Lubrication 101: Piston engine oil, its functions, types, and characteristics

Piston engine oil, its functions, types, and characteristics. By Barb Zuehlke Oil. Its basic functions within an engine include reducing friction, cooling, sealing, cleaning, and serving as protection for moving parts. But it's often...

Lubrication 101
Piston engine oil, its functions, types, and characteristics.

By Barb Zuehlke

Oil. Its basic functions within an engine include reducing friction, cooling, sealing, cleaning, and serving as protection for moving parts. But it's often taken for granted. This article will look at the basics of lubrication and the various types and characteristics of oil.

Lubricants provide a fluid barrier between moving parts to prevent friction and wear. As for cooling, oil provides up to 40 percent of an aircraft's air-cooled engine's cooling. Oil creates a seal between piston rings and cylinder walls. This helps to reduce wear, provide better compression, and keep contaminants out while improving fuel efficiency.

If oil is doing its job, it should be dirty. Oil treated with an effective dispersant suspends dirt, metallic materials, and unburned carbon. By monitoring an oil's condition through oil analysis you can establish operating trends to use it as a preventive maintenance tool. Check with engine manufacturer's recommendations, but the typical oil change interval rule of thumb is 50 hours for a filtered engine, and 25 hours if it is a screened engine. Along with the hourly interval, oil should be changed on a quarterly or seasonal basis. This process will help eliminate moisture from the engine and oil to help prevent corrosion.

Types of oils are derived from specifications developed by the military in the 1940s and later standardized by the Society of Automotive Engineers (SAE). The system classifies engine oils by viscosity grades. Oils are classified based on their measured viscosity at high temperatures for single grade oils and at low and high temperatures for multigrade oils. Multigrades have a high viscosity index (VI) and may fall into more than one SAE grade classification.

Aviation engines have used a different viscosity rating than that of automotive and SAE. They use 65 weight or SAE 30, 80 weight or SAE 40, 100 weight or SAE 50, and 120 weight or SAE 60. Aviation multigrades developed later adopted the SAE automotive system of classification and can be found in 15W-50, 20W-50, and 25W-60 ranges.

SAE standards for lubricating oils include J1966 and J1899. The SAE standard J1966 establishes the requirements for nondispersant, (straight grade) mineral lubricating oils used in four-stroke cycle piston aircraft engines. It covers the same requirements as the former military specification MIL-L-6082. J1899 establishes the requirements for lubricating oils containing ashless dispersant additives, the same as MIL-L-22851.

Following are some technical terms, characteristics, and descriptions of the various types of lubricating oils used in the aviation piston engine industry.

Viscosity is the measure of the oil's resistance to shear or flow. High viscosity indicates a high resistance to flow while low indicates a low resistance. It varies with temperature and is affected by pressure. Increasing temperature causes viscosity to decrease; conversely reducing temperature causes viscosity to increase. Higher pressure causes viscosity to increase which also increases the oil's film thickness. Viscosity is measured by shear and time. When measured by shear it is expressed in centipoise and is known as dynamic viscosity. Kinematic viscosity is expressed in centistokes and is usually given at two temperatures 40 C and 100 C. Kinematic viscosity is measured as the time required for an oil sample to flow through a viscosity tube at a standard temperature. This value is then converted to centistokes.

Pour point
This is the lowest temperature at which an oil will flow. Oils are usually selected to ensure the pour point is well below anticipated ambient temperatures.

Flash point
The flash point is the lowest temperature at which a lubricant must be heated before it's vapor, when mixed with air and exposed to a source of ignition will ignite but not continue to burn. It is used to determine the transportation and storage temperature requirements along with potential product contamination.

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