X-Mozilla-Keys:
Bill Freeman wrote:
This is my reasoning on my turbo set up.
The chart is for a TO4E "50" trim.
The max boost is 15 psi at sea level with a pressure
ratio of 2:1. The 2.6 L 13B, according to a chart
in Turbonetics catalog, will use 32 lbs of air a minute
at a 15 psi boost at 6000 RPM at sea level.
At 12:1 air fuel ratio that is 2.6 pounds of fuel
per minute or 156 pounds per hour. At a BSFC
of .5 that is 312 HP. I think this is a rather low
estimate for 15 psi boost at 6000 RPM but what
the heck lets go with it.
Let us say it is 400 HP. So I set the throttle
aprox one one half of the way open or about
200 HP and take off. At this time I do
not know what the manifold abs pressure or boost
would be. As I go down the runway I would
not be a bit surprised if the boost increased
slightly as the prop unloaded somewhat and the
engine RPM increased.
During TO the turbo is pushing very hard against
a half-closed throttle, inefficient and causes an
increase in induction air heating without benefit.
Why not a manual wastegate open at TO, close
down as rqd at altitude?? A proven system.
Why would it be max effort with half throttle? Would
not the exhaust energy driving the turbo be lower at
half throttle as well?
I gradually open the throttle to keep the
manifold pressure the same as I climb and at
30,000 feet the air density has fallen from
about .0019 pounds per cubic feet at sea level
to .0011 at 30,000 feet. At this point
I would need a pressure ratio of only 1.72 to
maintain sea level pressure. This appears to
be a peice of cake for this turbo. A max
pressure ratio of 3:1 appears to be
available.
The max PR of 3:1 agrees with what I know about
turbocompressors. Again, it seems that you
will be running around about 80% the time with
the turbo providing both exhaust backpressure
and a lot of inlet air heating for no benefit.
Only if I fly at sea level all the time. I am a
a high altitude cross country freak and we have
14K mountains out here unlike Kansas :-). PL
Seems to me that unloading the turbo totally with
a fully open wastegate (bypass) is more efficient
and keeps the turbo either stopped or spinning
slowly at low turbine temps for much of its life.
Turbos are a durability problem to start with, why
work it to death for nothing? The low volumetric
eff. measured from the compressor outlet to the
comb chamber also offends my sense of engineering
elegance.
As the RPM and air flow requirements of the
engine is reduced the operating
point of the turbo moves into a higher eff.
range of the turbo air flow chart.
Note: The RPM of the turbo started
out at only 80,000 or so and I have 126,000
to work with. I choose this trim just because
of the high max RPM.
The air temp has fallen from about 15 degrees C or
60 degrees F at sea level to -15 C or about
-27 degrees F if my conversion calc's are right.
This should help cool the mixture with the aid of
an intercooler. I will upload a chart on air density
and temperature next.
This is all speculation on my part and I could be
missing a lot. What do you think Jeff and Bill?
Paul Lamar turbflow.jpg
I also wonder if there are any compressor stability
problems that might occur since at TO you would
have the compressor running max effort against a
substantial restriction. Any possiblity of a surging
problem with this under these conditions?
A lot is going to depend on the A/R ratio which I hope
to talk to Bob Keller about later this week. PL
A simple butterfly valve made of SS321 in a thick walled
tube well downstream (lower temps) with a manual control
via a throttle valve would seem more elegant and less likely
to burn extra fuel and the turbine wheel during the most
common condition - less than max altitude.
This sounds a lot like the "calibrated leak" system used on
Mooney 231s and Turbo Arrows. Was an inefficient system,
and badly needed an intercooler. Had the additional disadvantage
of lowering the max altitude since the "leak" was fixed and
could not be closed.
Bill
You might be right about all this Bill. Nothing is cast in concreate
or SS at this time.
Paul Lamar
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