The alpha type Stirling Engine utilizes two pistons connected to a flywheel with one piston connected to the heat source and the other to the cold side.  The working gas is expanded and contracted in the respective chambers causing the pistons to turn the flywheel, causing mechanical work to be done.  One of the challenges is the use of a working gas and the ability to keep it at pressure  without it leaking out through the chambers.  Hydrogen has been used, but one of the problems with it is that it causes metal fatigue and failure, so choosing an affordable and readily available working gas is very important.

β-Type Stirling Engine

The beta design Stirling Engine has a single cylinder with a driving piston connected to a flywheel to do the work and inside the cylinder there is a loosely fitting displacer piston which allows the working gas to expand an contract in the cycle of the engine.

The gamma type Stirling Engine, is a two cylinder Stirling Engine which has a cylinder for the power piston and another cylinder for expansion and contraction of the working gas.  This design was developed to take advantage of small differential temperature between the hot and cold sides of the engine.

The Carnot efficiency of the stirling cycle is similar to that of a combustion of an internal combustion engine, but the advantage comes when you use it for cogeneration of heat and electricity.