Since the Stirling engine is a closed cycle, it contains a fixed mass of gas called the “working fluid”, most commonly air, hydrogen or helium. In normal operation, the engine is sealed and no gas enters or leaves the engine. No valves are required, unlike other types of piston engines. The Stirling engine, like most heat engines, cycles through four main processes: cooling, compression, heating and expansion. This is accomplished by moving the gas back and forth between hot and cold heat exchangers, often with a regenerator between the heater and cooler. The hot heat exchanger is in thermal contact with an external heat source, such as a fuel burner, and the cold heat exchanger being in thermal contact with an external heat sink, such as air fins. A change in gas temperature will cause a corresponding change in gas pressure, while the motion of the piston causes the gas to be alternately expanded and compressed.
When the gas is heated, because it is in a sealed chamber, the pressure rises and this then acts on the power piston to produce a power stroke. When the gas is cooled the pressure drops and this means that less work needs to be done by the piston to compress the gas on the return stroke, thus yielding a net power output.
In summary, the Stirling engine uses the temperature difference between its hot end and cold end to establish a cycle of a fixed mass of gas, heated and expanded, and cooled and compressed, thus converting thermal energy into mechanical energy. The greater the temperature differences between the hot and cold sources, the greater the thermal efficiency. The maximum theoretical efficiency is equivalent to the Carnot cycle; however the efficiency of real engines is less than this value due to friction and other losses.
Existing Configurations (sourced from http://en.wikipedia.org/wiki/Stirling_engine);
There are three most common basic configurations:
- Gamma type: This design also uses a mechanical displacer to push the working gas between the hot and cold sides of the cylinder, but the displacer is housed in a separate cylinder for easier mechanical fabrication
- Alpha type: This design has independent cylinders and a gas driven between the hot and cold spaces.
- Beta type: This design uses an insulated mechanical displacer to push the working gas between the hot and cold sides of the cylinder. The displacer piston runs though the power piston, for less “dead space”.
THE STRILING CYCLE;
The Stirling cycle comes closest to the Carnot cycle efficiency while having a higher work ratio. Despite the fact that the efficiency may not be practical in real fabrication and testing, the Stirling engine gives the best output.
The idealized Stirling cycle consists of four thermodynamic processes acting on the network of the fluid;
- 2-3: Isothermal expansion: the expansion space and associated heat exchanger are maintained at a constant high temperature and the gas undergoes isothermal expansion absorbing heat from the heat source.
- 3-4: Constant volume heat removal: the gas is passed through a regenerator where it cools transferring heat to the regenerator for use in the next cycle.
- 4-1: Isothermal compression: the compression space and associated heat exchanger are maintained at a constant low temperature so that the gas undergoes isothermal compression, rejecting heat to the cold sink.
- 1-2: Constant volume heat addition: the gas passes back through the regenerator where it recovers much of the heat transferred in (2) above, heating up on its way to the expansion space.
The Stirling cycle on a P-V & T-S diagram