This expansion cools the air which contracts, at the same time being pushed back by the weight of the displaced water column.
The U-tube version has no moving parts in the engine other than the water and air, although there are two check valves in the pump.
The simple pump is external to the engine, and consists of two check valves, one on the intake and one on the outlet.
In the engine, the loop of oscillating liquid can be thought of as acting as a displacer piston.
The liquid in the single tube extending to the pump acts as the power piston.
This is a Fluidyne variant with a solid displacer piston (3). In figure -a-, as the displacer moves from the cold compression space (2), to the hot expansion space (4) in figure -b-, the temperature of the gaseous working fluid is increased. This increases the pressure of the gaseous working fluid, and as it expands, work is done on the (red) liquid piston as it is pushed through the tube.
schematic of a U-tube type Fluidyne engine.
A concentric-cylinder Fluidyne pumping engine. Topologically equivalent to a U-tube design.
Test of a model Fluidyne engine.
Detail of a water level displacement in a leftmost vertical tube.
