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Yes, You are right. Including the note "It is different running an engine in a test cell than it is on an airplane"/It is different running an engine in a test cell than it is on an airplane, but doing these tests establishes reliability numbers for the engine and accessories as well as allowing the manufacturer to establish overhaul criteria.
Yes, You are right. Including the note "It is different running an engine in a test cell than it is on an airplane"/
From Pratt and Whitney R-2800 Double Wasp - USA :
" Perhaps the most outstanding example was the great R-2800 Double Wasp, which went into production in 1940 for the B-26 Marauder at 1,850 hp and by 1944 was in service in late model P-47 Thunderbolts (and other aircraft) at a rating of 2,800 (experimental) hp on 115-grade fuel with water injection. Of course, all engines naturally grow in power with development, but a major war demands the utmost performance from engines fitted to aircraft, whose life in front-line service was unlikely to exceed 50 hours' flying time over a period of only a month or two.
In peace time, the call was for reliability over a period of perhaps a dozen years. "
When you mess with R-2800, you mess with me
When we calculate in the cooling system, weight difference is negligible. We could talk about drag, yet P-47M -N were every bit as fast as anything powered by piston engine.
Then we could talk about resilience to battle damage, and radials come 1st there.
As for development speed and money invested, it was liquid-cooled/inline engines that were suffering big-time setbacks (Jumo-222 and a plethora of US engines - Chrysler and big Allisons for example).
Soif now we put aside details and look at general ways - we have three ways to increase power output of the aero-engine :
1) to give more boost (this connected with a) super- and turbocharger construction, b) quality of fuel and c) strength of engine details)
2) to increase compression ratio (connected with quality of fuel and strength of engine details)
3) to increase RPM.
So You'll see that for inlines to increase power output engeneers can use three ways but for radials - only two.
What is the reason? The main reason - Valvetrain.
It lead to two limitations :1) Limited engine speeds or RPM — OHV engines have more valvetrain moving parts, thus more valvetrain inertia and mass, as a result they suffer more easily from valve "float", and may exhibit a tendency for the pushrods, if improperly designed, to flex or snap at high engine speeds. Therefore, OHV engine designs cannot revolve ("rev") at engine speeds as high as OHC
2) Limited cylinder head design flexibility — overhead camshaft (OHC) engines benefit substantially from the ability to use multiple valves per cylinder, as well as much greater freedom of component placement, and intake and exhaust port geometry. Most modern OHV engines have two valves per cylinder, while many OHC engines can have three, four or even five valves per cylinder to achieve greater power. Though multi-valve OHV engines exist, their use is somewhat limited due to their complexity and is mostly restricted to low and medium speed diesel engines. In OHV engines, the size and shape of the intake ports as well as the position of the valves are limited by the pushrods
Now about first exception. It is such a radial as Bristol Hercules. It was forced to 1725 HP at Hercules VII, and its RPM was of 2900.
But - it has sleeve valves not pushroads and rocker arms!
REMEMBER -- NO engine is EVER going to run more than 80 to 150 hours before a major overhaul, so testing them for 3000 hours IS STUPID.
What about test of R-2800-63 (P-47 engine) run at WEP (2,700rpm / 2,600hp) for 7-1/2 hours when it wasn't to be used for more than 5 minutes.
...
You forget the calculate weight for wider cowling for a radial, its cowl shutter and its mechanization, for P-47 weight of turbo-charger and its tubes. So...
The inlines had longer cowling, so that evens things out. Radiators also had cowl shutter mechanization, again to even the weight.
P-47 put extra weight to a goo use, so nothing to put radials to the disadvantage there.
And, you know, cooling system of one aircraft differes from cooling system of another one. As you remerber, for instance, F4U-1 had an interesting system. Intakes for its intercoler and oil cooler were mounted in the roots of wings. And we have some extentions of their pipesto the engine in that aircraft.
Yes, I know
Crysler and Allison never had such goverment funding.
What about resilience to battle damage - I'm not going to argue this argument, but l suppose that degree of superiority of radials over inline engines to some degree overstated.
With one important system less to be hit, damaged, destroyed, radials had were in advantage.
Volume of oil system of large radials is as a rule greater than of inlines. one of the reasons - at large radials oil system played a role of cooling subsystem. And this role of oil system for radials is greater than for inlines.
Look at pipe net on R-2800 - http://www.aviation-history.com/engines/pr-2800-2.jpg
And you know, what happened to an engine if its oil system is damaged.
Hmmm...Engine stops?
# 2 is out. For any given fuel, say 100 octane, any ONE engine is going to have a limit on the allowable compression ratio/ boost combination it can use. Different engines have differnt limits on the same fuel. Going back to our "test" engine and the 100 ocatne fuel, if you raise the compression you have to lower the boost limit. The result is better fuel economy but less max power. On an existing engine you might be able to LOWER the compresion ratio, accept the worse fuel economy and raise the boost to get more power than the original engine.
Improving the fuel will let you raise one or the other or a little bit of both but higher boost will always give you more power than raising the compression.
Nope. The reason was all those cylinders acting on one crankpin. Or to put it another way, trying to increase revolutions on the big master rod and 6/8 link rods rapidly overloaded the Bearing. Forces/loads acting on the bearing go up with SQUARE of the engine speed.
The commonly quoted piston speed specification is not really related to piston ring failure but gives an idea of the loads on the crankshaft bearings. There is a formula for corrected piston speed that helps take into account piston weight. Under square engines get a slight reduction in piston speed while over square (large bore) engines get an increase in the rating.
V-12 engines, in general, Spread the load out over more bearings than a radial but had problems of their own. They are however, much more capable of higher rpm operation than a radial. They are also heavier than a radial of equel displacement.
All the Bristol poppet valve radial engines used 4 valves per cylinder. The Jupiter was licensed to around 17 countries? All WW II Mercury and Pegasus engines use 4 valves per cylinder.
Some overhead cam engines used truely terriable intake and exhaust ports and passages, see the Hispano V-12s
It also has a piston speed of 3,142fpm which is rather high for an aircraft engine. Hercules engines from the 1950s were pushed to around 2100hp but they kept the same rpm even though the the crankcase was redesigned with larger roller bearings to handle the increased loads.
A P&W R-2800 has a piston speed of 2800fps at 2800rpm.
the R-3350 has a piston speed of 3050.8 at 2900rpm.
THe old Pegasus had one of the highest at 3250fps at 2600rpm but then few other engines used a 7.5in (190mm) stroke. One that did was the Russian AM-38 engine used in the IL-2 but then they didn't rev that engine very high did they, inspite of the overhead cam and 4 valves per cylinder.
Small radials that reved high were the Bristol Taurus and the Gnome-Rhone "M" series. The Taurus hit 3100rpm, in part due to it's short stroke (143mm) which held the piston speed to 2,906. The Gnome-Rhone 14M hit 3030rpm but again it's short stroke held piston speed to a mere 2258fpm.
The Napier Sabre engine, a real Champ when it comes to RPM at 3850 kept it's piston speed down to 3,048fps due the 120mm stroke.
I will conclude this by noting that the Merlin had a piston speed of 3000fps and the Griffon had a piston speed of 3025fps.
It's true just for an engine improved up to the limits of its design. But almost all the late-war miltary aircraft engines (especially for fighters) didn't reach their limits (at least to the end of war, some of them were upgraded during after-war years), IMHO - the era of turbo-jets started earlier of that event.
Look at a couple of examples. First - BMW-801. BMW-801D has higher boost and higher compression, and has a higher power output and lower fuel consamption on the power regime than BMW-801C. But it's not a pure example because it consumed C3 fuel instead of B4 (for BMW-801C).
But we have a pure example - Homare 23.
Both Homare 23 mod.12 and mod.21 consumed fuel with octine number "92" (with ADI). But for mod.12 compression ratio was 7.0, boost - 45.7", max. power output - 1840 HP; for mod.21 compression ratio - 8.0, boost - 49.6", power ouput max. - 2050 HP (according to TAIC). Figures of fuel consumption is unknown to me.
It's rather interesting point of veiw. So - you are sure, that the problem called "floating of valves" is fabricated, aren't You? And tuning of input-exhaust cycle (I'm not sure that use a proper term, but I hope you guess what I mean) is matter of unimportance, isn't it?.
It's rahter interesting but as most of examles you took designs of one company - Bristol which are not tipical at all in design (if we talk about radials). And engineers from Bristol took part in development of Napier Sabre, as you know - they took care of improving its valvetrain
Moreover to use a term "piston speed" I suppose for sleeve-valve system is not completelly correct. Maybe "speed of opening\closing valve apertures?
If to speak about classical radials we should take R-2800 or R-3350, I suppose. With their one intake and one exaust valves.