What Is the Stirling Cycle?

The Stirling cycle is a type of regenerative thermodynamic cycle that is capable of using a self-contained working fluid, and an internal heat exchanging component, to either convert heat to mechanical motion or the other way around. That makes the Stirling cycle useful in engines, heat pumps, refrigeration, and numerous other applications. A number of different engine designs make use of the Stirling cycle, most of which contain either one or two cylinders. Regardless of the specific design, an engine that uses this cycle goes through the four steps of compression, the addition of heat, expansion, and the removal of heat.

There are a number of different external combustion engines, each of which uses a different type of thermodynamic cycle. Steam engines work under the principles of the Rankine cycle, which uses a working fluid, such as water, in both liquid and gaseous states. Additional fluids must be added from time to time, reducing the efficiency of these systems. The Stirling cycle, which was originally invented in 1816 as a rival for the Rankine cycle, makes use of a working fluid that is sealed within the system. In most cases, the working fluid used in Stirling engines is air.

The basic idea behind the Stirling cycle revolves around the heating, and subsequent cooling, of a single volume of working fluid. That causes the thermal expansion and contraction of the fluid, which can be used to perform mechanical work. In most cases, that is accomplished by attaching pistons to a flywheel. As the fluid within the system expands and contracts, the pistons are driven up and down, which causes the flywheel to turn. The Stirling cycle is referred to as a regenerative cycle due to the fact that the fluid is self contained, and the same volume of fluid is used in both the compression chamber and expansion chamber.

Use of the Stirling cycle is not limited to engines, since the process is also reversible. That means it is possible for a device that uses this cycle to act as a heat pump if mechanical power is supplied. In this case, external mechanical power is used to drive the pistons, which pump the working fluid between the expansion chamber and compression chamber. Depending on the way a device is set up, this reversal of the cycle can be used in a heat pump, refrigeration device, or any other application that requires the transfer of heat energy.