I recently made my first combustion spudgun. It's quite simple: It's fueled by a propane torch head (not direct injection) and the igniter is a simple igniter that creates a small flame. It worked fine.
However, I decided to inject propane directly to the chamber, and so I made this:
And so the whole gun looks like this (I changed the igniter to an electric one since the other broke):
But when I try to shoot the cannon by using direct injection (without even measuring the pressure) there's no combustion. When I open the cap at the end of the chamber sometimes some gas comes out and I'm able to create a small flame and sometimes there's not even gas there, and there's never any combustion noise. I've tried with the same amount of gas, less gas and more gas than with the torch head and got the same results. And yet when I use the propane torch head it works perfectly.
I think it has something to do with the temperature of the propane. When it comes out of the propane torch it's at a normal temperature, but with direct injection it comes out extremely cold (if you keep it inside the metering device for some time the tube looks like it's frozen).
Any ideas of what could be going wrong? Thanks in advance.
you fuel might not be mixing with air well enough to ignite,
id add a chamber fan to help with mixing
Also ensure the chamber is fully vented between shots.
Use GGDT to get the mix right. If you can't add a chamber fan, add a bouncy ball and shake it like a spray paint can to fully mix the contents.
With fresh air, proper fuel amount, and properly mixed, it should go boom when ignited.
I'd be worried about safety if I were you. What's under that duct tape, not hiding anything are you?
With those two large holes (fuel and igniter) drilled so closely together and through a single layer of ABS, the cannon is at risk of failing structurally. Since it is ABS, it should only tear and not shrapnel but it will render your chamber useless.
It's not hard to build an advanced combustion properly. If ABS is readily available, use that. Get some PVC, copper or galv. iron fittings are construct a proper fuel meter; not one held together by JB-Weld. Measure the volume of the chamber and meter using water and calculate how much pressure of propane you need in the meter so you can inject it in the cannon and achieve combustion upon ignition.
Apart from that, your issues are almost certainly linked to what Tech described in his post. Place a projectile in the barrel and start off by injecting a small amount of propane, try the ignition a few times and if it doesn't combust remove the projectile and vent the chamber with air. Repeat these steps but each time you should increase the amount of fuel injected by a small amount.
i never heard of the bouncy ball idea
Measure the volume of the chamber and the volume of the meter, and use that to calculate the correct pressure to fill the fuel meter to. Combine this with the bouncy ball idea (or, better yet, a tiny fan) and you should have no trouble with ignition.
*Upgrade in progress*
Current declassified projects:
ETG, ELGG, LGG, 1D gun model, CLGG. One of which is likely to be finished in the next year...
Thanks for the replies. I'll try to place a chamber fan (or the bouncy ball) and post the results. As for the structure of the cannon, I know how it looks but it actually works fine and I'm already planning on doing a proper PVC one (but I need this one to work with the meter first). I was also venting between shots, but I didn't remove the projectile so I guess it wasn't properly vented.
adding a chamber fan is really easy, just look around it helps for fueling and venting and increases performance
The reason your gas temperature is so cold, is probably due to the propane coming out of your can in liquid form. try to hold the cannister vertical when u measure the gas.
The other thing that strikes me, is that the measuring pipe u have there looks a bit large, what preassure do you use when filling the pipe ?
To make dreams come true, u must wake up.
I found a CPU fan which I could use for the cannon, but it doesn't fit in the chamber. Since the chamber is so narrow I'll just build a new cannon and test the metering system there. I'll post the results when I get this done.
i had the same problem with the fan i got for my one cannon, if its square i used a dremel to make the fan round and then put electrical on it to make it snug, and works fine
If you mean cutting the corners of the fan that make it square shaped, I could try but it wouldn't fit anyway. My chamber is really small so I'll just make a new cannon like I said.
Finally I've solved the problem. I made a new launcher and realised I had been using too much propane for combustion to happen. It seems that with the propane meter more gas comes out than with the torch head. So I just used less propane (without fan in the end) and the launcher worked. Thanks for the replies.
I saw you mentioned you had a flame when you ignited with the chamber open. That's what you'll see when you add too much fuel. It's almost like a blue liquid flame the way it seems to slowly splash around and pour out. Too much fuel will cause a visible fire, rather than a pop.
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