Gas Laws in the Stirling Engine

During this part of my ISU, I related the gas laws I read about earlier to the Stirling Engine. Similar to reading about thermodynamics, since I was applying the knowledge, I didn’t have to return to the internet or textbooks for much reading. My textbook suggested that the gas laws allow for the motion of the working piston to be explained. As a result, this was my focus during the ISU.

Shown on the right, during the first phase of the Stirling Engine the displacer does not move, while the piston is pushed upwards. A number of laws are needed to describe this motion. First, prior and throughout the phase the gas was heated. The Gay-Lussac’s law states that as the temperature of a gas is increased, the pressure also increases. As the pressure in the cylinder builds, the gas puts a stronger force against the working piston. Eventually this force is strong enough to push the piston upwards. While the piston is moving the temperature is kept at a constant temperature. This is explained by the Boyle’s law; the volume increases while the pressure decreases. However, in the Stirling Engine since the gas is still increasing in temperature the gas nearly maintains its pressure. My textbook references this motion as isothermal expansion.

Once the working piston has been pushed upwards the displacer falls, shortly followed by the piston. The third phase – the falling of the piston – is shown on the left. Just before this phase the gas had been cooling. The Gay-Lussac’s law states as the temperature decreases so does the pressure. As a result, the force the gas exerts on the piston decreases. Eventually the force of the gas will not be able to hold the piston upwards. This results in the piston falling. While the piston falls, the volume decreases which beings to increase the pressure and temperature. This leads into the fourth and final phase of the cycle when the gas is heated before starting the first phase again.

This portion of my ISU was also very easy to understand. I did not need to visit many internet resources beyond finding a good diagram of the motion of the engine. This diagram was found at animated engines, which also contains a variety of other Stirling Engines. These engines also work because of the same properties, with changes to the layout of the engine. From my understanding, the engine I have researched is the most basic and as a result the least efficient version of the Stirling Engine. The site is an interesting look at different ways to apply the same principles to different mechanisms.

Sources:

Hooper, C & Reader, T. G. (1983). STIRLING ENGINES. New York, NY: E. & F. N. Spon.

Animated Engines. (n.d.). Low Temperature Differential Stirling Engine. Retrieved May 27, 2014 from “Animated Engines”:www.animatedengines.com