Just as there are two basic types of aircraft engines, piston and turbine, there are two categories of aero engine lubricants, the piston engine oils and the jet engine oils. Each oil type is formulated using base-stocks and additives that reflect the requirements of the physical, chemical and performance characteristics imposed by the corresponding engine design and operating conditions. As a result of these differences, aviation piston engine oils can generally be blended from petroleum base-stocks, whereas the modern gas turbine oils are mainly of synthetic origin.

• Aircraft Piston Engine Oils
• Jet Engine Oils

Aircraft Piston Engine Oils

Aviation piston engine oils are of two types, straight mineral and those containing additives, irrespective of the viscosity grade or composition of the base stock.

The first petroleum based aviation piston engine oils developed were straight mineral oils. As their name implies, they contained no additives, but relied upon crude oil selection and refinery processing to impart the desired performance characteristics. Today, these oils may contain certain additives such as pour point depressants and approved oxidation inhibitors.

Straight mineral oils are still popular with operators of older engines. In addition, many engine builders and overhaul agencies recommend the use of these oils for break-in of new or newly overhauled engines.

However the straight mineral oils are not very effective in controlling engine deposits which are caused by the product of combustion (mainly soot, blowby reactants, and oxidation products). If these deposits are allow ed to build-up in an engine, they may prevent proper lubrication.

Unlike an automotive internal combustion engine, aircraft piston engines operate most of the time at a constant throttle setting, where the turbulence pattern in the combustion chamber is very consistent. Under these conditions, deposits tend to form and concentrate at points where there is little turbulence. As the metallic additives used in automotive engine oils promote deposits in these low turbulence areas, they are not acceptable in lubricants for aircraft piston engines.

To overcome these problems additive type oils were developed in the late 1950s, incorporating non-metallic dispersant additives. The introduction of these additives greatly reduced engine deposits, resulting in less wear and lower oil consumption. Some of the ashless dispersant additives also improved the viscosity index (VI) leading to increased oil flow at low ambients during engine start and simultaneously maintaining the desired viscosity at higher operating temperatures.

A dispersant is an additive that prevents the accumulation of particles of sludge, carbon, lead compounds (derived from the fuel) and dirt in one spot by holding them in suspension in the oil. The suspended particles are eventually removed from the engine as the oil is filtered, consumed, and drained.

Ashless dispersant type oils are now the most widely used for general, commercial, and military aviation piston engines. Engines that have run for an extended period on straight mineral oils may have built up a significant level of deposits which can become dislodged if the oil is changed to a dispersant type. Caution should therefore be exercised in such cases to prevent potential problems due to premature filter clogging.

In each of the straight mineral and dispersant categories a number of viscosity grades are available, designated now by the viscosity classes defined by the Society of Automotive Engineers (SAE) USA, in a manner similar to the automotive lubricants. Earlier the Defence / Military specifications had their own viscosity designation numbers, which were often incorporated as a suffix by the oil manufacturers to their product brand name for ease of selection.

The following table gives the summary of the types, governing specifications and viscosity grades of aircraft piston engine oils, with their equivalent designations. The most popular choice is the ashless dispersant type SAE grade 15W-50 which has a wide operating temperature range and is approved for use on most aircraft piston engines

Piston-engine manufacturers like Textron-Lycoming and Teledyne Continental Motors accept products conforming to the military specifications as indicated in their Service Bulletins 1014 and MHS 24 / MHS 25 respectively.

Piston engine oils originating from Russia are only of the straight mineral type classified
according to their viscosity measured at 100°C. The main products are :

 

RUSSIAN PISTON ENGINE OILS
DESIGNATION	SPECIFICATION	KINEMATIC VISCOSITY, cStAT 100°C
MS-14	GOST 21743-76	14
MS-20	GOST 21743-76	20
MK-22	GOST 21743-76	22

The major application for these grades is now for the turbo prop engine oil mixtures as the usage for piston-engine applications has declined, with the phasing out of such aircraft for military service.