Stirling Engines - Mechanical Configurations

The mechanical configurations of Stirling engines are generally divided into three groups known as the Alpha, Beta, and Gamma arrangements. Alpha engines have two pistons in separate cylinders which are connected in series by a heater, regenerator and cooler. Both Beta and Gamma engines use displacer-piston arrangements, the Beta engine having both the displacer and the piston in an in-line cylinder system, whilst the Gamma engine uses separate cylinders.

The Alpha engine is conceptually the simplest Stirling engine configuration, however suffers from the disadvantage that both the hot and cold pistons need to have seals to contain the working gas. There are a number of mechanical mechanisms which enable this type of engine to operate correctly with the correct phasing of the two pistons. An excellent animation of the V-type Alpha engine developed by Richard Wheeler (Zephyris) of Wikipedia is shown below:

One example of an Alpha engine is the SolarHeart engine currently under advanced development by Cool Energy, Inc in Boulder, Colorado. It is one of the major components of a complete solar heat and power cogeneration system for home usage including evacuated tube solar thermal collectors, thermal storage systems, hot water and space heaters, and the SolarHeart Stirling engine/generator.

Andy Ross of Columbus, Ohio has been developing small air engines with extremely innovative Alpha designs. He is the inventor of the classical Ross Yoke drive engine and more recently a balanced "Rocker-V" mechanism, shown below. Andy recently sent me a "youtube" video of his unique balanced double-V Alpha engine which avoids the use of a heat exchanger section stretching across the V.


Matt Keveney has done an animation showing clearly the principles of operation of the Alpha Stirling engine using the Ross yoke linkage. Andy Ross has published an article on the model Climax locomotive that he built using a small Rocker-V engine, and allowed me to maintain a copy of this article "A Class A Climax Locomotive".

The Alpha engine can also be compounded into a compact multiple cylinder configuration, enabling an extremely high specific power output. A schematic diagram of this configuration is shown below. Notice that the four cylinders are interconnected, so that the expansion space of one cylinder is connected to the compression space of the adjacent cylinder via a series connected heater, regenerator and cooler. The pistons are typically driven by a swashplate, resulting in a pure sinusoidal reciprocating motion having a 90 degree phase difference between the adjacent pistons.

One example of the swashplate 4-cylinder Alpha engine is shown below. This engine was originally developed by Stirling Thermal Motors and is currently being continued by Stirling Biopower of Ann Arbor, Michigan.

Whisper Tech Limited in New Zealand have developed a modern four-piston Alpha Stirling engine for multi-purpose power generation applications, as shown in their animated display of the unique wobble-yoke drive that they have developed for this engine.

A few years ago William Beale of Sunpower, Inc came up with an ingenious configuration combining a four cylinder free-piston alpha engine with a gas turbine output stage, as shown in the following schematic diagram:

The four cylinders are physically arranged with a 90 degree phase difference with each piston connected to a gas compressor. The gas compressors are then used to drive a gas turbine expander as shown. The main advantage of this system is the promise of a high specific power and most important, high reliability and life resulting from the absence of heavily loaded moving parts, since there are no side loads on any sliding bearings.

The sketch shows single acting gas compressors for simplicity, however the actual machine uses double acting compressors so that there are eight gas pulses on the turbine for each cycle of the four cylinder machine.

On to Beta Engines, Gamma Engines

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