels
On the fuels side of things, major efforts are underway to use more sustainable sources, and formulators are approaching the problem from a variety of angles. STLE Fellow Christopher DellaCorte cites mathematical models that have shown that using fuels made from renewable resources—soybeans, for example—to charge the batteries that drive an electric motor can be less energy-intensive and better for the environment than using electricity from power plants fired by coal or natural gas. “You could even take it one step further and say, what if you use solar-powered electricity on the ground to grow soybeans to make soybean oil and convert the soybean oil into jet fuel? In other words, there’s different ways of making sustainable fuels,” he adds.
A variety of factors, some of them not immediately obvious, can affect fuel consumption. For example, passenger aircraft are required to provide about 20 cubic feet per minute of air per passenger, with an equal mixture of fresh and recirculated air. The air coming in from the outside is at a low pressure because of the altitude, so the compressor brings the air up to the required pressure. Keeping the air inside the cabin at the right pressure accounts for up to 4% of an aircraft’s fuel consumption, Winter says. With the broader use of high efficiency particulate air (HEPA) filters as a result of the COVID-19 pandemic, this fuel consumption can be affected since more power is required to pull air through these filters.
Aviation engine efficiency has been increasing by about 1% per year over the past 80-plus years, says Winter. In 2019, the commercial fleet consumed about 100 billion gallons of fuel, he adds. He notes that the greenhouse gases commercial aircraft produced (mainly CO
2) contributed about slightly over 2% of all human-generated CO
2 emissions in 2019
(see Resources). However, as the automotive and electrical power generation industries reduce their carbon footprints, aviation’s contributions could represent a greater percentage of the total (possibly as much as 20%), assuming this industry’s growth proceeds at traditional rates. It’s not too soon to begin addressing this problem, Winter says; because it takes so long to develop and certify new engines and aircraft, changes that are proposed now might not come into use until 10-20 years from now.
RESOURCES
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Air Transport Action Group (ATAG) (2019), “Facts and figures.” Available at
www.atag.org/facts-figures.html.
•
International Civil Aviation Organization (ICAO) Secretariat (2019), “Introduction to the ICAO basket of measures to mitigate climate change,”
Climate Change Mitigation: Technology and Operations, Chapter 4. Available
here.
•
Aviation’s impact on the environment, Aviation Benefits Beyond Borders, 2018. Available
here.
Much has been written about the drive toward cleaner and more efficient aviation fuel consumption, but less has been said about the lubricants that contribute to this efficiency. Nevertheless, aviation lubricant formulations and applications are evolving toward better performance and more efficient usage and disposal practices.
Where we are now
It takes many different lubricants to keep aircraft flying (and landing) smoothly, says STLE Fellow Christopher DellaCorte, a former NASA engineer who is now director of the Akron Engineering Tribology Laboratory and Timken Endowed chair at the University of Akron. An airplane can require several kinds of greases, in addition to solid film lubricants and liquid lubricants. Each one performs in different environments, under different operating conditions. Components on the exterior of the plane—flaps, rudders and landing gear—experience high and low ambient temperatures during takeoff and landing and extreme low temperatures during flight. Jet engines start cold, then warm up to extreme high temperatures during operation.
Dr. Michael Winter, senior Fellow for advanced technology at Pratt & Whitney, notes that aviation lubricant formulations have become very specialized over the past several decades to stand up under the demands of high-performance jet turbine engines. The oils used for aviation lubricants must stay above their pour points but below their flash points over the entire temperature range that they are exposed to during normal operation. Also, as jet engines stay at higher temperatures for longer periods of time, lubricating oils can begin to pyrolyze, even if they remain below the flash point. Operating temperature ranges are continually expanding as jet engines run hotter to improve efficiency, and, in the future, liquid hydrogen fuel could push the lower temperature limit to a new extreme
(see Base Oil Basics).
Base oil basics
Today’s aircraft rely on synthetic oils, but “synthetic” is not necessarily synonymous with “sustainable.” For example, polyalphaolefins (PAOs) are derived from ethylene. Ethylene is derived mainly from natural gas (with its associated fossil-fuel and greenhouse gas issues), and PAO synthesis involves multiple energy-intensive refining and catalysis steps. Although some military aircraft and smaller planes use PAO-based fluids, hydraulic fluids for today’s commercial aircraft turbines are mainly phosphate esters or polyol esters.
Phosphate esters can be produced by reacting fatty acids or alcohols with polyphosphoric acid, phosphorus pentoxide or phosphoric acid anhydride.
A Phosphate esters are more fire-resistant than other types of hydraulic fluids (they have a slight edge over polyol esters) because of their resistance to oxidation, low vapor pressures and high ignition temperatures. When these fluids do ignite, they release relatively little heat, so the flame goes out quickly.
B
However, many industry sectors, including aviation, use polyol esters instead because they provide adequate fire resistance, better environmental performance (low aquatic toxicity, fewer toxic decomposition products and good biodegradability) and better compatibility with a variety of system components. Both phosphate esters and polyol esters are expensive, so they are mainly used in applications that require fire resistance.
C
The raw materials for ester-based fluids come from mineral or plant sources. Generally, even-numbered fatty acids (referring to the number of carbon atoms in the main backbone chain) come from plant sources, while the odd-numbered fatty acids come from mineral oil. The fatty acids used in aviation lubricants, which generally have five to 10 atoms in the carbon chain, are reacted with multifunctional alcohols to produce the ester base oils.
Most of the fatty acids from plant-sourced oils go into personal care products, with machine lubricants representing a much smaller fraction. Even here, though, lubricant manufacturers can contribute to sustainability efforts through their choice of suppliers. Several organizations, including nonprofit organizations and industry consortia, have established certification policies for tropical plant-sourced oils like palm kernel or coconut oils
D,E and other common oil sources like soybeans.
F,G These policies help to ensure that the agricultural operations that provide these oils are run responsibly, and that they do not participate in large-scale destruction of tropical rainforests.
REFERENCES
A.
Lakeland Laboratories, Ltd., Phosphate esters: Why? Which? Where? Available
here.
B.
Wright, J. (November 2009), “Phosphate ester fluids: Benefits and limitations,”
Machinery Lubrication. Available at
here.
C.
Need a fire-resistant hydraulic fluid? Power & Motion, July 13, 2017. Available
here.
D.
A global standard for sustainable palm oil, roundtable on sustainable palm oil. Available
here.
E.
Sector network natural resources and rural development Asia and the Pacific. Available
here.
F.
Soy export sustainability certification portal. Available at
www.usses.org/.
G.
American Soybean Association Soy Sustainability Assurance Protocol. Available
here.
Winter notes that conventional aviation engines have several bearing systems, each with its own lubrication system. For example, the bearings at the back of an engine work in an environment of extreme heat
(see Figure 1). The oil that lubricates these bearings must avoid conditions that produce sloshing and foam, and the lubricant’s formulation helps to manage this. At the front of the engine, the compressor helps to maintain the air pressure inside the passenger cabin. Here, the lubricants must not interact with the air streams and release volatile compounds that could affect the air quality, even if an engine goes into failure mode.
Figure 1. Schematic diagram of the operation of a centrifugal flow turbojet engine. Figure courtesy of Emoscopes, CC BY-SA 3.0.
Aviation lubricants are actually a very small, specialized business sector, about 1% or less of the size of the market for passenger car motor oil, says Ron Yungk, lubricants chief for Eastman Chemical Co. Even though the world’s commercial airlines account for some 25,000 aircraft, the total amount of lubricant used is relatively small, and today’s aircraft lubricant and hydraulic fluid formulations are already well on the way to sustainability. Most aviation lubricants today are synthetic, and they use ashless additives, he says, including dispersants and antiwear additives.
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Reducing waste is another area where aircraft lubricants are doing well. Aircraft lubricants are expected to have a much longer service life than lubricants for the automotive sector. Some greased components are sealed for the life of the part (which can be decades), while oil lubricants are topped off with fresh oil and additives, rather than replaced. In addition, today’s aircraft leak lubricants much less than in the past, DellaCorte says. He adds that, although grease is not recycled, aircraft engine oil is recovered or recycled when an engine is drained before undergoing a major overhaul.
Lubricants make up a very small part of aircraft emissions and oil usage, Winter says. He notes that each individual jet engine uses just a few gallons of oil, and this oil stays inside the engine for the most part. Even though changes in lubricant formulations might not contribute much to directly reducing aviation’s carbon footprint, he says, the fuel efficiency and extended equipment service life that they enable can have a major impact.
Occasional oil top-offs are necessary, even in systems with negligible leakage, however. Jet turbine bearings can rotate at thousands of rpms, creating a fog of lubricating oil, Yungk explains. The compartments containing the moving parts must be vented to maintain the pressure inside within specifications, but the oil mist coming out passes through a deoiler to coalesce it and keep most of it inside the engine. The small amount of replacement oil that must be added during routine maintenance is enough to replenish the additives and keep the oil within specifications. “It’s a very robust product,” he says, adding that commercial aircraft engines commonly achieve 25,000 operating hours over an eight-year period between shop visits.
This service life is all the more remarkable, given that the lubricant passes within inches of engine parts that are extremely hot. Commercial aircraft have turbines, even propeller planes (which are actually turboprops) and helicopters (turboshafts). Engine oil, in addition to providing lubrication, also transfers heat from the engine to the fuel, which keeps the engine from overheating and helps the fuel to burn more efficiently.
Ashless additives in aviation lubricants help to prevent a buildup of solid deposits that could clog filters and orifices and cause an engine to malfunction or fail. Thus, antiwear additives are typically organophosphate compounds rather than those that contain, for example, zinc or molybdenum, says Yungk. Lubricant specifications limit the allowable ash content to such a low amount (about 1 ppm) that it effectively puts metal-containing additives off limits—the amount is too low to allow these additives to be effective—he says.
Yungk notes that because of the extremely conservative nature of aviation, specifications for lubricants change very slowly. Much of his work involves working with aviation OEMs to identify future design trends that lubricants need to address, but this often requires looking ahead 10 to 20 years into the future because of the lengthy process required to certify a new formulation.
“We can’t change qualified formulations; we can’t change the recipe. The only thing we can do is change where we get some of the raw materials that go into the recipe,” Yungk says. He adds that, even then, a rigorous process is required to prove that a new source provides the exact same quality and purity as the qualified material it replaces. He notes that a lubricant still produced was approved almost 58 years ago. “It probably accounts for 15%-20% of what the world flies on—at least on commercial aircraft, and a much greater percentage in business aircraft.”
A sustainability silver lining
Despite its disastrous consequences, the COVID-19 pandemic also has played a role in moving sustainable aviation lubrication forward. Yungk notes that during the most stringent phase of the pandemic lockdown, commercial airlines put more than 10,000 aircraft into storage because of travel restrictions and passenger concerns about air travel. He and his colleagues worked with an airline to validate a new procedure for conserving lubricating oil during preparations for storage.
3
When airlines are preparing to put aircraft into storage, they typically drain the used oil from each engine and send it into the waste stream. Next, they add fresh oil with a corrosion inhibitor. They taxi each aircraft to the end of a runway and run the engines for about 15 minutes to circulate the oil and coat all the mechanical parts with the corrosion inhibitor. Then, they return the aircraft to the hangar and drain this oil before removing and packaging the engines for storage.
The large number of planes put into storage during the pandemic generated large amounts of waste oil. Yungk’s team worked with an operator to collect data on the condition of the oil at each step. They found that it was equally effective to add the corrosion inhibitor to the used oil already in the tank. A cart attached to each engine turned the engine electrically and circulated the corrosion inhibitor without the plane leaving the hangar, which eliminated the need to burn jet fuel
(see Figure 2). Lab studies established the circulation time and conditions needed to produce results equivalent to those from the conventional process.
Figure 2. Commercial airliner connected to a ground power unit. Figure courtesy of Petr Kadlec, CC BY-SA 3.0 CZ.
OEMs and airlines quickly adopted the procedure, which saved them roughly $2,000 per engine, Yungk says, during a time when the airlines were facing financial stress due to sharp decreases in air travel. “And oh, by the way,” he adds, “there was a sustainability gain because we kept an awful lot of used oil from going to the waste stream.”
Another means of reducing oil consumption and waste is to reclaim the oil used during routine engine tests. After an aircraft engine has completed a specified number of service hours, it is shipped to a maintenance facility for an overhaul. Before shipping, the engine oil is drained into the same waste stream as jet fuel that is drained from the fuel tank for maintenance purposes, Yungk explains. This mixture is sold to waste services that burn it to produce heat for various applications. Since each engine contains roughly five to 12 gallons of oil, and the fuel tanks are generally close to empty at the end of a flight, the amounts of waste produced are relatively small. The small amounts reduce the incentive to try to reclaim the oil at the airport.
However, Yungk says, it is possible to make better use of used oil at engine manufacturing and maintenance facilities. Here, each engine is run for a couple of hours with a full tank of oil during “green runs” as a quality control measure to make sure it is working properly. Afterward, the oil is drained, and the engine is packaged for return shipment. Yungk has worked with engine OEMs to demonstrate that oil used in limited engine testing can be reclaimed and reused in multiple engines rather than discarded after one use, as long as proper sequestration, storage, testing and quality control procedures are followed.
3
Evolution
For several decades, evolution in aviation lubricant formulations has been slow. In part, this is due to the stringent requirements for certifying new formulations, but it also is because today’s lubricants work very well. Newer formulations provide greater thermal stability for today’s hotter engines, but radical changes are not in the picture—yet.
Although change comes slowly to the aviation lubricant industry, there is an evolution toward higher-performing oils. Thermal stability is a big part of this. Oils that perform well at higher temperatures allow engine manufacturers to build hotter-running, more thermally efficient turbine engines. Today’s engines contribute to sustainability in several ways, Yungk says. Not only do hotter engines reduce total specific fuel consumption, but oils that stand up to the higher temperatures reduce the frequency of draining the engine for maintenance and replacing the oil afterward, which reduces an aircraft’s carbon footprint still further.
Better-performing lubricants mean fewer part failures, which saves on the resources needed to manufacture new parts and dispose of old ones. Condition-based maintenance also contributes to more efficient operations. On-board sensors alert maintenance crews to unusual temperature changes or vibrations that may signal that a part is starting to fail, and regular oil sampling alerts them to the presence of metal particles or other debris in the lubricant stream. Thus, aircraft parts are repaired or replaced, and lubricants are replenished or replaced, only when needed.
Certifying a new lubricant formulation—or even a new component or supplier—is a lengthy process. If a car or truck experiences a mechanical failure, the driver can pull over to the side of the road and wait for help, but the aviation industry will never tolerate a mechanical failure at 30,000 feet that could result in lives lost, Winter says. Thus, the lubricants that keep an aircraft running smoothly must be proven reliable using exacting standards.
Even then, however, all failure modes must be investigated, Winter says. He cites the example of a new oil that had just been put on the market, tested and approved for use. However, on one particular aircraft, the oil was exposed to the electrodes of the generating system. Because this oil was a good insulator, it built up a static electrical charge. The resulting electrical arcing tripped the circuit breakers and caused the plane’s electrical system to shut down. “With all new oils, there’s a full battery of tests that needs to be done to cover every eventuality now, across the industry,” Winter says. He notes that safety is the highest priority for any new mechanical design, fuel, lubricant or construction material.
Safety is paramount for new lubricants, but longer life (which reduces the frequency of top-offs and replacement) and better lubrication (which lowers fuel consumption) also are considerations. Winter notes that today’s customers are more attuned to the effects of climate change, adding extra impetus to efforts to increase efficiency. A lubricant’s thermal stability is a big part of this, because running jet engines at a hotter temperature increases their fuel efficiency.
Disruption
Better-performing lubricants enable technological advances already in use, and they may pave the way for bigger hardware changes to come. For example, a large geared turbofan increases the propulsive efficiency of a jet engine
(see Figure 3).4 A slow-turning fan at the front of the engine is most efficient, while a fast-moving turbine at the rear is most efficient. However, in conventional turbine engines, the fan and turbine are constrained to move at the same speed. In a geared turbofan, a gearbox between the fan and the low-pressure turbine enables each to spin at its own optimal speed. Winter’s company spent 20 years developing the sophisticated gear system at the heart of its new gearbox, which is capable of generating about 30,000 horsepower. They claim that the geared turbofan, which was first used for commercial passenger flights in 2016, is 16% more fuel-efficient. Also, because of reduced fan speed, these engines have a 75% smaller noise footprint than previous-generation engines. The increase in efficiency is dependent on the performance of the lubricants used for the gears and other components, Winter says.
Figure 3. A geared turbine. Figure courtesy of Pratt & Whitney.
Aviation lubricants manufacturers are looking ahead to even bigger changes just over the horizon. Electric, hybrid electric and hydrogen-fueled aircraft, still in the early stages of research and development, present a need and an opportunity for significant changes in lubrication and cooling fluids. Like the sorting-out currently in progress in the automotive industry, the aviation industry will probably try a variety of technologies before settling on the best-performing ones. Lubricant manufacturers will collaborate with OEMs to come up with fluids that provide heat transfer, the desired degree of electrical conductivity or insulation, lubrication or some combination of these properties.
Electrification is the biggest change coming to the aviation industry, DellaCorte says. In the Boston and Seattle areas, retrofitted turboprop planes making 20- to 30-minute “puddle-jumper” flights are testing the capabilities of battery-powered motors for propulsion.
5 Although today’s batteries are not capable of powering long flights, they are heading in that direction, he says. One challenge facing battery-powered flights is that the weight of the battery stays constant throughout the flight, even though it is discharging energy. In contrast, jet fuel is consumed during a flight, which reduces the weight of the plane and, thus, increases fuel efficiency during the latter part of the flight.
At present, batteries are about 40 times heavier than tanks of jet fuel for a given amount of energy, Winter says. The engines on a typical jet airplane produce about 30,000 pounds of thrust, or about 18 megawatts, he says, adding that today’s electric motors are capable of producing one or two megawatts each.
Like electric vehicles, electric aircraft will rely on high-performance coolants, which may be water-based because of water’s superior cooling capacity. Less power is required to pump low-viscosity coolants through the small channels in an electric motor, DellaCorte says. Water-based fluids satisfy this requirement, but water also conducts electricity, which can be problematic, he adds. Water-based lubricants also run into problems in extreme-pressure environments. Unlike oils, which become more viscous under pressure, water maintains a constant viscosity. However, water-based lubricants can be used in some applications, like ball bearings, he says.
Although battery-powered propulsion systems have a ways to go, electromagnetic actuators are under serious consideration to replace hydraulic systems in landing gear, flaps and rudders. The electric controls are less complex and lighter in weight than hydraulic systems—and they don’t leak fluid, says DellaCorte. He notes that NASA is working on developing shape memory alloys for components like flap controls to eliminate the need for lubricants on these parts, and they have tested these alloys for use with foldable wings.
6 It’s not inconceivable that in 20 years, large passenger aircraft might not have hydraulic systems, he says. However, even if all the hydraulic systems on an aircraft are replaced by systems that don’t require lubrication, other systems will still rely on gears and bearings, and these will need greases and solid lubricants, he adds.
Hydrogen-fueled aircraft, currently in the early stages of development, present several opportunities for improved sustainability. Hydrogen can be produced by coal gasification or steam reforming of natural gas, both of which rely on carbon-based feedstocks. However, hydrogen also can be produced by water electrolysis, which is more energy-intensive, but this energy can come from wind, solar or other renewable resources.
Hydrogen fuel can be combusted directly in turbine engines (with relatively few modifications from today’s engines) or it can be used in fuel cells that power electric motors.
7 Because of the weight and power limitations of electric motors, hydrogen combustion will be a more viable option for the foreseeable future, Winter says. Thus, hydrogen combustion engines will still need bearing systems, gearboxes and other mechanical components that need effective lubrication.
One advantage to using hydrogen fuel (aside from the fact that it’s not carbon-based), Winter notes, is its lack of combustion particulates. Hydrogen fuel is stored as a liquid at -253 C, but it burns at a hotter temperature than carbon-based fuels—and the various lubricants used must maintain their viscosity and lubricity profiles over this entire temperature range to which they are exposed. However, hydrogen combustion produces less radiant heat because there are no particulates to transmit this heat. Generally, he says, today’s sustainable aviation fuels that he and his colleagues have worked with tend to be cleaner, which helps the lubricants to stay cleaner.
Small sector, significant effects
When it comes to aviation lubrication, “Sometimes I feel like we’re the tail on the dog when it comes to trying to affect sustainability through design,” Yungk says. “We provide lubricants that are enabling what the engine manufacturers are trying to accomplish.”
For now, evolution in aviation lubricants largely focuses on thermal stability, which contributes to more efficient engine operation. Lubricants also are being required to hold up under greater loads and deal with greater sliding speeds in gearboxes. However, these lubricants must continue to comply with existing specifications for safety and performance.
Ultimately, Winter says, the biggest contributor to CO
2 emissions—and the resulting climate change effects—comes from burning fuel. And fuel efficiency is intimately dependent on the effectiveness of an engine’s lubrication. A gas turbine engine has some 40,000 parts, he says, and these parts are spinning at tens to thousands of rpms. At the scale of a gas turbine engine, even a fraction of a percent increase in friction manifests itself in the generation of heat, an increase in fuel consumption and an increase in emissions. “And so, across the industry, we’re always eager to have ever better lubricants,” he says.
REFERENCES
1.
Energy consumption by mode of transportation, U.S. Bureau of Transportation Statistics. Available
here.
2.
Turbine engine oil specifications: HTS and HPC oils: MIL-PRF-23699 (HTS) and SAE AS5780 (HPC), Shell Aviation. Available
here.
3.
Eastman Aviation Solutions white paper, EAS-TO-12255 EAS ATO related preservation considerations white paper.
4.
Pratt & Whitney’s Geared Turbofan™ Engine videos:
here,
here, and
here.
5.
Howe, B. R. (April 16, 2022), “The battery that flies,”
New York Times. Available
here.
6.
Verpraet, I. (August 22, 2018), “NASA tests shape memory alloy in foldable wing,”
Aerospace Testing International. Available
here.
7.
David Kramer, D. (2020), “Hydrogen-powered aircraft may be getting a lift,”
Physics Today, 73 (12), p. 27,
https://doi.org/10.1063/PT.3.4632.
Nancy McGuire is a freelance writer based in Albuquerque, N.M. You can contact her at nmcguire@wordchemist.com.