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Full auto idea

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Full auto idea

Unread postAuthor: daccel » Sun Mar 15, 2009 10:46 pm

I keep thinking that there has to be a way to create a full auto hybrid without a timing circuit and solenoid valves.

But I can't get around the problem of delaying re-fueling to prevent premature ignition.

Does this design look like it has a chance of working? It consists of a blow forward breech which also acts as the main valve, and a hammer valve to fuel the chamber. The piston would have to be manually pulled back and released to initially prime the chamber. The burst of air/fuel returns the piston just past the spark gap. Ignition is via a continuous spark from a stun gun, electric bbq ignitor or similar.
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Unread postAuthor: psycix » Mon Mar 16, 2009 1:56 pm

You gotta be damn sure that that A/F tank can handle the combustion in case the flame goes in....
But were not here to talk about the dangers of a pre-mixed bulk tank.

I think that the hammer valve closes too soon, causing either the piston to stop halfway OR the gas expands, lowering the mix number, thus needing a higher pressure on the bulk tank. Which comes with some dangers.
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Unread postAuthor: daccel » Mon Mar 16, 2009 4:07 pm

No, we're not, but worth mentioning at the beginning of the thread I suppose. Thanks for not dwelling on it. Or suggesting the concept isn't worth discussing, as I've got before. :)

Since it's similar to a common paintball gun diagram that has been posted here before, which cycles the action and fires the paintball, I assumed there would be enough air released. But I don't have any experience with hammer valves.. you may be right.

Some decrease in pressure would be acceptable, trading power for auto capability, but it would be nice to get a decent mix in there. Otherwise might as well just make a hp pneumatic.

What about something like in this revised diagram, to increase the open time of the hammer valve. Problem is, then you're decreasing the time to refueling, and increasing the likelihood of premature ignition.

There is also the possibility of not being able to find a balance between a spring that the hammer can open but that will stayed closed during combustion.

On the positive side, maybe you wouldn't need o-rings on the piston if a sear was added for initial cocking, because it would cycle fast enough for the leakage to be negligible. I think this was mentioned on a pneumatic blowback thread recently.

Edit: revised the diagram again to address my last two points. Less surface area on hammer valve, sear and cocking handle.
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Unread postAuthor: psycix » Tue Mar 17, 2009 10:03 am

That hammer valve in the third diagram is certainly better.

But with that cocking handle it woulnt be fullauto anymore. With a bit of mechanical fiddling, pump action could be achieved.
However, when doing pump action, why would one need a hammer striking on the hammer valve? You could simply attach the hammer valve to the pump mechanism so that it fills on a certain point in the pump cycle.

For doing it full-auto (without handle), maybe extend the rod that comes out of the hammer valve, and make sure the hammer valve can still be pressed ALL the way in.
This will cause the hammer valve to stay open until the piston is almost on its cocked position and thus the whole chamber is filled with pressurized fuel mix.
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Unread postAuthor: daccel » Tue Mar 17, 2009 9:40 pm

The handle would just be for cocking it before firing the first time. Otherwise how would you get the first burst of a/f to push back the piston? You wouldn't need to do it between shots because the last shot would cock it.

Well you can't extend the rod past the spark gap, or it would fire before fully pressurizing. But you're right, you'd want it to be as close as possible. And hopefully the piston in the hammer valve having less mass than the large piston means it would accelerate back to the right as fast as the large piston, so that it would never ignite before the hammer valve was closed.

What about putting vents to the right of the sear to maximize pressure drop in the chamber. In theory you could adjust the spring strength so that the projectile was clear before the piston passed that point. It would allow for a higher opening pressure in the breech valve, because you wouldn't have to worry about the breech closing before enough gases escaped, and not letting enough a/f in.
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Unread postAuthor: psycix » Wed Mar 18, 2009 3:24 pm

You could. But with a weak spring in the blow forward you wouldn't have to.
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