Peter Duniho wrote:

"Friedrich Ostertag" wrote in
message
...

On a turbocharged engine egt affects the enthalpy delivered to the
turbocharger turbine and in turn the power deliverd to the compressor.
This will result in a slight change in boost pressure and therefore
MP for
a given throttle setting.

Whether this will be a big enough change to be noticed under typical
operating conditions of an airplane engine I don't know.


Thank you for trying to save me.

You're welcome :-)

However, I have to admit a couple of
things: I wasn't meaning to restrict my (erroneous) comments to
turbocharged
engines; and your point, while an interesting take on the question, is
probably only valid for turbocharged engines with manual wastegates.

It depends on how the automatic wastegate is controlled and what are the
operating conditions. If you have an automotive-type control with an
spring controlled actuator operated by boost pressure, that should keep
the boost pressure (upstream throttle) fairly constant, as long as there
is enough turbine enthalpy. Once the boost pressure cannot be maintained
with fully closed wastegate, turbine enthalpy controls the boost. This
is exactly what's behind your observation mentioned below.

Of course, that second point requires qualification too: I have
noticed in
my own airplane (turbocharged engine, with an automatic wastegate)
that at
high altitudes, above the critical altitude for the turbo, RPM
becomes the
primary power control. It's as if at lower RPM, there just isn't enough
energy in the exhaust to keep the turbo working effectively.
Throttle at
full, then adjust RPM. Small adjustments to RPM can make significant
(1" or
more) changes in MP.

The RPM thing isn't really what you were talking about, but it seems
related
in context.

Exactly the same behavior is demonstrated by turboed automotive engines
- a lack of torque below a certain engine speed. The "critical" speed,
above which the rated torque can be produced, rises significantly with
altitude, as more turbine energy is required to boost the lower ambient
pressure to the desired MP.

The physics behind this are quite interesting, it basically comes down
to a contradiction between the piston engine being a so called positive
displacement machine and the t/c being a continuous flow machine. The
positive displacement machine likes a constant pressure ratio over a
wide range of massflow, while the turbine pressure ratio (without
wastegate) must rise with massflow.

To the exhaust gas, the turbine is just a hole, through which it needs
to pass. High massflow creates a high pressure drop (and therefore high
power output!), low massflow creates low pressure ratio and low turbine
power. But it is even worse: As the higher or lower turbine power
increases or decreases boost, it increases or decreases the massflow and
therefore in turn the turbine power even more.

Anyway, thanks for posting more to think about.

regards,

Friedrich

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