Gamma Type Stirling Engines

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 therefor used when the advantages of having separate cylinders outweigh the specific power disadvantage. 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" (Moriya Press, 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. 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 as shown in the animation produced by Matt Keveney. We will attempt to justify this approach when we consider the Ideal Adiabatic analysis.

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 modellers and hobbyists. The following is only a small sample of the many Gamma type Stirling engine models displayed on the internet.
 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 eleven engines, including the unique KS90R Black Ross Yoke LTD engine shown, a twin engine design KS90T Polished Twin LTD, and other uniquely innovative engines.

 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.

 

 The Fizgig by master modeller Mick Collins. This delightful engine comes with a complete set of drawings and instructions which can be downloaded from Mick Collins' website, together with performance characteristics, feedback from modellers who have built the engine, and other interesting information. (Note: Unfortunately Mick Collins' website is no longer available, however there are a number of youtube videos of those who built these machines such as Fizgig is alive, as well as an interesting animation.)

  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. This is an example of converting a regular piston IC engine to a Gamma type Stirling engine. Boyette used an external crank to drive the displacer, however one could conceivably use an electrical drive or gas pressure force.

 

 
 The Little Engine Pages by Roy Rice and Richard Egge include some delightful Stirling engines by these two very innovative modellers. The engine shown is a water cooled Ringbom Stirling engine "Aquatap" made by Roy Rice. The Ringbom engine is a Gamma type engine in which the displacer piston is moved by pressure forces, and has no mechanical connection to the crankshaft.

 

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