To reduce friction and help carry away heat and impurities, all piston engines are lubricated by oil under pressure. The working fluid is petroleum oil; conventionally refined, this is called mineral oil, or synthetic oil when the crude petroleum is refined by a more sophisticated process. But the starting point is the same: crude pumped from underground.
Aircraft engines have their specific oil needs and stick to mineral or semi-synthetic oils. They definitely don’t use automotive oil.
The typical aircraft engine contains several quarts—typically 8 to 12 quarts —of mineral engine oil. Gravity pulls the oil to the bottom of the crankcase: the sump. A tube connects the sump to the oil pump, which is a pair of rotating gears that both suck the oil out of the sump and push it through various passages cast or drilled in the crankcase casting—called oil galleries. These passages get oil first to the rotating crankshaft, which through internal passages then supplies the connecting rods where they bolt to the crankshaft. Other passages get oil to the cylinder heads where it lubricates and cools the valve train.
Rotax engines use a more sophisticated “dry sump” system where oil is pumped into an external tank instead of pooling at the bottom of the engine, but the general points are the same.
The cylinder bores are often lubricated by “splash,” which is a misnomer. It’s actually oil oozing past the main and connecting rod bearings (we’ll discuss those in a later article) and incidentally getting flung onto the cylinder walls that lubes the cylinder/piston interface.
On some airplane engines, small nozzles are provided directly from the oil galleries to spray onto the underside of the pistons. These so-called “piston squirters” are especially effective at cooling the piston and are a hallmark of endurance engines designed to hammer out significant power for long periods. Most big truck and marine engines use piston squirters, for example. Few automotive engines do.
Clearly, oil gets very warm and must shed heat before the petroleum oil cooks into a uselessly thick goo. Therefore, oil is pumped to an oil cooler. This is essentially the same thing as a radiator in your car or other liquid-cooled engine. The oil passes through narrow tubes and those tubes have foil-thin fins between them. The foil greatly increases the surface area, thus transferring more heat to the passing atmosphere. A thermostat is also provided to ensure the oil does not fall below a prescribed minimum temperature, typically 180° F. Aviation mechanics—A&Ps—often call the thermostat by its Lycoming trade name: Vernatherm.
Oil gets dirty from combustion byproducts such as carbon and bits of grit inevitable inside an engine: casting sand, minute metal shavings, and other wear materials. These hard particles rapidly wear moving engine parts, so they are strained out of the oil by the oil filter. Eventually, the oil gets too dirty, is cooked by engine heat, and uses up the sacrificial elements that are part of lubrication chemistry, and so the old oil is drained out and new oil poured in. The oil filter is changed at the same time. These oil changes are vital to the engine’s health.
With all of the (justifiably) scary press out there about the conversion to unleaded avgas, it’s worth mentioning that a big contaminant in our oil today is … lead. Once the lead is gone, the whole engine becomes a great deal cleaner. Open up a car engine today and see how clean it is compared to your aircraft engine. It’s the lead!
Not so fast MNvelocitypilot. I drive a car that was designed in the 60s and built in 1972. It’s engine is about the same state of the art as our aircraft engines, it was also designed for leaded fuel. Using unleaded fuel as I do now, the oil gets just as dirty as on any aircraft engine. Bore scoping shows about the same amount of carbon build-up.
But yes, modern cars have extremely clean combustion chambers but that is not from the unleaded fuel, it is from much tighter clearances and a very optimized combustion.
You knew that!
Uhhh, no. And don’t take my word for it. Do some research. Attached is just the first article I came across (out of many).
This claim of having a much cleaner engine without the lead is propaganda by some of those pushing for zero lead fuel, mainly George Braly. The contamination we see as soot in the oil and carbon in the combustion chamber is mainly from Hydrocarbons being burnt. As long as you have oil consumption and suboptimal combustion, there will be carbon deposits proportional to the severity of oil used and fuel wasted.
Lead oxide (the byproduct of burning leaded fuel is rather harmless, sometimes considered a lubricant.
I’ll remember that next time I’m chipping the lead out of my plugs….
It’s not the lead. Google more. Lead is a lubricant! It’s mainly health reasons why lead was removed from gas (as well as production considerations). Lead is simply toxic.
wing smith is 100% correct: Modern automotive engines are “cleaner” than their aviation counterparts because the combustion cycle is simply more efficient (due to tolerances, materials, sensors and software, i.e. technology).
And another Rotax difference — they don’t use Vernatherms. Many hope for the best based on flight altitude temperatures, throttle position, and judicious use of aluminum tape to cover the cooler fins during winter operations.