What it shows
A Stirling engine runs on any heat difference, even hot water against room air, which makes it suitable for solar and waste heat. The project builds a gamma-type engine from tin cans, a displacer and a syringe, and measures rpm against the temperature difference.
The science
Air in the sealed chamber is shifted by the displacer between a hot plate and a cold plate. Hot air expands and pushes the power piston, and cold air contracts and pulls it back. The piston and displacer are linked by a crank about 90 degrees apart so that the cycle repeats. The maximum efficiency is the Carnot limit, 1 − Tc/Th, which for 80 and 30 degrees Celsius is only about 14 percent, and real engines get a fraction of that.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two flat aluminium or tin cans with the same diameter (for example a cake tin and a lid), 10 cm wide
- A displacer disc of foam or thermocol
- A 20 ml glass or plastic syringe as the power piston
- Stiff wire for the crank and rod, a CD as flywheel, bearings from straws
- A mug of hot water at 70 to 80 degrees, ice water for cooling (optional)
- A digital thermometer or two
- A phone video camera for measuring rpm
Safety
For this project: Hot water scalds, so an adult must pour it. Cut cans carefully, with gloves, filing the edges smooth. Keep the engine on a tray to catch spills.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
12 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Study the gamma-type Stirling cycle and draw your engine with its parts and the crank phase.
Make the hot chamber from the lower can that sits on the hot water and fit a loose foam displacer inside with a clearance around its edge.
Seal the lid on top to form the cold side and make a tiny hole for the displacer rod with a loose seal.
Connect the syringe through a small tube to the sealed chamber to act as the power piston.
Build the crank on the flywheel so that the displacer leads the power piston by about 90 degrees.
Check that air does not leak by pressing the chamber and watching the syringe.
Place the engine on the mug of hot water and give the flywheel a small push to start it.
Adjust the displacer height and crank phase until the engine runs steadily.
Record the temperature of the hot and cold plates and the rpm from slow-motion video for water at 80, 70, 60 and 50 degrees.
Find the minimum temperature difference at which the engine just runs.
Plot the rpm against the temperature difference and calculate the Carnot efficiency limit.
Write the report with the cycle, test results and the potential for solar or waste heat applications.
What you should see
The flywheel turns at 20 to 120 rpm at a temperature difference of 30 to 50 degrees and stops below a certain difference. The graph of rpm against temperature difference is a rising curve.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the Stirling cycle?
A closed cycle of heating at constant volume, expansion, cooling at constant volume and compression.
2Why is a displacer needed?
It moves the air between the hot and cold sides without changing the volume much.
3What limits the efficiency?
The small temperature difference, friction and heat conducted straight through the engine.
4What is the advantage over an internal combustion engine?
It can run on any heat source and is quiet, though it has low power for its size.
Take it further
Place the engine in a solar cooker or under a magnifying mirror and measure the rpm with sunlight as the heat source.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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