WI: USA pushes for turbo-compound instead of turbo-compressors?

There was an earlier thread about this, but I didn't wanted it to be killed as a zombie:)

Premise of the thread - instead of developing turbo-chargers, the turbo-compund engines are developed for use in aircraft. The historical engines with integral superchargers (R-1830/2600/2800, V-1710 etc) retain those.
Benefits of turbo-compounds being more power on same setting (RPM, manifold pressure) and same fuel consumed, no intercoolers and no plenty of ducting needed; benefits of turbo-chargers being greater power at really high altitudes. USN/USMC did not use turbo-charged egines in ww2, however, apart from experiments.

What Allied AFs could possibly gain, what would they loose? Possible/feasible Axis reply (if needed)?
 
It's a great idea because you can feed as much power as the turbine can take without melting back into the engine all the time irregardless of altitude and with out any concern of over boosting as could happen with a turbocharger at lower altitudes. With turbochargers to avoid over boosting some exhaust has to be sent out the waste gate at lower altitude. You can't really fully utilize a turbocharger until you are above a certain altitude in the thinner air.

If you use a 2-stage engine driven supercharger ala Rolls Royce Merlin or late model Allison you still can have good high altitude performance while utilizing a turbo-compound.

That's the beauty of the turbo-compound. It's separate from the supercharging and simply recovers some of the wasted exhaust energy and feeds it back into the engine. It's more efficient because it can be fully utilized at any altitude. There is a huge amount of recoverable power in the engines' exhaust gasses and the trick is to recover as much as possible without melting the turbine blades or exhaust valves.

Allison made great progress in developing a turbo-compound V 1710 in 1945 but interest shifted to jet engines. Too bad they didn't twig to the idea in 1940.

www.enginehistory.org/Convention/2009/Presentations/SuperchargingAllison.pdf

Follow this link for more info about Allison's efforts. Figure 1 shows illustrates the huge amount of potential energy piston engines lost through the exhaust gasses.
 
Thank you for the link, in the name of gearheads, or at least the ones that feel that way :)
Looks like a 20-25% gain, vs. same but non-TC engine. The bombers/attackers look to me as most benefitting users, plus of course transports and all kinds of fighters. A time line might look this way:
- XP-38 might look as a 'classic' twin, like Hornet or Fw 187, but with turbine at the back of the nacele, akin to the Moonbat? Kesely is not lulled vs. possible carb icing, so the XP-38 actually makes it safely on the other side of the US, so it can be tested. No problems with intercoolers in P-38. Even more range/radius, leading edge can house fuel from day one. RAF is satisfied with their ATL 'Lightnings'?
- A-20 gets it too (instead of historical turbocharger). 2 x 1900+ HP for take off, instead of 1600, makes 360-370 mph at 13000 ft
- P-39 that actually can do 400 mph
- C-47 that carries more or at longer range
- B-26, early model but with 2 x 2200 HP, outpaces Zero, hits a carrier during Midway, Yorktown survives?
- P-51 that does 410 mph in 1942?
- P-70 (A-20 modified for night fighting) actually works?
- a really big one: earlier closing of the Atlantic gap due to better mileage ?
 
Not every aircraft could benefit from a turbo-compound engine. There has to be space found in the airframe to install the turbine and where a mechanical linkage from the turbine to the engine can be fitted. As well as the piping for the exhaust and a discharge port for the turbine.

Much easier to install in larger multi-engine aircraft. But a few fighter designs could utilize them without too much difficulty. As the US was able to produce 10s of thousands of turbochargers in WW2 the same technology would have allowed a similar production of the same kind of turbines to be used in a turbo-compound installation.

Because of a shortage of alloys I don't think there would have been much more the Germans could have done to counter somewhat faster and longer ranged Allied aircraft other than trying to push the development of their jet aircraft.

On the other hand would an earlier concentration of effort on turbines have accelerated Allied jet engine development?
 
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Don't forget, in the '50s Airliner mechanics nicknamed the Power Recovery Turbine (PRT), as Parts Recovery Turbine as for the increase in bits breaking.

While increasing efficiency, also increased heating and backpressure.


Now with heavy turbocharging and Recovery sections, you are far off from a jet engine, just one that is using pistons for the main compressor section
 
When we look at 5-6 big countries with developed industry in 1930s/40s, the jet engine was more a thing whether it dawned it on someone or not. Granted, jets will need a serious amount of resources and time to make for practical powerplants.
BTW - turbos themselves were pretty light items.

As for the reliability issues - a long development (10-15 years before ww2) should've adressed the most urgent things. We can also recall that R-3350 was not that a reliable engine in it's 1st years, thing that it shared with many other engines that pushed boundaries (BMW 801, Sabre, Homare, VK-107, ...). Pushing it from 2200 HP to 3500 in a decade probably had something to do with the problems, and the engines derated to 2880 HP were trouble free, with excellent engine life, and some are still flying today in that form.

Re. fighters with TC engine - yes, a clean sheet of paper design would be a good approach, as it was the case with OTL P-38/39/47. Hmm - a P-47 with 2500 HP in late 1942?
 
I'd be very twitchy about wider use of turbo-compounding, certainly in combat aircraft - what you gain in efficiency you lose in power to weight ratio. That's just fine in some applications (airliners, maritime patrol) but in most cases for combat you're better off with a turbocharger or just plain supercharger. Remember that you actually get quite a lot of jet thrust from the exhaust gases which you're losing by turbo-compounding - from memory towards the end of the war this was something like 30% of a fighter aircraft's effective horsepower.
The other issue is mechanical complexity - because you've got the extra mechanical links to the engine, there are that many more things to go wrong. That's ultimately what killed the likes of the Nomad - turboprops with much better power to weight ratio and with a much simpler design, even if a big more fuel hungry, were coming down the line and replaced the turbo-compounds. Big piston engines are horrifically complex and painful to maintain compared with a turboprop.
 
I'm afraid that Nomad is too late for this topic - any piston engine is bound to loose vs. a half-decent turbo-prop.
The weight penalty for the TC is no greater than it is for turbo-charging, while it enables much greater take-off power and power at low and medium altitudes. Absence of good deal of ducting means a more compact aircraft. It can do well without intercoolers, that is again god for keping the size within limits, and means less drag. Lower consumption allows for greater range, or the aircraft can have less fuel aboard that cancels any weight increase due to th TC unit installed. Exhaust thrust amounted to 10-15% of additional thrust, in well executed installations and depending on altitude. In case flame dampers are instaled (night fighters and bombers), the exhaust thrust was lower. It was still lower with engines employing collector exhausts, like it was the case with many radial engines' installations.
 
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