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.
Lubricating oil 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 lubrication the engine would fail within minutes, keeping a watchful eye on the instruments during flight is therefore important.
Engine oil comes in many forms: synthetic, mineral or a combination of both. Each with their own unique properties and the most important one to the pilot is viscosity, which determines its readiness to flow at different temperatures.
To enhance the properties, special formulated additives are added which contain friction reducers, high pressure and anti-wear compounds as ZDDP, Graphite and MoS2 to name but a few.
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.
Engine oil performs a number of functions in the engine: lubrication, cooling, cleaning, sealing, corrosion protection, noise reduction and propeller blade operation for constant speed models. The most important being lubrication. Without oil all moving parts of the engine would be in direct contact and wear out very rapidly. Oil forms a layer between the parts and reduces friction. You can visualize oil as millions of tiny, molecular size, ball bearings rolling between the moving parts of the engine. The size of these balls is determined by the clearances in the engine and dictates which viscosity the oil must have for a long service life.
To perform its task, oil must be able to withstand high temperatures, pressure and shearing loads. It also has certain properties as viscosity and contains additives to clean the engine like ZDDP and other compounds. It is either sourced from mineral (from oil wells), semi-synthetic (part mineral part synthetic) or made off full synthetic (man made) origin. Where each type has its own unique properties and specific purposes.
Additives keep any contaminants suspended in the oil and makes sure that these do not build up anywhere in the engine where it could cause oil starvation to bearings or other critical parts. Read more in our article on lubrication additives.
For us pilots the most important property is viscosity, defined as readiness to flow under different temperatures. During a cold start in wintertime, oil will be thicker than during a warm start in summertime. In both cases it is important that oil pressure is attained within 30 seconds after start to prevent any damage. So keep a watchful eye on that pressure indicator!
Oil is said to be of a certain viscosity or grade. Multigrade oils are capable of keeping their specific viscosity under a wide temperature range, for example: -10°C to + 40°C, important during startup of the engine. Oil with higher grades are used at higher startup/ambient temperatures and not really usable in freezing polar or arctic conditions where a synthetic multigrade like 0W or 5W would be best.
Aircraft engines used to use a single grade oil as a 80 grade (SAE40) or even a 100 grade (SAE50). Although multigrade (for summer and winter use) like 15W50 or 20W50 are more common these days. Some diesel engines and more modern gasoline engines (Austro, Centurion, Wilksch, Rotax, Subaru) tend to use the 10W40 or 15W40 viscosity oils, where Rotax even recommends a motorbike oil as it contains additives for gears. Which is needed to lubricate the internal reduction gear box.
When replenishing, you can add oil of a different viscosity but keep in mind that the final viscosity will end up in between the two. For example: 50% of SAE80 mixed with 50% SAE100 results in SAE90. Mixing equal amounts of multigrade 10W40 and 15W50 should get 12W45, the final result depends on the actual mixing ratio. Be sure not to mix mineral, semi-synthetic or full synthetic oils. Rotax mandates a semi-synthetic oil when using leaded AVgas as a fuel.
As oil is pumped around in the engine lubricating gears, bearings, pistons and valves its temperature rises. Especially near the pistons and cylinders. To make sure the oil stays within the operating limits it will need to be cooled by letting it flow through an oil cooler. Some of which are thermostatically controlled, which is a must, as it keeps the engine oil on a preset constant temperature regardless the ambient temperature.
The oil temperature indicator shows the temperature when it leaves the cooler and is about to enter the engine. It must be within a certain range so that all parts are cooled properly and do not overheat. Oil temperature which is too low isn't good either as any moisture collected by the oil needs to dry out. Furthermore, the engine is only operating on design specifications when at its proper operating temperature.
Using the correct ashless dispersant oil keeps the interior of the engine clean if used continuously after the first hours of initial break-in. These oils contain specific additives which keep dirt suspended so that the oil filter can collect them.
During the time the engine is running, oil is collecting combustion byproducts as: soot, coke produced by hot areas, blow-by gases add acids, water vapor and gasoline dilution from priming. All these products form their own composition as sludge, varnish and corrosive acids. Oil is capable to handle all of this without problem but it will need regular changes as the additives in new fresh oil are 'used up'. The aircraft maintenance program dictates how many hours can be flown before the oil and filter will need changing, it is usually every 50 or 100 hours or annually should you fly less.
After the engine is shutdown the oil will eventually drain and collect in the sump, thereby leaving a thin film on all internal parts preventing corrosion. But what if the engine is shutdown with oil that was in service for quite some time and isn't flown for the next couple of weeks or months? There is a slight change that contaminants could corrode the metals, this depends on the metallurgy used. It is wise to change the oil before putting an aircraft in storage or even using preservation oil in the cylinders to prevent any possible corrosion and lockup.
The thin oil films provide the necessary gas tight seals between piston rings and cylinder walls preventing gas blow-by. Lubricating oil on the valve train cushions the valves which open and closes at 20 times per second at 2400 RPM cruise power reducing this valve noise.