This question is related to single/twin piston engine aircraft. After flying in a Cessna 152 and a Piper Dakota I have noticed that they both consume a lot of oil. Oil check is an integral part of the checklist. Pilots carry extra bottles of oil in the back. Why is this so? Especially compared to car engines that are 'similar' and do not require such regular top offs?
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asked Jan 1, 2018 at 21:48
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18$\begingroup$ how often do you run your car engine at full throttle and for how long? $\endgroup$– user3528438Commented Jan 1, 2018 at 21:50
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10$\begingroup$ And those are inline engines, there's a saying about radials, that if it's not leaking oil, something must be wrong. $\endgroup$– DavidwCommented Jan 2, 2018 at 1:25
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2$\begingroup$ @user3528438 how often do they fill up oil in car racing? $\endgroup$– PlasmaHHCommented Jan 2, 2018 at 9:23
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10$\begingroup$ I am an A&P and thought the same thing before getting my university degree. 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 0-360 used in the C-172 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. $\endgroup$– jwzumwaltCommented Jan 2, 2018 at 15:01
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2$\begingroup$ @jwzumwalt That's very good information. You should add that as an answer. $\endgroup$– ShawnCommented Jan 2, 2018 at 15:51
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3 Answers
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Automobile engines are not similar. They are liquid-cooled and therefore can be built to much tighter tolerances with regard to thermal expansion and contraction.
Air-cooled aircraft engines must deal with a large range of operating temperatures and oil is consumed due to the relatively looser fit of the piston rings.
answered Jan 1, 2018 at 22:35
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1$\begingroup$ Thanks. 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. $\endgroup$ Commented Jan 2, 2018 at 0:28
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23$\begingroup$ Aircraft engines also are basically 1950s (if not earlier) designs. Many still flying were built in the '60s & '70s. Most 1960s car engines leaked & burned oil - after a few years, it was common to need a quart every few thousand miles. 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 200Kmiles, mine still doesn't leak or burn a measurable amount. $\endgroup$– jamesqfCommented Jan 2, 2018 at 2:19
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1$\begingroup$ @jamesqf : this begs the question why there weren't any significant improvements in aircraft piston engines during the last 50-60 years, in contrast to cars. $\endgroup$– vszCommented Jan 2, 2018 at 17:14
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$\begingroup$ @vsz I guess other problems took priority. 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. $\endgroup$– MastCommented Jan 2, 2018 at 17:56
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6$\begingroup$ @vsz: Three reasons that I can see. First, if I'm not mistaken, the majority of GA aircraft and their engines were actually BUILT in the 1950s through mid-70s. (Compare the price of a '60s Cherokee, say, against a new Warrior.) Second, simple economics: the cost of R&D & tooling for auto engines can be amortized over millions of units, vs thousands for aircraft engine. And finally, automotive emission controls and fuel economy standards forced the automakers to improve, while FAA regulations discouraged innovation. $\endgroup$– jamesqfCommented Jan 2, 2018 at 18:52
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As I think most folks here tend to be A&P's I'll add this. Aircraft horizontally opposed four cylinder engines also use specifically "asheless dispersant" oil because the oil is pumped to lubricate the top of the cylinder and a decent bit is burned in combustion. So yes, there is a line that can be considered "excessive" and that has a lot to do with wisdom in a small piston engine as opposed, to a jet where there is generally a one pint per hour limit, or something to that affect, where anything more than than that would require an oil consumption run. But as far as aircraft piston engines go, they do burn oil because they lubricate differently than an automobile engine.
Cheers !
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'Air cooled engines' is a popular misnomer... oil does the primary cooling, with the fins on the cylinders only dispersing part of the heat. The oil, in addition to lubricating, draws heat away from high temperature points, like the cylinder heads, bearing surfaces on the crankshaft, and to a degree, the cylinder walls when oil splashes on them.
For a variety of reasons, oil/air cooled engines tend to consume more oil than water cooled engines, from the wider temperature range the engines experience, to the oil doing double duty.
In addition, the flat cylinder layout can result in some oil loss on startup, from oil pooling in the cylinders when not running, which doesn't happen in vertical or V cylinder arrangements that are typically used in automobiles. This is especially true of radial engines, which have some cylinders inverted, letting oil pool on the pistons when not running, leaking past and getting into the combustion chambers. That's why radial engines tend to expel huge clouds of smoke when started, as that oil burns off.
This is also true of automotive oil/air cooled engines, such as the flat four used in Volkswagen vehicles of the 1950's - 1980's, and the flat six in Porsche 911's until the wide temperature range of such engines ran up against tighter emissions controls. Those engines were also noted for their higher oil consumption.
