Stirling engine efficiency
The efficiency of the stirling engine is among the highest of any engine ever invented, it can be compared to the highest efficiency diesel engines that are available on the market even today, such as the common rail and blue motion diesel engines which are highly efficient (using approximately 50% of the fuel for motion).
Stirling Engine Types & Efficiencies
There are several types of strirling engines, each of which works on slightly different principles and mechanical linkages and each of which has maximum theoretical efficiency values due to these differences.
Stirling engine types –
Alpha Stirling Engine Efficiency
An alpha stirling engine has two cylinders that are connected to each other by a tube. Each cylinder has its own piston, one cylinder is referred to as the hot side and the other as the cold side. The hot side can be heated by any fuel source, this could be a straightforward flame to concentrated solar energy using a parabolic dish. The cold side can be cooled by simply having cooling fins around the cylinder al the way down to literally having the cylinder embedded in snow. Engines like these are used in Iceland where to hot side sits in warm geothermal springs and the cold side sits literally under a bed of snow.
Alpha stirling engine efficiencies are approximately 40%. This is due to the lack of a regenerator which I will discuss in more detail later on.
Beta Stirling Engine Efficiency
A beta stirling engine differs from the alpha in that it only has one cylinder, both cylinders are housed within this single cylinder. The cylinder has two ends, a hot end and a cold end. As with the alpha the hot end can be heated by nearly any heat source and the cold can be cooled by any cool source, but generally just utilizes cooling fins. One piston is a tight fit within he cylinder and the other is a loose fit, the tight fitting piston is referred to as the power piston and the loose fit is referred to as the displacer piston. Any change in pressure within the cylinder wall will act upon the tight fitting power piston, the displacer simply serves to move the gases around within the engine – from the hot side to the cold side.
Beta stirling engine efficiencies are also approximately 40%.
Gamma Stirling Engine Efficiency
The gamma stirling engine, also known at the low temperature stirling engine is the stirling engine with the maximum theoretical efficiency available. In fact, the gamma stirling engine has the highest theoretical efficiency of any engine ever invented. However you must note that this is theoretical. Gamma stirling engines work in a slightly different way to he other two engine, you can view an animation showing how the am stirling engine works below –
The gamma stirling engine can incorporate a re-generator into the design. This is simply a heat sink that is built into the engine. What it does is to store some of the heat within itself which can then be used when required, this vastly increases the efficiency of the engine.
If you would like to learn more detailed information about the efficiencies of stirling engines you might be interested in the wikipedia page, however be warned, it makes for tough reading – stirling engines.
Inventors and engineers, both professional and amateur are constantly working on ways to improve the efficiency of stirling engines. Hopefully we have not seen the last of this magnificent engine and it soon takes it rightful place along with the other engines that we use on a regular basis