Aviation lubricants have been slow to evolve, but major changes appear to be around the corner. Introduction of new materials, engines and aircraft, along with demands for lower emissions, better fuel economy and reductions in noise, are putting pressure on the oils that lubricate turbine engines. Two experts told a recent conference that the industry needs big adjustments in oil formulation and lubrication systems.
Today aviation lubes are formulated from high quality synthetics base oils, and drain intervals can last from two to 10 years. As a result, they receive heavy scrutiny from original equipment manufacturers, according to an expert in gas turbine lubrication. For each oil and engine type, OEMs perform sophisticated and costly rig and engine tests. Consequently, in the past few decades oil formulations havent changed very often.
At the same time, aircraft and engine efficiency has increased significantly. For example, the wing tips are bent up on some of the newer Boeings and Airbuses, Susan Ardito, of the aviation lubricants department at ExxonMobil Lubricants and Specialties, told ACIs European Base Oils and Lubricants summit held in Prague in September. Called winglets, these additions reduce drag and improve fuel economy. Additionally, Ardito explained, the use of lightweight composite materials is a trend that we will see more often in the future.
On the engine side, the wider use of what are called high bypass ratio engines and improvements in combustion technology have greatly improved fuel efficiency. However, these changes also increase internal engine temperature, which in turn impacts the lubricants.
Also, higher pressure hydraulic systems have been introduced on Airbus A380 and Boeing 787 jumbo jets, improving efficiency, Ardito, who is based in Ocean, New Jersey, U.S., told the conference audience. These systems weigh less but do the same work as heavier systems. Weight in an airplane is a key factor for fuel consumption. According to some studies, one kilogram less weight per plane across a whole fleet can result in savings of hundreds of thousands of liters of fuel.
While the use of lower viscosity engine oils has helped improve the fuel economy of piston engines used in passenger cars, the same doesnt hold true in the gas turbines used in aviation engines, Ardito noted. The viscosity is already as low as it can be, and it has really small impact because bearings are most of what we are lubricating, she said.
Historically, aviation engines have been lubricated with a wide variety of products. In the 1930s, aircraft engines were lubricated by mineral oils. Later, after the 1940s, early turboprop and turbojet engines were lubricated by diester oils. In the late 1970s and afterward, low to medium bypass ratio engines were lubricated by polyol ester oils with high concentrations of additives.
In the most recent phase of their development, engines have changed drastically, and that evolution continues. Several stages of compression have been added, with more dramatic amounts of thrust delivered by the engines, Ardito said. These engines require oils with higher stability, creating a need for specialty base oils.
Specialties like PPEs [polyphenyl ethers] and PFFs [perfluoro ethers] have shown excellent results in oxidation resistance and deposit formation but failed in material compatibility and low temperature fluidity, she said.
This point was emphasized by Albert Cornet of Safran Techspace Aero, who also addressed the ACI conference. Aviation oil is primarily used to cool parts. The amount of oil needed for purely tribological effects is very low. In fact, a mist is sufficient. He added that the parts to be cooled include bearings, gears, and electrical devices such as alternators.
Cornet said that required oil flow is proportional to the amount of heat to be dissipated. Speed is a major factor in the amount of heat dissipated by bearings. Currently, this is in the range of 50 to 150 kiloWatts, twice what it was 20 years ago, and is continuously growing.
Future lubricant development has to focus on increasing temperatures, Ardito contended. It must address the needs of hotter and more powerful engines. The primary function of these lubricants is to remove heat from the engines.
Limits on the Horizon
Current lubricant technologies have their limits, especially with the new more powerful gas turbine engines. Operating temperatures in the newer engines have increased significantly, according to ExxonMobil, and polyol esters have reached their thermodynamic limit for oxidation.
Polyphenyl ethers – used by the military in Black Hawk helicopters – are very good fluids, but they require oil system heating because they turn solid at ambient temperature. Their drawbacks are limited supply and complex and costly manufacturing processes. Perfluoro ethers are good but very costly fluids with limits in addressing the demands of new engine designs because they are inherently incompatible with engine metallurgy. For example, they decompose when they contact aluminum at high temperatures, Ardito said.
Another problem is that existing additives are all designed for use in hydrocarbon base oils, and they are not soluble in the more advanced oils. A whole new additive platform has to be developed to address the challenges of new engines and new coatings and materials, she continued.
Aircraft and engine manufacturers are under tremendous pressure by regulatory authorities globally to reduce noise and emissions, which translates into reducing fuel consumption. [But], regulations aside, airline companies always strive to reduce fuel consumption because it is the single largest portion of their operating costs, Ardito noted, adding that they are highly interested in anything that could further reduce fuel burn.
From 1975 to 1995, emissions from the industry were reduced more than 30 percent, Ardito said, citing U.S. Federal Aviation Administration data. This all was done by combustion technology improvements, which led directly to the reduction of nitrogen oxides, carbon dioxide and carbon monoxide emissions, she noted.
However, a cautionary note was issued by a University of Cambridge study published in March 2012. It stated that the introduction of new aircraft every year and the ever increasing density of air traffic could offset CO2 emissions reductions by 2050.
ExxonMobil found that improved fuel efficiency has an impact on the future formulation of aviation oils. It leads to higher engine temperatures which requires oils with improved resistance to oxidation and coking as well as lower volatility. Achieving higher heat removal could be possible with oils improved heat capacity and thermal conductivity, Ardito said.
Improved sealing is another factor because engine manufacturers are trying to reduce oil consumption. Oil losses are caused by mist and vapor entrained in the air vented through seals and typically range from 0.3 to 0.5 liters per hour in todays engines, said Cornet. The average oil consumption in these engines should be 0.1 liters per hour, Ardito said, so engine designers have to work on better sealing.
Cornet noted that there has been a significant improvement in high-performance plastic materials, such as polyimides, polyetherether ketone, and polyamideimide. Better mechanical properties and temperature limits make them suitable for wider use in aircraft engines. However, he said, the introduction of high thermal stability oils several years ago led to a lot of leaks due to seal damage. Thus, a new seal material has to be found quickly, concluded Cornet.
For their part, lube designers have to look for formulations with less volatile components and the right antioxidation and antiwear additives, according to Ardito. Aircraft manufacturers are increasingly using seals, plastics and paints with organic materials, and this requires more adjustments by lube formulators.
Esters can react with many of these structures. The challenge is to formulate oils that will be more inert to the organic materials. New aviation oils have to be inert and stable at high temperatures with low deposit tendency and good low temperature properties.
Airlines would also like to have broad application of the same oil. They are looking for a single formulation for all hardware, and backward compatibility for all engines and accessories, but it doesnt look like a realistic request, she said.
In the short term, engine manufacturers would like to see incrementally improved polyol ester technology. There are upcoming specifications for them, much different compared to existing formulations, and in this phase we are not able to tell what may be required, Ardito concluded.
As a consequence of engine evolution, modern lubricants must have maximum heat capacity per volume to increase heat dissipation, said Cornet. In addition, oils with higher viscosity index are needed to provide efficient lubrication at high temperatures. Oils with high VI also enable easier cold start and provide steady pressure in heat exchangers for easier flow control.
Cornet added, Oils must be totally neutral toward high-performance plastic materials at all temperatures, and provide either long term stability or easy disposal. Other important properties include reduced vaporization and low nitrogen affinity. Current oils can contain a large amount of dissolved nitrogen, which causes pump cavitation, resulting in reduced efficiency, damage and wear. Dissolved nitrogen also causes foam when pressure drops suddenly.
Oil formulations are one part of the solution, as these fluids should be applied in new designed engines and lubrications systems. Eventually design of lubrication systems should come in for changes, too.