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I agree Dave.
This is of course all speculation about what
actually happened to John's turbo. But, at the moment, probably about the
best we can do. Having two failures that soon and with the same results -
loose compressor wheels - certainly indicates its probably just not a
random factor, but has some underlying common cause. I agree if the
compressor maps we are looking at are even close to the Mazda compressor
parameters, it is unlikely that John was in the surge zone. If he had been
according to what I have read, there would have been indications like rapidly
fluctuating pressure surges and sound and vibration associated. Although
with the engine at the back the sound may not have been heard and the vibration
could have been dampened - which a composite construct is good at. So,
apparently no evident to support the surge hypothesis.
It does appear that whatever is causing it, the
stress appears to be concentrated on the compressor wheel.
Surges stress the entire rotating assembly but does particularly stress
the compressor wheel. I need to sit down again and calculate the
Pressure ratio and flow on the compressor chart after adjusting for the pressure
ratio creep caused by altitude. But, it would seem to take a considerable
increase in pressure ratio to move him into the surge zone. The worst case
situation for surge appears to be in boost with low mass flow, in other words
operating toward the left on the mass flow axis of the chart. The further over
you are the more like to enter it. But again - no indication that is
happening.
Overheating could indeed weaken the metal, however,
its my opinion that with the water cooling - overheating should not be a
problem. I mean almost all aircraft turbos I am aware of do not have water
cooling and survive for much longer, hundreds perhaps close to a thousand
hours. So you would think that with water cooling the Mazda turbo would do
even better - if overheating were the cause. Can't rule it out, but I would put
it further down the list.
My prime candidate of the three would be
overspeeding of the rotating assembly.
One of the main reasons I lean toward overspeeding
is that the turbine housing of the Mazda turbo is designed and tailored for
low engine rpm boost. The Mazda SAE papers made a point of that.
Higher engine rpm would increase the exhaust mass flow which
would tend to spin the assembly even faster. Operating
at altitude with the pressure ratio creep also increases the speed
required of the rotating assembly to maintain the same differential boost
pressure. The twin scrolls driving
the turbine wheel are separated with one having a waste gate and the other
not. That means the one scroll never has its exhaust gas diverted by the
waste gate. Again a design feature to promote boost at low exhaust mass
flow. Then as I have speculated, I believe there is a chance that the BOV can
add to the risk of overspeeding. So given those factors and possibilities
my assessment would lean toward overspeeding.
However, that being said, I would expect to
see damage that would support that assessment. If no damage or little rotational
damage that would seem to indicate low rotation speed when the wheel
separated. So if overspeeding were the cause, I would expect to see the
compressor wheel and housing fairly well beaten up.
The turbine wheel is generally friction welded to
the shaft whereas the compressor wheel is held on with a nut. I believe
John indicated in one case the wheel was just laying in the compressor housing
and the in last case I believe he indicated it was wedged in tightly. I
don't recall how the nut is secured (other than torque). I'll have to go
down to the shop later and take a look.
I don't recall John mentioning how damaged the
compressor wheel was. I would assume that if it separated from the shaft
while spinning at 80,000 + rpm it would really tear up the wheel and
housing. On the turbine side I have seen photos where the wheel actually
tore through the cast iron housing as hard as that might be to
believe.
John do you have any photos of the wheels and
housing after the incidents??
I am still looking at the BOV and any possible role
it may play. I still appears to me that suddenly releasing the back
pressure across the compressor wheel when the BOV opens could open the
possibility that the compressor could be stressed. If the back
pressure across the wheel suddenly drops and lessens the load while
the turbine end is still getting the boost mass flow that there is the possibly
of overspeeding the rotating assembly.
I, of course, could be wrong and the
BOV effects not only be benign but actually helps prevent damage in an
aircraft operating environment as you and others have suggested.
If aircraft turbo systems had blow off valves then we would have another
data point, but of the several aircraft systems I looked on the internet
had either sized the A/r of the turbine housing or use a waste gate type of
bleed for the exhaust.
Like you say it could be a combination of
factors, but I do agree with the three you mention as to being the most likely
culprits.
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
Sent: Saturday, July 10, 2004 12:42
AM
Subject: [FlyRotary] Re: BOV more/less
air flow
Ed,
Great write up! In general I very much agree with you on all points but
I want to free flow some thoughts that may modify people's feeling about a
BOV. I am even less of a turbo expert than you, but here are some
thoughts to consider.
First, I think we have to figure out exactly what is causing John's
turbo's to fry. IMHO we have 3 general candidates 1) overheating,
2) turbo overspeading, or 3) compressors into the surge region. While it
is possible that there is a combined cause, and probable that more than one
factor plays a partial role, most likely ONE factor is
primarily responsible for JTD (John's Turbos'
Demise).
IMHO
that one factor is overheating. We know that heat is a factor
while according you the map you posted we are probably not in
surge or overspeed region most of the time. Also, turbo overspeading and
compressor surge are likely to present with very different symptoms like
surging pressures or a disintegrated turbo. In contrast, overheating
weakens the metals involved and would behave much like what John
Describes. In this case it seems to be turbine axle and possibly the
blades that are not up to the task.
There are 2 ways to improve the situation. Reduce the heat, of course,
and reduce the forces acting on the metals. The BOV can be
very instrumental in reducing the force acting on that axle, and possibly also
slightly reduce the heat transfer. My BOV (which I assume is fairly
standard) is not an on/off valve, but a spring loaded valve that will open
more or less depending on the pressure in the plenum, the ambient pressure,
and the pressure differential across the throttle body. There is no way
it will instantly drop the pressure significantly. In normal operation
it would never go from full closed to full open suddenly, but rather just
start leaking more and more air as the combination of those three pressures
changes. The only time it would suddenly open is when the throttle is
suddenly closed.
While on one hand it does seem very inefficient to compress air only to
have it bleed out before entering the engine, I believe it is far more
inefficient and potentially harmful to have the compressor constantly fighting
against a large pressure differential. That pressure differential is the
largest determinant of the force on the axle.
Further, if the turbine wheel is free to turn a little easier (faster)
it places less back pressure on the exhaust and less heat will be
transfeffed. It is true that the faster RPM increases the radial
stresses on the blades, but radial stresses are tolerated much better than the
axial stresses caused by high back pressure. (Since I have decided that
heating is the problem and not overspeeding.)
There is no doubt that we both agree on the BEST solution: a larger
turbo with an even larger waste gate placed perhaps a little further from the
engine. But until I change my set-up to fit such a turbo and waste
gate I think the BOV is a helpful addition.
Further, the BOV does provide some measure of protection against over
boost like what happened to Todd.
Dave
Leonard
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