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
What kind of efficiency have Starter-Generators on turbines?
I know that starting my Turbomeca Arrius on the EC120B helicopter pulls up
to 220 Amps from a 28 V battery, though when I get to the 200 Amps it is
down to about 17 -18 V. [4.8 HP gross] Even before starting the V-meter indicates only
about 25.8 V available with a draw of 10 Amps [.35 HP gross] shown for bare essentials
(electronics, no NavCom at this stage), before hitting the starter button.
At this time the electric fuel pump will run and the V's go down to roughly
24 V, Ampers stay at 10 or jump to 15 depending on the day (obviously the
scale is 5 Amps at a time...). Once the engine starts to pull on its own the
readings go back towards 120 Amps and the Back up to around 18 - 19 V,
before I let go on the starter switch at 50% Ng.
Thomas J.
Most electric motors and generators/alternators are between 80% and 95% efficient.
http://www.stanford.edu/group/solarcar/ev/vehicle.html
Paul Lamar
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