A Scottish minister named Robert Stirling invented the Stirling engine around 1816. Various manufacturers built Stirling engines over many years, but those engines usually had low power compared to their weight (low specific power). Although pressurizing the cycle can achieve a higher specific power, sealing of the gas often proved problematic. To overcome this difficulty, William Beale, a professor at Ohio University, invented the free-piston Stirling engine in the early 1960’s. He realized that with a proper design, the engine did not require any mechanism and could be easily hermetically sealed. In the early 1970’s, Beale founded Sunpower, Inc. to continue the work he started at Ohio University.

The theoretical Stirling cycle has the following four stages in its cycle as shown in the graph below:

• (1-2) Isothermal Expansion
• (2-3) Constant Volume Cooling
• (3-4) Isothermal Compression
• (4-1) Constant Volume Heating

Practical engines however have smooth continuous motions represented by the ellipse within these bounds.

One could build an engine which operates as shown in the diagram with alternate heating and cooling within a single space, but losses would exist caused by the alternate heating and cooling of the metal walls. There would also be significant stress and material issues with such an engine.

The video below illustrates how a Free Piston Stirling Cryocooler Operates.