Stirling engine, an engine cycle conceived of almost 200 years ago by Robert Stirling, a Scottish minister. It is highly efficient, and in these days of environmental concern there is renewed interest in its design.
The Stirling engine works by expanding and compressing a fixed amount of gas in a closed container. Its operation depends on the gas having different temperatures in different parts of the engine. Nonetheless, two of the processes in a Stirling cycle are essentially isothermal, and two occur at constant volume and do no work. The heat source for a Stirling engine is external. A Stirling engine is an example of an external combustion engine, as is a steam engine.
The efficiency of a Stirling engine results from the clever use of a regenerator that sits between the hot and cold cylinders in the engine. The regenerator absorbs heat when it is not desired, increasing its own temperature, and then gives up heat to the gas during another part of the engine cycle when it is desired. Typical regenerators are made of metal mesh through which the gas passes as it moves from cylinder to cylinder. When hot gas moves to the cold cylinder, heat flows from the gas to the regenerator. When cooled gas moves back, heat flows from the regenerator back into the gas.
WhisperGen Brochure: http://www.whispertech.co.nz/content/library/WTL026Brochure.pdf
WhisperGen specifications: http://www.whispertech.co.nz/content/library/AC_specBase.pdf