The following valve was described and built by an amateur radio enthusiast wayyyy back in 2005-2006 as a way to build a compact valve for launching tennis balls carrying a line, for erecting wire antennas.
The creator eventually wound up building a very compact version using a modified piston valve called the "DFTV", or "Darn Fast Turbo Valve". It was a standard outlet sealing piston valve with one major twist: when the piston moved backwards, it un-sealed a pretty large port at the back (bottom) of the valve, allowing the pilot volume to exhaust much, much more quickly than using a standard pilot valve.
The original creation can be seen here: antennalaunchers.com.
Since then, people have churned out a variant every few years or so.
Here's an animated diagram of the operation of a variant I was planning on building...
While not too many people have built pneumatic variants of this valve, its legacy can be felt strongly in today's hybrid launchers.
Most current "hybrid piston valves" use the DFTV design. How?
- Take your standard DFTV.
- Remove the pilot valve (though not the sealing apparatus and port).
- Fill the pilot volume with compressed air.
- Fill the chamber with a fuel/air mixture at pressure.
- Ignite it.
- The increase in pressure on the chamber side increases.
- The pressure in the chamber will increase until the force pushing back on the piston equals the forced of the compressed air pushing it forward.
- The piston creeps backwards, until the port at the rear is unsealed.
- The piston fully opens at this point, exhausting combustion by-products out the barrel.
spudpuzzer's, MrCrowley's, and SpudBlaster15's piston hybrids on Spudfiles are a few examples.