I have been thinking about the problem of turbochargers Overheating ,
Over running, in most set ups, in the AC rotary conversion . My
ideal is to locate the turbocharger farther Down Stream of the
exhaust manifold where's theirs less heat and pressure to contend
with . It's my understanding that the car manufacturers mount them
as close to the manifold. as possible to reduce Turbolag which isn't
as much a problem in AC as it is with cars .
Clifton
The P47 had the turbo in the tail cone.
Paul Lamar
Was the exhaust pipe pipe on the P47 insulated?
Turbos recover heat and mechanical energy from the exhaust. Moving the
turbo further downstream will reduce the heat energy recovery. I don't
know how efficient turbos are at recovering heat energy in this type of
application. How much cooler is the exhaust stream after running
through the turbo (as compared to running through a locked turbo)?
Regards, Matt
Good question Matt. I only have half an answer.
Dave Leonards exhaust after the turbo runs about:
> Second pic is MAP 37 Engine RPM is 5560.
> Dave has a 2.17:1 gear box. EGT exiting
> the turbo is 766 centigrade or about 1410 F.
So the regular NA 13B exhaust temps run 1600 to 1800 F.
Dave does not have a thermal couple on the inlet to the turbo
so I am not sure what his TIT is.
Paul Lamar
Clifton, your idea is correct, the turbo should be installed some
distance from the engine.
Locating the turbo directly on the engine à la Mazda has a few
advantages: 1. Minimized turbo-lag (not an issue in aviation)
2. Compactness of the whole powerplant
3. No primary exhaust manifold and pipe (which are under pressure and
can create a serious problem very quickly in case of a crack)
However, in aviation applications it has two huge drawbacks:
1. At our typically high power-settings, postcombustion is happening
within the turbine housing (no matter how lean a mixture you run... and
believe me, Mistral has been experimenting with the leanest possible
mixture settings!). While we have the same EGTs as Dave's (around 750°C
[1382 F]) just out of the block, the temperatures are way higher
(>1,000°C [1832 F]) within the turbine housing and will fry the wheel
within a few dozen hours. A turbo, with all of its mixing, is a very
efficient thermal reactor, but a bit of a sensitive and expensive piece
of equipment to be used as such!
2. The hot section could conceivably be built in exotic Ni alloys
resisting much higher temperatures than the materials used in regular
turbos. However, building a customized hot-section in high-T materials
is an outrageously expensive proposition.
The imperfect but sensible solution is to locate the turbo enough
downstream from the engine so that postcombustion is complete, and the
peak in EGT passed, before the gases enter the turbine. Not too close to
give the turbine a living chance, and not too far (I think the tailcone
is an exaggeration! :-) ) so as to recuperate a good portion of the
energy. How far is far? We'll tell you in a few weeks. It's a
question of volume of the "thermal reactor" (exhaust manifold + primary
exhaust pipe) and therefore of throughput time, more than a question of
distance.
We regret this change to our design (because of advantages 2 and 3
mentioned above), but it seems to be a fact of life and of physics.
Hard to argue against!
We'll keep you informed.
Francois Badoux (Mistral Engines)
Interesting data Francois. Sounds like you are on to something
significant. It never ever occurred to me the exhaust gas temp in the
turbo would be actually higher than the exhaust gas temp coming out of
the block. Thanks for that great info. Explains a lot of what we are
seeing.
BTW Here is a heat balance chart for
a 2nd gen rotary configuration. Note the high percentage of unburned
hydro carbons labled Latent Heat of Unburnded Gas. The RX8 engine with
it side exhaust ports should be better. The second chart for the RX8
engine is unfortunately given at a much lower power level (80 Km or 50
miles per hour probably only about 25 HP?) so it cannot be directly
compared with the first chart.
Paul Lamar
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Interesting. I hadn't thought about continued combustion of the HC's in
the exhaust stream.. I had a turbo Mercedes with a catalytic converter
(of sorts) mounted between the engine and the turbocharger. Other brands
may be like this. It hits me now that this is probably good
thermodynmically as well as for emissions. Encourage as much of the HC in
the exhaust stream to continue to burn prior to the turbocharger -
increasing the mechanical energy available to the turbo, and reducing the
heat energy abusing it. Of course, the cat on the Mercedes had a
reputation for short life (clogged up), but that may have been an
implementation detail (or operational problem - automotive cycles) more
than anything else.
Regards,
Matt-
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Copyright 1998-2005 All world wide rights reserved.