Stirling Engine (Sabrina Loesh)

Author

Sabrina Loesh, Olive Vista Middle School

Principles

• temperature

• heat flow

• thermodynamics

• motion of molecules

Standards

HS Chemistry - Chemical Thermodynamics

7. Energy is exchanged or transformed in all chemical reactions and physical changes of matter. As a basis for understanding this concept:

a. Students know how to describe temperature and heat flow in terms of the motion of molecules (or atoms).

Materials needed

• American Stirling Company MM-5 Stirling Engine

• Cup of hot water

Procedure

1. Build Stirling Engine according to the instructions from manufacturer.

2. Heat a coffee mug full of water in themicrowave to boiling.

3. Place the Stirling Engine on top of the coffee mug.

4. Gently spin the propellor to get the stirling engine going.

Explanation

The Stirling Engine requires a temperature difference between two plates in order to run. In this case the plates are made from aluminium. This is a a good choice because aluminium is highly conductive. In order to power the Stirling Engine, we need to have one plate hotter than the other. This is easily achieved by placing the Stirling Engine on a cup of hot water.The gap between the two plates is sealed, containing a fixed volume of air. As the bottom plate warms up, the air between the plates expands, pushing up the piston that seals a hole in the top plate. This piston is attached to the flywheel by a metal rod.

As the piston rises, the flywheel is turned. The movement of the flywheel in turn pushes down a second rod which is attached to a displacer sitting loosely between the plates. As the displacer moves downwards, it pushes air away from the hot bottom plate and up to the top plate, which is cooler. This causes the air to contract and the piston is pulled back down again, turning the flywheel further. The turning flywheel raises the displacer again, pushing air back to the hot plate, and so the cycle continues while there is still a sufficient temperature difference between the two plates.

Questions

1. Draw a diagram of the Stirling engine showing the flow of air through the compression chamber. (Students should draw an image showing warm air rising up to the top of the chamber, then becoming cool and sinking down to the bottom of the chamber)

2. What would happen to the engine if you placed it on top of a cup of very cold material? (The reverse cycle would occur)

3. Why are there two pistons on the engine?

Everyday examples of the principles illustrated

The Stirling Engine is a heat engine that is vastly different from the internal-combusion engine in a car. Invented by Robert Stirling in 1816, the Stirling Engine has the potential to be much more efficient than gasoline or diesel engines. But today, Stirling Engines are used only in some very specialized applications, like in submarines or auxillary power generators for yachts, where quiet operation is important. Although there hasn't been a successful mass-market application for the Stirling engine, some very high-power inventors are working on it.

Photos

References

MM-5 Stirling Engine assembly

Explanation of Stirling Engine