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What if Allison had developed two different displacement inline engines: The V-1710 and a V-2318 or V-2746?
(28 liters, 38L, 45L)
Assuming both engines used the same design technology and were available at the same time, how might have events unfolded differently with the larger engine available?
There is still the issue of the power section, I can't think of a clear reason why the emergence of a bigger Allison unit would have circumvented the problem. As SR pointed out, it would be heavier but with no appreciable advantage in altitude capability; with the inherent extra weight and size in the airframe needed to carry it, we're left with a low-altitude, heavy bird.The German philosophy (for inline engines) favored larger displacement, and they had excellent success doing so.
The US UK favored smaller displacement.
What if Allison were larger displacement as with German engines?
Rather than argue that it could not have happened, imagine if it had happened and what uses could have been made of the benefits and how events might have unfolded differently
Fluid drive supercharger coupling in factA larger displacement Allison wouldn't have needed a 2-stage supercharger to achieve good high altitude performance - the DB engines did just fine w/single stage supercharging.
The German philosophy (for inline engines) favored larger displacement, and they had excellent success doing so.
The US UK favored smaller displacement.
What if Allison were larger displacement as with German engines?
Rather than argue that it could not have happened, imagine if it had happened and what uses could have been made of the benefits and how events might have unfolded differently.
Also, V-3420 is a bit more radical than I had in mind.
Think DB-601, DB-603, etc.
A larger displacement Allison wouldn't have needed a 2-stage supercharger to achieve good high altitude performance - the DB engines did just fine w/single stage supercharging.
Germans used a different design philosophy.
An engines horsepower is directly related to how much air it can move through the engine per minute.
1. In general this is governed by the size (displacement) of the engine. How much air is moved each revolution.
2. the number of revolutions per minute.
3. the volumetric efficiency of the engine. this covers all the fiddly stuff like valve sizes. How effeicnt the engine is at getting the air in and out. Supercharged engines can have volumetric efficiencies of over 100%.
This where the German (and others) design philosophy differed from the British and American. The Germans went for larger displacement but slower turning engines. The slower RPMs meant lower stresses on the crankshaft, rods, pistons and crankcase which meant that they could be lighter. The Larger displacement DB 601 and Jumo 211 didn't weigh much different than the Merlin or Allison. But then a 34-35 liter engine (20% larger) turning 2400rpm (20%less) doesn't move any more air than a 27-28 liter engine turning 3000 rpm if the intake manifold pressures are about equal.
Another difference was the compression ratios used. The higher compression ratio the more power you can get from a given amount of fuel. BUT, the higher the compression less boost you can use before hitting the detonation limit. Less boost=less total power. An engine running 45in (or roughly 7.5lbs boost) will make 50% more power than an engine running atmospheric pressure in the intake manifold. Or an engine running 44 in will make 10% more power than one running 40in. everything else being equal.
Germans also went for fuel injection. This had a number of advantages and a few disadvantages. For now let us say that it gave better fuel economy and perhaps allowed a slightly lower grade of fuel to be used. It also meant that the fuel was not evaporating in the supercharger and cooling the intake charge. This lack of cooling effect also means that not quite as much boost can be used for a given grade of fuel. Sort of cancels out the the lower grade advantage mentioned earlier.
One advantage the Germans did have was that because their engines didn't use as much boost at sea level their superchargers were able to maintain the boost they were using to a higher altitude. Say, for argument's sake both superchargers could deliver a pressure ratio of 2.3 to 1 at a reasonable efficiency. If the British or American engine used more boost at sea level (say 1.5 at.) than the German (say 1.3 at.) then the German supercharger could delivery 1.3 at. higher than the Allied supercharger could deliver 1.5at.
Now what part/s of the German philosophy is Allison supposed to use in this what if?
Designing a 38 liter engine?
Bigger cylinders means less rpm, it is rather dependent on stroke. Few engines went over 3000fpm on piston speed. Allison V-1710 is right on the limit. Larger diameter means the cylinders are harder to cool. more volume (fuel burning) per unit of cylinder all area. The bigger engine will make more power but not the same power per unit of displacement. Peak RPM will help dictated engine weight (strength of parts). Those French 860-930HP 36 liter Hispano engines only weighed 1085lbs.
Larger cylinders means a larger engine. Will it fit in existing planes or not? British went to a lot of trouble to get the Griffon to fit were Merlins did.
Another thing affecting weight is the pressure in the cylinders. low boost means lower pressures. Lower pressures means lighter construction.
The large displacement, slow turning engine with a low amount of supercharge may allow the engine to carry it's rated HP to a higher altitude than the smaller more highly boosted engine but that may be one of it's few advantages.
SR - that was a really great summary of all the issues