Small remark on the gasses that drive the turbo: the driving force is pressure, not speed. Turbo's work due to a pressure differential with the outside air, while a turbo-compound uses the kinetic energy of the exhaust gasses. That's why the P47's critical altitude is a function of turbocompressor speed (roughly 20K rpm) because due to the lower pressure outside, the turbine was spinning faster, until it's constructional limit was reached (and the pilot warned).
Turbo-compounds use a "blow-down" turbine and as a result do not produce parasitical back pressure on the engine. In theory a blow-down turbine can be added sequentially to a turbocompressor, adding even more efficiency to a system, maybe by driving a generator or coolant pump or as in case of the Wright R-3350 the crankshaft via hydraulic clutches.
Interesting document by Curtiss-Wright Co. on the Wright R-3350 Turbo-Compound:
http://www.enginehistory.org/Wright/TC%20Facts.pdf
12-20-2010, 12:25 PM
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