Gamma type engines have a displacer and power piston, similar to Beta machines, however in different cylinders. This allows a convenient complete separation between the heat exchangers associated with the displacer cylinder and the compression and expansion work space associated with the piston. Thus they tend to have somewhat larger dead (or unswept) volumes than either the Alpha or Beta engines.
Furthermore during the expansion process some of the expansion must take place in the compression space leading to a reduction of specific power. Gamma engines are therefore used when the advantages of having separate cylinders outweigh the specific power disadvantage.
In the historical web-page of Robert Stirling by Robert Sier we notice that one of the engines built by Robert Stirling was a Gamma engine. It was donated to the University of Glasgow, and was later used by William Thomson (Professor of Natural Philosophy, who later became Lord Kelvin) to show that Stirling's machine worked on a reversible cycle.
A unique Gamma type engine is the Ringbom engine in which the piston is connected to a crankshaft, however the displacer is driven only by the gas pressure forces on a displacer piston rod. This engine is described in a book Ringbom Stirling Engines (Oxford University Press, USA, 1993) by James Senft, formally a professor of mathematics at the University of Wisconsin, River Falls.
James Senft also developed a revolutionary approach to low temperature difference Stirling engine design using the Gamma type engine as described in his book "An Introduction to Low Temperature Differential Stirling Engines" (Moriya Press, 1996 – Unfortunately out of print). In this book he describes how to make an engine that runs off the heat of one's hand - a seemingly impossible task. The major innovation of this approach is the extremely large diameter displacer as opposed to an extremely small diameter piston.
Because of the structural convenience of two cylinders that enables a completely independent sizing and construction of the displacer and piston assemblies, the gamma configuration is a favorite among modelers and hobbyists. The website StirlingSouth by Roy Rice and Richard Egge, both very innovative modelers, includes a large worldwide selection of engine modelers.
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The most amazing and innovative of the Low Temperature (heat-of-your-hand) Gamma type engines are those made by Kontax Stirling Engines in the Thames Valley, England. Their website indicates that they produce around thirteen low temperature engines, including the unique KS90R Black Ross Yoke LTD engine shown, a twin engine design KS90T Polished Twin LTD, and other uniquely innovative engines. |
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Another engine inspired by James Senft is the "heat of your hand" engine, the model MM-7 Stirling engine made by the American Stirling Company - will run on a temperature difference of only 4 degrees C! This is one of a number of innovative gamma type model Stirling engines that they describe on their website, including the model MM-1 Coffee cup engine. |
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The Fizgig by master modeler Mick Collins. This delightful engine came with a complete set of drawings and instructions which can be downloaded from Mick Collins' website, together with performance characteristics, feedback from modelers who have built the engine, and other interesting information. (Unfortunately the website of Mick Collins is no longer active) |
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Another approach is that of Boyette White, who converted a Briggs and Stratton engine into a Gamma type Stirling engine by adding the displacer and heat exchanger sections. Unfortunately, Boyette's website is no longer active, however I did want to indicate the option of converting a regular piston IC engine to a Gamma type Stirling engine. |
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The Thumper engine by Richard Egge is a Ringbom type engine in which the displacer piston is moved by pressure forces, and have no mechanical connection to the crankshaft. Richard also made unique double-Ringbom and quad-Ringbom engines. One of the engines made by Roy Rice is Aquatap, a water cooled Ringbom type engine inspired by James Senft. |
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Stirling Cycle Machine Analysis by
Israel
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