>>>Paul,
>>>
>>>What would be the sea level exhaust pressure just before
the turbine
>>>inlet in a turbocharged engine?
>>>
>>>I'm guessing maybe 18 psi to account for a little bit of back
>>>pressure
>>>due to the turbine?
>>>
>>>Regards,
>>>
>>>Gordon.
>>>
>>>
>>>I don't have any data on that. I wish I did.
>>>
>>>Paul Lamar ...No rotor no motor.
>>>
>>(Snip from my comments October '04... since I never received
any
>>feedback on these comments, I always assumed no one
understood the
>>real kernel of turbo wisdom here. This is how 1500 bhp/liter is
>>done. So, I'll put it out there again:)
>>
>>I have seen no mention of this technique in the newsletter,
so I
>>thought I would contribute some thoughts on turbocharger
selection.
>>One of the damnable aspects of turbo sizing is trying to
come up with
>>the best compromise for the dynamics of manifold pressure
>>generation: how much, how quickly, how efficiently, and how
costly.
>>Automotive turbo applications are driven by a much different
set of
>>goals, primarily their wide-range dynamic response issues. The
>>aircraft turbo application is a much happier equation to solve,
>>whether it be for all out power, or altitude compensation.
In fact,
>>there is an opportunity to closely optimize the performance
of all
>>the components so that some real magic begins to happen.
>>It's possible to size a turbocharger so that the engine, and
its
>>forced induction, as a complete system, operates in
'crossover',
>>that is, when intake manifold pressure exceeds the exhaust
manifold
>>back pressure driving the turbine. The result is a net positive
>>scavenge flow through the cycle and the result is
significant gains
>>in power. Combined with the right intake/ exhaust event
timing, BSFC
>>benefits as well. This crossover condition can only be
achieved in a
>>narrow band of operation, but it happens to be the same band
that
>>aircraft engines are looking for: Part throttle, medium
load, and
>>medium rpm ( as a % of max rpm)
>>
>>A program goes something like this:
>>Select the 'frame' size, or family of turbocharger so that
there is a
>>good selection of turbine wheels, turbine houngs, trims and
>>compressors available in the range of mass flows necessary. The
>>Garrett T04 is such a family. Then, generally on the dyno, the
>>turbine side of the turbocharger is selected for its swallowing
>>capacity, resulting in larger than normal A/Rs. The
objective is to
>>use all the gas flow to drive the turbine, not bypass
continuously
>>through a waste gate. (The waste gate and its sizing still must
>>function as a safety/ control device, and for temporary
demands for
>>higher manifold pressure)
>>
>>The compressor selection can now be optimized for its output
mass
>>flow and tailored for its best efficiency. The simplest way to
>>verify the component selection is to monitor the exhaust
manifold
>>pressure just ahead of the turbine inlet, in fact the port can
>>sometimes be coincident with the EGT probe location. A small
damping
>>orifice, around .020 seems to work well, will give steady
readings at
>>the gage. A similar set up on the intake side, both before and
>>after the inter cooler. Then you can go to work mapping the
relative
>>pressure results vs. a whole range of operating parameters,
load,
>>speed, temps et al. When you home in on crossover, even if
its only
>>a .5 psi differential, watch what happens to all the other
numbers.
>>If you can achieve a 2, 3 or 5 psi lower turbine inlet pressure
>>relative to intake manifold pressure, then you've really
done a good
>>job. You'll see directionally correct moves in the power,
temps,
>>the bsfc, the ignition timing, et al.
>>
>>The resulting turbo configuration will be a hybrid: an unusual
>>combination of turbine and compressor, most likely much
bigger on
>>both sides than the automotive application. And operating in
speed
>>and temperature ranges that are comfortable, safe and
unstressed. My
>>guess is that none of the stock automotive turbo components
will be
>>there in the end. The material specs are just not high enough
>>( anyone who flies without an Inconel wheel is asking for
it...) The
>>power and efficiency numbers will be dramatic, the turbo
application
>>is truly in its element once this balance has been achieved,
and the
>>near steady state operation of the IC engine is one of the
few times
>>we as engine development engineers can come close to this near
>>perfect condition.
>>
>>Reeves Callaway
>>
>>Thanks Reeves. So re wording this a bit a good system has
>>2 to 5 psi lower exhaust manifold pressure than intake manifold
>>pressure. I'll do an example. Lets say we have 10 psi boost
>>above atmospheric. Then the exhaust manifold will show
>>5 to 8 psi above atmospheric. Did I get it right?
>>
>>--
>>Paul Lamar ...No rotor no motor.
>>
>
>Now we're 'cooking with gas, as it were, Paul. Your example is
correct.
>
>And just to emphasize, even the smallest differential between
>turbine inlet pressure and compressor outlet pressure is the
goal.
>( As pilots , we really refer to all pressures in inches of Hg
>Absolute. So running 40" of intake manifold pressure using only
>39.5" (or less) driving the turbine is the goal.
>
>What scares me most about the turbo discussion on this list
are the
>signs of impending disaster... tips of turbine wheels
eroding, or
>gone missing! I can't tell you how wildly over-stressed the
>components have to be to do that. In the horsepower and duty
cycle
>ranges that you are shooting for, the 200-300 hp area, there
is no
>question that you should all be in the T04b Garrett frame
size. I
>know nothing about the std Mazda turbo other than it has to be
>selected by cost factors, not metallurgy. It certainly was
when we
>did the turbocharger design/build MazdaSpeed Protege for MNAO
>( BTW, we were on the hook for sharing the warranty on those
units.
>4 years and 6000 vehicles later, I can report that the warranty
>incident on the Mazdaspeed/Callaway turbo version was
significantly
>less than the norm.
>In the car biz, a turbo failure is a 'walk home incident'.
You know
>what it is here...
>Reeves Callaway
>
>The standard RX7 Turbo II is about a TO3 size. Other than
that we know
>little about it. We have had several turbine failures. I suspect
>mostly due to over speed. All rather benign without a
>total loss of power. I suspect they are running well over
100,000
>RPM at 12,000 feet and higher. I have not been able to talk
>anybody into installing a turbo tach so far unfortunately.
>--
>Paul Lamar ...No rotor no motor.
>
>Thanks for that great info, Reeves.
>
>I think most "car guys" are surprised at how big the turbines
and
>compressors are on aircraft engines. I know of at least one
Subaru
>guy who fried a turbine wheel in his Van's RV because it was too
>small to keep up at altitude.
>
>As you explained, the pressure differential between intake and
>exhaust is one of the really key factors that most people aren't
>aware of. I was just re-reading an old NACA paper on
>turbocompounding from 1944 that really drives this home. As they
>lowered the pressure at the turbine outlet -- to simulate higher
>altitude -- the power recovered from the turbine increased
dramatically.
>
>I'm going to attach a copy but I don't know if its too big to
>distribute on the list. The engine is an R-1340 making about 500
>horsepower and they hooked up an impulse turbine to the
exhaust from
>eight of the nine engine cylinders. A separate brake
dynamometer was
>on the turbine wheel.
>
>At the equivalent of about 30,000 feet altitude the trubine was
>making over 100 horsepower, about 21 percent of the engine power
>output. This is incredible. One can only guess what could be
>possible with a rotary where there is no exhaust valve to eat up
>half of the exhaust energy.
>
>Anyway, I got to thinking about why they chose an impulse
turbine,
>rather than a reaction turbine as commonly seen on
turbochargers. Is
>it possible that an impulse wheel is more efficient at this
rpm and
>power? Plus an impulse wheel can be spun a lot more slowly,
even 1:1
>with engine rpm if the wheel is big enough. (The wheel on the
R-1340
>was 11 inches and spun about 20,000 rpm.)
>
>I did a quick calculation of turbine pumping specific speed,
using a
>flow of 300 cfm (for a two-rotor), and an exhuast pressure of 18
>psi, and came up with about 1000. This would seem to point to an
>impulse turbine as I understand reaction turbines are better
in the
>100 to 300 range.
>
>However, I also calculated power specific speed which comes
out to
>about 500 if we assume that we want to make 50 hp with a wheel
>spinning at 6500 rpm. It would be higher if we spin the wheel
>faster, but I'm trying to see if it could work 1:1. That way you
>could just bolt the turbine wheel right to the e-shaft. No
worries
>about gears or drives -- and nothing to fail. If this can be
done
>with a wheel of reasonable size, say 12 inches, it could work
>nicely. The nice thing about an impulse wheel is that, unlike a
>reaction wheel, it is very efficient at low rpm.
>
>The choice of an impulse or reaction wheel for the power
recovery
>turbine could have significant implications for efficiency. In
>turbine engines, there is usually a combination of both, and
each is
>there for a specific reason. So if there is something to be
gained,
>it bears investigating.
>
>A company that makes both types for aerospace applications
>Barber-Nichols and I will contact them to see what they say:
>
>http://www.barber-nichols.com/products/turbines/
axial_impulse_turbines/default
>.asp
>
>
>There's a bit of elementary stuff on impulse and reaction
turbines
>in Wikipedia: http://en.wikipedia.org/wiki/Turbine )
>
>
>Regards,
>
>Gordon.
>
>It is too big to publish but I already have it.
>Thanks anyway Gordon.
>
>We discussed bolting a turbine to the e-shaft quite awhile
>ago. Here are some 3D's. Mark Wrathal did not think too highly
>of the idea as I recall :)
>
>Paul Lamar
Paul,
Yes this picture of axial impulse turbine picture is what got
me jazzed
about T.C. in the first place. How to isolate the heat from the
end
housing
and gearbox would be a challenge. We are talking over 1650
degrees here
right? Put on the other end, isolated, with double walled housing
and water
cooled shaft it might be doable. Space might then be the problem.
Hence we start testing with a less efficient but more compact
centrifugal
turbine. Perhaps as Matt suggested once we have some data we can
order up a
smaller axial turbine.
Reeves,
I did save your 10/2004 note but missed the significance of it.
Thanks for
re-sending it. I am curious as to your thoughts if the same
increase in
efficiency would happen at high altitude by engaging an Rotrex
traction
driven supercharger (running off of the front engine belt) and
devoting the
exhaust driven turbine solely to the function of turbo
compounding?
You mention the T0-4 Garret group of turbines is good but for
300hp
turbo
normalizing I believe Tom Parks is using a T0-6. Better?? In
addition my
brother and other A & Ps have recommending RaJay units as a
bullet proof
devices. I understand RaJay have been owned by everyone including
Garrett
at one time or other. I completely agree with you on using
made-for-aviation-turbos however. In the bench test phase cheap
auto
turbos
can provide some serious data in the preliminary tests.
Doug Fir
A reaction turbine can't overspeed, right? It will always be
limited
to the
speed of the exhaust gasses impinging on the "buckets", regardless
fo the
load on the compressor, so if the unit has been designed to
withstand
turning at the speed of the exhaust pulse, everything will be
fine.
Mark W.
The trouble is the speed of the exhaust gas can be super sonic
or close to it I think. Lets say it is 1100 FPS so
a 12 inch turbine could go 350 revs per second or
21,000 RPM. More than enough to blow up a 12 inch
turbine I suspect. As I recall most small turbo fans
the turbines are limited to about 10,000 RPM.
A J79 limit was 8000 RPM. The exhaust velocity of a
18,000 pound thrust JT3D -3b is 1560 FPS.
If you want to get a feel for this hold a ball bearing
by the bore race and take the air squirter on your shop
air compressor hose and direct it at the outer race of the
bearing. You will be amazed.
--
Paul Lamar ...No rotor no motor.
Mark,
I think you meant to say impusle turbine. As far as I
understand it,
the speed of the exhuast jet across the buckets is determined
by the
nozzles, which deflect the gas so it flows axially across the
wheel.
This turning of the flow obviously speeds up the gas somewhat, so
you don't want it to go supersonic because that will reduce
efficiency.
And at higher altitude where the speed of sound goes down, you
want
to make sure that the flow is still subsonic. I've included
some of
the text from that NACA paper (Report No. 786, 1944.)
Paul,
I think your calculation of wheel speed is based on radial flow
not
axial. The impulse wheel does not need to go the same speed as the
exhuast gas -- it can go whatever speed you want, so long as the
buckets and wheel diameter are sized to absorb the kinetic
energy in
the exhuast jet efficiently.
You'll notice the equation for maximum power output of the
turbine,
Pmax. The authors note that, "the conditions are almost exactly
satisfied by single-stage impule turbines, unless the mach number
becomes too high, at which time the buckets choke and the
instantaneous efficiency is reduced."
So I don't think it's a case of worrying about wheel speed, but
rather nozzle design so as not to speed up the flow too much.
Those
nozzles and buckets that they used are really simple and I think
more modern designs use more aerodynamic shapes.
Doug,
You can see the power output at 13,000 feet altitude in Figure 4.
That height is about 18 inches of mercury, so the turbine is still
making 80 hp there, over 15 percent.
We can actually use the equations in this paper to figure out
exactly how much turbine power we could make from a rotary
engine --
all we need to know are a couple of pieces of data that could be
measured, such as exhaust gas velocity and mass flow.
As far as exhaust pressure is concerned, we can figure that out
too,
using the first law of thermodynamics. I've found a good paper on
this where they used this method to estimate exhaust pressure,
then
compared the results to measured data. I've attached it here as a
pdf and I think it's under 200 kb.
It would be great to get the data needed to run through some of
these calculations -- and to see how much power we could
expect to
recover from a rotary. It would be a lot more than from a
piston engine.
Regards,
Gordon.
Doug,
Sorry for the late answer.
But just a comment on frame sizes in turbocharger families: The
extreme sizes in each of the families has some overlap with the
capabilities and throughputs of the adjoining families, as you would
expect. That is, the largest of the TO4 series will overlap the
smallest of the TE06 in the Garret range. So, its entirely possible
to get equally good selections out of either frame size.
However, the weight and size is another consideration. A TE06 starts
at about 35 lbs. mass.
A T04B is about 17. And to be realistic, the selection in the car
world is all about packaging. Given that in aircraft, we'd like to
choose by best performance followed by best package, and followed by
best cost, let's say we wind up with a TE06. Great! it can be made
to be a perfect size match I'm sure. And it will be under stressed.
And they have been around for 40 years... there should be plenty of
them from the rebuilders. But they were almost all Diesel units.
That means no inconel wheels in high numbers, and no high nickel
iron housings. Be careful with the spec. and get only the gasoline,
high-grade, good ones.
Just remember; its two whole frame sizes away from a T03- the car size.
Re. Rajay. yes they were once good... when they were aircraft
grade. But they went after the larger automotive market and
floundered. I would be very suspicious. One of them failed me at
Pikes Peak when I had a full minute on the field! Probably my fault
as I was no doubt over spinning it. At least I could walk home.
Reeves
Pikes Peak is 14,000 feet high.
Paul Lamar ...No rotor no motor.
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