Impact energy in a moving gas stream is the mass of the gas times the
velocity
squared just like the energy in a plane crash. That is why the FAA
limits the stall
speed of GA airplanes. I digress.
The kinetic energy in the exhaust is the mass of the gas moving at
some speed. The heat of the exhaust gas has nothing to do directly with the
kinetic energy. The application of heat increases the kinetic energy
of the gas as it expands the gas in a limited volume. The gas expansion
in an open ended exhaust pipe causes an increase in the kinetic energy as
the expanded gas speeds up trying to get out. If you choke the pipe down
with a nozzle the kinetic energy further increases at the expense of an
increase in back pressure.
Paul Lamar
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Paul, and Group,
I wanted to run something by you to see what you think of it. These
newer turbo-compound trucks are using the axial flow turbines because
they are more efficient in the RPM range where an engine might operate.
I had a thought about how we usually will be running a specific RPM in
cruise and only slightly more at WOT. My suggestion is this, why not
optimize an axial flow turbine for cruise, and then gear it directly to
the e-shaft?
At idle or low RPM the turbine might even be a drag on the
engine, though a small one. This wouldn't be a big loss though and the
muffling we would get from the turbine would be worth it. As the exhaust
flow increased the turbine would start to produce power. Recovery at
lower RPMs might be a fraction of what it could be with a variable speed
drive, but we wouldn't be operating in that range for long. As the power
rose to at least cruise RPM the turbine would be in it's near optimum
range and greatly improving BSFC with the minimum loss. Max RPM at WOT
wouldn't be much higher and the engine would still be recovering
considerable HP from the turbine, though not the optimum available
through a variable speed gearbox.
All the while we would need less of a
muffler after the exhaust was run through the turbine. The connection
could be simple like a multi rib flat belt or even a direct drive chain
or gear drive. We wouldn't be recovering much at low RPM's but as we
have mentioned so many times before we wouldn't be at those RPMs for
long. The losses wouldn't be any different from the losses caused by a
heavy muffler system used to quiet the engine anyway. This idea would be
like the really large turbine you have drawn in the past, but could be
done with a more conventional look to it. So what do you think of this
idea? It is nowhere near as elegant or efficient as the traction drive,
but in the optimized RPM band it might be almost as good with a very
simple setup. I believe it would work. Tell me I'm wrong.
Bill Jepson
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These turbo charger variable angle vane devices work on axial flow turbines
as well as radial flow turbines to broaden the turbine HP range over a
range
of engine RPM and power. It is very similar to changing the A/R on a radial
turbine. In effect they are choking the exhaust pipe. They are very
simple devices.
If you add a one way clutch to the turbo compound (TC) turbine output shaft
the TC turbine will not be a drag on the engine when the engine is RPM is
higher than the RPM the TC turbine output shaft can generate.
Paul Lamar ...No rotor no motor.
Paul,
I completely understand how the variable vane turbines work. What I was
referring to would be more like the axial flow turbine you showed us for
the new diesel systems where there is a normal turbocharger in the
exhaust stream followed by an axial flow power recovery turbine. I also
noted that the Napier Nomad turbo compound that you showed the cutaway
of used an axial flow turbine. (For the non-techies among us the axial
flow turbine looks like a modern turbo jet, without the compressor
section.)
My point Paul is that you could I believe use a turbine that
wasn't very efficient at low RPMs, that is too free flowing, but size it
for cruise and you would still see significant gains at cruise. I agree
that you could use a sprag if needed to prevent reverse loading the
turbine if that would even be a problem. The other point here is that
this simple method could be constructed easily for test purposes and
even used on a NA engine. This could be a way for us to test some of the
improvements of turbo compounding without terrific expense. Speaking for
myself if I recovered a few percent, say just beyond what it takes to
make up for carrying the additional weight of the turbine I would
consider it since it would make the lowering the exhaust noise easier. I
really believe even if you were throwing away some of the recoverable
work once you got to cruise you would get a quite noticeable increase in
efficiency. You would never see the 20% or more that might be possible
with a good variable speed drive, but if you recovered 10% at cruise
which should be possible it might make investors consider financing a
more through TC package. Think of this idea as a kind of "starter set"
for turbo compounding.
Bill Jepson
Hi Bill,
There may be a potential pitfall to your idea. The axial flow blade
shape (twist, airfoil shape, length) would have to be optimized for a
particular gas flow. That would mean a bell curve for efficiency. (There
would be down slopes either side of a particular point.) As you go to
altitude, less and less gas would be produced so a design optimizing the
bell curve high point that works well at sea level will not be the
optimum at altitude. Some compromise would have to be allowed for. The
most logical scenario would be to design for 8000 feet at whatever gas
pressure is available from the engine at that altitude. Above 8000
you're WOT and nature will decrease gas flow as you go higher , and
below that level the pilot can maintain gas flow if
the additional performance is required. The downside here is that to
receive 'compound' fuel efficiency benefits at say 3000 ft the aircraft
has to be operated at WOT which may not always fit the flight
mission. Dave M
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