Stirling engine refers to a family of usually sealed cousins to steam engines. Basically pistons are moved by the heating and cooling of sealed gases. They usually operate at lower and safer pressures than steam engines without the weight of the boiler but suffer from relatively low power and slow acceleration. One advantage is that they can run on so many combinations of heating and cooling such as: fire, solar, fission, geothermal, opposite air or water cooling.
This is one of many DIY Stirling engine designs available on YouTube. Could make an interesting SP project base.