DIY Stirling Engine:
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The author writes:

Stirling Engine Demonstration Model. It is a do-it-yourself construction out of simple materials like test tubes, Lego pneumatic cylinders and marbles.

The starting position of the engine is tipped to the left. The alcohol burner heats the air in the right area of the test tube; the air expands and pushes the pneumatic cylinder and thus the test tube upwards. The marbles roll to the right. The hot air is then forced to the left, where it gets cooled. The cool air shrinks, the pneumatic cylinder moves downwards and the test tube tips counter clockwise. The marbles roll left and the air is pushed right, to the hot side. The air expands and moves the piston again. And so on and so forth.

There’s a piece of steel wool in the bottom of the test tube (hot side). It’s both, heat exchanger and cushion (to avoid the test tube to break). In the test tube are placed five marbles that act as displacer piston as they move the air from the hot side to the cold side and backwards. As friction is a common problem in building Stirling engines, I decided to use a low-friction “LEGO” pneumatic cylinder as working piston. Furthermore the whole system has to be 100 % leak-proof; I used an original Edradour Scotch Whisky cork to close the test tube and finally silicon to seal all crucial parts. To make the engine finally run some fine tuning in terms of balancing the test tube and positioning the pneumatic cylinder and the heat source is necessary.

DIY Stirling Engine: [Link]