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Piston Hybrid

A place to ask general spud cannon related questions.
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Piston Hybrid

Unread postAuthor: saefroch » Tue Mar 30, 2010 7:47 pm

I've been thinking about constructing a piston hybrid (having experience with spray 'n' pray and pneumatic, thought i'd just jump a step), and at this time have two idea i'd like to bounce off the collective head of the community here.

1. Build a piston hybrid with a metal piston and use the kinetic energy of the piston moving back in a QEV to trigger combustion by
1a. Using the piston to ram backwards into a piezoelectric generator
1b. Using the piston as a conductor to close a circuit and spark from a capacitor or stun gun
2. Build a system that is a metered propane with a QEV right behind it. Some of the shockwave from the combustion goes to triggering the QEV without a pilot valve- this would probably require a very specific design

I am not sure if either of these ideas have been tried before, but I would like to know if either of them look horrendously stupid before I try either.
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Unread postAuthor: SpudFarm » Tue Mar 30, 2010 7:53 pm

You don't need a pilot if i remember correctly. When you ignite the fuel/air mix in the chamber (not on the pilot side of the piston) the pressure will push the piston back.
Imagine trying to fill a pneumatic from the chamber side of the valve.

Correct me if Im wrong :)
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Unread postAuthor: saefroch » Tue Mar 30, 2010 8:05 pm

So you can just fill a normal pneumatic with a propane-oxygen mix and set it off in the chamber and the QEV triggers?
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Unread postAuthor: chinnerz » Tue Mar 30, 2010 8:12 pm

SpudFarm wrote:You don't need a pilot if i remember correctly. When you ignite the fuel/air mix in the chamber (not on the pilot side of the piston) the pressure will push the piston back.
Imagine trying to fill a pneumatic from the chamber side of the valve.

Correct me if Im wrong :)


so....
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yes?
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Unread postAuthor: spudtyrrant » Tue Mar 30, 2010 8:49 pm

or maybe.......
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Unread postAuthor: saefroch » Tue Mar 30, 2010 9:24 pm

Not sure how the one that you posted works, chinnerz. I don't see a valve on it.

The one that spudtyrant posted is idea 1b. I've thought about modding my pneumatic to do just that, and i'm making a metal-wood composite piston soon so I will be able to.
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Unread postAuthor: SpudBlaster15 » Tue Mar 30, 2010 9:30 pm

All of your designs have the fuel mixture igniting after the valve has fully opened, which defeats the purpose of building a hybrid in the first place. With such a setup, the projectile will be long gone before any significant amount of fuel has reacted, which will result in low chamber pressure, low projectile acceleration, and lots of wasted energy.

For best performance, you want the valve to open at as close to peak combustion pressure as you can manage. Depending on the dimensions of the chamber and the number of ignition points, this is ~10-20ms after the ignition system fires.

Look at any of the piston hybrids on this site, and you'll see that they all use a standard piston valve design, with seals on the piston and a pilot chamber filled with air. This is an effective design, as the opening pressure can easily be set to achieve optimal performance.
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Unread postAuthor: spudtyrrant » Tue Mar 30, 2010 9:45 pm

saefroch wrote:Not sure how the one that you posted works, chinnerz. I don't see a valve on it.

The one that spudtyrant posted is idea 1b. I've thought about modding my pneumatic to do just that, and i'm making a metal-wood composite piston soon so I will be able to.

the one chinnerz has shown is a coaxial, 1b will be your best bet imo, it has the highest chance of working succesfully.
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Unread postAuthor: saefroch » Tue Mar 30, 2010 9:53 pm

I did some looking around on the forum and now know what i'm talking about in terms of a piston hybrid. The important thing to remember is that they do not use a solid piston, but something that looks like a barbell. This looks like it would be very fragile to me, but it might not be, given the speed at which the piston moves.

I now understand SpudBlaster15's post entirely. I shall begin working along those lines now, with my completed understanding of a piston valve.
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Unread postAuthor: chinnerz » Tue Mar 30, 2010 10:02 pm

well, all i was doing was converting spudfarm's idea into a picture. how it works is:

you fill it with fule air mixture, it is like a pneumatic co-ax right now.
ignition will cause the chamber to instantly increase in pressure pushing the piston back, and allowing the rest of the expanded gas to go down the barrel. as these is a change in pressure zones between the pilot and the chamber, a valve is not needed. if it was there, it would be redundant

personally i think the hybrid.. hybrid or pneumatic combustion design may work, however, the projectile will be half way down the barrel before ignition.. and that is IF ignition happens.

a burst disk is a much better idea, but if you don't want to place a new disk in place after each shot, thats where my idea comes in :P
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Unread postAuthor: ramses » Wed Mar 31, 2010 8:26 am

2 is a winner. IIRC, all piston hybrids on this site use that concept. Some use a pop-off on the pilot to release the air behind it, but mine compresses the pilot to get an air-spring type effect, so I don't need a substantial bumper.

no one has used a QEV, though. The piston in it might not be sturdy enough.
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Unread postAuthor: saefroch » Wed Mar 31, 2010 9:46 am

2 is a winner. IIRC, all piston hybrids on this site use that concept. Some use a pop-off on the pilot to release the air behind it, but mine compresses the pilot to get an air-spring type effect, so I don't need a substantial bumper.


Yah, i'm starting to notice that, and to think I came up with that by myself! I actually came up with a good idea for once.

Bumper? I get the rest of your idea, but what do you need a bumper for?

no one has used a QEV, though. The piston in it might not be sturdy enough.


I am planning to make fully cylindrical piston out of a slightly sanded aluminium nipple and a wood dowel heat-sealed into it. I might be able to come up with an idea for creating the plunger-type piston that is used in a piston hybrid. I would like to use a non-inline QEV design because those work the best for me, a great user of plumbing.
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Unread postAuthor: SpudFarm » Wed Mar 31, 2010 10:30 am

No need to be overkill. If I have understood you correctly you want to make a valve behind the piston open a slight moment after the combustion starts? The piston would already have moved back by that time.
If you have a good seal around the piston inside the housing the pressure behind the piston will be lower then in fron of it. And it will open.

You can regulate the opening pressure of the valve by pressurizing the area behind the piston higher then the pre ignite pressure in the chamber. Pretty hard to explain in a understandable matter if you have little experience with piston valves.
Hope you understood it :P

Once again I say correct me if im wrong since I am not totally confident in my "knowlege" before I get into it again.
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Unread postAuthor: saefroch » Wed Mar 31, 2010 3:35 pm

Yah, I understand the concept. You'd need two pressurized air fill locations then though, wouldn't you?
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Unread postAuthor: SpudFarm » Wed Mar 31, 2010 4:04 pm

saefroch wrote:You'd need two pressurized air fill locations then though, wouldn't you?


I don't think you have to, no. But if you do you have controll over the opening pressure so you can adapt it to your combustion pressure.

I don't think anyone have tried it though, just something i took from the top of my head but it would be a nice addition to a piston hybrid.
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