A Stirling engine is a closed-cycle heat engine that turns heat into mechanical work. It is an external combustion engine where the heat comes from an external source, such as fuel burners, geothermal heat, solar heat, and fossil fuel heat. It is similar to other familiar heat engines like steam engines, diesel, and gasoline.
The Stirling engine works on the principle that when air is heated it expands, and when it becomes cooled it contracts. It heats and cools air within a closed container at regular intervals, and uses the pressure changes this causes to run a piston.
This cyclical operation of heat is done by having one part of the engine kept hot and another part kept cold through thermal contact with an external heat source and an external heat sink. A mechanism then shifts the air from the hot side to the cold side, back and forth. The air cannot escape because it is in a sealed chamber. When the air goes to the hot area, it swells and pushes the piston upward. When the air returns to the cold side, it grows smaller and pulls down the piston. This occurs continuously.
Stirling engines are deemed reliable because they have no sparkplugs and valves. Unlike steam engines, Stirling engines don’t use a boiler, which has a high tendency to explode if not monitored well.
While the mechanism of Stirling engine is quite easy, understanding how specific models of Stirling engine work is difficult because there are hundreds of various engine designs that can utilize the Stirling cycle.
Some models of this engine use a piece of foam (like the one used as a filter in an air conditioner) to move the air between the hot side and the cold side. In this model, the air moves through and around the displacer from and to the different thermal areas, creating a power pulse.
When a side of the piston is exposed to the atmosphere, the process is a little bit different. When the sealed part of the working gas gets in contact with the hot part of the cylinder, it expands and works on the piston and on the atmosphere, as well. When the working gas goes to the cold area, the pressure of the air declines to below the pressure of the atmosphere; then the atmosphere would work on the piston and on the gas.
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