What if I told you that there is an engine out there that can potentially be much quieter and more efficient than a gasoline or diesel engine? A Stirling engine runs on an external heat source that can be anything from solar energy to a candle. They are currently only used for very specific and seemingly obscure purposes, like running a military submarine or powering scientific research stations in Antarctica, but hopefully, with enough research and experimentation, the Stirling engine can soon power some of our every day items.
To understand how a Stirling engine turns heat energy into mechanical work, we will create a Stirling cycle out of a tin can and some other household items. For this tin can Stirling engine, a candle will heat up air inside of a tin can, causing a displacement cylinder to rise and fall. The displaced air inside the can will cause two wooden rods to rotate a crankshaft, and the crankshaft will rotate a wheel.
For this experiment, you will need:
tin can (ours was a standard soup can–about 10.5cm x 7cm, but you can make other sizes work)
2 thumb tacks
wire coat hanger (thin enough to bend)
wood and a wood cutter (we just used pieces of scrap wood and cut them to the dimensions we needed)
The displacer cylinder is what goes inside the tin can to make movement out of heat. To make one, we will:
a) Cut circles of wood to fit inside the tin can. The diameter of the wooden circles should be about 0.5 cm less than that of the tin can (our can was 7 cm, so our circles were 6.5 cm). This will allow air to travel around the cylinder. We used balsa wood, which is ideal because of how lightweight it is.
d) Stack the circles into a cylinder. Use glue to stack the wooden circles to a height that is about 2/3 that of the can (our can was 10.5cm in height, so our cylinder was about 7cm).
The wooden frame will hold the whole engine together. The construction is fairly simple and leaves room for mistakes or improvisation.
a) Cut two wooden rectangles. These will be used for the walls and should be about 20 cm x 30 cm. We used thin pieces of scrap wood.
b) Cut four rectangular bars. These should all be cut the same length, about 20 cm.
c) Attach the bars to the walls. Use screws and a drill to attach a rectangular rod to each of the top two corners. For the bottom two, attach them so that the tin can can sit on top of them without falling through (our tin can was 7 cm wide, so the two bars were about 6 cm apart).
d) Drill a hole on each wall. This will be where the wire crankshaft connects. Drill it 10 cm from the top of each wall, and make sure the holes are lined up.
The measurements for the frame do not have to be exactly as stated, just as long as they’re proportionate with each other.
The crankshaft will take the movement from the displacer cylinder and create rotations. To make one, we will:
a) Bend a wire coat hanger. You will be creating three rectangular notches. Refer to the diagram for the dimensions.
(Photo source: RimstarOrg on Youtube)
The connecting rods will attach the tin can to the crankshaft.
a) Cut two connecting rods. The wood should be about 0.5 cm in width. Cut them to 10 cm in height.
b) Make a small hole at the top of each rod. We used a nail to create the hole 1 cm from the end of each rod. Make sure that the crankshaft wire can easily fit in the hole.
The heat energy produced by the candle will ultimately make the wheel turn. This is where you can get creative and decide the purpose for your Stirling engine. The rotating crankshaft has the potential to power an endless amount of things. For this project, we made a simple wheel to show the process of a Stirling cycle. If you would like to make the wheel, too, follow these steps:
a) Cut a circle out of wood. Ours was about 10 cm in diameter. Drill a hole in the center so it can connect to the wire crankshaft.
If not the wheel:
a) Get creative! Try making these things instead of a wheel:
a gear system
a Rube Goldberg machine (!!!)
Now we will assemble all of the parts!
a) Attach thread to the displacer cylinder. Push a nail into the center of the top of the cylinder. Tie thread around the nail and leave a good length for the next steps. (Our nail came loose so we ended up using three little nails.) Put the cylinder in the tin can.
b) Cut a balloon. You may want to blow it up first to stretch it out. (You will be cutting off the lip so that it will be able to stretch over the tin can.) Use a needle to push the thread attached to the nail through the center of the balloon.
c) Push two thumb tacks through the bottom of the balloon on either side of the thread.
d) Attach the connecting rods to the points of the thumb tacks. You will do this by pushing the bottom of the connecting rod onto the point.
e) Stretch the balloon over the opening of the tin can. Make sure that the thread is centered and the connecting rods line up well. The balloon should be air-tight and you can seal it around the rim with tape.
f) Push the crankshaft through the holes on the connecting rod. Assemble so that the connecting rods lie in the two opposite rectangular nooks.
g) Attach the crankshaft to the frame by pushing it through the previously drilled holes. Then, attach the wheel to the crankshaft on the outside of the frame.
h) Hang a paper clip from the middle rectangle on the crankshaft. Tie the thread to the paper clip and make sure that it is tight enough to rotate the crankshaft.
After it is all assembled, you can finally start your Stirling engine!
a) Place the candle underneath the tin can. You should be able to put it between the two bars on the base and place the tin can on top, but you can use books to elevate the frame higher if you need to.
b) Light the candle and watch the magic happen!
It should take a minute or two before the wheel starts turning. We made a few errors and had to restart with a couple of things, but that’s to expect! It was overall a fun project and we would love to continue experimenting with the many potential uses of a Stirling engine.