Algodoo

It doesn't work. Has a few circles because of 100 item limit for demo.

The alpha type uses two pistons.
The top piston is the working piston and the bottom is the displacer.

When the worker moves left, the displacer pushes the gas into it and the gas expands by heating up.

When the worker moves right, the displacer lets the gas to the cool side, compressing the gas by cooling it down.