How Far Could the Piston Aero Engine Have Gone?

After the development of turbojet and turboprop engines, piston engines were largely left with a niche in general aviation. Their use on the Canadair CP-107 Argus naval reconnaissance aircraft might be one of the last, if not the last, roles for which a large commercial or military aircraft was equipped with piston engines, and they used 1930s era Wright R-3350 Duplex-Cyclone engines. It's interesting to note that while production piston aero engines broke the one horsepower per cubic inch hyper engine limit near the end of World War II, production automobiles began reaching figures in excess of that by the 1960s and 1970s using significantly less advanced components, such as iron construction, pushrods (instead of the four valve overhead cam design of aero engines), and lower octane automotive fuel. In addition to standard aviation fuel, for example, the Western Allies tested 100/150 octane aviation gasoline during World War II, which is higher octane than modern aviation gasoline used for racing.

If piston aero engine development had continued, what kind of performance might have been possible? Also, how long might piston engines have been able to compete with jet engines? If they were more developed, might they even have found a niche in commercial and military aviation today, or would higher unit fuel costs and maintenance eventually lead to their retirement?

The big problem is vibration associated with large piston engines. The Republic Rainbow and the experience of piston fighters show quite nicely that jet equivalent performance is possible piston engines. Bearing in mind that a piston driven ducted fan is quite possible a supersonic piston driven aircraft is quite possible.

Ultimately the reason for a lack of a significant niche is not the limits of what can be built but the reality that large reciprocating engines quickly end up shaking themselves apart, requiring vastly more maintenance on a much more complicated piece of machinery than an equivalent turbine. Turbines are amazingly simple mechanically, even if they do require fairly high tolerances to manufacture. In terms of large aero-engines whatever gains exist in efficiency tend to be more than wiped out by the simplicity and smoothness of a turbine quite quickly. Certainly in terms of subsonic flight anything a turbine can do a piston can be designed to, it's just going to be a lot more trouble in just about every way except, possibly, fuel consumption. The fuel thing isn't even as clear cut as it might be once you consider that you are going to be using high octane av-gas of some sort while the turbines are quite happy burning kerosene.

The R-3350 was definitely 1940s technology, and it was used to power the Argus because it gave the best fuel burn for low speed at low altitude. When Canadair built the CL-44, they reverted to R-R Tynes because a transport can fly faster and cruise at higher altitude. Over the decades, the R-3350 has become a hyper engine, powering Rare Bear to a speed record. There is a size limit for pistons, and there is a complexity limit for number of cylinders, and there is a critical mach limit for propellers, which is well short of being suitable for powering aircraft at the structural size and performance limits, which Whittle perceived way back when. That car engines produce more power per capacity is moot. They are small by comparison. Turbines don't seem to have caught on for automotive use. There are still lots of fairly substantial niches around the world. Speaking of which the aircraft which flew around the world, unrefueled, was powered by piston engines. For flying time unrefueled, they beat out a '20s technology diesel-powered machine with only one valve per cylinder.

By time you add turbocharger to boost the engine, spray in water/alcohol mixes to cool that charge, pump in gasoline with direct injection to the cylinder, then use Power Recovery Turbines via fluid couplings back to the crankshaft, its almost as complicated as a turboprop.

But with more vibration, weight and volume.

The only think that could be added would be ceramic materials, better wearing bearings, and computer feedback and better ignition.

Magnetos have real limitations.

IIRC, with sleeve valve engines extremely high backpressure is ok. You can drive the mechanical supercharger with the engine and run a power turbine off the exhaust and actually get more power from the turbine then from the engine.

The problem was by the time the kinks were worked out, it was easier to use a gas turbine to feed the power turbine for Turboprop engines then use the blown ICE.

Andras said:

IIRC, with sleeve valve engines extremely high backpressure is ok. You can drive the mechanical supercharger with the engine and run a power turbine off the exhaust and actually get more power from the turbine then from the engine.

The problem was by the time the kinks were worked out, it was easier to use a gas turbine to feed the power turbine for Turboprop engines then use the blown ICE.

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That sounds similar to the Napier Nomad, a turbo-compound engine.

The Rolls-Royce Crecy was a more conventional design, being a two stroke email gasoline engine. It never flew, and in some applications such as the Supermarine Spitfire would have required detuning to avoid overstressing the aircraft.

For raw power you're looking at either a multibank radial or an X, W or H inlines.

MattII said:

For raw power you're looking at either a multibank radial or an X, W or H inlines.

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I'm thinking of a mixture of power, efficiency, and good power to weight and volume to power ratios. It's really more an exploration of what would be possible with piston engines in a scenario in which jet engine technology is adopted later and what kinds of limits piston engines would run up against, apart from the sound barrier of course.

Essentially, the limit for piston engines were the Turbo Compound versions of the Wright R-3350 radial engine found on later versions of the Lockheed Constellation and the Douglas DC-7. Small wonder why by the 1950's, the major airplane manufacturers were all involved in jet airliner projects or building planes with turboprop engines.

Delta Force said:

That sounds similar to the Napier Nomad, a turbo-compound engine.

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The one I had in mind was a Bristol predecessor to the Theseus. It's mentioned in the Jane's 1945/46 reprint copy I have.

Turbo-compounds are coupled back to the engine shaft. This one had a power turbine that was not coupled to the engine, but directly to the prop shaft.

Sleeve valves are limited to lower RPMS <3,000. If you have a high swept volume you can generate enormous amounts of gas exhaust for the turbine at lower rpms.

Andras said:

Sleeve valves are limited to lower RPMS <3,000.

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That's not true, is it?

... Turbines don't seem to have caught on for automotive use. ...[/QUOTE]

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The only ground vehicle powered by a gas turbine engine is the M1 Abrams tank. It uses a gas turbine to improve its power-to-weight ratio. Even with a large diameter recuperator, it still pumps out a massive heat signature. Infantrymen are afraid that if they try to use the "tank phone" they will get fried before they get within 100 feet of the tail gate!
Hah!
Hah!

Every other army uses huge diesel engines to power their armoured fighting vehicles.

Just Leo said:

The R-3350 was definitely 1940s technology, and it was used to power the Argus because it gave the best fuel burn for low speed at low altitude. When Canadair built the CL-44, they reverted to R-R Tynes because a transport can fly faster and cruise at higher altitude. Over the decades, the R-3350 has become a hyper engine, powering Rare Bear to a speed record. ...

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OTL Canadair designed one more piston-pounding plane during the 1960s. When they designed the CL-215 water bomber, they installed P&W R-2800 radial engines. Plenty of military-surplus R-2800 engines were available for cheap. Also considering the few hours per year flown by water bombers, avgas consumption was not a big issue.

As the supply of military-surplus R-2800s wore out, Canadair re-designed their water-bomber to accept P&WC turbo-prop engines.

Many other piston-pounding prop jobs continued in service for many years just because military-surplus radials were still cheaper turbo-props: DHC-2 Beaver, DHC-3 single Otter, DHC-4 Cariboo, Beech 18, Douglas DC-3. When the supply of military-surplus radials wore out, the airplanes were grounded.

riggerrob said:

Many other piston-pounding prop jobs continued in service for many years just because military-surplus radials were still cheaper turbo-props: DHC-2 Beaver, DHC-3 single Otter, DHC-4 Cariboo, Beech 18, Douglas DC-3. When the supply of military-surplus radials wore out, the airplanes were grounded.

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Some time ago, an AN-2 Colt Turbo flew over my head. A couple days before, there was a DC-3 with PT6As. It seems odd to see a turbo-prop biplane.

Years ago, I worked at DHC, and the DHC-2T, which I recall being known as the DHCT-2, or CT2, was a monumental flop for sales. The Turbo-Beaver took advantage of the reduced engine weight and increased power to extend the cabin volume forward, giving it load capability almost equal to the Otter. But it didn't sell well at all. It was too expensive because it didn't use cheap engines. Times, and markets change.

the fundamental problem with IC engines in aero applications is the reliance on propellors which limits the high speed potential

then as has been suggested turbo compounds add complexity when in a gas turbine the gas generator replacing the IC engine can be as simple as some burner cans and injectors as in a turbo compound you've got a compressor ( turbo or supercharger) and a power turbine ( driving the compressor and power recovery)... with the IC engine sandwiched as the gas generator

and it;s still driving a propellor

Just Leo said:

That's not true, is it?

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this is what wiki said about airplane engines
http://en.wikipedia.org/wiki/Sleeve_valve#Disadvantages
"A serious issue with large single-sleeve aero-engines is that their maximum reliable rotational speed is limited to about 3000 RPM."

A quick check of the specifications shows max power is reached around 2700-2800rpm.

zippy said:

the fundamental problem with IC engines in aero applications is the reliance on propellors which limits the high speed potential

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Not really though. Subsonic performance can be just about equivalent. The Rainbow I pointed out early on never went into production, but had performance very close to an early jet. The TU-114 was even closer, and is in essence a 707 equivalent aircraft driven by turboprops.

Add to that a ducted fan can be driven by a reciprocating engine and short of supersonic aircraft turbine or reciprocating power and prop or jet propulsion are largely separate issues.

riggerrob said:

Every other army uses huge diesel engines to power their armoured fighting vehicles.

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That's not true. The T-80 and most of its derivatives also use a gas turbine, and actually entered production before the M1 (not to mention the number of armies that use the M1 itself, but that's a separate issue). Diesel engines are more common, but there's a genuine argument there between logistics and raw capability. The United States has a strong logistics system, so taxing it a bit more but getting better performance out of it makes sense. Similarly, the T-80 was for elite divisions, so it wouldn't have been deployed in great numbers. Other countries face different strategic situations, so they deploy different vehicles.

In addition, there are apparently several hundred buses that use a very small gas turbine for power, along with a variety of experimental, racing, or small-scale use vehicles like Union Pacific's gas turbine locomotives. Gas turbines have certainly proved technically feasible in road and rail transport, but the economics tend to be rather situational.

Andras said:

this is what wiki said about airplane engines
http://en.wikipedia.org/wiki/Sleeve_valve#Disadvantages
"A serious issue with large single-sleeve aero-engines is that their maximum reliable rotational speed is limited to about 3000 RPM."

A quick check of the specifications shows max power is reached around 2700-2800rpm.

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The maximum tested rpm for Napier Sabre is 3850 rpm. It is a sleeve valve engine and it's quite large. Large poppet-valve engines were also often limited to 3000 rpm. not necessarily limited by the valves floating. It sounds like wiki dropped the ball again. The Griffon wheezed out at 2700 rpm. In the same class, the Hercules was 2800 rpm, the BMW801, 2700 rpm, and the Wright R-2600, 2600 rpm. These are wiki figures. They do not show that the sleeve valve engine was dis-advantaged.