Piston Engine

Lubrication System

Oil System

Purpose and function of oil system

• Lubrication of the engine’s moving parts. • Cooling of the engine by reducing friction. • Removing heat from the cylinders. • Providing a seal between the cylinder walls

and pistons. • Carrying away contaminants.

Oil System Oil system components

• Oil tank • Oil (Pressure) pump • Oil (scavenge) pump • Oil temperature gauge • Oil pressure gauge • Oil filter • Oil cooler • Oil bypass valve • Oil relief valve • Oil pressure regulator

Oil System

Principles of engine lubrication

•The primary purpose of a lubricant is to reduce friction between moving parts

•Because liquid lubricants (oils) can be readily circulated, they are universally used in aircraft engines

•Oil lubrication is based preventing metal to metal contact

•As long as the oil film remains unbroken, metallic friction is replaced by the internal fluid friction of the lubricant.

Other useful properties of oil

Cushioning between metal parts - this is particularly important for such parts as the crankshaft and connecting rods which are subject to shock loading

Oil absorbs heat from, particularly pistons and cylinder walls which are dependent on oil for cooling

Oil also aids in forming a seal between the piston and cylinder wall to prevent leakage of the gases from the combustion chamber

Oils reduce abrasive wear by picking up foreign particles and carrying them to a filter thus cleansing the engine

Other useful properties of oil

Even more useful properties of oil include:

Changing propeller pitch in variable pitch propellers

Moving turbocharger waste gates

Heating fuel

Protecting metal surfaces from oxidation and corrosion

Viscosity

The resistance of an oil to flow is known as its viscosity

An oil which flows slowly is viscous, ie. has a high viscosity

Viscosity of oil is affected by temperature, becoming more viscous as the temperature reduces

Most engine wear occurs during start-up, particularly if the engine is cold, because of the higher viscosity, and oil may not be pumped around to the engine’s moving parts

If the viscosity is too low especially at high temperature the oil film may break resulting in rapid wear of the moving parts

Viscosity

The oil selected for lubrication must be light enough to circulate freely, yet heavy enough to provide the proper oil film at engine operating temperatures

Since lubricants vary in properties and since no one oil is satisfactory for all engines and all operating conditions, it is extremely important that only the recommended grade be used

Some More Important Properties of Oil

Flash Point & Fire Point

The temperature at which a liquid will begin to give off ignitable vapours (flash), and the temperature at which there are sufficient vapours to support a flame (fire)

Cloud point and pour point

The temperature at which the oil’s wax content, normally held in solution, begins to solidify (cloud), and the lowest temperature at which oil will flow (pour)

Lubricating Oil Types

Straight Mineral Oil

Ash-less Dispersant Oil

Replaces mineral oil after the piston rings have seated, usually at 50 hourly oil change

Less tendency to oxidise to carbon and doesn’t form ash deposits. Keeps foreign particles in suspension until trapped in the oil filter rather than forming sludge.

Contains abrasive material necessary in seating new piston rings and cylinders. However prolonged use will rapidly wear engine parts

Oil Classification

Oils are classified under the Society of Automotive Engineers (SAE) which divides oils according to viscosity.

It is important to note that the classification given to aviation is different to other commercial applications which are usually half the number

80 grade aviation oil = SAE 40

100 grade aviation oil = SAE 50

Use only recommended type and grade of oils

Do NOT mix grades

Wet and Dry Sump Lubrication Systems

A wet sump engine is one where the oil supply is carried within the engine itself

A dry sump engine has the oil supply carried in a separate container pump out of the sump using an extra oil pump called the scavenge pump

Some engine designs don’t allow oil to be carried within the engine itself especially if sustained, inverted flight maneuvers are to be carried out.

Wet Sump oil system

Dry Sump oil system

Wet and Dry Sump Lubrication Systems

Although most aircraft oil systems are of the wet sump type, there are some disadvantages of this system:

•The oil supply is limited by the sump size

•Cooling of the oil is difficult because the oil supply is so close to the operating engine

•Prolonged inverted flight is not possible

The main disadvantage of the dry sump system is:

• Failure of the scavenge pump

Malfunctions in the Oil/Lubricating System

Incorrect oil types

Low oil pressure

High oil temperature

Faulty oil pressure gauge

High oil pressure

Low or fluctuating oil pressure

Gradual loss of oil

Engine Cooling

Although oil removes some heat of combustion, by far the majority of cooling in most light aircraft is by airflow over the engine parts, engine overheating can cause several problems, such as:

• Detonation

•Shortens the life of engine parts

•Impairs lubrication

Engine cooling system

Engine Cooling

Most light aircraft are air cooled by exposing the cylinders and their cooling fins to the airflow

The fins increase the exposed surface area to allow better cooling

As the cylinders are the hottest part of the engine, it is logical to place the cooling fins there

Air cooling is least effective at high power, low airspeed configurations

Engine Cooling

Cylinder baffles

These are metal shields designed and arranged to direct the flow of air evenly around all cylinders. This helps ensure all cylinders are reasonably uniform in temperature.

Cowl flaps

These are air vents that allow the pilot to control how much cooling takes place.

Opening the cowl flaps allows more engine cooling but the trade-off is a slightly slower airspeed due to increases parasite drag

Pilot Responsibility for Engine Cooling

In order to ensure that the aircraft engine is operated within approved temperature limits the pilot has several tools at their disposal:

•Cowl flaps

•Power setting

•Air speed

•Fuel mixture

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