Was reading a site were two guys took an old propane grill sized can and filled it with air and then used an air powered drill with chuck for a friction based model. They were riding together and were getting about a minute worth of ride per fill up.
Lots of no-no type stuff with this idea. The main one is that propane bottle/can will burst if punctured and be a bomb of sorts or a missle if the regulator gets knocked off in a crash.
Pretty sure the carbon based air tank will hold up better to a puncture ... may still have missle effect if the regulator gets knocked off. So far haven't fund a 4500 under $580! Not sure I like friction based idea either. I'm thinking another form of drive chain linked motor would be better. Haven't looked up the air powered motors yet. This may end up being more research than I'm willing to put into it. And may cost more than is practical for an alternate means of transportation. Plus I've got a few Q

on the life of the carbon based cans.
MERCY ... my heads hurtin' ...