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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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