SpudFiles

Well I'm gonna go ahead with the four cylinder design cause it's already 2/3 built, and think about the piston return issue some more. Thanks for the feedback. I'm not gonna use an air tool because if I wanted the most logical form of mechanically powered transportation I would just hop in my car, this is a hobby project that is not intended to be in any way practical but just to see if I can build a self-propelled CO2 vehicle from scratch.

And I've seen that CO2 engine animation, in fact that was what gave me the idea in the first place, but I eventually came up with a four cylinder design which I thought would improve reliability and be easier to make, since it wouldn't stall or require constructing a flywheel.

I agree with jack. Just get/make/buy/steal a gearbox or pulleys. on a small system, one could even use a bike's gears.

back on topic, you could rig the valves to inject a tiny amount of liquid CO2 at the very top of the stroke, and let it expand during the cylinder's "push"phase , but your PVC wouldn't like that.

The alternative would be to use the cam to open a valve. Do you have a lathe?