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Charlie writes:
>>I guess it's debatable whether or not it's compressor or
turbine. Results are the same. Compressor quits. As I understand it,
and the fix was to give the turbocharger more exhaust air volume at
lower engine rpm.<<
In turbosupercharger (aka turbocharger) technical terms it is called
"Compressor Stall" and is a component of compressor surge (like a stall
is part of a spin) and occurs at a threshold where the air mass flow is
insufficient for the desired pressure ratio or where the pressure ratio
is too high for the mass flow. It is a non-linear event that frequently
is accompanied by a loud noise or, in turbine engines, a flameout. Ever
notice how a fan will speed up when the discharge is blocked? Similar
effect but less dramatic. The delineation of the area on the compressor
performance map where stall will occur is called the Surge Line.
There is a very good book about turbochargers by Hugh MacInnes called
"Turbochargers", printed by HP books and available at may performance
auto stores and, I am sure, off the web.
Craig writes:
<<I do not know much about turbos, but since all of the exhaust
gas goes thru the turbine housing, the only way to generate more torque
to the turbine wheel would be to reduce the turbine inlet
size, increasing the gas back pressure, and therefore increasing torque
to the turbine wheel. This seems to me it would work up to the point
the gas was approaching super sonic speed at which time the flow would
greatly diminish. I'm curious... I wonder if some planes exhibit
turbine stall due to cabin leaks and therefore higher demand on the
turbos.>>
You have to think in terms of power (torque times RPM) and not
just RPM. Turbocharger speed is determined by compressor load and the
A/R ratio of the turbine. A/R ratio is defined as the area of the
turbine inlet divided by the radial distance from the inlet to the
turbine axis of rotation. The A/R ratio of a turbine housing is
frequently stamped or cast into the housing at the factory. The basic
A/R ration can be "trimmed" with an orifice or nozzle on the turbine
discharge but this is a "bandaid' fix for a poor turbo selection.
Turbocharger selection is a tricky business. So much so that when it
came time to select turbochargers for my Lycoming installation I hired
a retired Garret engineer to help. This was the same guy who designed
the Malibu installation (which was the precursor to the IV-P
Continental). Money well spent as he understood the issues with the
Continental engine and was able to make a good recommendation for a
compressor that had better aerodynamics.
Cabin leaks will NOT have a significant effect on engine performance.
Air for pressurization is tapped off of the deck pressure (after turbos
and intercoolers but before the butterfly) using a sonic venture. A
sonic venturi is nothing more than a hole with known flow
characteristics. The "venturi" part of the name refers to the tapered
bore which produces a vena constricta that is close to the physical
diameter of the orifice. This makes the performance of the sonic
venturi more consistent and predictable as compared to a sharp edge
orifice. The "sonic" part refers to the fact that the velocity of the
air throughout he orifice is the local speed of sound, the fastest rate
that a pressure wave can propagate in the medium.
Sonic venturis regulate flow regardless of changes in the down stream
conditions so cabin leaks have a negligible effect on pressurization
air flow rates. Think of it as a bucket with a hole in the bottom. Fill
the bucket with water and the water will flow through the hole at some
rate. How far the water falls after it exits the hole has little effect
on that flow rate. Similarly, the pressure difference across the sonic
venturi has little effect (in the range of pressures we are interested
in) on the flow rate through the sonic venturi. Flow rates into the
cabin of a IV-P should be about 40 SCFM (FAA says 10 SCFM per
passenger) which translated into a sonic venturi orifice diameter of
about 0.55".
Turbochargers and Sonic Venturis are simple devices with complex
dynamics and a detailed discussion of those dynamics is available in
the literature. I have simplified some elements for the purpose of
clarity and encourage you to research this topic, if interested. A good
source of compressor maps can be found at
http://www.turbocharged.com/main.htm.
Regards
Brent Regan
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