LUBRICATION 101: PISTON ENGINE OIL, ITS FUNCTIONS, TYPES, AND CHARACTERISTICS

Oil’s primary functions in an engine are to reduce friction, cool, seal, clean, and protect moving parts. It’s something that is often taken for granted. This article will discuss the basics of lubrication as well as the different types and characteristics that oil can have.

Lubricants act as a fluid barrier between moving components to prevent wear and friction. Oil can provide up to 40% of an aircraft’s engine’s cooling. The oil acts as a seal between the piston rings and the cylinder walls. This seal helps reduce wear, improve compression, keep contaminants out, and improve fuel efficiency.

Oil should not be considered dirty if it is performing its job. An effective dispersant is a substance that suspends dirt, metals, and unburned carbs in oil. You can monitor oil conditions and use them to help you prevent problems. You should consult the engine manufacturer for specific recommendations. The average oil change interval is 50 hours for a filter engine and 25 hours for a screening engine. Oil should also be changed at quarterly or even yearly intervals. This will remove moisture from the engine and help to prevent corrosion.  

The specifications of the military from the 1940s are used to determine the types of oils. These specifications were later standardized by the Society of Automotive Engineers. Engine oils are classified by their viscosity grade. The classification of oil is based on the measured viscosity at high temperatures in single-grade oils and low and high temperatures in multi-grade oils. Multi-grades may have a high viscosity (VI) and fall under more than one SAE classification.

The viscosity rating of aviation engines is different from that of automobiles and SAE. They are 65-weight or SAE 30, 80-weight or SAE 40, and 100-weight or SAE 50. 120-weight or SAE 60 is their maximum weight. Later, aviation multi-grades adopted the SAE automotive classification system. They can be found in the 15W-50 to 20W-50 ranges and the 25W-60 ranges.

J1966 and J1899. These are the SAE standards for oil lubricants. The SAE standard J1966 defines the requirements for non-dispersant (straight-grade) mineral lubricating oil used in four-stroke piston aircraft engines. It also covers the requirements of the MIL-L-6082 military specification. J1899 outlines the requirements for oils that contain ashless dispersant additives. It is the same as MIL-22851.

These are technical terms and descriptions that describe the different types of lubricating oil used in the aviation piston-engine industry.

Viscosity

The oil’s resistance against shear and flow is measured by its viscosity. A high-viscosity oil indicates that it is resistant to flow, while a low-viscosity oil indicates that it is less resistant. It changes with temperature and can be affected by pressure. Viscosity decreases with increasing temperature, while viscosity increases with decreasing temperature. Viscosity increases under higher pressure, which in turn causes oil to have a thicker film. Shear and time are two ways to measure viscosity. It is measured using shear and expressed in centipoise. This is called dynamic viscosity. Kinematic viscosity can be expressed in centistokes. It is typically measured at two temperatures, 40 C and 100 C. This value is then converted into centistokes.

Pour point

This is the temperature at which oil can flow. The pour point of an oil is usually set well below the ambient temperature.

Flash point

The flash point is the temperature at which a lubricant can be heated to make it vapor. It is measured in degrees Celsius. When a mixture of air and a source ignition is used, it will ignite but not burn. It’s used to determine storage and transportation temperatures, as well as potential product contamination.

Monograde or Single

Monograde oil, a petroleum-based oil with one viscosity level, is a lubricant that uses a petroleum-based base. Some single-grade oils are better for warmer temperatures, but they can’t provide enough flow for cold start-ups without the use of a heated hangar, engine preheating devices, or a heated hangar. One viscosity may be used in certain locations all year.

The debate over single-grade and multi-grade oils has been ongoing for years. One grade is preferred by some users because it offers better shear and high-temperature stability. Some users believe that only a multi-grade can provide the required performance.

Straight mineral oil is free of dispersants and is recommended by most manufacturers for the initial 50-hour break-in period on newly rebuilt engines. This allows for faster piston ring seating and allows for the accumulation of certain advantageous deposits that can lead to improved oil loss control.

Multi-grade oil

Multi-grade oils can be either full-mineral-based oils or synthetic blends. Multi-grades are designed for all-season operation and convenience. Multi-grades meet more than one SAE viscosity classification and can be used over a wider temperature range than a single-grade oil. Multi-grade oils have viscosity enhancers that reduce the oil’s tendency to lose viscosity and thin at different viscosities. You will also enjoy lower oil consumption and better fuel economy.

Ashless dispersant

These oils can be represented by multi-grades or the AD monograde. They are governed under SAE J1899. Additives that reduce deposit formation are called ashless dispersants. They don’t contain any metal compounds that could cause deposits in the combustion chamber. They prevent oil passageways from being blocked by contaminants. They prevent soil from forming sludge that could block oil passageways.

Synthetic

Synthetic oils are made from polyalphaolefins, which are produced by chemical synthesis and not the refinement of petroleum oil. Molecules are made uniformly in size and structure by refining them. Although these characteristics are dependent on the application, they include higher oxidation resistance or resistance, higher viscosity, a lower pour temperature, lower coefficients of friction, and longer life. The downside is the cost, which can be many times greater than for mineral-based oils.

Synthetic oils have been shown to cause seal degradation and solubility problems with leaded fuel. This can lead to deposits developing and passageways like arteries becoming clogged. Maintenance technicians are able to identify the engine oil based on the amount of buildup. Only one fully synthetic aviation piston oil has ever been released to the market. The product was later pulled due to the above reasons.

Additives

Refinery processes determine oil quality, but additives can increase overall performance. Multi-grades often introduce new technology to the market, which can include anti-corrosion and high-load characteristics. They can significantly improve the performance of older technology engine lubricants.

Textron Lycoming developed an anti-wear/anti-scuffing additive LW-16702 outlined in AD 80-04-03 R2. Many oils have been issued supplemental-type certificates since their introduction to the market.

Maintenance

No matter what grade you choose, whether it’s a straight or ashless dispersant, make sure that the oil change intervals follow manufacturer recommendations. To determine the right oil for your application and environment, consider your aircraft type, engine type, and flight profile. This will ensure that the oil you use meets the required standard.