Complete Cycle:

It is worth noting here that the cycle below is an idealized model of how how the Stirling Engine functions, in reality the four steps taken would be less concise.

Cycle Overview:

1
 2
 3
 4
Beta Stirling Engine Cycle Diagrams

Courtesy of emaze.com

There are four steps that (ideal) Stirling engines take to complete a cycle, they are shown above.

Steps:

1.
    The working fluid is initially compressed by the power piston, and the displacer is near the heat sink allowing the air to heat up as it is forced near the heat source. In this stage the pressure increases as the volume remains constant.

2.
    As a result of the increase in pressure, an isothermal expansion takes place. The Piston is driven back expanding the cylinder and decreasing the pressure at a constant temperature.

3.
    The Flywheel drives the displacer toward the heat source allowing the working fluid to cool down, all the while the power piston is held in place keeping the volume constant, which causes the pressure to decrease.

4.
    Due to the drop in pressure, the power piston is pulled in to decrease the volume in the cylinder, meanwhile the displacer is brought toward the heat sink.

Note: This process repeats itself so long as the heat source and sink remain at their respective initial temperatures (more or less). Often though when using metal fins and the surrounding air as a heat sink (as in the diagram below), eventually the system will overheat and step three and four of the cycle will not occur making the system come to a halt.


P-V Diagram

Cycle and Diagrams

Courtesy of emaze.com


    The Pressure vs Volume diagram, above, relates to the cycle described and gives us a better idea of what is happening inside the cylinder. This diagram is also helpful in showing at which stages heat is lost where it is gained and how much work is done.

    The Stirling cycle shown has four sections two of which (the vertical ones) are isometric processes wherein changes in pressure and temperature occur. No work is done during these parts. The other two sections involve an isothermal change in volume and pressure, these are the sections where work is done/required; however, the cycle does produce a net work output.

    In reality the P-V plot would not be so perfect, the diagram shows the ideal system. In reality the process is not exactly isothermal and the constant volume parts may not be so as they are controlled by the crankshaft which is constantly moving. But, due to the fact that each cycle occurs in a short period of time this idealization is a good model.

    If you would like to learn more about how this cycle produces work from heat (i.e. the math behind it), click here.