My understanding is that current GA piston engines from Continental and Lycoming are based on a design from the 1960's. There have been many evolutionary improvements, but only within the limitations of those 1960's designs. They are large displacement (~1.5L/cylinder), run at RPMs in the mid 2000s range so that they can spin the prop at optimal rates without using a gearbox, and they are air cooled to save the weight and complexity of a liquid cooling system.

What would a completely new, designed from the ground up GA piston engine look like today? Factoring the massive improvements that have been made in all sorts of engines over the years in non-aviation applications- for autos, trains, boats, generators, etc.

Would they still operate at such low RPMs? Yes reliability and durability are still major concerns, but manufacturing tolerances are so many times smaller now then they were in the 1960s (possibly allowing for reliable operation at higher RPMs), and they are now mounting fixed gearboxes to commercial airliner turbine engines so adding a fixed gearbox to a GA piston engine should be trivial.

Would they still be air cooled? Yes it can still save weight, but possibly at the cost of extra cooling drag, and definitely at the cost of much more finicky operation that requires the pilot to carefully monitor engine temps throughout their flight cycle.

Would they still be in a flat engine arrangement (flat 4, flat 6, etc.)? I would imagine so, I can't imagine an alternate piston engine arrangement with the smoothness, packaging, and scalability advantages of the flat architecture. Straight engines get too long, and V engines require many more changes to scale the number of pistons up or down.

Would they still fly on spark-ignited gasoline-type fuel, or could compression ignition based on jet fuel produce the power density and reliability required? I realize that the single largest factor in this area might be the simple availability (or lack thereof) of fuels, but I am talking about purely from an engineering perspective. Suppose we are talking about North America where both Avgas and Jet A are widely available, what would be the superior fuel for flying GA piston engines?

What other differences might there be that I haven't considered?

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    $\begingroup$ The low RPM isn't so much a function of the engine as it is the propeller. Propeller tips must be kept sub-sonic. There are clean-sheet designs in the new diesel/JET-A pistons being put in some aircraft. $\endgroup$
    – Ron Beyer
    Jan 18 '18 at 17:54
  • $\begingroup$ @RonBeyer Yes I addressed the prop-speed aspect of the equation, however could they not merely add a fix gear to allow the engine to run at much higher RPM? According to basic engine design principles, higher RPM should allow higher power density... of course many potential downsides, some of which I mention... $\endgroup$
    – Charles847
    Jan 18 '18 at 17:57
  • $\begingroup$ I'm most certainly not an aviation engine expert, but it seems to me that running the engine at a higher RPM then gearing the output back down to what is required for the prop is simply a recipe for burning more fuel to cover the same distance - not an appealing prospect. My car has 5 gears - at 2000 RPM in 3rd I can do 55MPH, in 4th, 65MPH and in 5th, 75MPH. I can change gearing to travel faster at the same fuel/hr, but that's not how it works in a prop aircraft. If anything, you'd want the engine spinning slower than the prop to save fuel (other considerations aside). $\endgroup$
    – FreeMan
    Jan 18 '18 at 19:39
  • $\begingroup$ @FreeMan: The difference is that at highway speeds, your car engine is producing only a small fraction of its rated power (all that's needed to overcome air friction & rolling resistance). Most of the potential power is just used for acceleration. (Which is why hybrids get better mpg.) An aircraft engine runs at a large fraction of max power all the time. $\endgroup$
    – jamesqf
    Jan 18 '18 at 19:47
  • $\begingroup$ A clean sheet design is the Jet Engine, no? Tbh, if you’re a 100lbs overweight, then the real cost savings is right under your nose. $\endgroup$ Jun 10 '18 at 9:19

It would have a gearbox between the crankshaft and the propeller. That is certain.

Other very likely details would be:

  • overhead camshafts
  • four valves per cylinder
  • water cooling at least for the cylinder heads
  • electronic ignition
  • single lever control

This would enable it to run constantly at about 4000 RPM and create the same power from a smaller displacement. The consequence would be a more compact and lighter core engine. Compared to a car engine, the cooling system would be more capable, so the weight would still be higher than that of a car engine of the same nominal shaft horsepower. But it should be lighter and allow for a more aerodynamic cowling than the current piston aero engines.

The Porsche flight motor (PFM 3200) was like that, and for a reason. Yes, it did not outperform the older engines in terms of mass, but it was noticeably less noisy. Starting from a dated six-cylinder boxer design, the engineers could not fully optimize the engine for its intended purpose.

Once a gearbox is fitted, it makes sense to reduce propeller speed, so the diameter of the propeller fitted to that engine would be a bit larger than that of a comparable propeller on a conventional engine.

Once Jet-A is widely available at GA airfields, a diesel version would also be very likely. Note, however, that the system weight of a piston engine (engine + accessories + fuel system + fuel) will be higher for a diesel on short ranges.

  • $\begingroup$ Thanks for these details. The changes you mention- overhead camshafts, four valves per cylinder, electronic ignition, single lever control- are these things that are impossible to integrate as evolutionary changes to the existing Continental/Lycoming designs? Regarding 4000rpm, is that just the current limit for reliable long-term operation? $\endgroup$
    – Charles847
    Jan 18 '18 at 21:12
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    $\begingroup$ @Charles847: Overhead camshafts and four valves per cylinder were already used in WW II aero engines, so they should be no problem to integrate. The only barrier is missing competition. Electronic ignition and single-lever control would be a no-brainer for every decent engine designer as well - same problem. The incumbents are protected by a massive wall of regulation which makes them lazy. The 4000 RPM are a conservative compromise between engine life and performance, but no fixed limit. $\endgroup$ Jan 19 '18 at 0:02
  • $\begingroup$ Why Jet-A rather than plain old automotive diesel? $\endgroup$
    – jamesqf
    Jan 19 '18 at 19:59
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    $\begingroup$ @jamesqf: Jet-A has a specified quality. Please read this answer for details. I could not explain it half as well. $\endgroup$ Jan 19 '18 at 20:40
  • $\begingroup$ @PeterKämpf From what I've seen around this place, if you say you couldn't explain it better, it must be a very good answer. $\endgroup$
    – T.J.L.
    Jan 19 '18 at 21:44

It would look almost identical mainly because the engines are designed around a use case not for purely engineering pursuits. You may see some different material choices, some changes in piping and head design as our understanding of these topics has improved and our production abilities have come a long way but ultimately little change.

You should read the answers here, and here. They are almost duplicates but more focus more on why modern car engines are not used in aviation. Although in some cases they are.

Piston GA engines with reductive gear boxes were attempted in the 80's on the Mooney M20 PFM ultimately the project was such a failure Mooney paid to replace all the engines with continentals.

As @RonBeyer mentions in the comments the low RPM is a function of the propellor not the engine.

Aircraft engines simply have a different use case than automotive engines which has kept them simpler over the years. The cost of certification (a huge factor in aviation) has also kept up rapid innovation at bay.

or could compression ignition based on jet fuel produce the power density and reliability required

Some modern piston diesels are already certified to run on Jet A.

This article says it pretty much as well as I can and is worth the read.

Something that is perhaps overlooked is that aircraft engines are not subject to the same regulations that automotive engines are. As such they are, in some cases actually more efficient. One major thing that impacts car engines efficiency is catalytic converters and mufflers, something GA planes dont really have. As such there has never been a reason to compensate for the performance loss as a result of regulations.

You also mention "more reliable", GA engines are quite reliable and the FAA/NTSB has reported that mechanically related accidents are on a general down trend if we look back at history. Modern machining and industrial practices can be used to make old engines in a more reliable and consistent way.

One major efficiency boost that has been realized on the ground is turbo charging. While there are plenty of turbo aircraft out there the prevailing attitude in aviation has always been to keep things simple. Less parts means less to fail as such there is simply a smaller market for a turbo aircraft engine. It is not that the technology is a mystery to the makers or the consumers its simply an unwanted complexity.

  • 1
    $\begingroup$ I just finished reading a fascinating article on the engine used in your mentioned Mooney M20 PFM, and I recognize all those downsides, but they seem to be downsides inherent to the application of automotive-optimized engines to aviation, rather than downsides inherent to the application of modern piston engine technology to purpose-built aviation engines. Shouldn't advancements in our technology level allow for aviation engines that are smaller, lighter, more reliable, more efficient, for the same power, just as our tech advancements have enabled such gains for auto/marine/stationary engines? $\endgroup$
    – Charles847
    Jan 18 '18 at 19:56
  • $\begingroup$ The answers linked mention all the non-technological reasons for the engine differences, but clearly imply new design would at least use higher RPM. It also probably would use all the computer control, which is now well tested in automobile operation. $\endgroup$
    – Jan Hudec
    Jan 18 '18 at 20:02
  • $\begingroup$ This is not the answer to the question. it's not about use of an automotive engine in GA. It's about comparing the progress that has happened in the automotive sector (which is huge), compared to the one that has happened in the GA sector. $\endgroup$ Dec 25 '21 at 20:40
  • $\begingroup$ Claiming they would look identical is like claiming that modern car engines look identical to car engines from the 60ies, just because already then they were designed for a car use case. $\endgroup$ Dec 25 '21 at 20:42

From a polish source Image source

It would look like the modern GA engine already in production, the Austro AE300:

  • a car engine basis, in the AE300 case a Mercedes diesel.
  • water cooling.
  • a 1:1.69 reduction gearbox.
  • running on diesel or kerosene (Jet A1).
  • $\begingroup$ Not sure about that. My (strong) assumption is that they use the AE300 as base because then they can save half the engineering development and testing by relying on proven parts, as a trade off to keep development costs down, especially in a small-volume market, where development costs play a larger role than in mass production market like the automotive industry. My assumption is - if you had a sponsor who's happy to pay anything - you'd get a different design $\endgroup$ Dec 25 '21 at 20:46

Rotax makes modern GA engines even for certified application. But they don't differ that much as car engines from aviation engines as they work and different requirements. https://www.flyrotax.com/produkte/detail/rotax-915-is-isc.html


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