Oil is the lifeblood of an engine. The correct maintenance of the oil system is the most important factor in its continued health and long life. The lubricating oil does considerably more than reduce friction, it performs six major functions:
- Reduces friction between moving parts.
- Provides cooling of the internal parts of the engine.
- Cushions moving parts against shock.
- Seals the piston ring cylinder gap.
- Protects the internal surfaces against corrosion.
- Keeps the interior of the engine clean and free from sludge, dirt varnish and other harmful contaminants.
The oil must perform these tasks with a high degree of compatibility so that each is carried out without detriment to the others. This is no small task and it has to be much more than just plain oil. Let us examine each function and see how the oil behaves and the subsequent effects on the engine.
1. Reduces Friction.
During normal operating, lubricating oil is distributed to all moving parts in the engine and the quantity and pressure vary according to engine speed and hence the loads imposed on the various parts. In all cases the oil reduces friction by forming a slippery film between the moving parts. There is a minimum thickness in which the oil can maintain its anti-friction film and still continue to flow. This property is regulated by the manufacturer in the refining stage and results in different grades or 'viscosities' of oils. The clearances between the moving parts in an engine dictate the viscosity requirements to ensure satisfactory operation and long service life.
We have all heard the phrase 'oil does not wear out'. Unfortunately, it does. Engines subject the lubricating oil to severe stresses and laboratory tests have shown that the oil undergoes constant shearing action from the various moving parts. In time this shearing action alters the original viscosity properties and the oil becomes less efficient in its anti-friction function. Once this happens the oil’s continued use could result in an increase in wear and a reduction in the service life of the engine.
2. Provides Cooling.
All moving parts generate friction which in turn produces heat. Although the lubricating oil virtually eliminates metal to metal contact, it is subject to its own friction from the shearing process. The oil is also in contact with the cylinder walls, pistons and valve gear which are exposed to extreme temperatures during normal combustion. In both cases the constant flow of oil to the various parts carries away the heat fast enough to keep the parts at a safe temperature.
Pilots should keep in mind that the oil temperature gauge does not measure the temperature of the oil in the engine but is the temperature of the oil entering the oil pump after passing through the oil cooler or oil tank. It is, therefore, not a reliable indication of the temperature inside the engine.
The 'hot' areas of the engine impose high temperatures on the oil while it is performing its cooling function and subject the oil to coking and oxidation. The coking tends to dirty the oil with carbon, and the oxidation causes the oil to break down and thicken. The full- flow type of oil filter fitted to modern high performance engines helps to remove much of the coking, however, the 'Auto-Klean' filter system fitted to the Gipsy Major engine allows much of this to pass through.
It is also essential that the oil does not become too cold. The viscosity of the oil varies with temperature and the correct anti-friction and flow properties are only available within the operating temperature range of the oil. The oil must also be hot enough to dry out the moisture in the engine that collects during the normal start and shut down operations. Moisture in the oil allows it to emulsify and hence reduces its efficiency.
A good example of the cushioning function of the oil is its use to lubricate the valve gear. Each valve is being opened and closed up to 20 times a second in short sharp movements. Every part of the valve mechanism is subject to reverse loads and much of the shock is cushioned by the lubricating oil. In this function it is easy to see the severe shearing and crushing action imposed on the oil. In common with the other functions, continued use of the oil past its recommended change period will lead to reduced efficiency and increased wear in the engine.
The thin film of oil on the cylinder walls not only lubricates but helps to provide the necessary gas tight seal between the piston rings and the cylinder wall. Unfortunately, a perfect seal is never achieved and some of the combustion gasses 'blow-by' the piston. Not only does this 'blow-by' contribute to the oxidation of the oil but it also contains the various acids and corrosive lead salts generated during combustion. These contaminants remain in the oil and are not removed by the filter. More contaminants are added each hour of operation and mix with the water vapour that condenses after shut-down. The result is a harmful corrosive mixture that can only be removed by changing the oil. Again, we see that it is false economy to operate past the recommended oil change period.
After engine shut-down the oil covers the whole of the interior of the engine. This thin coating of oil provides protection against rust, however, it will slowly drain off in time and the interior will eventually be exposed to the corrosive elements. Without fully inhibiting the engine, this protection can only be maintained by operating the engine at least once a month. But remember, the oil must maintain a sufficiently high temperature inside the engine to drive off the accumulated moisture. A ground run is, therefore, not sufficient, in fact it does more harm than leaving the engine idle. The aircraft must be flown for at least half an hour to enable all parts to reach the operating temperature and for the oil to 'dry out' the engine.
Remember also that during normal engine operation contaminants are continuously added to the oil. In time the protective additives will be used up and the oil becomes saturated with corrosive contaminants. The oil will no longer be able to protect the interior of the engine and, therefore, the only certain way to offer protection is regular oil changes.
All aviation piston engine oils are formulated to carry out efficiently the functions covered so far. However, only an ashless dispersant type oil can keep the interior of the engine clean.
Every hour of engine operation adds more dirt to the oil. This dirt comes from a variety of places: dust from the atmosphere, soot during starting and idling, coking from piston 'blow-by' and other hot areas, and minute metal particles from normal wear. With straight oil this dirt would normally settle throughout the engine and oil system when the engine is at rest and will clog up the oil ways and cause oil starvation to the many vital parts. With older engines the restriction in oil flow caused by the settling out of dirt was a major factor in the determination of engine life.
Ashless dispersant oils contain additives that cause the dirt to disperse throughout the oil and also prevent it from precipitating out and collecting in the engine when at rest. However, like all the other functions, we cannot have something for nothing. Continued operations will add more and more dirt to the oil which, once saturated, will not continue to clean the engine. Again, it is false economy to run past the recommended change periods.
MIXED BRANDS AND ADDITIVES
It is a misconception that different name brands should not be mixed. Provided that you use the same type of oil the different brands or makes should mix with each other satisfactorily. Aircraft lubricating oils are formulated to provide satisfactory lubricating properties under the specific operating conditions found in aircraft engines. This operating regime is completely different from that in which the automobile engine operates. Engine temperatures, loads and clearances are different. Therefore:
Do not use auto oils or additives in your aircraft engine.
By the same token aircraft oil should never be used in your car.
Oil is the life-blood of your engine and dirty, contaminated oil, will cause unnecessary and expensive damage.
DO use straight oil for the first 50 hours after either a full or top overhaul.
DO use an ashless dispersant type oil for normal operations.
DO change the lubricating oil as recommended: for the Gipsy Major, every 50 hours. Change the oil annually if the engine has a low utilisation of less than 50 hours a year.
DO NOT use automobile oils.
DO NOT use oil additives.
DO NOT mix different types of oil.