What if: Larger displacement V-1710?

[gjs238]

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?

 

[Shortround6]

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?

well. they were playing with the V-3420 at the Army's request but it went nowhere.

Allison V-3420 - Wikipedia, the free encyclopedia

1.To have the engine available in 1941 work would have had to have been started in 1935-36. Allison simple didn't have the engineering staff to any more than they were doing. Allison refused an Army request to develop fuel injection in the mid thirties because of a lack of man power. They also went with the side by side V-3420 because it was easier and would use more common parts than building an X type engine ( 4 pistons acting on one crank throw) that the Army originally wanted.

2. Who was going to pay for it? By the spring of 1939 GM had already floated over 1/2 million dollars to Allison to keep it going and the Army owed Allison over 900,000 dollars for work already done. Army got out of paying the money by swapping the debt for permission to export Allisons to France and England.

3. The only common parts are going to be nuts, bolts and some accessories like Magnetos, and vacuum pumps.

4. The larger displacement engines are going to turn fewer rpm and since a larger cylinder is harder to cool they aren't going to be able to use quite the same level of boost. They will make more power but not quite the same power per cu in of displacement as the V-1710.

5. The will weigh more.

What events were you thinking of?

And if you want a really big V-12 you might try to get Packard to modernize the V-2500 that they were making for PT-boats

 

[gjs238]

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.

 

[Colin1]

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

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.

Events may have unfolded something like this

i. the USAAC still doesn't have a single-engined fighter candidate for the ETO
ii. the new powerplant turbocharged in the P-38 would have been interesting, though not necessarily better
iii. the USAAC may well have found its very own Typhoon in any new bird equipped with the powerplant, hopefully without the Sabre's frailties, insofar as it is a heavy, low-altitude hot-rod of an unspecified performance
iv. outside possibility of repercussions for Republic Aviation, with the P-47A axed owing to the V-1710 being unable to haul the newly-requiremented fighter around the sky, the new powerplant may have been given the call

The USAAC Typhoon is an outside shot, a 2,000hp inline contemporaneous with the V-1710 is stretching it a bit

 

[gjs238]

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.

 

[Colin1]

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.

Fluid drive supercharger coupling in fact
If the V-1710 didn't have it, why do you think your larger displacement Allison would have?

 

[Shortround6]

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.

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.

 

[Shortround6]

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.

Not really. While they did do better than the single speed Allisons they didn't do better than most two stage engines. Merlins with their post Hooker superchargers would give the DB engines quite a run with even single stage 2 speed superchargers.

and you have to look at what they gave up to get it.

Allisons were quite capable of giving 1400-1500hp at WEP settings at low altitude. It was done by simply adjusting the allowable boost and using a fuel that would allow it. Perhaps a bit of extra enrichment of the mixture to act as a primitive ADI.

The Early German engines didn't seem to have this amount of stretch. In part because of their higher compression which does help altitude performance a bit but as mentioned above, limits the amount of boost that can be used below the critical hight.
As an example, some people have claimed that if the Allison had lowered it's compression ratio from 6.67:1 to 6.0:1 like the Merlin it could have used enough more boost to make 10% more power.
Allison did lower the compression of the last models built.

 

[drgondog]

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

 

[Shortround6]

SR - that was a really great summary of all the issues

Thank you.

I am not saying which was better

But they were different and many of the differences were interrelated so you can't quite pick and choose certain features as I am sure that you are well aware of.

 

[drgondog]

As you noted directly and indirectly - Everything is a compromise with selection for the broadest range of positive attributes...

Allison had some really good high altitude performance in the -117 range and would have been a nice engine in the P-51/51A had it been around in 1942.. but then the J-47 would have been nice for the P-80 when it first rolled out - lol.

 

[gjs238]

Much criticism of the Allison centers on the supercharger, although it could also be argued that the supercharger was adequate, and that if Allison had used design philosophies more akin to the Germans, the engine may have performed well at altitude relative to its contemporaries.
If indeed this had occurred, we would likely be having radically different discussions on this board.

 

[Shortround6]

Some of it wasn't up to Allison, They were making what the Army (and in the beginning) Navy told them to make.

Army was also obsessed with the hyper concept. They wanted engines that would produce 1HP per cubic inch (61 HP per liter) and were paying for R&D contracts in the early-mid thirties. In fact in some cases the actual engineering was done by the Army at Wright field and companies (like Continental) merely manufactured test cylinders to Army Specifications.

Please note that the Early DB-601s were no great shakes at altitude either. The very early ones using a single speed supercharger. The Hydraulic drive was a fancy way of getting a variable drive that had some teething troubles of it's own and offered a variation from about a 7:1 ratio to a 10:1 ratio.

Also note that the critical altitudes for the 601 engines went from 3700 meters for the A-O to 4900 meters for the N. The big disparity over the Allison doesn't show up until the 605A is put into the field.

Several things different could have been done to the Allison engine.

#1, and easiest in hindsight, beef up the supercharger drive to handle the 9.60 gear ratio sooner. This with some other modifications would have allowed for 1125Hp at about 4700 meters. Of course this does limit the low altitude power by about 100-125hp over what could be done with the 8.80 gears and might alter the ability of the p-40 to handle itself below 10,000ft?

#2, fit a two speed mechanical drive and a larger supercharger, a one off prototype was ordered in Oct 1941 and delivered (late) in April 1942. This could have pushed critical altitude at 1150hp to 4850 meters while increasing take-off and low level power.

#3, Fit a much improved, totally new supercharger similar to the one on a Merlin 46/47. This might have allowed 1100hp or a bit more at 22-23000ft. But it also would have limited low altitude HP to 1100-1200hp even at WER settings without a two speed drive.

You never get something for nothing. The DB 601s could not match the Allisons in low level performance and even the early 605s seem to be hard pressed to match it's WER ratings below 5,000ft.

Edit> The DB 601 E seems to be the first one that really showed a big advantage over the Allison, not the 605 although the 605 used most of the tricks used on the 601E and held onto or increased the margin over the Allison at altitude.

See: http://www.enginehistory.org/German/daimler-benz.htm

But please note that this large overlap cam timing is best used on an engine using direct fuel injection.

There is only plain air in the intake passages when the inlet valve opens. The DB 600 series was immune to backfires once fuel injection was adopted. American and British engines using regular carburettors or fuel injection carburettors had intake passages full of explosive mixture and suffered from back fires when every thing was running even almost right. Opening the intake valve that much earlier is just inviting backfires. Bad backfires can destroy intake manifolds, wreck superchargers and even blow the intake duct leading TO the carburettor apart.

Also please note that the DB 601E was on about the 4th different supercharger from the start of the 600 series. Talking about impeller design and not drive system.