Stirling Cycle

The Stirling cycle is a thermodynamic heat engine cycle that operates on having a temperature differential present. In the simplest of terms, when a fluid in a gaseous state has a rise in temperature it rises and becomes lighter, when it is then cooled it becomes heavier and drops. By having a temperature difference present, it allows us to create a cycle of heating and cooling a fluid which we can then get mechanical work out of with a variety of different Stirling engine designs.


http://www.icefoundry.org/images/stirling_engine.jpg



Lets look at the photo above and analyse all the parts of this low temperature differential Stirling engine so that we can further understand the Stirling cycle.

We have a two aluminum plates, one being hot and one being cold. Typically the hot plate is the bottom plate but the cycle does work in reverse. Notice that there is a displacer inside a fixed volume of air, this is key because if the volume is not constant the Stirling engine does not work. As we add heat to the system through the bottom plate, the temperature of the air increases, therefore increasing the pressure between the lower plate and the displacer. The displacer is pushed upwards by this pressure which moves the flywheel, directly turning the energy from the heat into rotational mechanical energy. As the flywheel turns it moves a small piston on the other side downward, increasing the pressure on the top of the plate. pushing the displacer down. Now the air has been pushed into the space above the displacer and below the cold plate. The air is cooled by the cold plate and drops, further pushing the displacer down. This in turn moves the piston the opposite direction which moves are below the displacer that this process may repeat itself. As you can see, the Stirling cycle is one more of the many ways we can directly turn the energy from heat into mechanical energy.

When analyzing this system, we should look at the Carnot efficiency of the engine which may explain why you do not see these engines used in real life applications very often. Most Stirling engines operate on low temperature differentials which in turn gives us very low efficiencies, generally below 10%.


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