Bill Eck, Herb Sanders and I had dinner at OSH and the main
topic of conversations was turbo controls. Bill has David Smith's
turbo Glassair in his garage and they are getting ready to
run it on a test stand.
When I got back I re-read all I had on the subject and I think
I have a handle on it to a certain extent.
The HP we can generate is directly proportional
to the absolute pressure in the intake manifold. Five pounds
of boost at sea level is 20 psi absolute in the intake
manifold.
First let me say that most automotive turbo controls are not very
useful for controlling an aircraft engine that is designed
to fly high. Lets look at the waste gate controller first.
A typical waste gate controller is a diaphragm and a spring.
The diaphragm is hooked to a valve that bypasses exhaust
gas around the turbine when it is open. One side of the
diaphragm is open to the atmosphere call it side A and
the other side call it side B is plumbed to the output
side of the compressor or the intake manifold.
What is frequently overlooked is the sea level atmospheric
pressure is added to the spring pressure to control the
opening of the waste gate.
Let us say for the moment the diaphragm is 3 inches in
diameter. The area of the diaphragm is about 7 square inches.
The sea level pressure is about 15 psi so we
will have 106 pounds of force on side A of the diaphragm
from sea level pressure alone. The other B side
of the diaphragm has that and the boost pressure of say 5 psi.
This is a total of 20 psi absolute and a force of 140 pounds.
The spring on side A then must have about 35 pounds of force to
keep the waste gate closed. The total force on the gate to
keep it closed is now about 140 pounds.
Now lets see what happens when we go up in altitude where
the air pressure is only 10 psi. The load on side A of the diaphragm goes
down to 70 pounds plus 35 pounds of spring pressure or 105 pounds.
On the other side of the diaphragm, side B we would like to have
10 psi atmospheric and 10 psi of boost or 140 pounds.
Unfortunately that will open the waste gate. In order to get
that pressure the compressor must now compress the air more to get
the absolute pressure up to 20 psi. In other words the pressure
ratio of the compressor must go from 1.33:1 to 2:1 at an altitude
where atmospheric pressure is only 10 psi.
Obviously it is not going to do that with the gate open.
We can no longer keep the waste gate closed and
essentially we have lost all boost. We are back to sea level power
more or less. So much for an automotive waste gate controller.
The simple solution is a manual control for the waste gate.
Greg Richter is not using a waste gate and the question came up
what is wrong with that? Greg uses a spring loaded pop off valve
instead. The same thing applies to a pop off valve on the intake
manifold. As you go up in altitude the absolute pressure in the
manifold will go down as it too partially relies on atmospheric
pressure to keep it closed. BTW a radiator pressure cap will work
as a pop off valve. Very cheap :-) The real downside to a pop
off valve only is an unnecessary amount of heat is going through
the turbine. You only want as much exhaust gas as necessary
to go through the turbine to minimize its temperature rise.
This is a simplified statement of what happens as the turbine will
speed up with less back pressure and if the pressure ratio of
the compressor does indeed increase so too will the temperature
of the charge which reduces the air density in the intake manifold
and consequently the HP. On the other hand the lower temperature
of the air at altitude helps. These are secondary effects however.
The ideal automatic controller would be a microprocessor that took
all these factors into consideration to: A. first protect the engine
from detonation B. the turbine from over speed while C. still maximizing
the HP as the airplane climbed.
Paul Lamar
The AirCraft Rotary Engine NewsLetter. Powered by Linux.
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