Thanks for the suggestion, but I can see a few problems with that you're proposing.
sgort87 wrote:As long as your valve will work in reverse directions, you can eliminate the manual fuel valve by arranging the port configuration as shown.
The manual chamber ball valve (Which is a 3KPSI rated unit) also serves the function of protecting the seals in the 5 way valve (150PSI rated) from the ~500-1000PSI combustion pressures, so it really needs to be there in order to prevent damage to other systems.
sgort87 wrote:Your fuel will fill the meter pipe while air is filling the chamber, and in the other position the meter will dump while air is pressurizing the piston. After the shot, it will be in position 1 already, so air will purge the chamber automatically.
I don't think this would work for a couple reasons:
1) Due to the maximum vapour pressure of propylene at room temperature being ~150PSI, for the mix range I'm using, the chamber pre-ignition pressure needs to be higher than or very similar to the meter pressure. As a result, the fuel must be injected before the air, otherwise the chamber pressure will simply backup into the meter when the volumes are combined, and the gases will never mix properly.
2) Maybe I'm not seeing your diagram properly, but it looks like you have the chamber being filled with air prior to the volume behind the piston being pressurized. This won't work - The pilot volume must be pressurized for the piston to seal, just like in a barrel sealing pneumatic valve.
sgort87 wrote:The only thing I can think of that may get in the way a little bit is that some of the fuel may escape while the piston is seating. With a little tweaking of the meter pressure, it's probably of no real concern.
Now I'm slightly confused. Are you suggesting that the air supply be shut off while the valve is initially in position one, then turned back on after the meter has filled and the valve is switched to position 2? That would make more sense than what I've interpreted from your description above, but it would require an extra valve on the air supply.
As for the fueling issue, the ~0.62PSI/mix that would be lost from the chamber could be compensated for, but I'm not sure the losses would be consistent.
sgort87 wrote:Also, you can get rid of the little ball valve at the piston by replacing it with a check valve.
That was the original plan, but the price quote my supplier gave me for a 1/4" high pressure inline check valve was well outside the range I was willing to spend. If I can find a cheaper one, the ball valve will probably be replaced.
sgort87 wrote:Awesome cannon. I hope this helps to improve it.

Thanks for the input, and I hope you can clear up some of my questions/potential misunderstandings.