Turbo Compound Engines

[Clay_Allison]

What obstacles prevented these from being developed earlier? It seems like this modification added to many existing engines (like the Allison) would have pushed it back into contention with the Merlin and 605. It's an amazing thing to me, a little turbine that gives free power by eating exhaust gas.

(Turbo Compound) it won't let me edit the title.

 

[Pugnator]

Turbocharger needs perfect balancing, good oil, technology to make shaft and wheels.... it revs to 100-150000RPMs... and i think it'd cost too much for mass production. Sorry for my English

 

[red admiral]

The exhaust from the cylinder is rather hot (600°C or so) so high temperature alloys are needed. Metals with performance at higher temperatures were available for gas turbines from 1940/41 but jet turbines are significantly larger. This turbine would have to be smaller from the lower mass flow which pretty much means higher rpm and greater stress in the blades.

The only wartime turbocompound I'm aware of is Allison's V-1710-127 which had an additional 900hp or so over the conventional versions but didn't work satisfactorily as the temperatures were too high for the turbine blades.

 

[syscom3]

The main problem is the additional increases in engine complexity and associated weight.

 

[HoHun]

Hi Clay,

>It seems like this modification added to many existing engines (like the Allison) would have pushed it back into contention with the Merlin and 605. It's an amazing thing to me, a little turbine that gives free power by eating exhaust gas.

"Free" is not entirely correct as the power generation is done at the expense of the exhaust thrust.

For fighters, power recovery turbines were not attractive due to this ... it was simpler to use the exhaust thrust directly.

Power recovery turbines were really good at improving efficiency, but not so good at improving performance.

Turbo-supercharging really harnessed the exhaust power in a similar manner as the power recovery turbines did, except that it avoided transmitting power to the engine crankshaft, thus reducing the complexity of the installation. It did free the crankshaft of the work to run a (big) supercharger so it gave more or less the same effect as a recovery turbine installation at full power, but in situations were the waste gate had to be opened, it was less efficient than recovery turbines.

I guess power recovery turbines mainly made sense for commercial and military long-range aircraft as they helped to descrease specific fuel consumption, obviously more so than turbo-superchargers (or they wouldn't have been used at all

Regards,

Henning (HoHun)

 

[Venganza]

I guess power recovery turbines mainly made sense for commercial and military long-range aircraft as they helped to descrease specific fuel consumption, obviously more so than turbo-superchargers (or they wouldn't have been used at all

Regards,

Henning (HoHun)

Looking at the postwar use of the turbo-compounds, and I'm thinking of the Curtiss-Wright R-3350 Turbo-Compound, they do seem to have been used on long-range aircraft. Think of the Argus, the Neptune, and the Constellation for starters (this is just off the top of my head - I'm sure if I did some research, I could come up with a much longer list). An interesting postwar engine was the experimental Napier Nomad, which would've been fitted to the Shackleton MR.4, another long-range airplane. I believe this engine used the exhaust gasses to run what was basically a small auxiliary jet engine.

Venganza

 

[cherry blossom]

I guess power recovery turbines mainly made sense for commercial and military long-range aircraft as they helped to decrease specific fuel consumption, obviously more so than turbo-superchargers (or they wouldn't have been used at all

Turbo-superchargers work best at high altitude. The turbo-compound gave more power for take off (exhaust thrust is also inefficient at low speed). Thus they may have simply allowed commercial aircraft to get into the air with more fuel (rockets might have been bad for passenger morale). The most important military long-range aircraft using turbo-compounds was the Neptune which patrolled at low altitude looking for submarines

 

[Sweb]

There were superchargers - single and 2-speed, turbo chargers (period nomenclature turbo-superchargers like the Bierman) and turbo-compound engines.

The first two have been explained pretty well on this and other threads but the turbo-compound engine is a mixture of the two.

The turbo-compound device is an exhaust-driven turbine on a common shaft with a reduction gearbox. The gearbox output shaft is in turn coupled mechanically to the crankshaft of the engine. The concept was to use the heat energy of the exhaust and convert it back into mechanical energy. Engines equipped in this manner usually had three such devices attached to the rear of the engine crankcase 120 degrees apart. Miter gears were used on the gearbox output shaft that coupled with a gear on the engine crankshaft. These types of engines were used on the Douglas DC-7C SevenSeas and Lockheed L-1649 Starliner passenger transports.

I worked with a few old salts who maintained engines of this type and they all agreed they were more trouble and costly than they were worth. The flight engineers I knew concurred and stated that there were restrictions on their use. As far as I know they all agreed that they never trusted the design and typically adjusted their flight altitudes where the devices could be used at experienced reliability settings. It was about this time Douglas and Boeing were introducing the DC-8 and 707 jetliners, respectively, and the era of piston engines for commercial air travel was at its end.

 

[ppopsie]

Below is a graph showing how the power recovery were made on the each system; PRT and turbo supercharging. I got this manual from my old pal who worked on the airline for very many years. He and some other ex-mechanics said, in general, in the same way with Sweb-sama's previous posting.

On the other hand some of my friends who flew P2V-7s in JMSDF in 70's spoke very highly of the reliability of the plane including the engine. They truly trusted the big and heavy engines and even say the P-2J, a modified Neptune having the T-64 turbine engines in place of the R-3350s, was not reliable as more officialy known. But they checked the spark plugs of the Wright engine one by one, 72 in all, by using the engine analyzer at the run-up area prior to taking off even in the middle of the summer.

When I told the above to the masters of the airline, they said that they couldn't do that because it was quite inpracticeable to check all 144 spark plugs prior to every take-off.

 

[peril]

Frankly, the reason given most often I see is Weight, the J4M (turbo) was 200kgs heavier.

Weight costs maneuverability and payload.

 

[FLYBOYJ]

Frankly, the reason given most often I see is Weight, the J4M (turbo) was 200kgs heavier.

Weight costs maneuverability and payload.

It also depends on the airframe its attached to.

 

[unix_nerd]

Hallo,

I would think that the materials needed for the turbines needed some time to be developed. As you might know the gas turbine was at the end of WW2 a new development.
In addition the all turbo charges needed high octane fuel to develop their full potential. This was also only available in quantity since beginning of the 40's....
other issues as given above like high altitude flight and reliability are valid as well. Took a while to make this things reliable...

have also a look at:
Wright Aeronautical Division
and
http://www.enginehistory.org/Wright/TC Facts.pdf
and
http://www.enginehistory.org/Wright/R-3350Int&Ex.pdf

regards