Roaring Becomes Elektra

Moderator: Mike Everman

Postby Anthony » Fri May 21, 2004 11:22 pm

Would it be a question of compression and expansion?
Last edited by Anthony on Fri May 21, 2004 11:39 pm, edited 1 time in total.
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Postby Rossco » Fri May 21, 2004 11:27 pm

That about sums it up Avenger, the wonderfull world of wave mechanics!

The intake on my test rig can run from full length, right back to very short. With the short intake, it runs at a noticably higher frequency! Whether its a matter of timing, sort of like advancing the ingnition timing, or its changing the running frequency i dont know.

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Postby Bruno Ogorelec » Sat May 22, 2004 9:57 am

Avenger wrote:Would it be a question of compression and expansion?


Basically yes. The Kadenacy oscillation is just that -- a slowly decaying cycle of compression of expansion. But, that is only the beginning. Forgive me if you already know the following, but I can see that many participants in the forum fail to appreciate the basic mechanisms behind the functioning of pulsejets.

In a pulsejet, the Kadenacy cycle is started by a combustion event. The cycle does not decay because the combustion events generate each other and repeat endlessly, in a kind of relaxational oscillation. You can see it in its purest form in the Reynst combustor (or a simple jam jar with burning alcohol on the bottom).

However, the cycle is very similar in many ways to a sound wave. If you measure the changes in gas pressure over time at any point in the combustor and plot the values against time, you get a sine curve. The same goes for gas speeds. This is the same kind of behavior you see in sound.

For that reason, you can superimpose acoustic resonance on the pulsating process by adding a vessel which will allow the formation of a gas column long enough to support a standing wave. Presto! You get an acoustic combustor. The reason to do it is that resonance amplifies the process greatly and imposes on it a frequency determined by the length of the gas column. The pressure swings are greater, which gives you greater combustion efficiency. The frequency is higher, which packs more bangs into a second, giving you greater power.

This, however, is where the real complication starts. In the simple kind of generation of sound, in which you have some kind of a mechanical starting impulse (e.g. a strike of a mallet on a wire or a hammer blow on a bell) the sound you have generated does not affect the impulse. Generally speaking, there is no feedback.

In pulsating combustion, however, the sound-like pressure waves generated by one combustion event greatly affect the next combustion event. They determine (or at least affect greatly) the extent to which the engine will purge itself of the remains of combustion – burnt gases. They determine the amount of fresh charge the engine will manage to suck in. They determine the amount of pre-compression the charge will receive before ignition and the amount of mechanical compression that will be added to the pressure of the burning gas.

In practice, the acoustic oscillation governs almost the entire behavior of your engine. It is one of the most important factors of all for engine performance, together with the fuel delivery (quality of mixing of fuel and air), ignition and thrust augmentation.

That is why all those little things are so important – the exact lengths, the tapers, the orifice sizes, the openings and closures and branchings – because they determine the exact shape of the wave train that will be generated and the exact kind of resonance. That is why it is not at all the same thing whether your intake or exhaust are here or there or anywhere, and whether there is one of five or none of each.
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Postby Anthony » Sat May 22, 2004 3:19 pm

I try to understand, but I will have to build one to do so fully hehe. I've got to read your documents over and over to get better understanding, it's easier each time! But there's nothing better than running one I guess!
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Video Clip - Elektra I Trying to Run ...

Postby larry cottrill » Mon May 24, 2004 5:31 pm

Here's a short video clip [snipped at several points] of the Elektra I prototype, just before I ran out of tape and then lengthened the pipe a couple more inches and got it to run self-sustaining. The clip was produced by my friend Kevin Day. He also did a longer clip with much better audio that I will eventually put on my site, and then I'll edit this post to show the link, but that one is significantly longer than the limit for streaming files on this site.

Items to note: [1] Me doing the Power Cord Polka [where did I put those lederhosen?]; [2] The noise level of the leaf blower, which is itself loud enough to cause clamping by the camcorder, totally disappears when the roaring combustion of Elektra kicks in [the blower was never shut down during this test run]; [3] The Dynajet-like frequency of this very long engine configuration.

I hope you enjoy this. I WILL still try to get some good video of Elektra free running to post later, after I get the intake flare perfected.

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ElektraI-2004-05-19.WMV [ 1.11 MiB | Viewed 5326 times ]

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Postby Mike Everman » Mon May 24, 2004 6:26 pm

Cool, Larry! Seems to me you need to rig for a trickle of starting air. Looks like most of the time you're blowing it out.
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Postby larry cottrill » Mon May 24, 2004 6:41 pm

Mike Everman wrote:Cool, Larry! Seems to me you need to rig for a trickle of starting air. Looks like most of the time you're blowing it out.

Actually, Mike, most of the time with the air applied, it was burning with a kind of gravel-voiced, gutteral sub-roar. The bursts of yellow flame came from when I'd get off it completely for a moment, then reintroduce the air.

Basically, you're right, though -- what I was finding was that getting it to roar, I was just gently tweaking it with air. When the pipe was lengthened just a couple inches more, I could hold the air nozzle about 8 or 10 inches away from the intake, and hardly worry about exact aim at all -- just turn on the air and start easing in the propane until it catches, then move it off and let it run. Simplicity itself.

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Elektra I, Really Running

Postby larry cottrill » Mon Jun 21, 2004 6:40 pm

All right, FINALLY -- video of Elektra I starting and running.

These videos are low quality for fast downloading, and to fit the forum size limits for streaming files.

The first video is just introducing the J-B Weld intake flare used for these run attempts. The second video is a good self-sustaining run with the Black & Decker Leaf Hog as the starting unit. The third video is an attempt to start the next run with a hair dryer. This failed to provide enough velocity for a start, but the sounds made by the engine, especially in the low frequencies, were fairly interesting, I thought.

In the middle video, the good run, note the 38.5 inch overall length[!]; also note the apparent frequency stability of the engine running. I should have let it run longer, of course, but I'd never have made it under the upload limit for a video file. I will eventually put these on the Elektra I site in higher quality format. Note that I am getting around on a "sprained" left ankle, which is the main reason I haven't been doing much tinkering or testing for the last three weeks -- I've had a bone scan now, but don't have the results yet.

All files were produced by my friend Kevin Day.

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Attachments

ElektraI_flare_2004-06-14_Lo.wmv [ 1.93 MiB | Viewed 5176 times ]

ElektraI_good_run_2004-06-14_Lo.wmv [ 1.96 MiB | Viewed 5176 times ]

ElektraI_dryer_attempt_2004-06-14_Lo.wmv [ 1.01 MiB | Viewed 5175 times ]

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It works

Postby Graham C. Williams » Tue Jun 22, 2004 3:47 pm

Dear Larry.
You don't know what a joy it is to see your motor work.
Great
Best Regards
Graham.
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Postby Tom » Tue Jun 22, 2004 3:57 pm

*adds elektra to list of things to build some day*
Wooh : oD, It's alive Igor, IT'S ALIVE. Long day, sorry for the delusional ramblings.

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Re: It works

Postby larry cottrill » Tue Jun 22, 2004 4:03 pm

Graham C. Williams wrote:Dear Larry.
You don't know what a joy it is to see your motor work.
Great
Best Regards
Graham.

Graham -

Thanks much. Coming from you, sir, that means a lot.

The joy is really what makes it worth it, isn't it? You plan it, you argue about it, you hack it together, you try it, you mess with it and hack some more, you finally get it to kick in and go, you stand there and watch it -- and something way, way back inside your head says, "How can it do that?"

Larry
Last edited by larry cottrill on Tue Jun 22, 2004 4:08 pm, edited 1 time in total.
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Postby larry cottrill » Tue Jun 22, 2004 4:07 pm

Coffee wrote:*adds elektra to list of things to build some day*
Wooh : oD, It's alive Igor, IT'S ALIVE. Long day, sorry for the delusional ramblings.

Tom

Thanks, Tom -

Not a world beater, at the moment, by any means. But, hey -- at least it's a project you can put together with no cost overruns!

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Postby Bruno Ogorelec » Tue Jun 22, 2004 4:27 pm

Heartfelt congratulations! I am really glad to see a totally new configuration work, rather than just iterations of the familiar ones. This is what this forum has done best, I think -- encouraged a number of people to try something completely new.

You are one of the people in the forum who deserved success the most, so I am doubly glad, for I have also seen justice done.
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Postby Anthony » Tue Jun 22, 2004 4:37 pm

Congratulations, superb work Larry!

I'm wondering, if you change the rectangular combustion chamber for a spherical one, would you get better performances?
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Postby Bruno Ogorelec » Tue Jun 22, 2004 4:47 pm

Avenger wrote:Congratulations, superb work Larry!

I'm wondering, if you change the rectangular combustion chamber for a spherical one, would you get better performances?


No. He'd get a bong, though.

Seriously, all other things being equal, I don't think the shape of the chamber has a notable influence on performance. I will wager that -- much to Larry's horror -- not even the shape of the transition from the chamber to the tailpipe has a great influence. One of the best performing pulsejets I know of has a straight pipe sticking directly from a near-cubical box. No cone, notheing. Just a box and a pipe sticking straight out at right angles from the rear side. How about that?
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