What would be a good homemade air motor using an air supply of 15 psi and 15 cfm?

This was sounding easy until I got to "positive displacement" the pressure is modest, and the fairly high volume suggests a rather simple fan like turbine, but for positive displacement it gets a lot harder to build an air motor.

Do you have access to metal working machinery? at least an engine lathe, a milling machine would also be very helpful, plus the knowledge to use them. I suspect a vane type motor will be the least hard type to build, but I still wouldn't call it easy. A piston motor is not going to be easy to build at all.

I'm thinking of a small gear pump, such as an automotive oil pump. If you force air through it, you should be able to spin the impellers and obtain power from the hex drive shaft. Another idea is a squirrel cage automotive heater fan. If you excite the d.c. motor and drive the fan, you can generate electricity with it. Although the latter is not positive displacement. The main problem is that 15 psi is not much pressure.

Not sure of your goals or skills and resources, but this page offers some ideas, especially the coaxial valve arrangement, which could be adapted to a small piston engine. You can block the ports on a two stroke for example, and use these valve arrangements. .

It seems the total power is not going to be great.