SpudFiles

Do you think this will work

So you let it charge up then whack it into a door, causing the piston to slide down, squashing the propane finally tripping a switch, igniting the compressed mix and blasting the piston out again with more force.

Could work but I'm sure theres a reason why the police stick to swinging a bloody great lump of steel at your door instead.

Quieter and less dangerous p'raps.

*edit whahaha first post by seconds

Well, for starters, the valve would have to let air is as well, not just propane. Also, the piston would have to connect to a cam-shaft like in a car (I'm assuminig you want to make an engine-style device) in order to push down after the combustion stroke is over.
I also see a potential fuel loss problem when the piston is up because the exhaust port and the fuel valve are open at the same time.
I suggest moving the exhaust up to the very end of the piston's stroke (up in your diagram), and putting the propane/air valve at the bottom your cylinder.
Another idea would be to use a peizo-electric igniter as the spark. Think about this: when the piston hits the igniter's button as it ends its compression stroke, then it will cause the spark.
It looks like perhaps you are going with an electric valve? I suggest making it mechanical so that the top "touch valve" pshyically pushes open the propane/air valve via an external shaft.

Hope this helps,
-goose_man

EDIT: Damn, beaten to first post!

thanks for the help i might try to build it soon i'll post pics when it's finished

if you get it to work you shold hook it up to a kind of piston that pushes air though a 6mm barrel and you got a airsoft. machine gun [/b]

Why not use a solid propellant?
[ugh forum rules]
Alot easier in my opinion

If you make it i bet you end off worse then the door. The recoil of the piston/ram hitting something solid will be insane. Be very careful when testing this you could do your self some damage.

You are better off building a normal pneumatic or combustion and shooting out the door handle.

check out the design of the Tippmann C3 it has some of the aspects that you are interested in (i.e. propane combustion)