Engine is fired by denatured alcohol burner . Heat is applied to the end of the glass cylinder . The cylinder is air-tight. Therefore air in the cylinder is permanently sealed inside the
engine. This fixed amount of the air is heated & cooled , causing it to expand & contract , this driving the piston.
As the displacer moves up , the air in the cylinder moves down to the hot side , so the air pressure inside the cylinder increase , pushing up on the piston .
As the displacer move down , the air in the cylinder moves up to the cool side , so the pressure inside the cylinder decrease, pulling down on the piston.