Curiosity question on WW2 piston engine designs (1 Viewer)

[imipak]

I know the Merlin was approaching the fundamental limits of its design, by the time of the Merlin 130 and the RM17SM (which reached 2600hp in branch tests).

Back of the envelope calculations suggest that if you were to build such an engine today using modern materials and modern fuels, you'd probably only squeeze 2800-3000hp if you wanted reliability, and Voodoo (which used an earlier design and upgraded a few things but not everything) topped out at 3100hp under racing conditions.

This puts the Merlin in 1945 as being respectably close to what the design was physically capable of, and respectably powerful for a piston engine even back them.

So, my question: Just how close did WW2 piston engine designs get to their theoretical upper limit?

 

[Sisu]

Much depends on the time period you are looking at. After 1964 some very good minds and well-financed development programs produced Merlins that would live for a while at 3600 and more horsepower. This took Allison connecting rods, custom pistons and such, and much more. We are talking 160 octane fuel, 3600 rpm at 140 inches, and an airport runway available right under you, so far from operational WW2 conditions.So what limits are we talking about, and when?

 

[imipak]

Much depends on the time period you are looking at. After 1964 some very good minds and well-financed development programs produced Merlins that would live for a while at 3600 and more horsepower. This took Allison connecting rods, custom pistons and such, and much more. We are talking 160 octane fuel, 3600 rpm at 140 inches, and an airport runway available right under you, so far from operational WW2 conditions.So what limits are we talking about, and when?

Thanks, that's an excellent question, apologies as this is a little complex

The TLDR version: I'm looking for the absolute upper limit of what they could have built in 1944/45 with the engine design they had but not necessarily the same materials, merely materials that were available.

To me, the absolute theoretical limits of a design are the best that engine design can do, regardless of the materials used. You're going to get declining returns at some point, as you obviously can't get more power out than the fuel has - you'd need to change the design, such as increasing the size of the engine, at some point.

There's then a second maximum, the theoretical maximum you could achieve with the materials available in the first half of the 1940s, assuming metallurgy remained fairly constant over such a short time - an assumption that might not be valid, but is necessary to make it the question can't be answered.

I think I'd be happy with either theoretical maximum (the first tells me how good the design itself is, the second tells me how good the combination of design and materials is), but I'm really aiming more for the 1944/1945 conditions.

 

[Tony Kambic]

Try purchasing this book and reading it. Should have some answers for you: The Secret Horsepower Race: Western Front Fighter Engine Development by Calum Douglas.

 

[pbehn]

I know the Merlin was approaching the fundamental limits of its design, by the time of the Merlin 130 and the RM17SM (which reached 2600hp in branch tests).

Back of the envelope calculations suggest that if you were to build such an engine today using modern materials and modern fuels, you'd probably only squeeze 2800-3000hp if you wanted reliability, and Voodoo (which used an earlier design and upgraded a few things but not everything) topped out at 3100hp under racing conditions.

This puts the Merlin in 1945 as being respectably close to what the design was physically capable of, and respectably powerful for a piston engine even back them.

So, my question: Just how close did WW2 piston engine designs get to their theoretical upper limit?

I would say that the Merlin exceeded its original perceived upper limits by a long way. The status quo in 1935 decreed that for a 2000 BHP engine you needed much more displacement than the Merlin had, hence the Vulture and Sabre. In fact the Griffon made as much power as anyone needed and as far as I know much of the development of the Merlin was not to produce more power, which is the easy part it was to make the Merlin as reliable with extra power enabled by high octane fuels as it was before. The de rated Vulture produced just a little more power than the two stage Merlin/ Packard type fitted to P-51s and Spitfire Mk !X,

 

[MIflyer]

The curious thing about the Merlin was that by 1941 everyone knew it was too small in displacement to power a single engine front line fighter plane. At 1653 cu in it was smaller than the V-1710 and even smaller than the A6M3's 1700 cu in radial. It was almost 200 cu in smaller than the R-1830 and R-1820 that powered inadequate fighters such as the F3F, F4F/Martlet, P-36/Hawk 75, Brewster F2A, P-66. The demand of air combat known by 1941 demanded fighters of more range, better firepower, better altitude performance, better armor protection, the capability to carry bombs, That demanded more power and more power at altitude, and that of course all that made the longer range problem even worse.

The Germans did not start WW2 with engines as puny as the Merlin. The DB601 was 2069 cu in, the DB 603 2715, the Jumo 211 was 2136, and the BMW 801 was 2562. Even the Hisso 12Y was 2196 cu in. Everybody knew that a single engined fighter needed at least 2000 cu in to be at least competitive, and that was the bare minimum.

The USAAF planned to handle this problem by using two V-1710 or larger or the R-2800 or larger, combined with its Secret Weapon: Turbosupercharging. The British thought that Griffon and Centaurus were their answer; they never even bothered something along the lines of a P-38 with two Merlins, like a souped up Whirlwind.

But the Merlin, by adding a two staged supercharger combined with a liquid cooled inter/aftercooler proved everybody wrong. It was the Little Engine That Could Kick Ass.