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Combustion Power Plants


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#1 PVC Arsenal 17

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Posted 17 July 2011 - 03:47 PM

I expect this to be shunned but in the event someone here is interested, I'd like to share another one of my projects. Earlier in the summer I spent some time investigating propane combustion power plants for Nerf. My hope was to create something that would provide performance similar to spring-piston guns without the cocking effort and construction time. Using some scrap parts and materials, I built a volumetrically metered test rig to see if the idea had any potential.

Those familiar with spudguns know that a volumetric propane meter typically uses two ball valves connected to a regulated propane supply. The first valve lets propane into a small reservoir of specific volume. That valve is then closed off and the second one is opened, allowing the propane inside the small reservoir to enter the combustion chamber to mix with fresh air inside. The spudgun is then ready to fire. My test rig does essentially the same thing, but consolidates the metering system into one little 3-way valve with some convenient features. It has a threaded portion below the button and it exhausts air (in this case propane) around the button. This valve is mounted at the back of the combustion chamber using those threads such that the button is inside the chamber. The test rig has a floating barrel portion/piston with a rubber seal. When the barrel is pushed back within the combustion chamber, it impacts the valve's button and causes regulated propane to enter a tiny reservoir. When the barrel is pulled forward again, the valve then releases that propane into the chamber to mix with fresh air that enters via a check valve and aided by vacuum caused when the barrel is pulled forward (assuming it has been loaded with a dart). The rig is then ready to fire. The spark is provided by a piezo igniter.

The video should explain this process more clearly: http://s237.photobuc...nt=MVI_2467.mp4

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Note: That messy looking pipe nipple sticking out from the valve is NOT the reservoir. The schrader valve is not part of this either. I just put those on there to protect the ports from epoxy when I sealed everything off. The actual reservoir exists between the perpendicular port on the valve, and a hex plug that fits flush inside it. The reservoir is ridiculously small in reality. Propane is injected at 45psi. So far the rig has fired several hundred shots and the propane tank is still quite heavy.

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(Actual fittings shown here)

It should be obvious how the firing cycle can be adapted to a bolt-action configuration with that barrel/piston plugging into a barrel extension on the forward stroke and exposing a breech on the backward stroke. When I have time, I will complete the rig and post more pictures. With an effective barrel length of only 3 inches, the rig gets consistent ranges of about 60 feet held horizontally four feet from the ground.



DISCLAIMER: This project involves PROPANE, HIGH VOLTAGE, and FIRE! Be aware of all risks involved when starting a project and DO NOT hold others [ME] responsible for whatever bad things may happen.

Edited by PVC Arsenal 17, 18 July 2011 - 04:11 PM.

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#2 PVC Arsenal 17

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Posted 17 July 2011 - 03:52 PM

Edit: delete

Edited by PVC Arsenal 17, 17 July 2011 - 03:52 PM.

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#3 taerKitty

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Posted 18 July 2011 - 12:01 AM

No shunning here. I'm sure this won't be allowed in a war, but not all mods are for wars. I like seeing new things, safe things, and 60' seems quite safe to me. I'm sure some yahoo can (or might) overpower it to stupid level, but that doesn't invalidate the design.
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#4 Exo

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Posted 18 July 2011 - 12:01 PM

I like seeing new things, safe things, and 60' seems quite safe to me.


Ditto. Just so long as said "New things" are done cleanly and not whacked all over the place. Nice job. Not quite sure what the duct tape is for, but the design sounds like it works. Try getting some larger barrels, then you an determine exactly how much power is coming from the gas. then you can scale it down if it's getting a lot of power. and if it's scaled down, then it'll be even safer, if it's possible to use propane safely. And be careful how you store that stuff.
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#5 PVC Arsenal 17

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Posted 18 July 2011 - 03:40 PM

Thanks. The duct tape was temporarily holding the two halves of the combustion chamber together to facilitate adjustments to the spark gap. That should give an idea of how weak the combustion pressures are here.

Don't quote me on this, but I recall seeing that typical propane combustion pressures peak around 120psi and of course it's only for a brief time. That said, I feel comfortable holding this in my hand. There are no indefinitely pressurized plastic parts to shatter when dropped. The propane line is regulated to a very safe 45psi and assuming it's sealed properly (which it is) there's no risk of leaks catching fire. Flashback is also impossible. If I ever adapt this to a handheld blaster, I'll take additional safety precautions including containment of the propane (or smaller - butane) tank in a padded case.

The advantages over spring-piston blasters are still minimal, though. If I could automate the firing cycle with actuators and solenoids somehow, I'd be on my way to having a blaster with the A2's abilities at far less operating cost.
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#6 taerKitty

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Posted 18 July 2011 - 04:42 PM

As a proof-of-concept, this looks fine. If you're going to turn it into a blaster and subject it to both regular use and the usual wear-and-tear, I'd suggest storing it in another hard tank so if it does crack, it's contained (somewhat).
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#7 TantumBull

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Posted 18 July 2011 - 07:40 PM

The advantages over spring-piston blasters are still minimal, though. If I could automate the firing cycle with actuators and solenoids somehow, I'd be on my way to having a blaster with the A2's abilities at far less operating cost.

I would venture to disagree. I'd imagine that the force required to draw propane into the tank is significantly less than that required to compress a [k26]. What you may consider a disadvantage (lack of automated firing) may actually be an advantage depending on how you look at it. The fact that you have to move the barrel back and forth mimics the motion of priming a springer, and thus gives a time delay between shots. Semi-autos are unfair at your typical nerf war just because they can fire so damn fast.

So essentially, you have something that has the ROF of a pump action springer, but doesn't require any of the arm strength (something that the younger nerfers will usually lack) to get good ranges. That's pretty fucking cool in my book.

Edited by TantumBull, 18 July 2011 - 07:40 PM.

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#8 PVC Arsenal 17

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Posted 30 July 2011 - 04:01 PM

@Tantum- I definitely didn't expect to hear that.

I finally had time to test this out with a 12" barrel and I got just over 100 feet held 2 feet off the ground firing 1.5" washer darts. The flat trajectory amazed me. I will now go ahead and turn this into something usable.
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#9 Curly

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Posted 30 July 2011 - 06:22 PM

That's pretty cool, too bad it requires me making the difficult decision between working barbeque and gun at any given time. On account of no war host letting you near them with that thing, I thing you could try stepping it up a notch. I would definitely recommend partially shielding the plunger skirt from heat with tinfoil or something.
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#10 TantumBull

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Posted 31 July 2011 - 12:57 AM

Have you tried firing this thing repeatedly before? I'd be curious see how many shots in rapid succession it takes to get the PVC to a disconcertingly high temperature. That could pose a bit of a problem in a war setting if the blaster started warping on you. Would it be possible to create some sort of heat sink or radiator that could draw heat from the combustion chamber?
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#11 Archangel45

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Posted 31 July 2011 - 01:04 AM

Have you tried firing this thing repeatedly before? I'd be curious see how many shots in rapid succession it takes to get the PVC to a disconcertingly high temperature. That could pose a bit of a problem in a war setting if the blaster started warping on you. Would it be possible to create some sort of heat sink or radiator that could draw heat from the combustion chamber?



I don't know if he does have heating issues with it or not- however a good radiator system for it would be to use a liquid cooling system from a pc(where he would connect it and or power a fan to help cool it I have no idea).
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