>>> 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
> turbo compounding 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 turbine 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 exhaust 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
>
> Thanks for the drawings, Paul.
>
> Btw, when I guessed 18 psi exhaust pressure at turbine inlet, I
> meant without boost. Obviously with any increase in boost, the
> exhaust pressure will rise by a similar amount. Reeves makes a good
> point about keeping the inlet pressure above outlet by a few psi (or
> inches of mercury).
>
> About that big turbine wheel, I notice that you have joined the two
> exhaust stacks before bringing them into the wheel.
>
> Joining the stacks defeats one of the big advantages of this type of
> setup, which is called a blow down turbine -- namely that the
> maximum kinetic energy is gained from each individual exhaust
> stack. When you join two stacks, you lose kinetic energy as heat,
> due to the pressure differences.
>
> "For a given turbine outlet pressure, the maximum net power of the
> engine and the blow down turbine can be attained by so discharging
> the exhaust gas from each cylinder as a separate jet that no
> interaction of exhaust events occurs and thus permitting each
> cylinder to exhaust to the turbine outlet pressure."
>
> The NACA guys paid particular attention to this. They did pair up
> two cylinders because they had nine individual stacks to work with,
> which was a little bit of a fabrication challenge. But they admit it
> cost a little power.
>
> With a rotary though, you have only two stacks (maximum four stacks,
> depending on how many rotors you have). This is very workable with
> having individual stacks go right to the nozzles.
>
> And speaking of nozzles, where are the nozzles in your inlet box?
> The nozzles are what convert the pressure to velocity, which drives
> the wheel, so you've got to have them.
>
> And it might make more sense to mount the wheel at the accessory
> end, if that's workable.
>
> What was the objection to the big turbine wheel? I would bet it
> would make more power than a turbocharger-type turbine and reduction
> gearing. Only thing is you would have to probably fabricate
> everything from scratch. Could get expensive in a hurry.
>
>
> Regards,
>
> Gordon.
>
> Gordon please try screen capture on those pdf files to illustrate
> your points. Makes things much more understandable.
> Enlarge the graph until it fits the screen. Then press
> the Print Scrn key somewhere on you keyboard. Load MS Paint.
> Under "Edit" choose "Paste" The screen capture will appear.
> Save as a jpg and attach the jpg to your message.
>
>
> Here are some images from the NACA paper you are referring to.
>
> --
> Paul Lamar ...No rotor no motor.
Paul,
So with that engine we could recover about 9% power at 8200' but we
would
have to go to 33,000 feet to obtain the 21% increase in power.
So at 15,000 feet perhaps at least 13% is possible.
Doug Fir
I expect much better numbers than that.
That was 70 years ago. By the time the R3350TC was developed
10 years later things were much improved. I think the aerodynamics
of turbines have progressed somewhat since then. Also 2/3 of the
kinetic energy useful for driving the turbine was dissipated by
the exhaust valves. The 2nd chart is at sea level.
Do I need to point out again the rotary has no exhaust
valves? :)
Paul Lamar
What??!! No exhaust valves? That's just crazy.. :)
Matt-
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