Do you see anything wrong with this pulse jet?

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BigR77
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Do you see anything wrong with this pulse jet?

Post by BigR77 » Tue Sep 20, 2016 9:40 pm

Hello all, just built my second pulse jet with a friend, we both work at a fabrication shop and have used remnants of materials to make this pulse jet, so it is made out of 12 GA mild steel, which is about .100" thick. Anyway, we tried to get it to start today, only tried briefly, but we never got sustained running, got some good cycles, and its pretty loud. Just curious if you guys see something immediately wrong with it that we should fix, or if we just need to mess with it longer to get it running. We were also trying to get this to self start, but the spark plug doesn't seem to be igniting the gas (propane). so we're going with the good old fashioned air blower in the intake. This jet was designed around the parameters laid out in the sticky at the beginning of this forum.
Thanks!
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metiz
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Re: Do you see anything wrong with this pulse jet?

Post by metiz » Tue Sep 20, 2016 10:40 pm

Open up the spark gap a bit to get the gas to properly ignite. What kind of injector are you using, and is the propane bottle full and unrestricted?
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BigR77
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Re: Do you see anything wrong with this pulse jet?

Post by BigR77 » Tue Sep 20, 2016 11:16 pm

Propane tank probably 1/3-1/2 full, unrestricted,(orifice in hose drilled out) the injector can be seen in one of the pictures, its a small rod with a 1/8" hole drilled in the cap, pointing back toward the combustion chamber, i think in the picture its pointing to the side, but it was just for photo purposes. The propane is injected into the intake at the elbow, and the injector hole is in the middle of the air stream, to get the best mixture. We didnt even come close to maxing out the propane, we could turn it up way further....that may be the issue, cuz when it did cycle briefly, it would blow itself out...say the cycling was 1/2 second.

metiz
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Re: Do you see anything wrong with this pulse jet?

Post by metiz » Wed Sep 21, 2016 3:55 pm

Ah now I see. the small version of the picture made me believe that was a sparkplug. I think that is the crux of the problem though: the motor doesn't like the injector at that point. You want a tube pointing down the intake, depending on type, 0-50% of the length. The fuel is now delivered at the wrong place, inproperly mixed and considering the size of the tube you are using, it's actually choking the airflow. Try a thin walled Rosscojector, 4-6mm internal diameter, 10-40% inside the intake.
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BigR77
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Re: Do you see anything wrong with this pulse jet?

Post by BigR77 » Wed Sep 21, 2016 4:39 pm

Truthfully we were looking for a more elegant solution than a rosscojector, is there any way around using one? could we just move the injector further forward in the intake and not have it protruding into the pipe as far? We want to convert this to run on liquid fuel as well, so it would auto start on propane, then switch to gas. I was kind of hoping to choke the air slightly because after reading some of that sticky on designing pulse jets I found some info that rear facing intakes generally like to be smaller than a straight through design, as i discovered this after we'd built most of the jet, i figured choking some air wouldn't hurt. the spark plug is on the front face of the combustion chamber, centered.

If we were to move the injector further up, we're still injecting fuel rear facing, so wouldn't that accomplish the same thing as the rosscojector?
The reason I dont like them is that you just take some tubing and crimp the end of it to make it a smaller orifice, and I'd like something a bit more technical, and measure-able...for instance, i know the exact hole size that is in my injector, and its no problem to duplicate it exactly, or resize it. Maybe that's just because I'm an engineer.

If we were able to just move our injector...where would be the best place? halfway down the total length of the intake, or just the straight part, if we were to pull the injector closer to the sidewall of the tube(IE not insert it as far), would the bottom be the best?

BigR77
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Re: Do you see anything wrong with this pulse jet?

Post by BigR77 » Thu Sep 22, 2016 3:13 pm

Well, we built a proper coil driving circuit, and opened up the spark plug gap. She doesnt self start, but it is infinitely easier starting this with a spark plug, Got a couple vids of it running if you're interested.

Coil driver
https://www.youtube.com/watch?v=Ml4rSI1p3ec

Starting
https://www.youtube.com/watch?v=BNEamrecyBw

Running
https://www.youtube.com/watch?v=9El0ndkRDEI

Mark
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Re: Do you see anything wrong with this pulse jet?

Post by Mark » Thu Sep 22, 2016 3:30 pm

Protect your ears each and every time.
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BigR77
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Re: Do you see anything wrong with this pulse jet?

Post by BigR77 » Thu Sep 22, 2016 3:54 pm

Yea we got a little excited with it working...looking into good earmuffs for the next run!

metiz
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Re: Do you see anything wrong with this pulse jet?

Post by metiz » Thu Sep 22, 2016 10:14 pm

I'm not sure about our injector suggestion - I have never done an injector like this. I don't think you'll be able to start automatically like this. just try with a hand hold Rosscojector to see if it starts easier. Once running, switching to liquid fuel should be doable.
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