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Engine Oil

Engine Lubrication, Friction

With the correct oil, friction losses in an engine are reduced to a minimum. This is done by taking into consideration circumstances as engine usage, ambient temperature, time of year and climate, location and engine design. The engine manufacturer usually recommends a certain type of oil to use, taking all of these circumstances into account.

Engine oil contains additives known as friction modifiers/reducers and anti wear additives which are soluble and are used in the lubrication industry for many years now.

You as an experimental aircraft builder are probably using them without the full appreciation for the properties of these additives. For example: application of wheel bearing grease and refilling engine oil is usually done without much thought. On this and the next page we touch on a complicated subject explaining why and how these additives are used.

Reducing Friction & Wear

In the 1970s certain gear oil additives were used and found to reduce frictional heat, temperatures under high loads and reducing clatter at the same time. Before long these additives were used in engine oil adding to the lubricity of the base oil. Early variations were made from animal or vegetable fats and later synthesized in labs from alcohols and basic chemical compounds.

Desired Lubrication State

Hydrodynamic Lubrication

The preferred state of lubrication is of course the hydrodynamic lubrication state. This is to be followed by the friction modified mode of operation, followed by an AW/EP regime. When high speeds or low loads are present, it is easy to maintain the hydrodynamic regime.

For example: when the engine RPM drops, however, or the load rises above a critical point, the hydrodynamic regime breaks down and then it would be very desirable to be able to glide smoothly into a friction modification mode of operation.

If no friction modification has been provided, the oil defaults to an AW/EP regime. So friction modification and AW/EP is a logical method to broaden the range of effectiveness of the lubricating film. Friction Modification depends much on the mechanism of contact (geometry) and molecular construction of the friction modifier.

AW / EP & FM

These acronyms stand for Anti-Wear / Extreme-Pressure (the same) and Friction Modifiers. Although it sounds like they are all three the same thing they do operate in different parts of the lubrication regime. AW and FM operate both in the boundary lubrication regime whereas FM only operates in the desired hydrodynamic lubrication regime.

Additive types

Materials we know as Zinc Dialkyldithio Phosphates (ZDDP), sulfurized fats and esters, organometallic compounds (such as Molybdenum Dithiophosphates, Molybdenum Dithiocarbamates, Antimony Dithiocarbamates) have shown their ability to build and maintain a very good boundary lubrication film under severe load conditions and heat encountered in internal combustion engines.

Other additives used in the industry are: Titanium (TI), Graphite, Tungsten Disulfide (WS2), Teflon (PTFE), Chlorine (CP) and Molybdenum Disulfide (MoS2). Although each and every one has their own specific application and reduce friction and wear. Some of them are not suitable for use in aircraft engines due to corrosive reaction with the materials used in the engine.

Mechanical Properties

The critical difference between AW/EP additive films and FM films is in their mechanical properties. AW/EP films are semi-plastic deposits which are hard to shear off. AW/EP films work by protecting the mating metal surfaces from asperities physically gouging the opposite surface. When a hydrodynamic film of oil is ruptured, this layer of AW/EP material protects the mating surfaces from catastrophic failure.


Another huge benefit is that when oil is lost from the engine the additives provide short term dry running properties. For an aircraft this means that the pilot still has a running engine capable of helping him to land during this emergency.

More information about anti-friction from MolyKote in their Anti-friction Coatings Selection Guide.


Written by EAI.

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