>>4245723This is an Alpha Stirling engine. (with a lot of bells and whistles apparently).
>>4239912This is a Beta Stirling engine.
They are heat engines, or external combustion engines, meaning they use the expansion and compression of a gas (usually air or hydrogen I think)
In a sealed piston.
The beta has two pistons: one hot, one cold, the alpha shares a piston but uses a "displacer" piston.
In case anyone cares.