Proposed Design: Elektra II - Revised Version

[larry cottrill]

At the time I proposed Elektra I, I also roughed out the idea for Elektra II, a supposedly improved version. Here is an updated version of Elektra II, incorporating lessons learned from Elektra I as well as the geometric ratios advanced by Bill Hinote in his response to my problems getting the Fo Mi Chin engine running [see my 'For a Close Shave ...' thread].

In this one, the exit port from the intake is moved to a point just in front of the tailpipe entrance, by cutting into the top of the tailpipe, but not set as far rearward as in the Fo Mi Chin prototype. The intake pipe is radically shortened from the Elektra I, to set up the intake at a wave path length of L/3 from the front wall of the chamber with a full length L = 23 inches, just a hair longer than a Dynajet valved engine. The intake flare, however constructed, will need to be about the same diameter as the tailpipe.

The 4.6 inch dimension should actually be the wave path length in the intake pipe, including the bend; however, the bent portion is so short that the error should be negligible. The engine in this proposed prototype form has calculated ratios as follows:

Datum 1 (intake mouth station): 7.6 / 23.0 = 0.330 = 33%
Datum 2 (intake tube length): 4.6 / 23.0 = 0.200 = 20%
Datum 3 (intake port station): 3.0(est.) / 23.0 = 0.130 = 13%

See Bill's discussion of the criteria in his posts to the 'For a Close Shave ...' thread on this forum.

So, what do you all think?

L Cottrill

[hinote]

At the time I proposed Elektra I, I also roughed out the idea for Elektra II, a supposedly improved version. Here is an updated version of Elektra II, incorporating lessons learned from Elektra I as well as the geometric ratios advanced by Bill Hinote in his response to my problems getting the Fo Mi Chin engine running [see my 'For a Close Shave ...' thread].

In this one, the exit port from the intake is moved to a point just in front of the tailpipe entrance, by cutting into the top of the tailpipe, but not set as far rearward as in the Fo Mi Chin prototype.

So, what do you all think?

L Cottrill

Hi Larry:

Since almost everybody here is shooting (PJ) ducks in the dark, I can only say it looks like a good effort to me--give it a try--and if it doesn't work, you can blame me!!

One detail I particularly like is that the intake is moved into the back of the combustion chamber (somewhat like the Chinese); I've been wondering if the location of the intake intersecting the "output tube" of the Fo Mi Chin might have been a poor choice--the potential for high exhaust velocities there may have been contaminating the intake resonance that appears to be an important function of these valveless designs.

I can only hope that in a couple more years we're going to be able to look back on these times as primitive, in terms of PJ design capabilities. With all the software development occurring out there I can only hope that Uflow is just the first of many programs we may be able to access, allowing us to accurately design the pulsejet of our dreams.

Meanwhile, (as M. would say) Vive la Pulsion!

Bill H.
Acoustic Propulsion Concepts

[Stuart]

I think I'm going to start wrapping my fuel line around the tail pipe for a little heat recovery. Doesn't make sense if you are trying to evolve to liquid fuel, but would give you a little extra Gluey poop from propane.

Also, if you are using propane, nozzling your copper fuel line tip (with a pinch) will do a little.

I know these things aren't for the purist, but if you are trying to get the bugger to run - well, anything goes.

[larry cottrill]

Hi Larry:

Since almost everybody here is shooting (PJ) ducks in the dark, I can only say it looks like a good effort to me--give it a try--and if it doesn't work, you can blame me!!

One detail I particularly like is that the intake is moved into the back of the combustion chamber (somewhat like the Chinese); I've been wondering if the location of the intake intersecting the "output tube" of the Fo Mi Chin might have been a poor choice--the potential for high exhaust velocities there may have been contaminating the intake resonance that appears to be an important function of these valveless designs.

Bill, it may very well be that this is a better location. One good feature is that you're not forced to severely flatten the inside end of the intake pipe. I show it flattened slightly just because it is so close to the tailpipe port, where everything comes together.

However, the location of the port in the Fo Mi Chin is quite intentionally placed in the high velocity zone. This is to take advantage of the 'zero excess static pressure' mode of that flow through most of the expansion cycle, theoretically eliminating mass ejection out through the intake. Of course, the pressure wave will not be so accommodating, so the intake is still bent fully rearward.

I can only hope that in a couple more years we're going to be able to look back on these times as primitive, in terms of PJ design capabilities. With all the software development occurring out there I can only hope that Uflow is just the first of many programs we may be able to access, allowing us to accurately design the pulsejet of our dreams.

Meanwhile, (as M. would say) Vive la Pulsion!

Bill H.
Acoustic Propulsion Concepts

UFlow1D really has its limitations, in my case, because it is impossible [I think] to rearrange these engines into one-dimensional form, since they aren't symmetrical around the central axis. I think the 'least wrong' approach is to model them as if the intake pipe just didn't exist, but that's obviously not very close to reality, especially in terms of wave action.

And what about the Elektra I business of the basic frequency not being determined by the engine overall length? How does one explain that kind of behavior, really?

I personally believe that the only way to accurately model such things will be to model the gases as a bunch of 'giant molecules' filling the space and behaving the way molecules of ideal gas really do, responding to pressure, temperature, collision, etc. and then feeding back their resultant temp, pressure, velocity, etc. to the whole, iterated many times around. It isn't equations that drive our pulsejets, but rather, intermolecular forces -- that's all. A "perfect" model would know that, and work accordingly.

L Cottrill

[yipster]

downloaded UFlow1D from http://capella.colorado.edu/~laney/uflow.htm sure looks like it has its limitations but like to try anyway. maybe it opens a IMM pic. have to play with it a little, is there more free or low cost software on this?
thanks for mentioning and Larry, shouldnt "volume" be next to pressure in the list also?

[hinote]

And what about the Elektra I business of the basic frequency not being determined by the engine overall length? How does one explain that kind of behavior, really?

Here I go again, shooting ducks in the dark:

I think someone has mentioned that these engines can run at F2; I'm betting you're getting a major harmonic (F1 or F2) that has a smaller frequency superimposed over it--maybe subtractively. This would be because the resonant lengths of the acoustic components aren't matched to each other properly.

Does that make any sense at all?

Bill H.
Acoustic Propulsion Concepts

[Stuart]

I don't usually follow the acoustic arguments much, but I am curious if anyone "beats their pipes" - I know that may have a couple meanings. But, tapping the engine and finding out the frequency (when it isn't running) should be related to the operating frequency in some way.

[hinote]

I don't usually follow the acoustic arguments much, but I am curious if anyone "beats their pipes" - I know that may have a couple meanings. But, tapping the engine and finding out the frequency (when it isn't running) should be related to the operating frequency in some way.

I've found a similar but possibly more effective way to do this (but don't do it while somebody else is watching--they'll think you're crazy!):

SING into one end of the pulsejet, and vary the tone up and down; at some point the engine resonates along with your voice, and the tone becomes noticeably louder.

Try it--it really does work.

Bill H.
Acoustic Propulsion Concepts

[larry cottrill]

I don't usually follow the acoustic arguments much, but I am curious if anyone "beats their pipes" - I know that may have a couple meanings. But, tapping the engine and finding out the frequency (when it isn't running) should be related to the operating frequency in some way.

I've found a similar but possibly more effective way to do this (but don't do it while somebody else is watching--they'll think you're crazy!):

SING into one end of the pulsejet, and vary the tone up and down; at some point the engine resonates along with your voice, and the tone becomes noticeably louder.

Try it--it really does work.

Bill H.
Acoustic Propulsion Concepts

Bill -

I tried this a few months ago with the Dynajet. And you're right -- it is surprising how close it seems to the frequency of the engine running.

But, how can this be? When you're doing this, the engine is full of cool, dense air -- nothing like the gas condition in the pipe while the thing is running!!!

L Cottrill

[Viv]

I don't usually follow the acoustic arguments much, but I am curious if anyone "beats their pipes" - I know that may have a couple meanings. But, tapping the engine and finding out the frequency (when it isn't running) should be related to the operating frequency in some way.

I've found a similar but possibly more effective way to do this (but don't do it while somebody else is watching--they'll think you're crazy!):

SING into one end of the pulsejet, and vary the tone up and down; at some point the engine resonates along with your voice, and the tone becomes noticeably louder.

Try it--it really does work.

Bill H.
Acoustic Propulsion Concepts

Bill -

I tried this a few months ago with the Dynajet. And you're right -- it is surprising how close it seems to the frequency of the engine running.

But, how can this be? When you're doing this, the engine is full of cool, dense air -- nothing like the gas condition in the pipe while the thing is running!!!

L Cottrill

While developing the FFT method originaly for the BCVP project we used a loadspeaker driven by white noise mounted on one end to excite the engine then ran an FFT from a microphone at the other end.

All though more accurate I still want to hear Bill singing to his pulse jets:-)

Its a good way to test overall resonace but remember you have to allow for the temperature if you try back tracking with your calculations against a running engine.

Viv

[Nick]

another quite effective trick we used was to blow up a balloon inside the jet until it burst and recording resultant "pop", then analyse it. Still got to watch the heat thing as Viv says but i reckon it gives you a sound closer to the real workings of a PJ, a snapshot of one ignition.
Plus there is no danger at all of setting fire to your valuable audio kit in the process.
Having said all that , i want to hear Bill sing too! :-)

Nick

[Mike Everman]

I've been yodelling into Kazoos since Christmas! Or perhaps we should call it a "bi-labial sine sweep".

[mk]

I've been yodelling into Kazoos since Christmas! Or perhaps we should call it a "bi-labial sine sweep".

I'd call it an "Alphorn", you'd then be the "Alphornblaeser".
Can't await the "One-Man-Tour across Europe"...

[larry cottrill]

Here is yet another stab at "final dimensions" for the Elektra II pulsejet. The only part fabricated so far is the bent intake pipe, and the new dimensions are based on the total acoustic length of this pipe, which I'm calling 5.1 inches. This is significantly shorter than the intake pipe created for the Elektra I [6.5 inches, approximately], but the 3/4-inch nominal ID is used [3/4-inch EMT "conduit"]. This time, I formed the flare in the end of the tube with an old ball peen hammer head, and I think it came out fairly decent, with a flare OD right at 1.25 inch. Photos in a week or two.

The remainder of the dimensions derive from the Hinote Criteria and the assumed L/8 "Logan Point". The critical dimensions are shown in the drawing below. At Bill Hinote's request, I will make every effort to get the engine built precisely to the dimensions shown, unlike Elektra I which was just thrown together however things would fit and "looked about right". I think I can hit any of the dimensions shown within one or two hundredths inch, maximum. Hopefully, Bill will find this satisfactory.

Working with these dimensions, UFLOW1D came up with the pressure graphs as shown. I think the 'diamond' layout of the chamber came up with better curves for good performance than either the Elektra I or the Fo Mi Chin engine [see my post from earlier today in my 'A Speculation: The Hinote Criteria ..." thread for a side-by-side comparison of the three engines]. This one has better damping of the initial standing waves created in the chamber, and a more powerful pressure wave due, I think, to the improved nozzling of the expansion flow from the chamber into the tailpipe. Also, here I'm using a 1.25-inch pipe all the way, rather than a nominal 1-inch diameter [as in the Elektra I], so the gas mass per unit length is higher.

See what you think ...

L Cottrill

[larry cottrill]

While running a UFLOW1D analysis on a tailpipe flare added to the Elektra I showed only a tiny difference, doing the same thing to the 26-inch Elektra II brings out a visibly better low-pressure trough, in theory. Compare these curves, especially the L/8 curve, to the earlier ones.

The flare added is 8mm long [about 1/3 inch] and expands the tube from 1.25 to 1.5 inches.

NOTE: The lowest number on the vertical [pressure] scale is incorrect on the earlier post -- the correct number is 0.0152, not 0.0423. The other numbers on the scale are correct. That set of graphs and this one are indeed at the same scale! My apologies for any confusion.

L Cottrill