What is the theoretical maximum RPM of a typical aircraft piston engine?

Question

Given a standard 6-cylinder, direct-drive, horizontally-opposed engine like the Continental IO-550 or Lycoming IO-540, the maximum RPM for those engines is 2700 RPM. That's due to keeping the propeller tip speed under the supersonic range.

But what is preventing these engines from turning at a higher RPM for another, non-aircraft use?

I'm thinking if they would work up to around 5000 RPM or so. Maybe the fuel mapping must be modified? Will the valve-train start to float at that RPM? The application would be automotive, so the load would be far, far less from a duty cycle standpoint than an aircraft application: basically, 5000 RPM would only be seen for a second or two at a time under full (or nearly full) load.

Is there anything else I'm missing? I realize this is a weird question, just doing some thought experimenting.

Answer 1

There are three factors which limit the possible speed of piston engines:

1. Flame speed in the fuel-air mixture,
2. relative speed of moving parts, here the pistons in their cylinders, and
3. Valve inertia and valve float.

If the engine runs too fast, the flame front originating from the spark plug will not have traveled far enough to have burnt most of the fuel by the time the piston moves down again. This puts a fundamental limit to the growth in power output over speed in piston engines.

Also, if the relative speed between moving parts is too high, the lubrication will fail and the parts will overheat quickly. In order to run at the highest possible speed, rotary engines used castor oil, which in turn motivated early aviators to wear long scarves over their mouth so their digestive system would not be compromised by sitting right in the exhaust stream of their engine. Today, lubricants have improved but still set a clear limit to the maximum speed at which a piston engine can be run. Cooling can be managed by pressurizing the cooling system and improving radiator and pump performance while inertial loads on the engine components can be dealt with by lightweight, high-strength materials.

Reducing the bore will allow a higher speed before the ignition limit bites while reducing the stroke will reduce piston speeds, again allowing a higher engine speed before lubrication fails. An optimimized engine will balance both so that the possible speed can be fully exploited. For the IO-360 and IO-540, those dimensions are 5⅛" and 4⅜", respectively. This places them below the last generation of big aviation pistons and closer to the typical interwar engines.

With higher speeds, the valves need to move faster, too. If the force of the valve springs are not sufficient to close the valves quickly enough, compression will suffer, limiting RPM and power. In order to reduce the inertia of each individual valve, the number of valves can be doubled or even tripled.

I could not find reliable data on the engine speed of Reno Air Race winners, which should come close to the maximum sensible engine speed at that size class. The best I could find was the 3,700 RPM of the Jumo 213 E, an engine with 5.9" bore and 6.5" stroke, so I would guess by using good fuel and lubrication, the Lycoming might end up somewhere between 4000 and 4500 RPM. I'm sceptical that much useful power can be extracted at 5000 RPM and am sure that the lifetime at this speed will be measured in minutes, not hours.

Answer 2

The size and weight of the pistons is going to be a major hurdle. With a 5.25 in bore and a 4.25 inch stroke, the pistons are massive and don't change directions very easily—there's only so many Gs you can put a shape this large through at the top and bottom of each stroke before the metal starts deforming. Bounce an open umbrella up and down really fast, and you'll start to get the picture.

Cooling would also be a major factor for non-aviation use. Aircraft engines (more or less rightly) assume that there's going to be a large fan out front and/or 100+ mph of airflow. Liquid cooling would allow the heads to maintain a more constant temperature, but would also add weight and complexity.

If you want a faster-revving flat six that will put out more than 100hp and is suitable for automotive use, I might suggest pulling an engine out of a Honda Goldwing or Valkyrie. Given the edited description, though, I'm guessing you're thinking of a drag racer.

Answer 3

I agree with the other posts with the general limiting factors being failure at high rpm and cooling being a problem. Assuming reliability isn't a concern for you... what else will be the limiting factor?

Reliability and cooling aside no-one seems to have mentioned intake restriction or valve train.

The first limiting factor will be how much fuel can your engine injest and burn. So is the engine normally aspirated or forced? NA engines are limited by how much fuel and air they can draw in. So: carbs, intake manifold, cam and valves need to be able to flow enough to produce the power you want. Forced induction makes this a bit simpler.

But as revs rise to produce big HP numbers you are going to hit the glass ceiling of how fast your valve train can function. At some point the valve springs will not be able to close in time (known as valve bounce) and can simply stay open. This affects the compression and combustion strokes and may result in the piston hitting the valve shortly followed by very unpleasant noises.

My answer was given with a stock aero engine as the starting point, thinking about what limitations you would hit as revs rise. Each limitation can be overcome but the solutions get more complex and expensive as revs rise and you chase bigger HP figures.

Answer 4

There is nothing physically limiting it but the engines are designed to run optimally in the given range so they may simply not run very well. Of course you could always change the cam timing, spark timing etc. For the record your run of the mill aircraft engine is not all that different than the early Porsche/VW air cooled flat car motors (which were even used on aircraft at one point). You would need to cool it at least partial with an oil system and partially with an air fan much as Porsche did on their pre 996 era cars if you did not want to leverage water cooling (and you dont need to). For the record large bore air cooled engines can develop a lot of horse power and rev very high if built correctly.

For what its worth aircraft engines are far more expensive than their non FAA certified automotive counterparts and you are better off using a car engine simply from a cost perspective.

Answer 5

There are mechanical limits to rpm and volumetric efficiency limits to engine RPM. Mechanical: Piston speed- damaged piston, rod Piston acceleration - ring flutter failure Valve float- catastrophic valve failure Increase RPM results in higher oil temps Volumetric: Ability to intake air is limited by: Airflow limits of intake system: carb airbox Intake tubes Intake manifolds, and ports, valves, cam design and timing. All thees things determine the amount of air the motor is able to intake and that determines the rpm the motor will work best at. More details can be found by looking in a superflow flowbench manual available for free on the net.

Exceed mechanical limits and experience catastrophic failure. I the world of racing I often found mechanical limits by exceeding them. Exceed volumetric limits and see loss of torque.

Answer 6

One could come up with a higher revving aircraft engine and gear it down for subsonic prop operation. That is what Porsche did with its PFM 3200 aircraft engine: took it's 911 flat six and geared it down. While it was a very good engine, with about twice the HP of similar displacement aircraft flat sixes, and was used to equip the Mooney M20L, it was also expensive to buy and had a fairly short TBO. Supposedly, the PFM 3200 sounded magnificent on takeoff.

Porsche exited the market in 2007 after $75 million in development costs to produce and sell around 80 engines. Supposedly, they bought back all of the PFM 3200 engines in use for liability reasons.

In contrast, the Continental and Lycoming flat sixes are much simpler, slower turning pushrod engines, built with an emphasis on reliability, not power to weight. Low RPM means no gearbox to gear down for prop operation, and no gearbox to maintain or fail. They are intended to operate at high power output for long periods of time without failing, as opposed to automotive engines that tend to loaf along at low power output, with an occasional burst of full power. Loss of power in an aircraft is a bit more serious than loss of power in a car, hence the emphasis on reliability.

So, if you want a flat six with lots of HP for non aircraft use... don't look at aircraft engines, unless you want an engine that turns slow, that you can floor for an hour or two regularly.

Answer 7

Niels responds to an old question.

Pushrod valvetrains in big engines (~500 cubic inch class) limit out at about 5000 RPM. Going faster than that usually requires an overhead cam and might yield ~6000 RPM, but past that the reciprocating mass tends to make the engine explode unexpectedly, so to go still faster (and make more power) you reduce the piston size and the crankshaft stroke and add more pistons to maintain displacement.

So instead of a 6-cylinder engine you now have a 10-cylinder engine, and you might make that monster spin faster with dual overhead cams instead of singles. But now you run out of lubrication capacity in the main bearings, so you re-engineer the engine for roller bearing mains.

Now you have to re-engineer the heads for higher heat-rejecting capacity so you do not overheat the exhaust valve seats, which means water-cooling and perhaps also forced-oil cooling.

Now the engine requires supercharging to overcome its pumping losses, so you bolt on a blower to make it spin faster and make more power... but all of this produces a big problem, as follows:

Every crank revolution wears the piston rings, bores, valve train components and crank bearings a tiny little bit. The useful lifetime of the engine is set by how many crank turns you can accumulate before the wear limits are hit. And this means that if you gear the engine down to spin the prop half as fast, you'll accumulate crank turns twice as fast per hour of flight time and you cut the TBO in half- and now you have to overhaul the gearbox and the supercharger as well as the internal engine components in 1000 hours instead of 2000 hours.

Drag-racing engines can push to 7500 RPM or sometimes 10,000 RPM and produce a thousand horsepower or more out of 500 cubic inches, with a TBO of several minutes (no kidding!)- and crowd-pleasing (i.e., truly spectacular) failure modes if any little thing happens to go wrong. So...

High reliability and long life dictate minimizing RPM's and engine complexity, so in internal combustion aircraft engines you stick with big pistons, direct drive, natural aspiration and you live with half a horsepower per cubic inch of displacement at sea level.

Answer 8

But what is preventing these engines from turning a higher RPM, for a non-aircraft use?

Overheating.

It is important to remember that aircraft engines must be reliable enough to maintain enough airspeed for flight. It is also important to understand that thrust is proportional to the square of RPM, so the operating RPM range is not going to be nearly as wide as a motorcycle or a car.

it is the drag of the prop that limits RPM.

For example, an aircraft may maintain level flight at 1500 rpm, but doubles its engine output torque requirement at 2300 rpm to climb.

A more practical solution to boost power output is the turbocharger, allowing far more horsepower at a lower RPM, while the "Genesis" motorcycle is screaming 12,000 RPM to make 120 horsepower.

Using an aircraft engine for a high RPM application is a bit like bringing a Clydesdale to the track and racing it against a Thoroughbred. Generally not a good idea.