A Sterling engine is a heat engine that operates by cyclic compression and expansion of air or other gas, the working fluid , at different temperature levels such that there is a net conversion of heat energy to mechanical work .
The engine is like a steam engine in that all heat transfer takes place through the engine wall. This is traditionally known as an external combustion engine in contrast to an internal combustion engine where the heat input is by combustion of a fuel within the body of the working fluid.
Unlike the steam engine's use of water in both its liquid and gaseous phases as the working fluid, the Sterling engine encloses a fixed quantity of permanently gaseous fluid such as air or helium . As in all heat engines, the general cycle consists of compressing cool gas, heating the gas, expanding the hot gas, and finally cooling the gas before repeating the cycle
There are two major types of Sterling engines that are distinguished by the way they move the air between the hot and cold sides of the cylinder:
• The two piston alpha type design has pistons in independent cylinders, and gas is driven between the hot and cold spaces.
• The displacement type Sterling engines, known as beta and gamma types, use an insulated mechanical displacer to push the working gas between the hot and cold sides of the cylinder. The displacer is large enough to insulate the hot and cold sides of the cylinder thermally and to displace a large quantity of gas. It must have enough of a gap between the displacer and the cylinder wall to allow gas to flow around the displacer easily.
An alpha Sterling contains two power pistons in separate cylinders, one hot and one cold. The hot cylinder is situated inside the high temperature heat exchanger and the cold cylinder is situated inside the low temperature heat exchanger. This type of engine has a high power-to-volume ratio but has technical problems due to the usually high temperature of the hot piston and the durability of its seals. In practice, this piston usually carries a large insulating head to move the seals away from the hot zone at the expense of some additional dead space.
Action of an alpha type Sterling engine
The following diagrams do not show internal heat exchangers in the compression and expansion spaces, which are needed to produce power. A regenerator would be placed in the pipe connecting the two cylinders. The crankshaft has also been omitted