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
Doug,
I would suggest you start from scratch on the turbine. As Monty has pointed
out, this will be far more likely to to bear fruit. There are at least three
persons/firms making small production runs of vaccuum cast superalloy
turbine blisks for the model gas turbine world. The standard 66mm turbine
for most model engines costs about $120. Join the gas turbine builders
association or the yahoo group
http://groups.yahoo.com/group/homebuildturbines/
I suspect that automotive turbo's survive the high EGT's because they are
only exposed to a relatively small mass flow and therefore low heat flow.
They are able to reject this heat flow fast enough. As you up the duty
cycle, although the EGT is not increasing, you are flowing more heat than
the turbine can reject. Thus the need for Inconel or superalloy (MAR-M,
RENE... etc). The combination of Wankel EGT's, a high duty cycle, and manned
aircraft expected TBO and reliablity may well exceed the material
capabilities of the super alloys and drive you to a cooled turbine (thus
ending the project). Gas turbine engines only combust a fraction of the
cycle air, and cool the cumbustion products significantly before they are
passed across the first stage nozzle. We are not afforded that luxury.
Replacing the shaft on an off the shelf turbo is not trivial. The turbine
wheel is normally shrunk onto the shaft, and it's removal damages the wheel.
Calculate the energy stored in a heavy radial inflow turbine at 100000 rpm,
and the velocity/ inertia of the parts if it lets go as two even parts. Bad
idea using a damaged turbine.
For a given mass flow and fresh gas temperature (which you have) , you have
a single optimum nozzle throat area. Work out what that is and you can play
around with the turbine diameter and blade length. Too large a diameter and
the blades are too short and the tip losses dominate, too small and RPM is
excessive.
If you design to a single point, you will still have to go through a few
iterations to optimise it, and that will be at your test altitude.
For your design point I would see if Tracy could provide the following:
At sea level, 65% RPM, the OAT, density, fuel mass flow, and air flow ratio.
With this you can work out your mass airflow, and thus the HP the turbine
delivers. You can then optimise the trade off between increasing turbine
diameter simplifying it's design (and increasing it's cost) or higher speed
gearbox (with higher cost).
Why not separate the tasks and use an aircraft turbo, and an axial flow free
power turbine?
Regards,
Mark Wrathall
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