I was going through the hangar trying to get a handle
on my collection of junk and throwing out a lot
of stuff.
I ran across this book called "Magnesium Overcast" about the B36
by Dennis Jenkins. Therein he wrote about turbo compounding
the R4360.
Here is an OCR.
Paul Lamar
------------------------------------------------------------
Nevertheless, even proponents of mechanical superchargers were
aware of the drawbacks of the concept. The largest of these was
that it took a considerable amount of the engine's power to
drive the internal supercharger. The 600-hp Wasp engine required
44 hp to drive its supercharger - the 3,500-hp R-4360 required
435 hp. In theory, for the Wasp Major to produce 4,000 hp at
40,000 feet, it would need to divert over 1,200 hp to the
supercharger. The power required went up quickly the higher the
altitude. There was another disadvantage to the internal
supercharger - engineers could not cool down the intake air
prior to using it. On aircraft with external superchargers, it
was already common practice to use intercoolers (or
aftercoolers) to cool the compressed air before it entered the
cylinders. The cooler the air, the less likely it would
pre-detonate ("ping" or "knock"), allowing more spark advance
and hence greater power.19 Compounding [??? turbo charging?]
appeared to solve both of these problems.
By eliminating the internal supercharger, the
R-4360-VDT instantly gained 435 hp from the current production
engine - unfortunately, it also lost the 1,930 hp that was
provided by the supercharger. The engine itself would have to
make up the resulting loss of 1,500 horsepower. In retrospect,.
it is hard to see how the engineers intended to accomplish this.
Since compounding compressed the air outside the R-4360, it was
possible to run the air through a large aftercooler prior to
using it in the cylinders, providing much cooler air. This
allowed the engine to run more efficiently, providing a few
hundred horsepower boost. The jet thrust from the turbine could
add another 300 hp or so, but this still left 1,000 hp
unaccounted for.
As envisioned by Pratt & Whitney engineers, compounding had four
major design goals: (1) provide more power for takeoff to get
heavier loads into the air, (2) give more power for continuous
operation at high altitudes, (3) fuel savings, and (4) be less
complex.
To accomplish these goals, Pratt & Whitney investigated four"
compounding" concepts. The simplest was one where the exhaust
gases were passed through a conventional exhaust gas turbine
that transmitted power back to the crankshaft through a suitable
mechanical drive.
This was the approach taken by Wright for the R-3350 Compound.
As proposed for the R-4360, this type of compound was confined
to low altitude operation because its maximum-altitude
performance was limited by the capacity of its internal-engine
supercharger.
Another compounding arrangement was to add conventional
exhaust-driven superchargers (turbochargers) to the combination
just described. This partially compensated for the altitude
restrictions of the first concept, but offered little advantage
to the standard R-4360.
The third concept was to duct the hot exhaust gases from the
R-4360 directly into the burner cans of a conventional jet
engine. A compressor on the front of the jet unit supplied
additional air to mix with the engine exhaust and complete the
combustion of residual fuel.
The resultant hot gas mixture was then passed through the
turbine unit to produce power for operating the compressor.
Excess power was transmitted back to the R-4360 through suitable
shafting and the exhaust was discharged rearward to provide
additional thrust.2o , The fourth concept was the variable
discharge turbosupercharger (or turbine, depending on when,the
description was written).
HA! I was at the Udvar-Hazy Smithsonian Annex last Saturday
and saw the B-36 engine! There is one on display and the
description makes reference to the book. THEN, I went to
the Air and Space Museum Sunday and saw something else about
turbo compounding with reference to the Magnesium Overcast book.
You may be onto something here... ;-)
Kevin in Denver (well, Washington DC for a few weeks)
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