Paul Lamar wrote:

Turbo compound Francois is the answer. Sixty year old technology tried and true. CW R3350TC
Turbo compound engines held all world unrefueled distance records until the Rutan Voyager
went around the world. A Lockheed twin CW R3350TC P2V flew from Australia to Ohio and held
the record for years. Lockheed Connies routinely flew from San Francisco nonstop to London in
the late 50's. It was years before the 747 could equal that.

The Achilles heel of the piston turbo compound was the exhaust valve. If it broke up it would
take out the turbine on the way out. The RX8 engine with the side exhaust ports reportedly
has a cooler exhaust gas.

The trick is to open the waste gate controlled by the exhaust gas temp to prevent damage to
the turbine at full power. At altitude leaned cruise where the exhaust temp is lower the BSFC
will be optimized and improved by 20%.

While turbo-compounding will get the exhaust energy to the output shaft, I wonder if the
engineering challenges of implementing it for the Wankel are worthwhile.  Paul, you of all
people should be able to understand that.

I've been musing about taking an intermediate step in that direction which I *believe* might
have fewer engineering headaches: couple a standard turbocharger to an alternator designed for
high speed operation.  Use a second pad-drive alternator to get the "dual alternator" setup.
Use a load controller for the two alternators.  The objective is to make the turbo-alternator
carry all of the load if possible.  This might eliminate the need for a wastegate, and would
return a couple HP to the output shaft (since it wouldn't need to drive the loafing pad-drive
alternator).  And the failure mode is softer than the traditional turbo-compound setup (no gear
train to crap out).

Russell Kent

A 100 amp alternator at 14 volts is 1,400 watts or about 1.9 HP. Given
an 80% efficient alternator that would be about 2.3 mechanical HP to drive the
existing alternator at full load. Unfortunately the electrical load is way less than
that. Probably about 20 or 30 amps on the average for pumps and IGN/EFI.
This lowers the mechanical HP requirements or savings to drive the alternator
to about 30/100 X 2.3 or only about about 3/4 of a horse.

What we need and what we are looking for, to make significant improvement in the
BSFC, is 20% of the engine cruise HP or about 30 additional HP. There is no
way around the turbine gear box.

BTW you would still need a waste gate with the turbine driven alternator
as full power would still over rev or over heat the turbine driving the
alternator. Turbines are narrow band devices.

Paul Lamar

How about a compounded engine that uses a turbo-generator to power a motor connected to the drive shaft the motor could take the place of the starter and you
wouldn't need an altnator. this would off set somewhat the weight consideration such an approach would generate. Seams to me this could be easier to do than
some of the other things being consider. I wonder how efficient such  a configuration would be.

cliff

As you can see above you would need a very heavy 30 HP electric motor and generator. If the generator 
was 80% (mechanical to electrical) efficient and the motor was 80% (electrical to mechanical)
efficient than the overall conversion efficiency is only 64%. Hybrid cars suffer from the same problem. Hybrid cars also suffer a 750 to 1000 pound weight
disadvantage only some of
which is due to the extra sized battery.

Diesel electric locomotives are arranged like this but weight is an advantage 
in a locomotive as you need the traction to get a mile long train moving and to climb hills.

Therefor in an airplane, engine weight disadvantages on this order, would far outweigh 
the modest reduction in fuel burn.

Paul Lamar


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