How Piston Valves Work

Watch carfully 

This was posted in the Spud Tech forum a while ago.

Phase 0: Basically just the gun in its pre-use state. No pressure in the gun. 

Phase 1: An air source is connected behind the piston. Air enters the pilot chamber and the pressure pushes the piston against the rear of the barrel or chamber port. 

Phase 2: Air continues to flow through the input and leaks around the outside of the piston or through an equalization hole into the main chamber. Once desired pressure is reached, the input flow is cut off and the gun is ready to fire. 

Phase 3: The exhaust valve is opened and the pressure begins to fall in the pilot chamber. The exhaust must exhaust faster than the equilization hole can leak air back into the pilot. Once the force pressing on the back of the piston falls below the force acting on the front of the piston, it begins to slide back. Suddenly there is more surface area exposed on the front of the piston, and the jump in force slams the piston back leaving an opening for air to flow into the barrel to accelerate the projectile.

The exhaust valve is closed and a new projectile loaded. The gun is then back to it's original condition, ready for the cycle to repeat.

The main thing to notice between the different types are the forces caused by pressure. Of course forces could be changed by changing the changing the piston/sealing port diameter, but in the general senario like the one shown, with a constant barrel diameter between all types and a tee slightly larger than the port, the barrel sealing valve will have the same forces as coaxial in the closed position. Barrel sealing will have the highest opening force, coaxial will be in the middle, and chamber sealing will have both a low closing force and a low opening force.

Coaxial Piston Valve

Barrel Sealing Tee Valve

Chamber Sealing Tee Valve