If Reciprocating engines were still made (Read)

wuzak said:

A gas turbine with the same power as a regular Chevrolet Corvette engine would be considered a microturbine!

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Not quite micro, but definitely on the small side.

When I was involved in some cold temp starting tests of the ALF-502, the starter was peaking out at over 600 hp.

PWR4360-59B said:

I'm just glad there are some folks that aren't stuck in a box, else inventions and refinements would be nonexistent. Even your precious turbine would not have come to fruition. I like how the one fellow above says they are way more efficient than piston engines, how come my little 50 mpg car doesn't use one?

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You keep repeating the phrase "stuck in a box" like just by saying it you can magically make real world considerations go away.

Then you bring in ridiculous examples like dragster engines running on nitromethane as an 'example' of possible.

When we try to explain that, aside from the whole engine lasting seconds thing, that the power level is reached using a fuel that is totally useless for aviation you drag the "stuck in a box" label out.

Nitromethane weighs around 4 times as much as gasoline does for the same "energy". It works as a racing fuel because it carries a lot of the needed oxygen itself and you can burn over 8 times as much per second as you can gasoline. Now even if you back off considerably on the power level so the engine/s will last at least few hours you run into the problem that you can't get enough fuel into an airplane to perform a useful (money making) flight.
An old Convair 340 airliner

could hold 1750 US gallons (10,500lbs) max fuel and needed 1157 US gallons (6,942lb) for a 2000 mile range with reserves.
It doesn't matter what the power to weight ratio of the engines are if the fuel needed for that 2000 mile trip weighs over 27,000lbs does it?
Empty weight of the Convair 340 was 29,486lbs with a max gross weight of 47,000lbs.

Real world engine design for non-racing engines is constrained by available fuels and no amount of "thinking outside the box" is going to change that.

You could run either a piston engine or turbine on hydrogen gas, the problem, especially for aircraft, is storing it in a weight efficient way.

" Hydrogen has more energy per unit mass than other fuels (61,100 BTUs per pound versus 20,900 BTUs per pound of gasoline). The problem with hydrogen is that it is much less dense (pounds per gallon) than other fuels. A gallon of gasoline has a mass of 6.0 pounds, the same gallon of liquid hydrogen only has a mass of 0.567 pounds or only 9.45% of the mass of gasoline. Therefore one gallon of gasoline yields 125,400 BTUs of energy while a gallon of liquid hydrogen yields only 34,643 BTUs or 27.6% of the energy in a gallon of gasoline."

and you can't store either liquid hydrogen or highly pressurized hydrogen (5000lb per sq in) in integral fuel tanks (tanks that are part of the aircraft structure) anywhere near as simply as you can store gasoline or diesel/jet fuel in integral tanks. Airliners use integral tanks because they are lighter than using seperate tanks. Needing 3-4 times the volume of gasoline (let alone diesel/kerosine) for your fuel tanks is going to play havoc with aircraft design.

This consideration trumps power output on hydrogen. It is a non starter for most aircraft from the start. (short of Mach 5 wonder planes that cruise in subspace).

Any commercial fuel is going to have to be easy to store and transfer, high in energy density (both weight and volume) and relatively non toxic in normal use.
See; Zip fuel - Wikipedia

for problems the US Air Force ran into with Boron laced fuel in the 1950s. Then imagine trying to introduce it today.

There are real world problems with many alternatives to fuel or engines that cannot be wished away by thinking outside the box or octagon or whatever.

Ran across these Wiki articles:

first gas turbine patent 1791
John Barber (engineer) - Wikipedia

and short history of the internal combustion engine.
Internal combustion engine - Wikipedia

There has been a lot of thinking outside box over the last 160-230 years and as a result the "box" has been stretched and squashed number of times.
New materials have been introduced but that has pretty much come to an end as far as basic elements go. Any additions to the periodic table will only exist for fractions of a second inside a nuclear collider. Aluminium has been being used as an engine material for over 100 years so I don't see any major changes there. Magnesium has been being used for for over 80 years.
I remember as a teenager (or early 20s?) being promised ceramic engine parts (not coatings).......still waiting over 40 years later

What you can do with stationary power plants is different from what you can do with mobile power plants and especially what you can do with aircraft power plants.
I drive a Hyundai Tucson, hardly an awe inspiring vehicle. it has a curb weight of around 3300lbs and a gross weight of around 4300lbs (I will have to check the manual). but even small SUVs like this one can play games with engines and drivelines that aircraft cannot. A Cessna 206 has a gross weight of about 3300lbs.
Cars and SUVs don't have to worry anywhere near as much about centers of gravity and the only thing you have to do to some vehicles to change the GVW is change the tires and maybe put in heavier springs (although bigger brakes might be a good idea). Many cars/SUVs/pickups are operated well outside the listed load rating and/or weight placement (CG) as any of us who have seen vehicles on the road with rear bumpers dragging and headlights pointed at the sky can attest to.
Load a plane that way and it crashes.
There are ways to increase fuel economy in a number of engines, some are just not practical in an aircraft engine due to weight/volume.

If one looks at the history of IC engines you can find all manner of thinking outside the box. Most engines built today are inside the box because none of the old outside the box engines offered any real advantages and in some cases some real disadvantages.

Fairchild Caminez engine, actually flew in the late 20s, may have even got an ATC number. The figure 8 cam acted like a gear and you got four piston stokes (2 up and two down) for every revolution of the figure 8 cam which the prop was attached to.
Vibration was horrendous.
The Germans had a rotary engine (meaning the cylinders rotated) in WW I where the crankshaft was NOT stationary but rotated in the opposite direction as the cylinder assembly. they were geared together and the combination used a 4 bladed propeller as the "engine" rotated at about 1/2 the rpm of a normal rotary of the same power.
Curtiss built a two row six cylinder radial in the late 30s, a large number of aviation museums in the US have one in fairly good condition because any pilot who could afford to do so replaced it as soon as possible because the vibration shook the whole plane.
The list could go on.

The properties of just about any conceivable liquid fuel are already known. The spectrum of petrochemicals has been gone over. Alcohols have been used for over 100 years. Even peanut oil has been used in gas turbines (more just to see if it would work than any real intention of build peanut fueled vehicles).

Some of the boxes that exist for internal combustion engines exist for reasons. Improvements can be made (and will be made) but radical changes are very, very unlikely.

Like guns and smokeless powder, I am still waiting for my rocket pistol or my rifle with liquid propellent where I can dial up the velocity I want for the bullet and the gun will inject the right amount of propellent into the chamber.

"but even small SUVs like this one can play games with engines and drivelines that aircraft cannot".
"The list could go on."

Certification costs and product liability insurance of unorthodox items tend to get ridiculous and often find little acceptance within the aviation community.

The auto industry investigated, among other things, direct-injection, stratified charge, blower-scavenged two-strokes, and spent several tens of millions of dollars before giving up.

Outside the box, there are dragons.

swampyankee said:

Outside the box, there are dragons.

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Outside the box there are dozens of examples of how not to do things in addition to a few dragons.

Based on what I see here, I'm tempted to suggest that this thread be renamed, "What would have happened if all the nifty development work that went into developing high power piston aviation engines during WWII continued at the same rate for more advanced piston engines, instead of going into turbine development". As I see it, that's what this is really all about. Piston engines developed tremendously as all the powers tossed dollars, marks, and pounds at the problem and assigned some of their best mechanical engineers to it.

So, in some parallel universe, the jet engine never gets invented (or proves to be impractical for some reason), and Postwar/Cold War money gets spent making more advanced non-turbine engines to turn props. What do these engines look like? What can they do?

I'm not sure that I agree that the piston engine (or rotary, or whatever non-turbine you prefer) had been developed to near it's limits by the end of the war, but like all such things, it developed along an S-curve. Development is flat at the beginning (think turn of the century), then the pace of growth picks up exponentially as multiple tinkerers try out new ideas (think 1910s and 1920s). Eventually development hits it's maximum rate of increase and stays there for some time, proceeding along more or less predictable paths and fueled by corporate and/or governnment dollars (1940s). Eventually, you hit the top of the "S" and the development curve flattens out again (postwar and 1950s). Things have been taken to their limits until some new enabling technology comes along, whereupon a new "S" curve begins (turbine engines, so far as aircraft are concerned).

My take is that the curve had not flattened by 1945, but was starting to run out, starting to flatten. I think we could have gone a few stages further had their been compelling reason to develop even more powerful piston engines for aircraft. But I don't think we'd see anything near to 11,000 hp engines weighing a few hundred pounds, as in fuel dragsters (the Top Fuel and Funny Car classes essentially use the same engines).

Charles Fayette Taylor of MIT had written the bible on the internal combustion engine by then, and the latest ediction of his 2-volume opus is still the bible today. It keeps getting updated with things like stratified charge and variable valve timing, but the basics haven't changed in decades and are unlikely to.

Power is the product of piston speed and brake mean effective pressure. BMEP is limited in normally aspirated engines by atmospheric pressure, and good modern engines are within a few percent of the theoretical max. BMEP in a forced induction engine is limited by the amount of fuel and oxidizer that can be crammed into a given volume. Nitromethane is just a way of cheating the oxydizer problem, since instead of compressing air to ridiculous pressures, you just pour in a sort of "liquid oxygen" in the form of nitromethane. There are fuels that are even more efficient for this purpose, but they are outlawed in drag racing...you can imagine the expense and safety problems involved.

For numerous reasons, a "modern" propeller-driven fighter aircraft wouldn't be using such exotic fuel. Top Alcohol motors run on methanol and are somewhere in the neighborhood of 5,000 hp. Pro Mod motors run on super-high octane racing gasoline, run up to about 1,000 cubic inches for a large-block, aluminium V8, and develop about 3500 horsepower. In many ways, these motors are more advanced than NASCAR or Formula 1 motors. For example, the current record for highest piston speed is held by a Pro Mod "Mountain Motor", turning at about 8,000 rpm. Because the stroke is so much longer, the piston speed is higher than in a Forumula 1 motor turning 18,000 rpm (not a misprint!).

At the outer limits of BMEP for a forced induction engine, the enemy is detonation and its cousin, pre-ignition. This is caused by the exhaust valve being so hot that no spark is needed to set off the fuel charge, which does so explosively instead of in a controlled, very high speed burn. Detonation will destroy an engine in a few seconds, a minute or so at most. Factors that cause detonation that a designer can play with:

Compression ratio - limited by detonation
Supercharger/Turbocharger boost ratio/final boost pressure - limited by detonation
Piston diameter - limited by detonation

Of course things like intercooling between the compressor and the engine, and water-methanol injection to lower combustion temperatures helps here, but even the engines being developed at the end of WWII were starting to hit the limits. Modern combustion chamber design helps some. Race and street/race turbocharged motors are running compression ratios of 10:1 or so, 20 years ago much more than 7:1 in a boosted motor was unheard of. But I don't see how we're going to get cylinder diameters appreciably larger than about a Wright Cyclone at 6.125 inches, or boost pressure much more than about 50 psi. If we went to Diesel technology, we might be able to do staged turbocharging (one turbo feeds another to get higher boost than is possible with a single turbo) like the Pro Stock pulling tractors...but then we might run into weight problems. Audi had great success with diesels at Le Mans, but diesels got a break on both displacement (more allowed) and boost (more allowed) compared to spark-ignition motors.

As for piston speed, it's been well known for decades (going back to Mr. Taylor) that engine life (TBO for us) is directly related to piston speed. NASCAR and F1 motors last for a weekend (practice, qualifying, and race), and then must be torn down and rebuilt. Pro Mod motors might go 3-4 passes between overhauls, as they aren't subjected to the absurd forces at the piston surface that Top Fuel motors are. The curve is very exponential, back down on the speed just a little bit, and TBO goes up quite a bit. Modern materials do help, we're seeing speed vs. TBO times with the latest alloys that were unthinkable just a few years ago, but progress per year on this front since 1945 has been fairly slow. Don't expect miracles from materials, or for unobtanium to show up any time soon.

More displacement per cylinder doesn't help. As F1 has learned, you are limited by piston speed, not stroke, and taller motors weigh more. As the cylinders become more and more oversquare (bore much larger than stroke), induction becomes a problem. First 4-valve per cylinder becomes mandatory, and eventually exotica such as pneumatic valves (there are no valve springs because they weigh too much to close the valves at those rpms, compressed air will always be lighter than a steel spring). If you don't mind a little more engine weight, you don't have to be that extreme (Pro Mod motors use 2 valves per cylinder and pushrods). But F1 is a class that will literally spend $1 million to move 1 pound from the engine down to a weight plate at the bottom of the chassis. They also had to outlaw beryllium engine parts.

So, cylinders the size of a Wright Cyclone, boost pressures and piston speeds on the order of a Pro Mod Mountain Motor. What's left in our quest for more power? Only more cylinders. Frontal area needs to be kept manageable, so we're probably looking at water-cooled 6-row radials or X-32 layouts (we'll assume we can lick the vibration problems with the 8-cylinder long crankshaft, after all, straight-8s were eventually made to work). Bore, 6.125, stroke, 5.875, rpm, 7000 redline to keep TBO to no more than once per sortie. In an X-32 layout, 15,000 hp might be possible.

Where does that take us? We still don't have supersonic propellers, and probably won't, at least not with the power levels we can supply. If we can get up to around 15,000 hp, we can probably achieve Tu-95 levels of performance (approaching 500 knots), but we may have to swing a 20-foot counterrotating prop like the Tu-95 to do so. We'll probably have less payload, since our engine will probably weigh more than the 6400-lb turboprops that power that beast.

You want a speed record? Two big X-32s in a fore and aft layout like the Do-335 Pfeil. Shape the fusalage and the wings just right, and maybe she'll survive going supersonic in a dive. Maybe.

Thank you for that long and detailed post.

I would note that while a eight throw crankshaft is certainly not impossible it was very difficult. Most passenger cars with straight 8s were relatively low powered and the power impulses of the rear cylinders could be controlled by a relatively heavy crankshaft and block.

The racing straight 8s of the 20w, 30s, 40s, and early 50s often used two 4 cylinder blocks and sometimes used a central drive. Large gear in the middle of the crankshaft drove a an output drive shaft to one end of the engine where the flywheel and clutch were located.

The Mercedes M196 racing engine of 1953-55 used such a set up and used 10 bearings for it's eight cylinder crankshaft.
The Chrysler V-16 aircraft engine of WW II also used a center gear in the middle of the crank so the engine acted like two V-8s placed end to end to end.
The old engineers figured the added length and weight still came out ahead of a heavier crankshaft and crankcase that would handle the torsional vibrations of a straight 8 or V-16 with the power drive on one end.
Perhaps with modern computer power to figure vibration and modern materials the drive could be placed on one end?
The Mercedes used a built up crank and roller bearings but with modern bearings and lubrication that might not be needed.

In regards to fuel I would note that back in the early 50s Esso offered Mercedes 25 different fuel blends of which Mercedes tested 7. What they choose to use, based off of both power and economy was 45% Benzol, 25% methyl alcohol, 25% 110/130 octane gasoline (possible misprint?), 3 % acetone, 2% nitro-benzene and 0.03% by volume tetra-ethyl lead.

After the big oil crises of the early 70s a lot research went into alternative fuels. I doubt very strongly that some miracle fuel is going to show up at this late date
to replace the hi PN number fuels of the late 40s and early 50s and that is going to limit the BMEP that can be reached to some extent.

Thanks for the detailed post eljefe.

Just to clear some things up about F1 engines.

From 1988 to 1994 the engines were 3.5L unsupercharged engines, maximum 12 cylinders (had V8s, V10s and V12s).
From 1995 to 2001 the engines were 3.0L unsupercharged engines, maximum 12 cylinders (had V8s, V10s and V12s).
From 2002 to 2005 the engines were 3.0L unsupercharged V10s.
From 2006 to 2013 the engines were 2.4L unsupercharged V8s.
From 2014 to the current day the engines have been 1.6L V6 turbocharged hybrids with energy recovery systems - kinetic (eg braking) and heat (power drawn from turbocharger's shaft.

By 2005 the 3.0L V10s were making 900-950hp and revving to just above 19,000rpm.
The 2006 2.4L V8s could rev as much as 20,000rpm in dyno testing. For 2007 the engines were homologated and restricted to 19,000rpm. A few years later the restriction was reduced to 18,000rpm. This left power in the 750hp - 800hp range. By 2013 the limit was 8 engines per season.

The current V6 hybrid turbos are a different breed altogether.

The rpm limit is 15,000rpm, but that is rarely used, with the normal operating speed being between 10,000rpm and 12,000ropm. This is mainly due to the fuel flow restriction, which increases linearly with rpm to 10,500rpm and then remaining constant until redline. This value is 100kg/h.

Two motor/generator units are also added to the engine. One is connected by gears to the crankshaft (known as MGUK). This is limited to 120kW/160hp. This recovers energy during braking, lift and coast and, probably, during acceleration where the tyres are traction limited. A limit of 2MJ of energy can be transferred from this MGU to the battery (directly) and 4MJ from the battery to the MGU.

The other is connected to the turbocharger. Two manufacturers (Mercedes and Honda) have the MGU between turbine and compressor, which are mounted at each end of the engine block. The other two (Ferrari and Renault) have a more conventional turbo layout with the MGU ahead of the turbo, sitting in the vee of teh engine.

This MGU (aka MGUH) can drive the turbo (spool it up) or recover energy from the turbine, controlling the turbo's speed. This energy can be sent to the battery or to the other MGU directly. There are no limits to the MGUH power or energy it recovers.

By sending recovered energy directly from the MGUH to the MGUK the engine becomes, effectively, a turbo-compound.

The compressor runs at a pressure ratio of ~3.5 to 4:1. The engine's static compression ratio was fixed at 18:1 a couple of years ago, so as to limit development, and spending, in that area.

The engines run extremely lean - air to fuel ratios in the region of 1.5 times greater than stoichiometric, maybe as much as 2 times.

The best engines (Ferrari and Mercedes) are approaching 1,000hp (includes power from the battery) in qualifying and have peak thermal efficiency approaching 50%.

The V10s of 2005 had similar power, but were using twice as much fuel.

Oh, and for this season the teams are allowed 3 engines per car for the 21 race season. Each race is approximately 300km and probably as much as that again for practice.

Some have gone over that limit, but others will finish the season without extra units.

as a caution to comparing car/motorcycle engines to aircraft engines I would remind everyone that airplane engines don't have displacement limits.

The Mercedes FI engine in the early post was 2 1/2 liters and put out 290hp at 8,500rpm and had a very respectable (for 1955) 116.2 hp per liter.

However it weighed 451lbs (dry) and needed liquid coolant and a radiator.
In 1956 438lb could get you a Continental O-470 that made 230hp at 2600rpm and used 80 octane gas, not some witches brew.
464lbs would get a Lycoming GO-480 with 295hp at 3400rpm (and that weight includes the reduction gear for the prop) on 100/130 fuel.
Another 34lbs (498lbs) would add a supercharger and get to 340hp at sea level at 3400rpm.
This explains why designers were not trying to use high performance car engines as a basis for an aircraft engine. Those 3 engines were about 7.8-7.9 liters for their 438-498lb weights or under 1lb per cubic in.
These mid 50s aircraft engines did not enjoy the longer life that later Continentals and Lycomings did. This is a generality as each basic model and vast quantities of sub models. For instance the 1956 reference book lists 9 different GO-480 engines with powers of 270 to 290hp.
Lycoming later dropped the gear reduction and built engines with a longer stroke and more displacement that ran at lower rpm and lower piston speed.
An O-540 (same bore as the O-480) weighed just under 400lbs and was good for 290hp on 80/97 octane gas from it's 541 cubic in (8.86 liter) displacement.


AS a further note I have mentioned my 2017 Hyundai which is supposed to have a 2 liter engine that makes 164hp at 6200rpm which is certainly a very good advance on my old TR-3 and TR-250 sports car engines. However the better gas mileage needs a bit of looking at. The old sports cars ran at about 1000rpm for every 20mph of speed so 60mph was 3000rpm and 80mph was 4000rpm, neither of my cars had overdrive. They were cruising pretty much right on the torque peak however so hills or mild acceleration could be handled by simply pushing down on the gas pedal.
The Hyundai has a 6 speed automatic and in high gear goes about 25mph for every 1000rpm. or 2500rpm gives about 75mph which gives very relaxed cruising but means the thing downshifts to maintain speed on a Florida highway hill (overpass) as the cruise rpm is well below the torque peak. granted I am a lot older but I don't think in over 20,000 miles of driving it has ever gone over 5000rpm. Engine should last quite some time as it never gets close to max power (cruise speed of 75mph burns about 3 gallons an hour. so it is making 45hp or less at that speed. I have no idea what it weighs but I doubt if converted to aircraft use it would be rated at anywhere near 160hp.

Aero engines most certainly DO (effectively) have displacement limits.

Bore size is limited by combustion considerations, and stroke is limited by the mechanical limit of the resultant "mean piston speed".

How many of these "units" of acceptable bore/stroke can be accomodated is strictly limited by

1) The aircraft design weight (eg range and general performance), of which the engine is (in the case by example of mk1 spit) 33% of the total unloaded weight.

2) The frontal area, which will be dramatically altered by the choice of Vee, Flat, Radial or In-Line radial (eg Jumo222) layouts.

3) Cooling, experience has shown that anything longer than a twin-row radial is exceptionally difficult to cool properly, this limits displacement.

4) Vibration characteristics, for an in-line engine, anything longer than a V12 will produce very unpleasant torsional vibrations, which DB discovered
with the DB609 (a V16).

As for why designers were not trying to use high performance automotive units as the basis for aero engines, this is also incorrect, as my last post
illustrated with the RED diesel aero-engines all based on Toyota Formula 1 engine technology. Auto-Union was also doing Aero-Engines in WW2,
these did not fail because of any intrinsic problem with the concept. Its rather obvious that Dragster engines are a pointless comparison.

Essentially I dont really understand either of these threads, which were supposed to be about how you would make the best piston engine, but
have decended into a pretty fruitless turbine/piston mud-slinging contest - which is about as useful as arguing about if apples are better than carpets.

This thread could have been very interesting, but has been basically ruined. Why not start a new thread and call it "Pistons or Turbines" ?

eljefe said:

Where does that take us? We still don't have supersonic propellers, and probably won't, at least not with the power levels we can supply.
. Maybe.

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I guess you haven't studied that very well. They did supersonic props in the 50's.

Snowygrouch said:

Aero engines most certainly DO (effectively) have displacement limits.

Bore size is limited by combustion considerations, and stroke is limited by the mechanical limit of the resultant "mean piston speed".

How many of these "units" of acceptable bore/stroke can be accomodated is strictly limited by

1) The aircraft design weight (eg range and general performance), of which the engine is (in the case by example of mk1 spit) 33% of the total unloaded weight.

2) The frontal area, which will be dramatically altered by the choice of Vee, Flat, Radial or In-Line radial (eg Jumo222) layouts.

3) Cooling, experience has shown that anything longer than a twin-row radial is exceptionally difficult to cool properly, this limits displacement.

4) Vibration characteristics, for an in-line engine, anything longer than a V12 will produce very unpleasant torsional vibrations, which DB discovered
with the DB609 (a V16).

As for why designers were not trying to use high performance automotive units as the basis for aero engines, this is also incorrect, as my last post
illustrated with the RED diesel aero-engines all based on Toyota Formula 1 engine technology. Auto-Union was also doing Aero-Engines in WW2,
these did not fail because of any intrinsic problem with the concept. Its rather obvious that Dragster engines are a pointless comparison.

Essentially I dont really understand either of these threads, which were supposed to be about how you would make the best piston engine, but
have decended into a pretty fruitless turbine/piston mud-slinging contest - which is about as useful as arguing about if apples are better than carpets.

This thread could have been very interesting, but has been basically ruined. Why not start a new thread and call it "Pistons or Turbines" ?

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You make some good points. However even in the late 30s the engine designers had options open to them that a formula 1 designer (or motorcycle engine designer) did not.
Aircraft designers (the users) didn't care if the Hispano V-12 was 36 liters vs the Merlins 27 liters or the DB 601s 34 liters. There was no 27 liter or 32 liter or other number limit on displacement for a certain "class" of fighter. The Russians went as far as sticking a 46.66 liter V-12 in the Mig 1 & 3.

If you are building a 750 cc race engine or a 2000 cc race engine for a ground vehicle you have certain limits and to get around them you have certain options.
You can make the engine heavier in order to use more complicated valve mechanisms for instance, or you can use a heavier crankcase and crankshaft to stand up to higher rpm (or both). Sometimes racing rules have a weight limit on the vehicle as whole not on the engine, usually a minimum weight as the few times they tried a maximum weight it didn't work well for restricting speeds or making for competitive racing.

You essentially had two schools of thought. Large slow turning engines or smaller faster turning engines, all weighing close to each other (the big russian engine aside and that forced compromises elsewhere in the design) for a similar output.

Yes maximum cylinder dimensions had an upper limit, at least without going nuts.


The Sunbeam Sikh had a 7.09 in (180 mm) bore and 8.27 in (210 mm) stroke. The engine's total displacement was 3,913 cu in (64.1 L), and it produced 800 hp (597 kW) at 1,400 rpm. The Sikh had a dry weight of 1,952 lb (885 kg).
This is from 1920 and some of the problems with really big engines can be seen (6 valves per cylinder and 4 spark plugs per cylinder).
Gasoline, as you well know but perhaps other posters do not, has pretty much a fixed flame speed (perhaps the wrong term?) so as the cylinder gets bigger in diameter the spark has to timed earlier so that the combustion is pretty much done by the time the crankshaft and piston has gone past 20 degrees top dead center. This is one reason for dual ignition in addition to safety. With flame fronts starting from different places (and please note on large aircraft engines the plugs are usually spaced a distance apart) more of the mixture can be burned in a shorter period of time. This also comes into play in higher rpm engines. The piston goes up and down faster but the fuel mixture burns at the same speed.
Aircraft engines generally are set up with a much narrower rpm band than car engines for number of reasons. One is that some experimental engines aside, the output shaft was either the same speed as the crankshaft or had one fixed gear ratio. Any variation in speed of the aircraft compared to a fixed rpm of the engine was done (from the 1930s on) with a variable pitch propeller and some of those had some restrictions on the amount of pitch change. I may not be explaining that well.
Allisons and Merlins maxed at 3000rpm but could cruise down around 1600rpm. Trying to use a 6000rpm engine and cruise at 2000rpm might exceed the capacity of the propeller to provide a suitable pitch? Or the 6000 rpm engine may not have a wide enough power band to provide sufficient torque at 2000rpm to suit the propeller? Yes you could up the cruising RPM but then you losing some of the advantage that the car/motorcycle enjoy. The ability to use high rpm to make lots of power when needed and the ability to cruise at a much lower rpm when only a fraction of the power is needed and use multi-speed transmissions to suit the engine speed (power band) to the task at hand. My rather plebeian Hyundai Tucson for example turns 5.33 time the rpm in 1st gear than it does in 6th gear per mph, which certainly helps the 2 liter engine get the 3300lb vehicle moving without winding the snot out of the engine. If you take 1st and 6th gear away you have a spread of 2.58 times between 2nd and 5th (which is 1.00 to 1.00) which would make for both harder starting from a standing stop and higher rpm on highway and worse gas mileage.

There have been a number of improvements that could be used on a "new" aircraft engine but trying to use special racing engines to prove a point doesn't really get us anywhere.
A modern ignition system (as in computer controlled with sensors) would probably help quite a bit as the ignition timing could be changed to suit current conditions and prevent detonation in real time while providing the optimum ignition timing for either power or economy with a given fuel.
WW II engines and vast majority of private/light plane engines used fixed ignition timing. Not even vacuum or spring/centrifugal advance.
Same timing for idle, cruise or full power. Some engines could be retarded for starting. Some of the very last commercial or military transport engines had variable timing but it was under the control of the flight engineer who could monitor not only engine temperature but exhaust temperature and adjust timing and mixture accordingly. Fighter and private/light plane engines are generally set up so that worst case real life conditions still fall in the safe zone for detonation which tends to limit both ultimate power and best economy.

Again this is not directed at you Snowygrouch as you understand these things better than I do but a few posters/readers may not.

PWR4360-59B said:

I guess you haven't studied that very well. They did supersonic props in the 50's.

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They didn't work well then and nothing since then suggests they are going to work any better.

Throw in the noise limits in force at any but the most remote government air base and the chances of supersonic propellers showing up at a commercial airport near you are just about zero.

And that is for propellers with the tips going much beyond supersonic. Chances of a plane driven by propellers going supersonic in level flight are also zero.

Shortround6 said:

Chances of a plane driven by propellers going supersonic in level flight are also zero.

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I guess you haven't studied it either. How about Mach 1.2 ? All in the 50's all old school tech if they did it then just think now. Also I guess you haven't heard about the latest engineering that is working on quiet sonic booms for small supersonic business class jets. Yeah this is modern times we are in now, not in the 1700's .

PWR4360-59B said:

I guess you haven't studied it either. How about Mach 1.2 ? All in the 50's all old school tech if they did it then just think now. Also I guess you haven't heard about the latest engineering that is working on quiet sonic booms for small supersonic business class jets. Yeah this is modern times we are not in the 1700's now.

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From Wiki.

"Although The Guinness Book of Records recorded the XF-84H as the fastest propeller-driven aircraft ever built,[21] with a design top speed of 670 mph (1,080 km/h) (Mach 0.9) and 623 mph (1,003 km/h) (Mach 0.83) during tests, this claim has been disputed.[14] The unofficial record speed is also inconsistent with data from the National Museum of the United States Air Force, which gives a top speed of 520 mph (840 km/h) (Mach 0.70), nonetheless, making the XF-84H the fastest single-engine propeller-driven aircraft[11] until 1989 when "Rare Bear", a highly modified Grumman F8F Bearcat reached 528 mph (850 km/h) (Mach 0.71)"

care to list a reference for that Mach 1.2 propdrive aircraft?


As this didn't make it either. It was faster than the Thunderscreech but 540-550mph is still well short of Mach 1.2

"The speed of sound is not a constant, but depends on altitude (or actually the temperature at that altitude). A plane flying Mach 1.0 at sea level is flying about 1225 km/h (661 Knots, 761 mph), a plane flying Mach 1.0 at 30000 ft is flying 1091 km/h (589 knots, 678 mph) etc."

Care to post a link to this 1950s wonderplane.

PWR4360-59B said:

I guess you haven't studied it either. How about Mach 1.2 ? All in the 50's all old school tech if they did it then just think now. Also I guess you haven't heard about the latest engineering that is working on quiet sonic booms for small supersonic business class jets. Yeah this is modern times we are in now, not in the 1700's .

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Let me guess you are talking about the XF-88B? If so you might want to study it a lil more. Sure it had a turboprop up front driving a propellor, but it also had 2 turbojets. That's how it achieved Mach 1.2 in 1953.

DerAdlerIstGelandet said:

Let me guess you are talking about the XF-88B? If so you might want to study it a lil more. Sure it had a turboprop up front driving a propeller, but it also had 2 turbojets. That's how it achieved Mach 1.2 in 1953.

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That would not make sense to say they flew the propeller assisted like that, the other engines were part of the plane and helped it get to altitude for the test flights as well as for safety . If they were wind milling it then there would be no need for an engine to power the prop.
The Fighter Writer: Ron Easley's Aviation Blog: Supersonic Propeller Research: The XF-88B Voodoo

PWR4360-59B said:

That would not make sense to say they flew the propeller assisted like that, the other engines were part of the plane and helped it get to altitude for the test flights as well as for safety . If they were wind milling it then there would be no need for an engine to power the prop.
The Fighter Writer: Ron Easley's Aviation Blog: Supersonic Propeller Research: The XF-88B Voodoo

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They were not windmilling anything. Read your article you posted.

They used the jet engines only to get to test altitude. Then they turned on the turboprop and flew it with all three engines. The turbojets were never shut off.

And your notion that it is all "old school" tech is BS as well. A turbprop is a turbine as well.

Oh that's right, no one else heard about it...