Engine Lubrication, Part VII
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 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. Application of wheel bearing grease and of course engine oil is usually done without much thought. On this page we touch on a complicated subject explaining why and how these additives are used.
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.
AW / EP and 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.
The preferred state of lubrication is of course the hydrodynamic film. 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. 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.
Materials we know as Zinc Dialkyldithiophosphates (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 the engines.
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.
Zinc Dialkyldithiophosphates were used in relatively high concentrations in engine oils as an anti-wear additive for valve trains, flat tappet camshafts and high tension valve springs in high powered engines. The use of catalytic converters in cars saw a reduction of ZDDP in oils as the phosphorus compound reduced the efficiency of these converters. There was no immediate increase in engine wear since so it seems to be doing its job but rumors say that cam shaft wear increased.
Conclusion: added by oil manufacturers, current levels probably a bit too low, adding more will help.
Grahpite is a solid lubricant and is formed in so called graphene layers in a honeycomb structure. This and many other metallic materials are often used in some other lubricating applications are less suitable. Graphite is a carbon
compound and the use of a graphite lubricant can contribute to a much faster erosion due to its conductive properties. When two different metals such as copper and iron alloy, come into contact in the presence of atmospheric moisture and excellent conductor, (like carbon graphite), galvanic electrical currents are to some degree produced, resulting in metallic oxidation or corrosion and pitting.
In short: not recommended!
Molybdenum Disulphide, MoS2
Molybdenum Disulfide (MoS2) is a naturally occurring relatively inert, chemically stable, practically noncorrosive, nontoxic mineral. MoS2 has a particular affinity to adhere to metals with a peculiar mechanical thermal chemical bonding. Under the microscope it can be seen as tiny plates. And it is these plates forming a coating, filling minute imperfections that slide over each other during lubrication creating a smooth surface. Replenishing with half the normal dose (3-5 %) will be needed every oil change.
Diamond Aircraft is as per their announcement at AERO 2016 filling the Austro Diesel engines for their aircraft with Liqui-Moly 5W40 oil.
Tungsten Disulphide, WS2
Tungsten disulphide was developed by NASA as a lubricant for the Mariner deep space probes, where the high vacuum and friction coefficient temperature render conventional lubricants unsuitable for the missions. It adopts the same layered structure as MoS2 and is used as an additive extending gear box life, ball bearings and sliding surfaces. It is chemically inert and can withstand temperatures up to 650 °C. Very expensive and hard to get in additive form for engines.
Engine oil containing additives as ZDDP, MoS2, Ceratec from Liqui Moly or WS2, is therefore preferable above any other.
PTFE / Teflon
Polytetrafluoroethylene, or in short PTFE, is widely known as Teflon. This is a solid (micrometer size particles) that is the active ingredient in many aftermarket oil supplements. Some claims include reduced wear and reduced friction. PTFE and other suspended solids have been shown to compromise oil flow and increase wear under some conditions. Teflon does not adhere to metals by itself so it remains in suspension in contrary to WS2 and MoS2.
They can (read: might, maybe) reduce friction but OEMs do NOT approve of their use because of the associated risks. Dupont, the highest quality source for PTFE, no longer sells PTFE for use as an oil additive or supplement. This makes any supplement containing PTFE even more questionable. Slick 50 or FIN25 are known to contain PTFE.
Chlorinated Paraffin's are soluble additives that are very common in aftermarket oil supplements. They are used as extreme pressure additives and friction modifiers. They can be effective in this role. But, they can also be corrosive to some engine metals and are NOT approved by OEMs. TSL (Tri Star Lube) and Militec are some examples of this kind of additive.
Basically: stay away from PTFE and chlorine based additives!
More information about anti friction from MolyKote in their Anti-Friction Coatings Selection Guide.