Principles of Aircraft Engine Lubrication
By Harold Tucker
July / August 1998
At this year's Professional Aviation Maintenance Association's annual meeting which took place in Kansas City, Harold Tucker, Lubricants Technical Director for Phillips 66 Company offered a presentation to maintenance personnel from around the country on some of the basics of aviation engine oil. The following is an overview of that presentation:
What major functions do lubricants perform in aviation engines?
Lubricants are used to reduce friction and wear, whether it's in an aviation engine or the wheel bearing on a car.
Other major functions of a lubricant include cleaning, cooling and sealing, in addition to helping fight corrosion and rust in the engine.
Airplanes that are used infrequently especially need the corrosion and rust protection that good aviation lubricants can provide. Unused aircraft have a high potential for rust and corrosion, among other downtime problems. The more frequently and consistently an airplane is flown, the easier it is to properly maintain and lubricate.
What are the benefits of using a lubricant that cleans the engine?
All aviation oils clean. When we say an aviation oil cleans, we think of removing sludge, varnishes, and grunge accumulations in the oil pan, on plugs, or in the screen. However, when a lubricant keeps your airplane engine clean, it also means a clean ring belt area and better control of the combustion process. When those rings are able to move freely, your engine operates at higher efficiencies, has better ring seal, produces less blow-by, and consumes less oil.
A dirty ring belt restrains the movement of the rings within the grooves and they can't seal. This may create pressure between the ring face and the cylinder wall — leading to wear, scarring or scuffing.
How do aviation lubricants keep an engine cool?
Air-cooled aircraft engines rely on their oil for cooling far more than water-cooled automotive engines. Automotive oil typically accounts for about 40 percent of the engine's cooling capacity. In aviation engines, the oil must carry off a greater percentage of the engine's heat.
Oil is a heat-transfer medium which flows through the crankcase and oil coolers, and dissipates the heat from moving parts, thus constantly cooling engine bearings and piston rings.
Without the cooling oil film on a cylinder wall, the rings wouldn't have a good heat transfer path. This can lead to melting, galling, or scarring problems. Oil also cools the valve springs and the whole valve train.
How does oil seal an aviation engine?
Aviation oil not only provides a seal between the rings and cylinder walls, but also helps seal the gasketed areas and the rubber or synthetic seals for the crankshaft. When oil washes around those areas, it helps retain a seal. Thus, aviation oil must be of a blend or formulation that is compatible with the seal materials so that the seal itself lasts longer.
What about the job we think of first when we think of oil — lubrication?
Lubricating properties are among the most important physical characteristics of aviation oil. Proper lubrication requires a strong enough and thick enough oil film between moving parts to keep friction and wear to a minimum.
Oil properties can include boundary or mixed film, dynamic, hydrodynamic, and elastohydrodynamic forms.
Boundary or mixed film lubrication is found in the upper cylinder area in the outer boundary of an aircraft engine. This is the most remote engine area to lubricate because the oil rings scrape most of the oil film off the cylinder walls before it reaches the upper cylinder. However, there must be a residual amount of lubrication in the upper cylinder to protect the engine on startup. Also, if an engine has been sitting idle for a month, some lifters have been pressed against cam faces and loaded under maximum spring pressure. Most of the oil has been squeezed out of that junction. When the engine is fired up, it takes a while to get oil to all those surfaces again. So, for that crucial moment, you need good boundary or mixed film strength at those critical boundary areas. Oil film retention is not as critical on startup in cam and crank journal areas.