Gas expands when heated, and contracts when cooled. Stirling engines move the gas from the hot side of the engine, where it expands, to the cold side, where it contracts.
When there is a temperature difference between upper displacer space and
lower displacer space, the engine pressure is changed by the movement of
the displacer. The pressure increases when the displacer is located in the
upper part of the cylinder (and most of the air is on the hot lower side).
The pressure decreases when the displacer is moved to the lower part of
the cylinder. The displacer only moves the air back and forth from the hot
side to the cold side. It does not operate the crankshaft and the engine.
In other words, the connecting rod to the displacer could be a string in
this engine and it would still work.
When the engine pressure reaches its maximum because of the motion of the
displacer, a power piston is pushed by the expanding gas adding energy to
the crankshaft. The power piston should ideally be 90 degrees out of phase
with the displacer piston. The displacer type Stirling engine is operated
by the power of the power piston.
A special thanks to Koichi Hirata for the excellent illustrations!