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I'm sure this question could apply to many other plane engines, but specifically I'm looking at the Piper Cherokee PA-28-140.

Surprisingly, it appears that the engine used, the Lycoming O-320, has ~320 cubic inches of displacement but only puts out 150hp.

When comparing the horsepower per cubic inch of displacement (CID), the ratio is quite low. We have the technology (and already used it in vehicles in the 50's, 60's etc.) to achieve a better efficiency than that.

Why is this engine (and perhaps other models) so inefficient?

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  • $\begingroup$ There seems to be a general pattern I've noticed that small GA aircraft engines tend to produce 1 hp per 2 cubic inches of displacement. This isn't too far off that typical pattern. $\endgroup$
    – PJNoes
    Sep 9, 2016 at 17:05
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    $\begingroup$ Remember that the IO-320 is a 4 cylinder, 5.24L engine, that's a huge 4 cylinder, with a compression ratio (max for that engine series) of 9:1 (or as low as 7:1). Auto engines use compression ratios between 10:1 and 15:1 (or higher for diesels). $\endgroup$
    – Ron Beyer
    Sep 9, 2016 at 17:33
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    $\begingroup$ Because the engine speed is limited. Car engines easily have twice the RPM of an aviation piston engine, so they can produce twice the horsepower from the same displacement. Add turbocharging and the gap widens even more. $\endgroup$
    – Peter Kämpf
    Sep 10, 2016 at 18:27

4 Answers 4

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Aviation engines run at near max RPM through out the flight. A car on the other hand doesn't use the full RPM spectrum except in bursts.1

If a car engine was utilized the same way an aviation engine is, it won't last long.

So an aviation engine is sturdier, heavier, and weaker (hp) for the same displacement, but also provides higher torque (big cylinders).2

Formula One engines have small displacement, very light weight, yet deliver near a 1,000 hp. But they don't last long either. Something like 15 hours of racing and practice sessions.

Further reading: Do Car Engines Make Good Airplane Engines?

1 Piston planes run at near max RPM because they don't have and don't need gearboxes.

2 Same era alloys and technology.

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    $\begingroup$ I didn't find that article very convincing. It starts off stating that car engines aren't suitable, continues by saying that we actually don't know what would happen when we put a car engine in a plane, and concludes with some successful examples of manufacturers who did it anyway... $\endgroup$
    – marcvangend
    Sep 10, 2016 at 9:05
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    $\begingroup$ Not sure I find the constant near max RPM argument very convincing, either. I'm not an engineer, but I'd assume that it's much more difficult to make an engine efficient&sturdy if has to operate over a wide range of speeds, than if you can basically assume a constant RPM. $\endgroup$
    – leftaroundabout
    Sep 10, 2016 at 10:19
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    $\begingroup$ "But [Formula One engines] don't last long either. Something like 15 hours of racing and practice sessions." That used to be correct, but the current F1 engine regulation allows each driver to have only 4 "power units" (engine + other components) per season, so each unit has to last averagely for five GP weekends (three practise sessions, one qualifying session and one race). Still a short life, but longer than you assumed. $\endgroup$
    – gilbertohasnofb
    Sep 10, 2016 at 11:10
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    $\begingroup$ @leftaroundabout It's not so much about constant rpm but more about a constant high rpm. The mean rpm of an aircraft engine is closer to its max rpm than for a car engine. $\endgroup$
    – curious_cat
    Sep 10, 2016 at 13:27
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    $\begingroup$ One thing I forgot to mention is that the Lycoming is about half as heavy as the Mustang 5.0. That is a big weight reduction and an advantage that probably shouldn't be ignored. $\endgroup$
    – Shane
    Sep 10, 2016 at 19:42
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One reason is not so much can you build a reciprocating engine with higher horsepower or better efficiency, but can it do so with a very high reliability for extended periods. Most automobile engines are run on average at 20% power with very brief higher power outputs whereas aircraft engines are routinely operated at 65-85% power and are expected to have a mean time between overhauls of around 2000 hours at these power settings. To accomplish this, most aviation engines operate at slower speeds and much greater displacements than traditional automobile engines do.

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  • $\begingroup$ Aircraft engines operate at those high power levels because they produce so little power, no? If you double or triple the power it put out per displacement (so that it matched car engine levels) then they would only need to operate at 20% power. $\endgroup$
    – Shane
    Sep 10, 2016 at 19:09
  • $\begingroup$ What happens to the unused 80% in a car? Same thing here. What do you mean about it not things not being free? Either you have an efficient engine and you only use a fraction of its power. Or you have an inefficient engine that runs at max power. Torque is generally lowest at low RPM. Highest around the middle of the power band, then tapers off. HP is mostly linear, but drops off quickly after peak. Look at the power curves for some engines services.edmunds-media.com/image-service/media-ed/ximm/…. $\endgroup$
    – Shane
    Sep 10, 2016 at 19:34
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    $\begingroup$ You're looking at this problem all wrong; its not that an automobile engine 'wastes' the other 80% available power; it just doesn't need to operate at those higher power settings. So a car with a 240 hp engine in it only needs to use 40-60hp during typical operation on public roads at sanctioned highway speeds. And though a typical automobile engine redlines around 6000-7000 rpm, it's usually operating at 2000-3000 rpm while driving. The faster the engine operates at, the more stress it's parts are subjected to and the shorter the service life is. $\endgroup$
    – Carlo Felicione
    Sep 10, 2016 at 19:44
  • $\begingroup$ it's also far more critical that an aircraft engine be more reliable than an automobile engine as the consequences of an engine failure are more dire for an aircraft than an automobile. $\endgroup$
    – Carlo Felicione
    Sep 10, 2016 at 19:47
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A modern aircraft propeller must be designed so that the tip speed is less than supersonic so as to avoid issues with noise and performance. Most small plane propellers are about 6 feet in diameter and are thus limited to about 2700 RPM. The engine designer may thus connect the crankshaft directly to the propeller and limit engine speed to about 2700 rpm, or use an intermediate gearbox to allow higher crankshaft speeds.

Because the propeller has very high rotational inertia and wants to turn at a constant speed while the engine supplies power in a few pulses during each revolution, the wear and tear on a gearbox may be quite extreme. As a result, a high-speed engine plus robust gearbox is often not worth the extra complexity as compared to a simpler direct drive solution. (see "Continental Tiara" and "Porsche PFM 3200")

Of course, as engine power is proportional to the product of the torque and speed of rotation, the slower turning engine produces less power for a given displacement. But experience has shown that slower-turning direct drive units are often as light and more durable for a given power output when compared to geared units.

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  • $\begingroup$ For reference: a 2-foot diameter car wheel at 60mph/~100km/h is about 840rpm. At 100mph, 1400rpm. (According to google's hit on a quora question; I didn't check the math but those sound reasonable). So the car's inertia has less mechanical advantage on the drivetrain than a prop has on its drivetrain. (I assume there are other huge differences in drivetrain that I don't know about. Car engine flywheels help with this some, right?). $\endgroup$
    – Peter Cordes
    Sep 9, 2016 at 21:51
  • $\begingroup$ @PeterCordes (840 / minute) * pi * 2 feet in mph — yep, that works. And 24" is a reasonable enough tire diameter. $\endgroup$
    – hobbs
    Sep 10, 2016 at 1:07
  • $\begingroup$ I think this argument makes much more sense than ymb1's: the specific output is low because the RPM is low (absolutely speaking), not because it is high relative to the engine's rated maximum. $\endgroup$
    – leftaroundabout
    Sep 10, 2016 at 18:28
  • $\begingroup$ Well the truth is plenty of aircraft do use high speed engines which are geared down to drive a propeller. Take for example a PWAC PT-6 Turboprop engine: The power turbine typically operates at 4500 RPM and drives a reduction gearbox with an output to the prop at 2000-2200 RPM. PT-6s have logged more than 4 million flight hours and are very reliable. The Rotax 912 is another high speed reciprocating engine, popular with ultralights and LSAs, which uses a reduction gearbox for it's shaft output. Gearbox reliability is not a problem if design correctly. $\endgroup$
    – Carlo Felicione
    Sep 10, 2016 at 19:57
  • $\begingroup$ You can certainly design gear boxes to be reliable (especially when they provide a single, fixed reduction ratio). I'd note, however, that 4500 RPM isn't particularly fast by automotive standards. It's faster than a typical car engine runs most of the time, but red line on a reasonable high performance car engine is typically closer to seven or eight thousand RPM (and F1 cars can hit around 12.000 or so). $\endgroup$
    – Jerry Coffin
    Sep 11, 2016 at 0:35
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RPM, RPM, RPM. If you want 1,000 HP out of 1.5L, you run it as 10,000 rpm. An earlier poster referenced prop tip speeds being a limiting factor - true. However, there are geared aircraft engines which permit the engine to run at higher RPM without overspeeding the prop. These are the same displacement as their non-geared brethren but make more horsepower for the simple reason that they are burning more fuel/air and making more heat. They are also more expensive to maintain and are seen on larger twins where they remain cheaper than adding another engine.

Not related directly to horsepower, but the design of A/C engines have a greater bore to stroke ratio then auto engines - typically 4:3 while auto engines are closer to square or with even longer stroke.

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  • $\begingroup$ You'll rarely see a stroke longer than the bore on a modern automotive engine, and 4:3 isn't particularly rare/out of line either. Even truck engines can be in the same range (e.g., GMC used to have a 430 CID engine with a 3.47" stroke and a 4.44" bore). $\endgroup$
    – Jerry Coffin
    Sep 11, 2016 at 0:38

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