Why do aircraft engines burn oil

Multi-weight or multi-grade oil, say 15W, is a thin 15 weight oil with viscosity improvers added to it. The VI compounds literally coil into tiny balls at low temperature and uncoil into longer strands at high temperature. In our 15W example the oil pours like 15 weight oil at 0F and 50 weight oil at F. This helps because, like everything else, oil has an operating temperature range. Engine preheating is a great answer, but a multi-viscosity oil with greatly improved flow at low temperatures is a big, very convenient help, too.

Cold oil, no matter what type, is a real concern. Besides flowing poorly until it gets a bit of heat into it, thick oil causes meaningful drag on engine internals. This makes life difficult for the starter motor and drags down the battery.

It also robs engine power and wastes gasoline overcoming the excess drag. But the worst issue is rapid metal-to-metal engine wear due to no or low oil flow. Short of preheating, a multi-viscosity oil and warming the engine in the run-up area until movement is seen on the oil temperature instrument are the practical answers. At the other end of the thermometer, excessive heat is fatal to mineral oil. As temperature ramps up, mineral oil breaks down, cooks, burns, call it what you will, but it permanently turns into a non-lubricating goo.

Clearly Goldilocks oil temps are the goal: F to F. Given an hour of flight time, this is warm enough to burn off the copious water contamination formed by combustion, but not so hot as to break down the oil.

All motor oils are augmented by additives chosen by the oil manufacturer. They get used up by engine operation, and either more additives must be poured into the crankcase not unknown in over-the-road trucking or industrial engines, but not done in aviation or automotive applications , or the oil must be replaced.

Typical motor oil additives address high-pressure lubricity the camshaft-lifter interface is the big player here , but aircraft engines are also heavy on anti-sludge additives to combat the gray goo formed when leaded gasoline, water, and loose engine tolerances get together, along with acid neutralizers. Then there are the well-known ashless dispersant additives.

Ash is a combustion byproduct formed in the combustion chamber when engines burn oil there. The big players in ash formation are detergent additives, so unlike automotive engines with their essentially oil-tight combustion chambers, aircraft oils avoid detergents.

Ashless dispersant additives hold what ash that does form in solution so it can be scrubbed out by the oil filter, or amazingly failing an oil filter, until the oil is replaced. Lycoming and Continental provide for both too-cold and too-hot oil temperatures. A thermostat, called the vernatherm on Lycoming engines , is set to open at F. It shuttles cold oil directly through the engine and hot oil through an oil-to-air oil cooler before letting it go through the engine.

Thus, oil temperature on these engines is a minimum of F, except from between a cold engine start and when the oil warms to F. Few pilots seem to have the discipline to avoid cold-oil engine operation, and low-performance standard category applications and their Experimental equivalents seem to survive such barbarity.

But as engine performance goes up, avoiding high-load, cold-oil operation makes a difference in engine longevity.

Maximum oil temperature is controlled by an oil cooler, and on aircraft, these are inevitably oil-to-air radiators. As Experimental aircraft builders, we are responsible for everything, and dealing with the many variables in constructing an efficient oil cooling system is a major creative area for us. This may mean air bubbles are being fed to critical bearings rather than lubricating oil, causing unnecessary wear.

I once delivered a new Cessna T-Stationair to Sydney, Australia, and on the initial hour overwater leg from Santa Barbara to Honolulu, the engine burned 10 quarts of oil. All temperatures and pressures remained normal, and the big Lycoming ran great for the whole trip.

The next morning, when I tried to check the level, it was off the bottom of the dipstick. Cessna pulled the engine and replaced it in Honolulu, and I kissed the ground for my good luck. Had the leg been 15 hours…? Oil temperature is another critical concern, and both too hot and too cold can be preludes to trouble. Every pilot knows to watch for high oil temps, but how many pilots have seen temperatures off the bottom of the gauge?

I was flying the leg from Bangor, Maine, to Goose Bay, Canada, in January, and the digital oil temp on the Garmin G was showing me readings consistently between 60 and 70 degrees F. Most aircraft flight manuals prohibit even runups until oil temperature reaches degrees F. It turned out there had been a service bulletin issued by Continental requiring part of the cowling air intake to be blocked in extreme cold weather. On the surface, aircraft oil seems such a simple product, but my friend Ed Kaston, taught me differently.

Already have an account? Prashant Saraswat Prashant Saraswat 1, 8 8 silver badges 24 24 bronze badges. High oil consumption is a sign of an overly worn out engine or neglected engine. If an engine has excess oil consumption something is wrong and it is unsafe - period. There are also quirks such as the Lycoming used in the C will blow out any oil above 6 or 7qt even though the mfg recommends 8qt - pilots learn not to put 8qt in. Jet engines have increased oil consumption as they wear and that is one criteria used for overhaul for them too.

An aircraft engine should burn no more oil than a well maintained car. You should add that as an answer. Show 9 more comments. Active Oldest Votes. Improve this answer.

Mike Sowsun Mike Sowsun I assume you are saying that because the piston rings are looser fit the gap must be filled by oil. Part of this oil gets burned during the normal internal combustion cycle. Hence the increased oil consumption.

The lose fit is needed because of higher operating temperature range of the air cooled engine compared to the liquid cooled engine. This makes sense. The oil used was commonly 30 weight or thicker. Modern car engines, like the one in my Honda Insight, are built to tighter tolerances made feasible by modern tooling and run a much thinner 0w20 oil.

After Kmiles, mine still doesn't leak or burn a measurable amount. Engines and airplanes have definitely been improved much over the years, but you can't solve everything at once. I'm wondering about actual numbers though, to put all myths to bed. Feel free to make a new question. Compare the price of a '60s Cherokee, say, against a new Warrior.