Highest RPM of any WWII Piston Engine
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wells
Airman
Napier Sabre, 3700 or 3800 rpm
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Like the highest speed achieved with a piston or the speed the piston moves up and down?
Regardless I'm looking at some overboost figures that seem to indicate 4000 rpm, have they ever exceed that particular speed using some form of wartime power setting?
It is stating the obvious but often overlooked. In every revolution the piston comes to rest at TDC and BDC. between that the longer the stroke and the higher the RPM results in higher piston speeds. The problem of producing power comes when the max and or mean piston speed is faster than the flame front in the explosion/ combustion. RPM in itself means little, I have ridden small cc Honda fours which revved freely to 16,000, they just didn't make much power doing it.
wuzak
Captain
I believe the Dagger got up to 4,200rpm.
AMCKen
Senior Airman
Wright R-2160 Tornado ran pretty fast too. It helps that it had a rather short 3 5/8 inch stroke. 
"2,350 hp (1,752 kW) at 4,150 rpm for takeoff "
Wright R-2160 Tornado - Wikipedia
"2,350 hp (1,752 kW) at 4,150 rpm for takeoff "
Wright R-2160 Tornado - Wikipedia
AMCKen
Senior Airman
Their Rapier was rated at 340hp at 4,000 rpm.
Something to consider is that propellers usually like one speed and engines another. Up to a point, that encourages the designer to lengthen stroke rather than increase RPM, since power = piston speed X brake mean effective pressure and for the same RPM, the longer the stroke, the higher the piston speed.
But the specific horsepower (hp/weight) of a long-stroke engine is lower than the specific horsepower of an engine with the same piston speed and a shorter stroke (and higher rpms). At some point, that advantage becomes irresistable, and the designer signs off on a costly (and heavy) gearbox to match the ideal motor rpm with the ideal propeller rpm.
Mostly that's a moot point for late-WWII piston engines, as gearboxes became more common. But it certainly was a factor in the evolution of the aviation engine, and also of the propeller, as technology jumped over the "gearbox" hump.
The highest piston speed of any engine today is in normally aspirated "mountain motors" for drag racing, large automotive V8s of about 1,000 cu. in. that spin up to about 10,000 rpm during a 6-second pass. Had the turbine engine never been invented, we might have gotten to similar piston speeds in prototype military engines by the early 1960s.
But the specific horsepower (hp/weight) of a long-stroke engine is lower than the specific horsepower of an engine with the same piston speed and a shorter stroke (and higher rpms). At some point, that advantage becomes irresistable, and the designer signs off on a costly (and heavy) gearbox to match the ideal motor rpm with the ideal propeller rpm.
Mostly that's a moot point for late-WWII piston engines, as gearboxes became more common. But it certainly was a factor in the evolution of the aviation engine, and also of the propeller, as technology jumped over the "gearbox" hump.
The highest piston speed of any engine today is in normally aspirated "mountain motors" for drag racing, large automotive V8s of about 1,000 cu. in. that spin up to about 10,000 rpm during a 6-second pass. Had the turbine engine never been invented, we might have gotten to similar piston speeds in prototype military engines by the early 1960s.
Great post, however isn't drag racing a niche that very high piston speeds can fill? Who ever proposed an engine that only had to run for 6 seconds. I know that is experimental frequently becomes the norm but even at that low threshold many do not last for the whole six seconds I believe.
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wuzak
Captain
Powerful military and civil airline engines had long since gone to gear reduction systems by the time WW2 started.
Even with fixed pitch wooden props it was desirable to use reduction gearing.
That said, an alternative power plant proposal for the Me 264 was a steam turbine of 6,000hp. Two propeller systems were proposed - one would use a reduction gear and be large in diameter. The other would be smaller diameter and run directly off the turbine (6,000rpm). I'm sure that similar systems were considered for piston aero engines.
The other development before and during WW2 was for 2 speed reduction gear systems, though I'm not sure any ever flew.
wuzak
Captain
Hmmm I might look into piston speeds. The radials piston/rods don't seem to care about been attached to a radials crank but the forces inside with that enormous counter weight flinging around seem scary!
The R2800 "C" engines ran up to 2800 RPM. However the R2800, though very forgiving of over boost, did not feel kindly toward over RPM for any period. Some of thins may be due to stresses on the master rod, whereas an inline engine distributes these stresses over the length of a crankshaft. Indeed large amounts of power can be extracted from a bunch of small cylinders going zzzzzzzzz at high RPM. For production, reliability and repair fewer parts, fewer cylinders make sense. Gear reduction, present on most high HP piston aircraft engines continues today in turboprops in which even higher ratios are necessary. However even the ratios possible at the time required serious machine capacity in mfgr. In wartime ship construction steam piston are diesel or turbo electric drive was often used though less optimal due to gear cutting being a production bottleneck.
As was pointed out earlier, successful engines such as the R2800 (still in commercial use) used large pistons (155 cu inch each) and the speed of the flame front put a definite limit on the speed at which each piston could act.
Despite all the experimentation in deriving more HP/cu inch the old saw about there being no substitute for Cu inches held true in the long run.
Interesting stuff!
As was pointed out earlier, successful engines such as the R2800 (still in commercial use) used large pistons (155 cu inch each) and the speed of the flame front put a definite limit on the speed at which each piston could act.
Despite all the experimentation in deriving more HP/cu inch the old saw about there being no substitute for Cu inches held true in the long run.
Interesting stuff!
Elvis
Chief Master Sergeant
...and the Rolls Griffon would have a higher piston speed, due to it's .600" longer stroke, and because I couldn't resist...P&W - 1350 ft./min. Rolls - 1512.5 ft./min.
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Deleted member 68059
Staff Sergeant
- 1,058
- Dec 28, 2015
High mean piston speeds are not a very intelligent thing alone, what is intelligent is high crankshaft speeds with as short a stroke as possible to enable very high power outout with managable mechanical stresses. Mean piston speed is an irrelevance to power production, the power goes out through the crank, crank speed going up whilst maintaining torque is what good practise is, thats the maths ! Power (Watts) = Torque (N.m) x Crank Speed (6.28 x RPM/60)
High mean piston speed just means you are running far too high a stroke for the target engine speed, which is usually what happens when you run out of power and have no choice left but to raise the crankspeed on an old design.
Mean piston speed is just a reflection on the mechanical stresses which will be imposed, someone posted something saying it had relevance to flame propogation and therefore power production- which is sadly, total nonsense because burn rate goes up basically in line with crank speed because burn rate is related to turbulence, which goes up as crank speed does because port gas velocity goes up with revs, because you are moving through the same volume of air down the same diameter pipe in less time!
(this is not quite the case with diesels because you run into difficulties to vapourise and burn the heavy involatile fuel in time to burn it all as revs go up, which is why diesels dont rev, you`ll see forums all over the internet saying its because diesels are long stroke and so reving them makes the mean piston speed too high, but thats putting the cart before the horse....)
High mean piston speed just means you are running far too high a stroke for the target engine speed, which is usually what happens when you run out of power and have no choice left but to raise the crankspeed on an old design.
Mean piston speed is just a reflection on the mechanical stresses which will be imposed, someone posted something saying it had relevance to flame propogation and therefore power production- which is sadly, total nonsense because burn rate goes up basically in line with crank speed because burn rate is related to turbulence, which goes up as crank speed does because port gas velocity goes up with revs, because you are moving through the same volume of air down the same diameter pipe in less time!
(this is not quite the case with diesels because you run into difficulties to vapourise and burn the heavy involatile fuel in time to burn it all as revs go up, which is why diesels dont rev, you`ll see forums all over the internet saying its because diesels are long stroke and so reving them makes the mean piston speed too high, but thats putting the cart before the horse....)
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I knew you would bring some sense to the thread, but as I understand it the maximum crank speed and maximum piston speed are (obviously) connected.
Deleted member 68059
Staff Sergeant
- 1,058
- Dec 28, 2015
They key take-away is that crank speed and piston speed are related but NOT fixed across engines, because they are totally dependant on the stroke. Therefore a very high crankshaft speed can be achieved without a particuarly high mean piston speed by virtue of running a short stroke (this makes all sorts of other horrible things happen which are expensive to get right, because the valves then spend more time being close to banging into the piston crown....). Instantanious piston speed is basically the <Stroke/2 x Crank Speed> (its actually a horrible big formula but thats the basic working of it).
