Why won't my Pulse Jet run?

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SkeptiCol
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Why won't my Pulse Jet run?

Post by SkeptiCol » Sun Jun 23, 2013 9:46 am

I have a combustion chamber, there are no leaks, the inlet is shorter than the exhaust, the inlet is flared (roughly), but the bastard just refuses to run. I am using LPG for the BBQ, have a valve, and have tried to use my compresser, but she still won't run.

I have watched youtube vids on how to light them, but mine just 'WOOOOP's and then goes out, we reduced the fuel and it popped a few times but won't sustain.

I will attach some photo's, they are a bit shite, my phone is shite sorry. Will try to find others.

Col
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metiz
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Re: Why won't my Pulse Jet run?

Post by metiz » Sun Jun 23, 2013 12:10 pm

I think your combustion chamber is too small and your (conial?) exhaust too fat. Also, I think I see a propane regulator on your tank. Get rid of that first before you try anything else. If you think you're giving it too much fuel, you're probably don't give it enough. Why didn't you use prover plans for a small engine instead of designing one yourself?
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Re: Why won't my Pulse Jet run?

Post by SkeptiCol » Tue Jun 25, 2013 8:33 am

I guess that my main reason for scratching one together was that I was impatient and cheap. I had these bits of stainless in my shed and it all looked like it would go together in roughly the right sort of shape, so we did it. I don't have a heap of cash to spend on this thing so these bits had to do. It's been six months now, with a few alterations over that time. I did stick to a few rules;
1) intake shorter than the exahust
2) intake diameter smaller than exhaust
3) flare the intake
4) jet the gas into the front of the intake.

When we light it, it all just burns out the exhaust in a huge flame, and does nothing else, that is why we are certain it's too much fuel so far. It has made some great noises and convincing pops and bangs, but has always needed air to be pushed into it to keep it going.

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Re: Why won't my Pulse Jet run?

Post by PyroJoe » Tue Jun 25, 2013 5:54 pm

What does your injection line tip look like, is it crimped?

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Re: Why won't my Pulse Jet run?

Post by SkeptiCol » Wed Jun 26, 2013 5:19 am

the one going in the intake isn't crimped, it just looks like it, it's 1/4in gas line. The one at the back of the combustion chamber is blanked off, that was the place of our original, and it was easier to leave it there and blank it, than try and remove it.

gettylee
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Re: Why won't my Pulse Jet run?

Post by gettylee » Wed Jun 26, 2013 11:37 am

Hello,my 1st build was Larry Cottrills little FWE, it is a great 1st engine! I just can't say enough good about that little fwe. It can also be converted to a hpx later on. I have dim. Of my hpx under successful hpx plan. I built Larry's fwe 1st & got so hooked on PJs!!! It has been such a great hobby&I have several nice PJs now. For your lpg. I use a turbo torch set up,& removed the big rosebud burner head, & attached my injector line to it, the big turbo torch/weed burner set, works great for me&allows plenty of psi. These PJs get tricky sometimes! But don't give up" it gets very fun!!!!!! :)

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Re: Why won't my Pulse Jet run?

Post by gettylee » Fri Jun 28, 2013 2:54 pm

I've found pyro joes recipe for beginners,to be very helpful.

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Re: Why won't my Pulse Jet run?

Post by PyroJoe » Fri Jun 28, 2013 8:39 pm

The injector pointing into the intake should be crimped to look like this picture. Also the regulator will definitely need to be removed. The injector should hiss slightly and have a flow that isn't laminar.
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Re: Why won't my Pulse Jet run?

Post by SkeptiCol » Mon Jul 01, 2013 6:12 am

not sure what removing the regulator will do as when i have tried to light it full open as it currently is, has resulted in t 3ft flame out the exhaust, like a huge bunsen burner!! i'd love to get this one going, basically due to the romance of having built and run a jet in my shed etc etc.

I have attached another pic, from a iphone, so you can see it a bit better, and offer some other ideas.

SkeptiCol
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photo (1).JPG

metiz
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Re: Why won't my Pulse Jet run?

Post by metiz » Mon Jul 01, 2013 12:12 pm

I'd say your exhaust is too fat and short. As for the restrictor: get rid of it. It might look like you're already giving it too much fuel, but you'd be suprised how much these engines take. With a restrictor, you might be able to get it running if it was build properly, but only on idle.

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gettylee
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Re: Why won't my Pulse Jet run?

Post by gettylee » Mon Jul 01, 2013 12:47 pm

Hi,I'd recommended adding about 3" to your intake&put a flair on it. & for starting air use low psi. Approx.10 to 20 psi. Approx 1/2"away from intake, try to get burbleing sounds, & increase your gas when you find that burbeling sound.

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Re: Why won't my Pulse Jet run?

Post by SkeptiCol » Tue Jul 02, 2013 9:23 am

Cheers guys!

Sorry if I sound a bit ungrateful, i'm just frustrated with it is all. I've seen some really ugly ones on YT run fine, so i'm just frustrated and jealous.

next thing check list:
1) Crimp the gas line
2) remove restrictor (my guess it will venturi in more air that way? or force more mixing when going in? close or not even?)
3) increase length of intake (already flared (very ugly though))
4) pray!

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Re: Why won't my Pulse Jet run?

Post by gettylee » Tue Jul 02, 2013 6:00 pm

Hello,if it helps,I've built a lot of beautiful PJs that were useless! Got so frustrated @ times!" I've heard the term simple pulse jet" lol,not quite!!! Lol. My hats off to these guys like Larry cottlil,etc...who figure these things out,&design such works of art!!! Are amazing!!! It is puzzling@times, but I guess that's what makes PJs so addictive" it can&will be very rewarding eventually!:)

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Re: Why won't my Pulse Jet run?

Post by gettylee » Tue Jul 02, 2013 6:03 pm

gettylee wrote:Hello,if it helps,I've built a lot of beautiful PJs that were useless! Got so frustrated @ times!" I've heard the term simple pulse jet" lol,not quite!!! Lol. My hats off to these guys like Larry cottlil,etc...who figure these things out,&design such works of art!!! Are amazing!!! It is puzzling@times, but I guess that's what makes PJs so addictive" it can&will be very rewarding eventually!:)
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Re: Why won't my Pulse Jet run?

Post by Jutte » Wed Jul 03, 2013 9:44 am

Hi Skepticol,
First get rid of that damned restrictor...Dude get rid of it!
OK - like metiz says - exhaust is too fat and short ...the intake looks way short as well.
I had trouble with some of my earlier jets...so here is what I would suggest.
Cut the flare off the intake first - the main mission is to get the thing running-
once you got it going then you can put a flare back on it.
Now get some lengths of pipe ( one slightly larger dia than the intake and the other slightly larger dia than the tail pipe) - doesn't matter if it is mild steel - the idea here is to tune the pipe( ala trombone ) by varying the lengths - you slide the pipes to change the length.
You will be surprised how long a length you will need in the tail pipe - I have had some rediculous lengths that worked.Try extending about the same length again on what you have for the tail pipe and the same for the intake .
This system is what I used on my first pulse jet.
What you a looking for is the flame out the tail pipe being visiably being 'sucked' back.At some stage the flame will get stuck and sort of 'bloom' on the end
- which shows that you're getting close.You will have a lot of mucking about
trying different lengths but hang on in there and you'll get it running.
Once the thing is going take your measurements - note them - and then weld
the correct lengths on and you're done ( do the intake flare then).
Just remember that with smaller pulse jets the critical lengths can be only a few mm difference between success and just plain suck -which of course means that a hot engine runs great with the thermal expansion but behaves poorly when cold.
So hang on in there - search the site - there's heaps of info and have fun!
Hope this helps.

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