Characteristics of a Semi-free-piston Stirling engine are that the output power can be used directly from the oscillating motion of the power piston, and it does not have or need a flywheel. We have studied the Semi-free-piston Stirling engine for the power source of a fish robot.

What is a Semi Free-Piston Stirling Engine?
What is a suitable power source for the fish robot? Here is a discussion on the application of a semi-free-piston Stirling engine.
An Experimental Seme Free-Piston Stirling Engine
In order to disscuss if the engine is suitable for a fish robot, we developed a small experimental engine.
A Seme Free-Piston Stirling Engine 'S-FPSE 2000'
Here is our latest creation, the S-FPSE 2000. It is a model boat that uses a semi free-piston Stirling engine to power a fishlike propulsion system. It was just developed in September 2000.