  A Stirling engine of the type used for demonstration purposes has been
outfitted with a pair of sensors that measure pressure and piston displacement
when the engine is operating with a small temperature difference between the
hot and cold reservoirs. Measured variables are compared against computer
generated output based on a simple theory that involves nonlinear equations of
motion. Theory and experiment are found to be in reasonable agreement.
Temperature dependence of the graph of pressure versus piston displacement, for
different directions of flywheel rotation, permits a better understanding of
the physics of heat engines and refrigerators in general.
