This is an interesting variation on the Stirling engine, which uses three glass marbles as the displacer. It's also unusual in that it uses a ratchet mechanism instead of a crank to convert the linear motion of the power piston into circular motion of the driving wheel.
A glass test tube forms the displacement cylinder and also encloses the power cylinder. The power cylinder consists of a small free moving syringe. The rear end of the tube is heated by a single wick meths burner. The heated air expands and pushes the power piston forward. As the power piston travels forward, the piston rod rides up over the ratchet wheel, causing the displacement cylinder to tilt upwards at the front. This allows the three marbles to roll back towards the rear end, displacing the enclosed air forward into the cold part of the tube. As the air cools, the power piston retracts, causing the tube to tilt in the other direction and the marbles to roll back down towards the rear, displacing the air back into the hot part.
The model was purchased as a kit and assembled by me.