spudchucker wrote:I think you should make it a pneumatic for the simple reason of free fuel because after a while wouldn't it pay for itself?
No. Any pneumatic engine like this will require huge gas consumption.
I had designs for such a thing ages ago, but I had to give it up because even under it's most efficent arrangement it would have used up the air sources I can reasonably use incredibly fast.
It would have been capable of catapulting whatever vehicle it was on with impressive acceleration, but not that fast overall because the supply would have been gone before you got up to speed.
The high density (if not pressure) of stored propane or butane as well as the much lower amount of it needed for each stroke of the engine makes a huge difference to how far such a vehicle could go.
However, unlike a pure pneumatic engine, a propane powered engine couldn't do stationary starts, and will need both a clutch that can handle the torque - and some way to start it.
It's not that simple building an engine from scratch. I've got a nice set of plans for a moderately powered propane engine, but I've "stalled" on finding a way to make the clutch. (Sorry, very bad pun)