SECTOR FOCUS


Aviation derived ester technology for the industry


How performance esters for industrial lubrication have taken advantage of the development of aviation lubricants


Eric Piveteau, General Manager and Siegfried Lucazeau, Product Manager Industry & Automotive, NYCO, France


The challenges of jet engine lubrication Aircraft gas turbine oils are used in aircraft engines to lubricate and cool the bearings of the compressor and turbine shafts. The main constraint placed upon the oil is the extreme thermal and oxidative stress, due to the fact that the bearings are located in the core of the engine, and are directly exposed to the heat radiated by the flow of hot compressed air (in the compressor section) and the combustion gases (in the turbine section). The temperature of the compressed air typically exceeds 350°C at the outlet of the high pressure compressor, and the turbine inlet temperature exceeds 1200°C. Whilst some thermal insulation and sophisticated cooling techniques (using relatively cool air blanketing) are in place to protect the bearing chambers from such extreme temperatures, the oil temperature in the bearings is continuously in the range of 200 to 230°C. The heat dissipated by friction in the bearings is also a significant contributor to the generated heat.


The thermal stress on the oil can also be greatly increased by inadequate engine shut-down procedures. A sudden shut-down, without any idling period, cuts off the supply of circulating oil and cooling air in the bearing chambers. At this point, the heat accumulated in the hot engine parts (blades, vanes) will sharply increase the temperature of the bearing chamber up to 340°C, leading to a rapid oxidation and, eventually, coking of the residual quantity of oil in the bearing chamber.


Jet Engine - 24,000 h Standard performance oil (STD) High temperature oil (HTS)


Earlier generation jet engine oils, especially for military engines, were based on diesters, i.e. esters of diacids. Diesters of sebacic acids, in particular, have been used as base fluids for the MIL-PRF-7808 Grade 3 turbine oils. However, diesters break down through the ß-elimination mechanism to olefins and acids, because of the presence of a hydrogen atom on their alcohol


Continued on page 16 14 LUBE MAGAZINE NO.143 FEBRUARY 2018


Formation of coke is therefore a major concern, as coke build-up may eventually block pipes or oil injection nozzles, leading to engine failure and a potentially catastrophic fate for the aircraft. Not surprisingly, the improvement of thermal and oxidative stability has been a constant focus of aviation oil manufacturers over the past decades.


Low temperature viscosity is also an important requirement, because during the certification process, aircraft and engines must demonstrate their ability to start at temperatures as low as -40°C. For this reason, the maximum viscosity at -40°C of aviation oils is limited to 13,000 mm²/s. Some military aircraft have a more demanding requirement: to operate at -54°C, in which case a lower viscosity oil, complying with MIL-PRF-7808 Gr.3, is used.


Advances in commercial engine technology enable airliners engines to operate for a very long period of time - as much as 40,000 h (more than 10 years) under certain conditions – without any oil drain. In contrast, military engines are operated for a much shorter period of time, but in very high power and temperature conditions.


The advantages of neopolyol esters In order to comply with such extreme requirements, aviation turbine oils use very carefully selected and manufactured high performance components. They are typically composed of high quality polyol ester base fluid (93 to 95%), and anti-oxidant, anti-wear and corrosion inhibiting additives (5 to 7%).


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