Free Piston Compressor for Turbofan? ("pulse turbine")

[schwee]

Has anyone on these forums heard of using a free-piston compressor to boost the compression ratio of a turboprop or turbofan?

The concept would be to refit the combustion chamber of an existing jet engine with a cylinder, valves or ports, and a free piston, using the "air cushion" on the non-combustion side of the piston to provide the compressing force. Oscillations could be introduced by allowing a portion of the escaping exhaust gases (thrust) to press against the air cushions, which would then spring back and compress a fresh charge once the "stroke" is completed.

The result, it seems, might be a "pulse turbine," where a multi-stage turbine would act as a compressor for low compression, high volume, ambient density air, and a free piston oscillating at high rpm (10,000+ ?), would further compress the now lower volume, higher density, higher compression air, and combust it intermittently. There would still be no connecting rods and no crankshaft -- the addition of a free piston to the combustion chamber would simply be an attempt to increase the compression ratio, and hence the fuel efficiency, of the turbofan/ turboprop while preserving its characteristics of light weight and high power.

Variations on this scheme might include one or more dual-ended free pistons, in which there would be no air cushions but rather combustion on each end; or, alternately, a single cylinder with two opposing free pistons, each with its own air cushion. This latter variation might be useful in balancing the moving weight of the pistons and thus minimizing vibration.

I would welcome any speculation on the effects that a "pulsed" type of thrust vs. continuous combustion might have on the functioning and longevity of the (rear) turbine blades.

 

[tbuelna]

schwee:

Here's a nifty way to get high pressure ratios in a single compressor stage:

http://www.ramgen.com/tech.html

Regards,
Terry

 

[schwee]

Thanks, Terry -

Intriguing. On the other hand, I hear myself saying the kinds of things I myself hate to hear -- "if it's so good, why haven't they built it?" "Why are there 'projected,' rather than measured, efficiencies?" It sounds like it's an expensive project, and they don't have the money. That sounds distressingly familiar.

My big puzzlement is with the basic concept. Compression is one of the things you want, and that's achieved in this model by generating supersonic shockwaves from the "waffleated" rim of a rapidly spinning disk. But the real thing you want in an engine is to process as much air at a time as you can. I guess this does that, if it has a compressor. But no -- I don't think it does. It'll compress air all right, I guess, but if this is continuous where is new air (the "air flow&quot coming from? And how do they, . . . .well, I don't know, but I can't see it. I mean, even a piston is at least generating a vacuum to pull in air -- I don't see where this is happening, here.

It's an interesting idea, but I'd sure want to see a prototype in action before I'd invest. Don't think I'd trade 'em even up. Thanks for the link, though. ).

--Paul

 

[JayMaechtlen]

My confusion about the free-piston compressor is similar to yours regarding the ramdisk.
Where is the energy obtained to drive the fpc?
regards


Jay Maechtlen

 

[schwee]

Hi, Jay --
Interesting you should ask, because I'm trying to dope that out right now.

I'm thinking the power to drive the free piston compressor comes from the expanding gases during combustion. I mean, I know that's where it comes from. This was all straightforward to me once, but now it's not. I know it DOES work -- people have done it, but it's not easy to find stuff.

OK, well, first, you constantly charge your air bouncers with the compressor, if they need it, due to leakage around the rings, say. Put in a reed valve so that when bouncer pressure is less than compressor pressure, probably most often at TDC (pistons together in the center), reed valve lets in fresh compressor air. This also keeps the 2 bouncers evenly charged -- a very important thing. To bleed off fumes and other undesirables, we're gonna have to periodically vent junk outta the bouncers (analogous to a crankcase). can't do it while the bouncers are compressed, though -- that would be bad. Ideas? Periodic ground maintenance? It's a diesel, so inadvertent combustion of the bouncers shouldn't be a problem.

Now, you must valve intake and exhaust (thrust). Intake seems not too bad -- I'd put a reed valve nearish the center, to open when compressor pressure is greater than the chamber pressure. Simple -- no cams or timing needed, it runs itself. Might could do it with strategically placed ports, too -- but I can't see it right now.

Exhaust is harder. You can't just use a port, because the oscillations of the pistons are not consistent. Well, maybe you can -- put a long ish slot or gash in there round and about the bottom of the stroke. Or not even that long. Piston covers up the exhaust hole during compression, exposes at bottom of stroke. the more I think about it, this should work -- the piston will pretty much always make it to the hole, because expansion stroke is always much stronger than resistance of bouncers (equal to compression). At low throttle, piston will just barely expose hole; at full throttle, hole is completely open. The bigger hole lets gases escape faster and slows the careening pistons. Pistons halt; chamber pressure is now below compressor pressure; intake reed valve opens for a nanosecond or so; pistons begin their upward ascent; voila.

Compression depends on how far the bouncers have been compressed, but this doesn't change as much as one might think, because the expanding gases escape faster the harder the piston drives down from combustion.

As with any two-cycles, the crucial time is the bottom of the expansion stroke, where you've gotta evacuate the exhaust and take in a fresh charge in an instant. Plus, since I think this thing needs to run at ridiculously high rpms to get power competitive with conventional turbofans, (40k-60k?? More??) I'd prefer not to have valves, if at all possible. Valve floating would be uncool.

Does that help? I hope so. In fact, I hope you understand it better than I do, 'cause I could use some fresh ideas.

--Paul

 

[JayMaechtlen]

Ok- you are thinking of using the fpe not just as a compressor, but as a high-pressure section of the machine, including combustion and initial expansion. Right?
Then the exhaust would continue expanding out the power section of the turbine, delivering shaft power and/or thrust. The compressor side of the turbine would essentially be a supercharger for the fpe.
Have you looked at the flow rate of any particular turbine, then looked at what size fpe would flow the same amount of air?
You propose a compression-ignition fpe in this instance?
What kind of injectors would function at 40kHz? or even 10 kHz?
regards



Jay Maechtlen

 

[schwee]

Jay --
Right, that's true, it's a compressor combined with combustion chamber.

Yup, the compressor is the biggest, hairiest supercharger around. Not sure it's advantageous, energy-wise, to have it so big.

No, I haven't looked at the flow rate of a particular turbine. Maybe Williams has something. that is an important question for me right now -- how fast and how big?

I'm proposing injection, because it sure seems to me compression ignition is problematic. Narrow power/speed range, perhaps? Also dirty -- it's much neater in my mind if you intake fresh air rather than fuel/air. You can even force a bit of clean air out after expansion/exhaust to make sure it's all outta there.

Dunno what kind of injectors will function at 60 kHz. That's an arresting thought. certainly no commercial ones, but I'm not thinking of building in my garage. I guess the question becomes, if you spend $5 million, can you develop a really fast injector, and make it cheap?

All has to do with flow rate, you're right. I'd prefer small and fast instead of big and slow. Perhaps the speed limitations determine the displacement.

Paul

 

[tbuelna]

Scwhee-

the interesting part about the Ramgen technology was that they were proposing to use it as a stationary ramjet engine. I saw a design review from NASA regarding the concept, and the simulation numbers looked pretty good with regards to SFC and emissions. In fact the SFC was about 10% better than a 1 MW Solar turbine used as a baseline.

I have no idea if it will actually work as advertised, but it sure sounds intriguing!

regards,
Terry

 

[schwee]

Terry:

I wonder if they could put reed valves on it (can you tell I love reed valves? Evidently that's what the German WWII V-1 (and V-2?) buzzbombs did.

Here's a note of interest: a "howstuffworks.com" response to a several-years-old query about, I think the correct phrase is, "turbine backpressure." Quey seemed to be exactly what I want to know -- then I realized this was because I was the guy who asked the question, way back when.
). I do not agree with the responder, which is why I have been keeping at it.
--------

ME: Why doesn't the Exhaust in a jet engine come out the front?

I have been reading about jet engines for a long time. I would like to know how the air that comes from the compressors doesn't force its way back through them (out the front of the engine) after exploding in the burning chamber. In a four-stroke engine (for example), there are valves to close the chamber just before burning. The pressure in the jet burning chamber is much higher than that of incoming air flow, so what makes the exhaust air flow only to the exit (turbo) fan?

ANSWER:
One thing to keep in mind when thinking about the flow of air through a jet engine is the "path of least resistance" concept. When you burn the fuel in a jet engine, the air and combusted fuel expand considerably. These exhaust gases are looking for a way out of the combustion chamber.

When they "look" forward toward the compressor, what they "see" is a wall of air at something like 10 times normal atmospheric pressure moving at several hundred miles per hour. When the gases "look" back toward the end of the engine, what they "see" is a nearly clear path toward normal atmospheric pressure. The only thing in the way is the turbine fan, and that is nothing. The path of least resistance is clearly toward the back of the engine, so that's where the gases go.

That is true in any jet engine that has a compressor. A Ram jet is essentially open at both ends, so it suffers from the problem you are suggesting. It actually has to be moving through the air at a fairly good clip before it will work. There has to be enough pressure in the incoming air to make the back of the engine the obvious path of least resistance.

In WWII, Germany used pulse jets in its V-1 missiles. These missiles were known as "buzz bombs" at the time. They actually had a valve like you suggest -- at the front of the engine was a set of shutters. These shutters would open to let in a slug of air, and then close. The engine would inject fuel into this slug of air and ignite it. With the flaps shut, the exhaust gases had only one way to go. Then the flaps would open and the cycle repeated. This allowed buzz bombs to start from a standing stop (unlike a Ram jet) without having the complexity of a compressor.

----
Also:
Jay -- just came up with an idea for an extremely high speed, variable flow fuel injector. Oughta run at least 100 Hz -- 200 Hz-- whatever you want, really. Should cost about $10 in volume production for the injector itself, more for the control (integrated with a lot of other stuff). Even I could make one now in my garage.
Also have very high speed, durable, ridiculously simple one-way valves that should be very resistant to fatigue.

--Paul

 

[tbuelna]

schwee:

the first thing that came to my mind when I read the description of your compressor/free piston reciprocator/turbine arrangement, was the infamous Napier Nomad. It was fairly close to what you propose, except for the fact that the reciprocator was a flat-12, conventional piston engine. It had a 12 stage axial compressor and a 3 stage axial turbine compounded back to the crank with a Beir CVT, with the piston engine in between.

http://www.enginehistory.org/NASM/NomadE145-1.jpg

I just love oddball engine projects myself. If you're interested in seeing what I'm working on in my garage, send me your email and I'll forward some illustrations.

Regards,
Terry

 

[schwee]

Terry --

Yikes. "one of the most complex and fuel efficient engines ever built." I LIKE it already. I read some more about it on a different site -- any idea why it didn't make it to commercial viability? (how many answers could there be for THAT one?)

In comparison to mine, I'd say, well, "WHAT crank?" and "WHAT cvt?"

I am definitely interested in what you are working on in your garage. The topic du jour in my brain these days is cooling, especially for the pistons.

I'm at pschwiesow@interfold.com.

--paul

 

[Michael996]

Piston compressors are not more efficient than axial compressors. The only reason they are used anywhere (including cars) is for cost and speed issues.

 

[schwee]

Um, Michael, if that's a challenge, I refuse to answer it. Evidently our sources of information differ. I'll choose to believe mine.
--Paul

 

[tbuelna]

Michael996:

Piston compressors are most definitely more efficient than axial compressors. The difference resulting from the fact that a piston compressor is a positive dispalcement device and the axial compressor is a dynamic device.

However, you are correct when you state that piston compressors are more costly and suited only for relativey slow speed operation.

Regards,
Terry

 

[jdsewell]

I don't understand how a piston compressor can be more effecient than an axial compressor. A piston has to change directions twice to complete a cycle, having to work against inertia. An axial compressor doesn't have this issue. Is the efficiency of compressing the fluid in a closed enviroment that much more effecient than the axial compressor's more open system?
Please enlighten me.
As a side note, has anyone attempted to use a "scroll" compressor in an engine?

 

[JayMaechtlen]

axial vs. inline-
I believe that the above post contains a misconception or two.
"having to work against inertia": the motion of the piston is halted by the compression of a gas* - a combustion event then occurs which dives the piston vigerously in the other direction. The kinetic energy of the piston is stored in the gas as pressure and temperature. If there were no combustion, the piston would (if launched and released) eventually stop due mainly to sliding friction. (there would be losses due to gas turbulence, etc., also slowing the piston)
In the axial compressor there are aerodynamic losses from each blade in each fan. There are typically 4-7-(more?) fans in the compressor section of a jet engine to get the total compression ratio high enough.
One way to compare piston vs. axial is
piston = low flow, high pressure
axial = high flow, low pressure.

Right?
* compare gas to a spring- costs energy to compress, but get almost all of it back.

regards


Jay Maechtlen

 

[schwee]

I've seen an (advertised) compression ratio of 7:1 in a 16-stage axial commpressor in a turbofan. Compare to typical piston engines at anywhere from about 7:1 to 50:1 or more. I agree with your assessment.

 

[Michael996]

Yes, we are looking at different information. The differences are very pronounced at the mass flow rates of turbine engines.

Just the inlet designs available to the two different types of compressors rule out any piston/valve compressors, in the efficiency arena alone.

 

[Michael996]

FYI the Allied Signal G250 APU Compressor peaks out at 82% efficient (with a pressure ratio of 5.5). According to A/S Data book on the AUP.

 

[schwee]

Michael 1996 --
Sorry to be touchy, it's just that sometimes a blunt statement of fact without additional explanation seems like an attack, to an "inventor" type, or at least one who'd like to fashion himself so.

I've been trying to bone up a bit on compression in turbofans, and it seems the nature of my confusion is mostly in terminology. When people quote "pressure ratios" in a turbofan of as high as 50:1, I believe they are referring to "overall pressure ratio," a measure of pressure difference between inlet air and point of highest pressure (?) just prior to reaching the expansion turbines.

For my proposed device, I believe the applicable concept is "compressor pressure ratio," or CPR. Would this be accurate? These numbers appear more consistent -- 5.5, as you point out, or perhaps 7. NASA Glenn's UEET project appears to have a "highly loaded turbomachinery" group seeking to work on mostly materials technology to achieve a, as they put it, "radical" CPR of around 12.

I've recently come across a paper that is very close to what I'm proposing -- the direct use of expanding gases from a free-piston compressor/combustor to power a turbine. Here is the link:

http://www.itk.ntnu.no/ansatte/Johansen_Tor.Arne/klc_struc4.pdf

As you point out, mass flow rate is the critical issue for a thrust/weight ratio suitable for a turbofan application. It appears that any free piston device must operate in the neighborhood of at least 15,000 cycles per minute, and perhaps more like 30,000, from various inputs I've had.