Principles of aircraft engine lubrication

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...

Dynamic lubrication is produced through the pressure generated by an oil pump and this pressure provides an adequate flow of oil to the lubrication system. Hydrodynamic lubrication is like water skiing — it provides a smooth surface for any moving part to ride on and prevents any direct contact between moving parts.

Hydrodynamic lubrication is full-film lubrication that keeps moving parts from contacting one another.

In true hydrodynamic lubrication, as with water skiing, contact pressure is much lower and is spread over a large surface area. A constant supply of oil is required between the parts for hydro-dynamic lubrication.

When everything is operating properly in an aircraft engine, there is a constant lubricating film between any parts that might rub together. Any wear that the lubricant flow itself could cause is so slight that it would take several lifetimes to wear out a component — like a river wearing away the rocks.

If that's true, why do engines wear out?
Your biggest problems are on surfaces where there is no oil. That usually happens after an engine has been sitting for a while.

You need the right viscosity and the right velocity between moving parts to keep oil where it needs to be. Think about what happens inside your engine whenever you do something like a cold start. If it's very cold when you fire up your engine, there is maximum velocity between metal parts and maximum oil viscosity. The oil isn't going to provide good hydrodynamic lubrication until the engine warms up.

With bearings, the clearances are so close and so contained that they will sometimes keep a good lubricating film on that bearing for years.

In elastohydrodynamic lubrication, an oil can act like a solid — as in areas of very fast, extreme force, such as where the rocker arm contacts the valve stem. The contact happens so quick that the oil can't get out of the way. When engine parts hit that fast, the oil literally acts like a solid. Elastohydrodynamic lubrication provides effective protection for the instant it's needed. The oil acts as a shock absorber, and hence, exhibits elastohydrodynamic properties.

What does viscosity have to do with lubrication? All of these lubrication types — the mixed film, dynamic, hydrodynamic, and elastohydrodynamic, all relate to and depend on oil viscosity. Viscosity is a measure of a fluid's resistance to flow. All fluids flow better when they are warm — cold oil is thick, but thins and flows better as it gets hot.

Oil viscosity is more important in an aviation engine than in an automobile engine. The fewer additives in the oil, the more dependent it is on its viscometrics (viscosity properties). Straight, untreated base oil can be limited in its lubrication without supplemental additives. Aviation oil will assist in boundary or mixed film lubrication, detergency and other lubrication aspects.

Ash cannot be added to aviation piston engine oils. Regulations prohibit the use of ash-bearing detergents and anti-wear, zinc-dithio-phosphate that are used in automotive or diesel truck engine oils because they may cause pre-ignition or detonation in an aircraft engine.

What is an oil's viscosity index?
While viscosity is an oil's internal resistance to flow, its viscosity index is simply its resistance to changing flow characteristics due to changes in temperature. If an oil's viscosity changes very little, despite significant temperature changes, the oil has a high viscosity index.

Viscosity index is an arbitrary numbering system. Higher numbers mean an oil's viscosity changes little with temperature, and lower numbers means it changes more. Single grade oils typically have a viscosity index of 90 to 110.

Multi-grade oils, with a viscosity index of 150 or higher, can tolerate extreme temperature changes and better retain their viscosity characteristics. Some automatic transmission fluid is so multi-graded that it may have a viscosity index of 200. Multi-grade oils are common in applications such as aviation oil, automatic transmission fluid, power steering fluid, gear oil, and hydraulic fluids.

How can an oil's viscosity index be improved?
Viscosity index can be increased by adding viscosity modifiers, or viscosity index improvers, to base oils. Several types of polymers are used to change the viscosity index of aviation oils.

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