Actually one could limit the RPM at lower altitudes to eliminate an
additional gear reduction sacrificing some early mission climb rate.

Cheers,
Brian
--------------------

Brian Feeney
DreamSpace and The da Vinci Project
bfeeney@davinciproject.com
http://www.davinciproject.com

Turbo systems are not nearly as hard to design as most people will
make them out to be. Series turbos are just an extrapolation on a
basic turbo system. Figuring for sizing and plumbing isn't too hard
but you really, really need to have turbo and wastegate reliability
nailed down. If a wastegate fails closed at low altitude, it becomes
very easy to destroy an engine in a hurry (rotary or otherwise).

I'm very comfortable sizing turbos for specific needs and know a
little bit about designing the system as a whole if you want any
advice.

-Chris

Do you have any data on the HP generated by turbo charger turbines?


Paul Lamar

Paul,

That stuff is almost never released by turbocharger companies. While
it would be invaluable for turbo-compounding when converting
off-the-shelf turbo parts, the data is not very useful in normal
turbocharging applications.

Look at it this way: automotive turbos are a careful compromise. For
75% of the powerband they are bleeding off exhaust gas to prevent
overboosting because turbines are intentionally under-sized to give
better response at low RPM. When you're wasting that much potential
work, turbine efficiency isn't very important. Practically speaking,
we don't _want_ all of the available exhaust energy in a sea-level
turbocharging application because the centrifugal compressor doesn't
need much power to develop more boost than the engine can handle. I
have contributed to a test where the wastegates were fixed shut on a
race car and 35psi was available VERY quickly. That engine was built
to handle it and was producing approximately 800hp. So in normal turbo
applications, we don't need to put all of the engine's exhaust energy
to work.

Another angle:

A popular method in automotive turbocharging a few years ago was
"clipping" the turbine -- grinding an angle off the outermost edge of
the exducer blades to allow more mass flow at a given pressure, or a
lower pressure for the same mass flow. Obviously this damages turbine
efficiency, but the tradeoff is apparently worth it. So worthwhile, in
fact, that Garrett in the newest turbochargers they sell "casts in"
some clip to the blades already. More mass flow at the same pressure
*without increasing turbine size* minimizes turbine inertia, which is
a worthy consideration in transient response - "lag" - in an
automotive applications. The efficiency penalty goes unnoticed because
there is exhaust energy to spare in that application.

Long story short, they don't publish turbine efficiency because it's
not important to the application. If you look at a turbocharger
turbine next to any gas turbine engine, you can see exactly what would
make efficiency mediocre to begin with. Stator/guide vanes are
completely absent in the radial inflow turbine of a turbocharger, and
the whole unit is designed for packaging and manufacturability rather
than efficiency.


Something you might be interested in:

http://www.nyethermodynamics.com/ta30-nt-xx/index.html

Automotive turbocharger as exhaust-driven electrical generator.


-Chris

Very well put Chris. Thanks. I nominate you as our new resident turbo
expert.
Obviously to me you know what you are talking about. We have been around
and around on this subject for years and some of the people on here have
destroyed more than one automotive turbo when trying to adapt it to the
rotary aircraft engine.

What do you think of the idea of loading down the typical automotive turbo
charger turbine with larger compressors to slow them down?

BTW I have a new book coming from Foyles in London by mail on turbines that
I have high hopes for. Gas Turbine Engineering Handbook (3rd Edition)

* comprehensive treatment of gas turbines from design to operation and
maintenance.
* a special maintenance section dealing with the advanced gas turbines.
* special diagnostic charts have been provided that will enable the reader
to troubleshoot problems he encounters in the field.


It is also available in the states a lot cheaper than what I payed for it
in London. Like half. ISBN 978-0-7506-7846-9             1006 Pages.
http://www.knovel.com/web/portal/browse/display?_EXT_KNOVEL_DISPLAY_bookid=1887

Paul Lamar

Thanks. =)

What would be the point of fitting a larger compressor to slow down
the turbo? Larger compressors have a higher "comfortable" flow rate
and function very poorly if this rate isn't carefully matched to the
engine it is supplying. A bigger compressor would allow the turbine to
extract more work from the exhaust gas, but the extra airflow this
generated would have to be wasted or it would literally back up and
flow out the compressor inlet several times per second. This "surge"
really beats up the turbo and just means you're asking the compressor
to operate in a pressure-flow regime it is incapable of.

One thing they did in the F1 turbo era was to use very large
compressors to circumvent the poppet-valve release used to control
boost pressures. The story as I remember it is that at some maximum
permitted pressure, a valve on the intake would open and vent charge
air to the atmosphere until pressure dropped below the max allowed
value. Teams selected turbos that would outflow the poppet valve,
which would have looked rather something like you described: a big
compressor wheel fitted to a smaller exhaust wheel, possibly lacking a
wastegate altogether. Naturally this uses ALL of the exhaust energy
the turbine is able to reclaim, but the extra work is still being
wasted.

If you wanted to use an air motor to drive the crank, you could use an
exhaust-driven turbo to supply it for some Rube Goldberg-style turbo
compounding. The frictional losses would likely be obscene. =D


I have a PDF version of the 2nd edition of that book. I have not
exactly read it cover-to-cover though.

-Chris.

Our biggest problem is the turbine blows up after a few hours.
The built in waste gate is too small. Even when wide open.
True the heat is high but the excessive RPM is the main culprit as
the load on the blades is m*V^2/r. A pop off valve on the intake manifold
would be no problem. So far a bigger compressor seems to be working almost fine
on Dave Leonard's turbo 13B in his RV6. My guess is the turbine is three or four times more powerful than needed for continuous operation. The excess power is there to accelerate the compressor as quickly as possible.

Dang now you tell me :) Can you email me the pdf?

Paul Lamar