In the conventional Stirling engine, a fixed amount of helium is compressed in a cool chamber and then transferred to a chamber heated by an external burner.
Spend a day there (or three, as I did while we waited for the boat's engine to be fixed), and you will fall in love with the friendly, beach bum lifestyle.
Instead of stepped gears like a conventional transmission, it has a fixed planetary gear, and changes engine speed by adjusting the generator electric load.