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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?

Prashant Saraswat
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    how often do you run your car engine at full throttle and for how long? – user3528438 Jan 01 '18 at 21:50
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    And those are inline engines, there's a saying about radials, that if it's not leaking oil, something must be wrong. – Davidw Jan 02 '18 at 01:25
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    @user3528438 how often do they fill up oil in car racing? – PlasmaHH Jan 02 '18 at 09:23
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    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. – jwzumwalt Jan 02 '18 at 15:01
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    @jwzumwalt That's very good information. You should add that as an answer. – Shawn Jan 02 '18 at 15:51
  • @user3528438: That's actually not so relevant. As a designer, you optimize for the typical case. So a car might have excessive oil leakage when running at max RPM, and an aircraft when running at low RPM, but neither should consume much oil in the opposite case. – MSalters Jan 02 '18 at 16:28
  • "Oil check is an integral part of the checklist." That, of course, being in part because having too little oil in the engine at 8,000 ft AGL and quite possibly miles from the nearest suitable landing site, let alone the nearest airport, can be a little unsettling to the pilot and any passengers. It's a fairly quick check on the ground that can save you plenty of headache during the flight; the same can be said for quite a fraction of the checklist items in that class of aircraft, especially those in the preflight check checklist. – user Jan 02 '18 at 16:32
  • @jwzumwalt: The question, though, is what constitutes excess consumption? My Insight & Miata (both with over 200K miles) don't consume a noticable amount between oil changes - a period about equivalent to 100 hours on an aircraft engine. My '80s pickup will leak about a quart in the same time. '60s cars I've driven might use a quart in 1000 miles. – jamesqf Jan 02 '18 at 18:59
  • @ jamesqf "The question, though, is what constitutes excess consumption?" Where did you read this? 100hrs=200k? Redo your calcs. 2000hrs is roughly equivalent to 50,000-75,000 miles.(30-35mph * 2000hrs = 70,000mi) – jwzumwalt Jan 03 '18 at 03:26
  • @jwzumwalt: I think you misread what I wrote. I meant that the oil change interval - 7500 miles - is about equivalent running time to 100 hours (figuring mostly highway driving, of course). I do change the oil &c at recommended intervals. I mentioned 200K miles, parenthetically, to show that they're not new cars: they have a reasonable amount of wear on them. So for a typical aircraft engine - say an O-360 - what's expected oil consumption, and what's excessive? A quart in 10-20 hours? – jamesqf Jan 03 '18 at 07:08
  • @ jamesqf: Mfg do not specify, but the University of Iowa used 12 planes to established the average oil consumption at TBO (1500/2000hr) was 1qt per 13hrs. For a car, that's about 1 qt per 450mi which for anybody that has the misfortune of owning a clunker is in pretty bad shape. Which makes sense because most people accept a rule of thumb that a car is on its last leg when it uses 1qt@500mi. This happens to correlate with my Mitsubishi SUV at it's present 140k mi. – jwzumwalt Jan 03 '18 at 13:34
  • @PlasmaHH in the more serious car racing leagues, engines are completely rebuilt (which involves draining the oil and replacing with new) after every 1-2 races. Generally no more than 24 hours of runtime between, and more often only a couple hours. In top fuel drag racing, the engine is rebuilt after every 3-5 second run. – JakeRobb Jan 10 '18 at 19:01
  • @Davidw As far as I can see, most GA planes (and, in particular, the ones mentioned in the question) don't have radial engines. I think radials were mostly used on larger aircraft: even things like the Spitfire and P51 Mustang were using V-12s – David Richerby Jun 13 '18 at 17:10
  • @DavidRicherby Which I did not suggest was the case. Also, a majority of US Navy fighter aircraft up until the jet age were equipped with radials, including the relatively small F3F and F4F. – Davidw Jun 14 '18 at 03:21

3 Answers3

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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.

Mike Sowsun
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    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. – Prashant Saraswat Jan 02 '18 at 00:28
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    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. – jamesqf Jan 02 '18 at 02:19
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    @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. – vsz Jan 02 '18 at 17:14
  • @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. – Mast Jan 02 '18 at 17:56
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    @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. – jamesqf Jan 02 '18 at 18:52
  • @jamesqf : so, it's basically because there is not enough demand for new GA aircraft, as there are enough old ones to saturate the market? Makes sense, as hull losses are rare enough, and existing aircraft are maintained regularly with new parts. The club I was flying at, had both gliders and tow-planes built in the 1960's, and parts are still manufactured to keep them airworthy. It seems most GA aircraft are like Theseus's ship today. – vsz Jan 02 '18 at 19:46
  • @vsz: I wouldn't say that it's lack of demand, really. It's really lack of supply: there aren't that many people who're able (or able but willing) to spend upwards of a quarter of a million US dollars on a new plane: http://www.planeandpilotmag.com/article/single-engine-piston-planes/ Especially when the 1960s equivalent to that plane can be had for 10% of the price. – jamesqf Jan 03 '18 at 01:14
  • I guess this answer forgot about the VW and Corvair. They did not have any more oil consumption than any other automobile when maintained correctly. – jwzumwalt Jan 03 '18 at 03:12
  • @jwzumwalt: Not my experience. While I've never owned a VW or Corvair, it was quite usual for cars of that era to consume oil. – jamesqf Jan 03 '18 at 07:19
  • Prior to 1979 when Chrysler/Lee Iacocca made computer balancing an industry standard, the average car lasted about 50k on dirt roads and 75k on highways. They came apart from vibration not design. After 1979 and computer balancing, autos immediately began to last 150-200k miles. Aircraft engines are still not normally computer balanced. – jwzumwalt Jan 03 '18 at 13:47
  • This begs the question: why aren't aircraft engines liquid cooled? – JakeRobb Jan 10 '18 at 19:02
  • @JakeRobb weight and complexity – acpilot Jan 14 '18 at 01:52
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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 !

William Midwood
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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.

tj1000
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