Douglas Fir wrote:
Paul.
Vance's dissertation on the exhaust properties was very enlightening. Here
are a few other of the more coherent opinions on 'turbocompounding' from the
Half Bakery website.
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Like many others have said, it is FALSE to say that turbo's take "wasted"
power from the exhaust and use no power of [its] own.. If this was true than
your idea would be a great one. The reason that a normal turbo "super
charges" the engine rather than having a belt that adds power to the crank
on the engine are basic laws of physics. The same reason the perpetual
motion doesn't work (on earth). As long as you have resistance (friction) of
any kind, there will be a loss and not a gain. The reason for this is
because of exhaust back pressure. Try taking an 800bhp racing engine and
plum it through a stock Honda exhaust. It would probably blow the Honda
exhaust to shreds due to the back pressure, but before it blew up you would
realize huge power losses due to the restriction. Turbos get their power by
robbing it from the exhaust system thus causing back pressure. This is still
efficient and only screw, centrifugal, and scroll super chargers can match a
turbo in efficiency. They are also much more expensive than a turbo. ??
?????? -DL
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enro2, Aug 19 2004??
?????? The idea here is to use a positive displacement supercharger at
the intake, and an exhaust-driven turbine to add power back into the drive
train via a belt-and-pulley arrangement in place of the original centrifugal
compressor. ??
?????? WWII aircraft had this fully baked. The P-38 Lightning is one
notable aircraft that used this method, and it was so successful that it was
declared top-secret and photos of the Lightning from any angle that showed
the turbine exhaust were prohibited. Although the system was geared, not
pulley-driven, the functional principle was the same. The B-17 engines also
had a similar system fitted, as did several other radial engines. It was one
of the mission-critical systems that allowed the B-17 and P-38 to extend
their range so far. ??
??????
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Interesting bit of info on the P-38s. I didn't know that.
In conclusion, it seems like a engine can only be turbo-compounded if at the
same time it is being turbocharged. Otherwise the back pressure increases
the load on the engine pulling down the rpms.
This applies whether the excess power (think of it as secondary power
output) from the turbine is going to a mechanical gearbox or a less
efficient mechanical-electrical-electrical- mechanical system. Caterpillar
research shows that with diesels engines, the higher the rpm, the larger the
difference between the power curve needed for turbocharging the engine and
power output of the exhaust turbine. Paul could you post that graph? It was
a in a PDF file introduced to you a month back. That Delta HP, if you will,
between the power curves is all that is available to us to grab. Higher
elevations will help as the turbine speed can increase 2% for each 1000 foot
of elevation. At 8,000 the rpms will increase 16%. I suspect the Connie
airplanes fly a lot lot higher thus taken advantage of cooler air and lower
outlet pressures.
So with the engine design capable of being boosted to 300hp but operated
with a smaller amount of boost at 150 hp in midrange cruise at 8,000ft I
wonder what amount of 'excess' energy can we effectively bring back to the
shaft? 10Hp?
Doug in Japan.
What is the URL for this web Half Bakery web site Doug?
BTW the B17, P47, B24 and a lot of other WW II engines used a turbo charger
AND an engine driven superchargers. The Turbo compound engine never
made it into the war. The incoherent statements above to the contrary.
I think the Half Bakery web site is half baked.
I never said the turbocharger caused no back pressure on the engine.
What I have always maintained is it uses less HP from the engine for
an equivalent amount of boost than a supercharger. Most of the HP
it uses to drive the compressor comes from the waste kinetic energy in the
exhaust. How many 400 to 800 HP 13B's are out there using superchargers?
Near zero. 999 out of 1000 use turbo chargers. Why? turbo chargers
use less HP from the engine to drive the compressor. In this Merlin
and other V12 engine heat balance chart, 9% or 140 HP of the output
HP is used to drive the supercharger. Here too is a heat balance chart
of the rotary at full chat and one at low power 80Km per hour.
The low power situation is very bad and typical of all internal combustion
engines. It is about time the world woke up to this fact. One can get some
of the waste power out of the exhaust by exploiting the kinetic energy and a lot
of the rest by making steam and injecting it into the turbine. Nobody is
talking about perpetual motion here nor any lack of friction or even
a 100% conversion of any kind. Every time you convert something to
something else it cost. If one can recover half the waste energy in the
exhaust one is doing extremely well.
Since the HP numbers are so low in a car cruise situation you could use a
very small turbine and a small heat exchanger and a small condenser.
I think the heat exchanger can be as simple as a counter flow concentric
pipe inside the all ready long exhaust pipe. The condenser can take
the place of the aft muffler. If one is really clever the cooling
system could be in the loop for additional heat recovery. It just
takes money for R&D.
Paul Lamar
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