Piston Valved Hybrid
[this is under construction; currently nothing links here. I'll finish it eventually -fnord]
A piston-hybrid is a generic name for a hybrid that uses a valve other than a traditional burst disk, the most common of which is a piston valve. However, like burst disk designs, they typically rely on the sudden increase in pressure upon ignition to actuate and work properly.
There are several advantages and disadvantages to this design:
- Fast reset time
- No tools or burst disk material needed to reload
- Potential for semi/full auto firing
- High mechanical stresses(large valves may require custom machining)
- Burst-pressure tuning is not always straightforward
- Typically higher weight
The easiest hybrid valve to construct is a traditional barrel-sealing piston valve, as it can be made from off-the-shelf fittings. Other options include snap valves, toolies-style pistons, and chamber-sealing valves. You should have a thorough understanding of pneumatic piston valves before attempting a hybrid valve.
General Design Considerations
Hybrid valves need to be built very tough in order to function safely. Materials such as steel, copper, brass and aluminum are preferred. PVC or other plastics are generally not acceptable or safe for such conditions.
Pistons need to be extremely impact resistant; pistons made from pvc or other brittle plastics may crack on the first use. Sealing face separation may also be an issue, so an ideal piston would be constructed entirely from neoprene rubber roundstock or similar materials. If a heavy metal piston is used, a good bumper is necessary. A metal piston actuating can easily crack steel fittings if it gathers enough kinetic energy
If o-rings are needed, seamless pipe and non-flammable grease should be used.
Safety pop-off valves are commonly thought to be a necessity when constructing a hybrid valve. This is a MYTH, though they may be useful in certain designs. A pop-off valve used as a safety device in a hybrid is useless, as the chamber should be able to contain your planned mix without sustaining damage. Pop-offs can THEORETICALLY be used to cause a piston valve to open at a "lower than normal" pressure, but they have never been tested scientifically. They will also exhaust the air cushion that forms behind the piston, causing it to slam back harder.
Building a Valve
The type of valve you should choose depends on the type of gun you're constructing. Snap valves are good for small bores, pistons are good for general use, and toolie valves are good where compactness/low weight is necessary.
When you build a valve, it should be constructed with high opening pressure in mind. In a burst disk hybrid, opening pressure is increased by thickening the disk, but in a valved hybrid it is increased by adjusting the force holding the valve shut. Tuning your valve to open at the right pressure is critical to efficient operation, and is discussed (link)
It should also be noted that the d/4 rule does not necessarily apply to hybrids. In pneumatics, this rule is used to find how far a piston must move back for maximum flow through the barrel, with 1/4 of the barrel diameter being optimal. In a hybrid, air trapped behind the piston will resist backwards movement since there is nowhere for it to go. This may limit the travel of a d/4 compliant piston enough to cause significantly choked flow, as well as causing the valve to re-seal while pressure remains in the chamber. The easiest way to fix this problem is to increase the travel distance. A travel distance approximately equal to the porting diameter has been shown to work well.