With the correct oil, friction losses in an engine are reduced to a minimum. This is done by taking into consideration circumstances as engine usage, ambient temperature, time of year and climate, location and engine design. The engine manufacturer usually recommends a certain type of oil to use taking all of these circumstances into account.
Lubrication plays an important part in the life of the engine and during maintenance it will be replaced, on certified aircraft the pilot can only replenish it. Without it the engine would fail within minutes, keeping a watchful eye during flight is therefore important.
Having a basic understanding of engine oil is a must for the professional and private pilot, here we can only scratch on the surface of a very interesting subject.
The oil system in an aircraft engine is very reliable and needs little maintenance in the form of changing oil and filters on regular intervals plus visual inspection. The pilot must keep an eye on levels, pressures and temperatures during operation of the aircraft.
Oil maintenance you say? Yes. Regular changes form the basis of good preventive maintenance. Oil should be changed during each 50 hour check (some have 100 hour intervals (Rotax)), it also gives the maintenance engineer the opportunity to have a look under the cowling (as some pilots would care less) and see if things still are as they should be. And if you are doing it the proper way, have an oil sample analyzed. This will give you insight in what is going on inside of the engine related to the wear and tear of bearings, pistons, cylinders, valves and more.
Engine oil has a number of important functions:
Oil must carry out all these functions under harsh conditions like low and high temperatures, pressure and shearing effects without any side effect. In part I of Engine Lubrication we discussed all off the above but cleaning. During an oil change, the oil together with suspended contaminants is being removed from the engine and it forms a very important part of preventive maintenance.
Normally ashless dispersant oils are used after the first 100 hours of engine run in. These contain additives capable of holding dirt in the engine suspended so that they can be collected by the filter, if large enough, or removed during an change. This is very important since there are many small passages in the engine which could clog up and cause oil starvation.
During engine operation the oil collects dirt from several places, from the atmosphere through the air filter, soot during start and idling, hot areas cause coke (burnt oil), blow-by gases produce sulfuric acids and water vapor is attracted after engine shutdown, overnight parking and startup.
Acids are corrosive, but only in combination with water. Thus it is very important that the oil reaches it correct operating temperature during each flight so that any water gets 'boiled off'. The oil dries out and the vapor leaves the engine through the crankcase breather or water/oil separator. The amount of water condensation depends on the humidity of the ambient air, the higher the temperature (summer) the more water vapor it can contain and naturally the location of the aircraft, near sea or large body of water.
Any good brand ashless dispersant oil contains additives as acid neutralizers, zinc and more to combat contaminant, sludge, dirt and varnish. There is a reserve of additives in new oil which is 'used up' during the time the engine runs. Just adding extra additives will not work as the oil itself is subject to very high temperatures and shearing action which have their effect on quality. Oil therefore need to be changed after a certain amount of time.
A filter can only remove particles of a certain specified size and larger. No filter is able to remove everything 100%, for such a filter would even block the oil. For example a 10 micron filter should remove all particles of 10 micron and larger, but what usually is forgotten is the effectiveness of the filter. And the longer the filter is on the engine, the more dirt it collects the more it starts loosing its effectiveness and it will need replacement which is done during the 100 hour or annual check.
Every engine is run-in or broken-in at the factory on a test stand, but it is generally accepted that the first 100 hours are considered the final break-in period. In the first 25 hours straight mineral oil should be used (or until consumption stabilizes), this is oil without the additives found in ashless dispersant oil. This aids with the break-in. If normal ashless dispersant oil would be used during the break-in period the additives would cause the break-in to fail and the piston rings will never properly seat in the cylinders resulting in a higher than normal consumption (blue exhaust gases) and possible a higher cylinder wall wear rate and the engine not reaching its recommended TBO at all.
When on a cross country flight and the engine needs an oil top-up, don't worry about the brand. Just make sure that it is the same type: straight mineral or ashless dispersant. The viscosity should be the same you would use, but if it is not available, you may mix a 20W50 with a 15W50. The result will be a 17W50, depending on the ratio of the mix. Just make sure not to mix synthetic oil with mineral oil.