How NOT to Build a Stirling Engine

Stirling Engines are beautiful delicate machines that transfer a difference in temperature into mechanical movement. In theory this is how it works:

The temperature difference is created when the cylinder is heated and cooled, this will make the air pressure inside of the cylinder goes up and down respectively. The cylinder contains a displacer which helps to move the air up and down faster inside of the cylinder. Furthermore, a piston is added to the top of the cylinder to make it airtight, the piston will move up and down depending on the air pressure generated inside the cylinder through the temperature difference .

The piston and the displacer drive each other, for that reason the displacer should be at the top of the cylinder when the piston is at the halfway point, this will make that the air at the bottom of the cylinder expands and therefore the piston will go up. When the displacer is at the bottom the air is on the cool side at the top of the cylinder, this will make the piston be pulled down. In order to keep this as a continuous cycle, a flywheel is added in order to create a momentum.

This instructable is about how to not build a Stirling Engine. We tried building a Stirling Engine using candy tins as a starting point. However hard we tried, we could not get it to run.

Despite this fact we wanted to publish the results of this experiment. Knowing what can go wrong is valuable information when building something complicated. This Instructable shows the steps we took and some advice for improvement.

This Instructable is part of an assignment for the course TfCD from the faculty of Industrial Design and Engineering.

Materials

• Two candy tins (Diele)
• Clear plastic bottle; the diameter should be the same size or larger than the diameter of the candy tins
• Piece of metal tube; diameter 27mm, length 15mm
• Balloon
• Elastic band
• Mechanical Pencil
• Eraser
• Wood; 80x80x5mm
• Foam; thickness 20mm
• Cardboard; thickness 0.5mm
• Iron wire; thickness 1.3mm
• Small safety pin
• Thread

Tools

• Ruler
• Superglue
• Thin double sided tape
• Pliers
• Needle
• Hammer
• Saw(ing machine)
• Nail
• Electric drill
• Drill bits, diameter 4mm and 2.1mm
• Utility knife
• Scissors
• Sanding paper
• Permanent marker
• Cup of hot water
• Tea bag (optional)

Paste a stroke of paper onto the bottle. The height of the stroke of paper should be height you want your cylinder to be. In this case it is 45mm.

Use this piece of paper as a guide to cut the plastic with the utility knife.

Check if your plastic cylinder fits your candy tin. It should be relatively airtight. If your cylinder is too big you can make it smaller by cutting it and taping it together at the right diameter.

Tape your cylinder to the bottom half of your candy tin with double sided tape. Check if the lid fits the cylinder as well but do not tape the lid on yet.

The displacer can be made out of foam. Because the displacer has the function of moving the air in the cylinder, it should have a smaller diameter than the cylinder it is moving in.

Draw a circle (diameter 65mm) on the foam with a compass. Cut the foam with the sawing machine. You could also use a hot wire or a heated knife.

Cut a long piece of thread and thread your needle. Put your thread through the centre of the displacer and tie a knot at the end. Make sure the knot is big enough that it does not disappear in the displacer.

Use your ruler to find the middle of the tin lid. Use a centre punch to mark the spot and drill with the 2.1mm drill.

Mark the hole for the second cylinder. Take the piece of metal tube, fit it on your lid and mark the centre. Use a centre punch to mark the spot and drill with the 4mm drill.

To make the flywheel, use your ruler to find the middle of the second tin lid. Use a centre punch to mark the spot and drill with the 2.1mm drill.

Saw the frame for the flywheel by making a rectangle of 50x80mm and two legs. Drill a hole (2.1mm) in the rectangle at the height you want the centre of the flywheel to be. Make sure the flywheel hole is placed high enough so the wheel will not touch the mechanical pencil tip and metal tube.

Saw the tip off the mechanical pencil. Glue it over the centre hole in the middle of the lid. Let it dry. Place the foam displacer inside the cylinder and use the needle to pass the thread through the pencil tip.

Cut a strip of cardboard of a few millimeters high. Glue it around the rim of the metal tube and let it dry. Cut the balloon and put it over the metal cylinder. Secure it with an elastic band.

Glue the tube over the second hole in the lid. Glue the wooden frame on the lid, on the opposite of the second hole. Make sure you leave a little bit of space between the frame and the pencil tip.

Cut a piece of iron wire of about 100mm long. Put the axis through the hole in the frame and use the marker to indicate the places where it passes over the pencil tip and the centre of the balloon.

Use pliers to bend the axis according to the picture.

To stabilize the axis, cut a piece of cardboard and fold it into a small box. Pierce the box twice to make room for the iron wire to pass through and glue it to the back of the fame.

Make a loop in another piece of iron wire and cut it to length. It should fit between the end of the axis and the top of the balloon. Put the eraser of the mechanical pencil on the end of the iron wire and glue it to the balloon.

Cut four small parts of the eraser to function as spacers on the axis.

Put the small safety pin over the axis in between two eraser spacers. The safety pin should be located at point C.

Put the flywheel over the axis and glue it at point B. You can stabalize the wheel with a piece of cardboard, similar to the frame.

Put the wire connected to the balloon over the end of the axis at point A. Secure the axis with two pieces of eraser over the ends.

Tie the thread to the small safety pin. When it is tied at the right length, the displacer should be up when point C is at its highest point.

Fill a cup with hot water and place the stirling engine on top. Wait a minute to give the bottom of the cylinder time to warm up and spin the flywheel.

As mentioned in the introduction, we could not get our engine to work. Here are some tips to make your design an improvement on ours.

How do we think you could get your engine to work?

• Making sure the axis is staight. Our axis had several imperfections because it was bended by hand from iron wire. Maybe using a needle as the axis for the flywheel and attaching the cranks as seperate parts could solve this problem.
• Stabilizing the axis. Having another frame design can improve the Stirling a lot. Perhaps a frame which holds the axis in more than one point.
• Trying out different sizes of displacers.
• Attaching the flywheel properly to the axis. This means that the axis goes straight trough the middle of the flywheel and the axis and wheel turn together without slipping. Making the hole in the flywheel exactly the diameter of the axis and/or using the right type of glue could fix this problem.
• Lessen the friction of the axis. Different spacers that cause less friction could be used. They could be attached to the axis using glue.