How many things could be wrong?

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MB
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How many things could be wrong?

Post by MB » Sat Sep 30, 2006 7:43 am

Ok. I've got a head scratcher for the big dogs.

We’ve built a big PJ (we expect over 100 lbf), and the closest thing to running we can get is what sounds like an idling nitro-burning funny-car engine (besides a really great boom) and of course, big billowy plumes of fire.

The dimensions are these:
  • Combustion Chamber - 9 in. dia., 13 in. len.;
  • Exhaust Tube - 6 in. dia., 36 in. len.;
  • Reduction Cone - 16 in. len.
The valve assembly is a 6-inch-square twin V grid -- so I got 4 rows of 2 valves each. That gives me 8 sets of 3 inch valves, covering 3 - 3/4 in slots each (which equals 32 slots total). The valves are backed by curved retainers to allow a maximum opening of about a half inch.

The fuel is propane; and is injected inside the chamber by two jets.

We've tried:
  • .006, .008, and .010 in. thick spring-steel material for the valves;
  • fuel in both gaseous and liquid forms;
  • fuel pressure from 0 to 100 psi;
  • with and without continuous (gas powered) leaf-blower air;
  • extending the exhaust tube length with a "trombone" sleeve another 12 inches;
  • shortening the exhaust tube length with metal cutters by 6 inches;
We’ve tried all kinds of combinations of these variables, and except for getting nothing at all, there didn’t seem to be much improvement towards a running state.

The best we get is by choosing a pressure, say about 20 psi, turn on the fuel, hit the spark, and give it the air blower. If we give it the air right away, we get something like, BbBbbbBBbBbbBBbbbB… (though it only lasts for a few seconds). Or if we hold off on the air for a couple seconds, and when we give it we get a great BOOM! In any event, it just ends up an over-achiever’s barbeque flame dispenser.

We figure there has to be something fundamentally wrong because there is such a wide margin of “as good as it getsâ€
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evildrome
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re: How many things could be wrong?

Post by evildrome » Sat Sep 30, 2006 11:20 am

>>>> Hi,

We’ve built a big PJ (we expect over 100 lbf),


>>>> Been there

and the closest thing to running we can get is what sounds like an idling nitro-burning funny-car engine (besides a really great boom)

>>>> Done that


and of course, big billowy plumes of fire.

>>>> And that.


The valve assembly is a 6-inch-square twin V grid
-- so I got 4 rows of 2 valves each. That gives me 8 sets of 3 inch valves, covering 3 - 3/4 in slots each (which equals 32 slots total).

>>>> I believe that my (and others) earlier attempts failed because the valve length was too long. I take it your valves are 3" wide, not high? How high are they? All the DIY grid valve systems I see look way too high.

I've seen an Argus sized pulsejet at the Deutsches Museum and the valves are about inch and a half high. For a CC diameter of about a foot.
As the only moving part, the valve is critical. I'm guessing there is a window for its height that can't really be mucked about with.

What I'm going to do is make a set of grids in 0.25" height steps from 1" to 2.5" to test on my PJ.

Cheers,

Wilson.

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re: How many things could be wrong?

Post by Mark » Sat Sep 30, 2006 12:15 pm

Have you tried adjusting the play in the reeds? Sometimes if they can swing open too much, they don't get closed in time to catch or build the feedback. I found I got cannon shots until I limited the play. Also the retainer or reed stop affects the snap back on the reeds, so not only the limit of lift, but also the arm and spring effect, depending on how the retainer is bent or shaped.
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re: How many things could be wrong?

Post by Jonny69 » Sat Sep 30, 2006 4:33 pm

I agree with mark, sounds like the valves are opening too far and not getting shut in time. Behind the valve you need a curved retainer to limit how far they open, I'd say no more than 3mm (or 1/8" in old money)

Basically if they don't open far enough it just limits the top power and you can experiment by opening the retainers up until it won't run properly but if they swing wide open it just blows out and won't resonate.

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re: How many things could be wrong?

Post by pezman » Sat Sep 30, 2006 4:58 pm

The recommendations on valving are probably correct, but the fact that results are more dramatic after a long delay suggests a mixing problem too.

Maybe deliver fuel by using a great many small jets -- for example a manifold consisting of a steel brake tube with a bunch of small holes (#60?) drilled in it, fastened to the peak of the valve grid and squirting propane at right angles across the openings.

*O*
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re: How many things could be wrong?

Post by Eric » Sat Sep 30, 2006 7:08 pm

Valve retainers and better mixing will probablyl help a lot, a first pulsejet of any kind can take some practice to get used to starting. Once you learn how it becomes second nature.

How much fuel are you giving it? Just vapor? Valved engines typically dont like to be throttled as low as valveless engines.

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MB
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re: How many things could be wrong?

Post by MB » Sat Sep 30, 2006 8:13 pm

Hmm... we thought that we needed more air, so we have been thinking about fabricating a more spacious valve grid. It didn't occur to us to try limiting the opening further. We'll give it a try.

BTW, each of the 8 valves are 3 in. wide and about 2 1/5 in. high.

I'm not sure of the jet orfice size, the guy at the propane supply shop said this should be just the ticket. We'll experiment with some variablity in the nozzling.

I'll post some pix and results when I get some.

Thanks for the feed back. You guys (gals?) - people - are awsome.
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Re: re: How many things could be wrong?

Post by Eric » Sun Oct 01, 2006 1:45 am

MB wrote: Thanks for the feed back. You guys (gals?) - people - are awsome.
Unfortunately pulsejet gals are far to rare, or any area of aerospace for that matter.

It sounds like it should work well once you have the retainers made up. With pulsejets its all about building resonance, when its running the small valve area will behave like a much larger valve area due to lower than atmospheric pressure in the engine.

Pulsejets need a lot of fuel, if theres any kind of restriction its probably not getting enough fuel, unless you are using liquid propane.

At 100 lbs the engine should go through a 20 lb propane tank in 6 minutes. With vapor, on a warm day, you might be able to get 20 lbs thrust from vapor only, if the engine will run that low.

How long is the intake duct length? The intake duct length is often ignored but it is quite important.

Eric
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Re: How many things could be wrong?

Post by El-Kablooey » Sun Oct 01, 2006 3:23 am

MB wrote:
The best we get is by choosing a pressure, say about 20 psi, turn on the fuel, hit the spark, and give it the air blower.

This may not solve your problem, but you should definately lose the regulator. There is no need for it, and it is likely giving you problems. I doubt you'll ever get an engine of that size to run with a regulator on the line, at least no propane regulator I've ever seen.


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re: How many things could be wrong?

Post by Irvine.J » Sun Oct 01, 2006 4:51 pm

Mate just a quick query, what metal are you using to cast that plate, and how did you melt it down? Is it mild steel?

Also, what is the name/brand of the red molding rock you are using there?
Bloody nice job if I may say so.

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re: How many things could be wrong?

Post by Eric » Sun Oct 01, 2006 6:11 pm

Ah theres your problem! You dont have an intake tube, and looks like its not getting enough fuel.

If you make an intake tube, blowing air in with the leaf blower will build pressure across the valve grid instead of just blowing air off perpendicular to the grid so you will get a decent volume of air into the engine.

It is also very important when the engine is running. You can get an engine running without one, but its not goign to work very well, and a tuned length intake stack can greatly increase performance and ease of starting.

Eric
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re: How many things could be wrong?

Post by Eric » Sun Oct 01, 2006 6:19 pm

Oh and another thing. If its a cinder block you are holding the engine back with, you should really upgrade to something much heavier. When it gets shooting big single shots the peak thrust could be over 400 lbs and if it catches it could take off with whatever you have infront of it. Maybe use some lag screws to screw it down right into the pavement.
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re: How many things could be wrong?

Post by MB » Sun Oct 01, 2006 6:52 pm

Eric, What, no girls? I'm out-a-here!

Seriously. Our plumbing is configured (and sized) so we can experiment with a great variety of fuel flow. We wanted to be able to achieve and measure both ends of the performance curve varying as many parameters as possible.

We presumed the intake duct was for pre-valve mixture containment. The upward end of our valve grid is smiling in the sunshine. What roll does it play? What should it be designed to do? None of the PJ design-dimensions-calculators I’ve seen specify any dimensions for intake ductwork. Do you have any suggestions?

El-Kablooey, We are using an industrial regulator, that shouldn't restrict fuel flow, even at full pressure. We would certainly ditch it if it looked like it was a problem. We are using two tanks on a manifold to assure we can provide enough flow as we want. And we are geared for both gaseous and liquid media.
Last edited by MB on Sun Oct 01, 2006 6:57 pm, edited 1 time in total.
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re: How many things could be wrong?

Post by MB » Sun Oct 01, 2006 6:53 pm

Evildrome, I love your test-site! We do our test-firing out in the desert, where there is nothing to catch fire but dirt. You got a pretty good pop in your clip there. Ours was so big I felt the need to put my foot on it (the fixture, not the engine) to assure it wasn’t going anywhere. We are trying to figure out how to convert our old-school 8-mm video camera feed into a post-able format so we can show it. Gimme a couple-a days; we’ll figure something out. My test-fixture is similar to yours, though, in that the valve grid is exposed on the front of the engine, and we are simply blowing right into it.

Eric, I think we need to put some scales under the fuel tanks (or something) to see just what the flow rate is. My seat-of-the-pants has no real idea what it should be, I guess.

Oh and yes. We’ve got that sucker nailed to the ground. We didn’t think about “peak thrustâ€
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