Douglas Fir wrote:
Everyone
Turbo compound project. Report #2
With the turbo 13B engine safely in my workshop I am starting to plan the
research project for turbo compounding it. After rebuilding the engine with
new seals, and putting Tracy's EC-2 unit on it. I'll leave the engine in
its stock condition (yes running on gasoline) with only the alternator and
water pump, connected during primary testing.
The first dyno testing (after I assemble the dyno) will be to determine
baseline BSFC data for this particular engine at different power settings.
I already have a small 25 channel data logger made by PICO in England that
interfaces to the PC through the RS-232 port.
The second phase of testing will be to design and make a turbo compound
system using the standard turbo and see if I can document an improved BSFC
with the same test variable loads.
Please jump in with your thoughts and correct me if I am wrong or help fill
in the ? marks.
IMHO:
It is safe assumption to say that the majority of homebuilt aircraft owners
are very concerned with improving fuel economy.
A standard turbo is sized to provide boost and waste the exhaust gas in the
high MAP situations and hopefully before the engine starts knocking.
Common aircraft economy cruise ranges are from 55% to 75% power.
At these power settings not a lot (if any ?) exhaust energy leaves via the
waste gate. We need to find a way to measure the actual torque real time
that is leaving the turbo shaft and would like to hear any ideas. Turbo
tach and torque sensor. transducer?
Stealing energy from the standard turbo/compressor, diverting it through a
CVT and delivering it to the e-shaft via a belt drive will become a
lose/lose/gain? situation. In short many variables come into play at almost
real time.
Lets say we steal 20% of the energy from turbo and send it to the e-shaft.
We still have 80% to use for the boost compressor though. When lowering the
boost by 20% it lowers the air/fuel amount but perhaps not by 20%?. For a
moment we could say the Hp is also lowered and assuming we didn't touch the
C/S prop setting, the engine rpm and hence turbo torque would go down.
However simultaneously we are applying 90% of this 'borrowed' torque (10%
loss through the CVT) thus adding a boost to the e-shaft. Logic would
dictate we are borrowing from Paul with interest to pay Peter. We are in a
losing proposition and it would seem that our system would rapidly
degenerate into becoming an engine brake as the rpm decreased even more. Of
course rpm, Hp. and boost curves are not perfectly linear. We may find a
sweet spot. But likely not.
Our experiment will fail. Fine but we expect that. Now we look at the data
and see where we failed the least. Even with the standard turbo there is
bound some turbo range that is a little less bitter in defeat. We then know
what to improve. For starters I would think we would have to get the CVT
operating well over 95% efficiency.
Moving on:
Logically the best rpm to transfer power to the e-shaft would be at the rpm
or manifold settings that causes the turbo waste gate to open. High power
setting? Not so good for economy though because of the high drag effect on
our airplane at high speeds. Forget for a moment we may be above 10K' where
the thrust/drag ratio becomes more beneficial to BSFC in some planes.
Thus:
The ideal situation will be to use an over sized turbine that has energy to
waste so to speak. One that is specifically designed to take advantage of
the rotary engine's dynamic boost. In short a very efficient turbo motor.
The unit will be determined by size, weight, performance. It might? need
moveable vanes in the intake to improve the compressor's compression ratio
at lower speeds or keep a boost setting through a wider (slower) RPM range.
With the oversized turbo we will at least have more wasted energy to play
with. We can then have our cake and perhaps nibble it too. Turbo
compounding is not the technology to let us go fast and sip fuel. At best it
will let us have our full boost climb out and full throttle dashes with a
small addition of torque to the eshaft.
I am betting it is somewhere in the cruise ranges that this idea will
shine. With a CVT we can dial in what percent of the turbo motor power goes
to the compressor or e-shaft. No more waste gate 'wasting' an available
resource. Best 55% cruise might have only a small air boost with most of
the energy going through the turbo compound? Don't know. Have to test it.
Bound to be some surprises. At what point does a low BSFC happen in your
airplane with this device? That will depend upon your altitude, power
setting, type of airplane, prop and rpm setting.
For me that life saver cruise speed could be lower than 50% power when
encountering headwinds more than halfway into a 2000 mile leg over cold
waters.
With some success and a lot more failures we can make an educated guess on
how to size the turbo-compound system for the 20B, which is the engine I'll
eventually use.
Doug in Japan
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