What is a Sterling Engine?
A Sterling Engine is a lesser known engine that runs off of a temperature differential, rather than a conventional fuel source. Simply put, one part of the engine simply must be kept hot, and another part must be kept cold. The hot side can be maintained by any method; from burning fossil fuels under it to collecting the heat of the sun through a solar ray. If done correctly it can be very renewable and small engines can operate on a temperature difference of as little as 100 degrees F. Above is a heat driven engine found in Black Engineering Hall at Iowa State University.
Step 1: How Does a Sterling Engine Work?
Much like a standard internal combustion car engine, a Sterling Engine works by utilizing the fact that hot air expands and contracts as it is heated and cooled. Air is heated in a compartment containing a piston. As the air expands, it pushes the piston out, which in turn compresses a cold piston. This act shoves cold air back into the system and causes the air to compress and return to its beginning location. When the air is heated again, the process then repeats. See the video for an example of a simple Sterling Engine. As you might guess, the red side is where hot gasses expand, and the blue side is where the heat is dissipated to repeat the cycle.
Step 2: Math (You May Skip This Step If You Are Not Interested in the Theory)
I have modeled this engine using thermodynamic principles to estimate the likely outcome of the engine. As you can see, it does not generate a large amount of power, however it does have enough energy to convert the potential temperature energy into kinetic energy. The images included are of an excerpt of the EES (Engineering Equation Solver) code used to model this process, and a graph of the predicted output.
Step 3: Materials
For this project you will need the following items. Most of the supplies can be found at the local hardware store or re-purposed from recycled materials found around the house.
2 tin cans
1/4in. Copper Pipe
19 Gauge Steel Wire
3/4in. Steel tube
15 Min. Epoxy
2.5in.X3in.X3in. Aluminum block
2in. Screws (9)
1/4in. Drill Bit
3/4 in. Drill Bit
1/6in. Nails (2)
Step 4: Prepair the Cans
1) Remove labels from both tin cans
2) Use a can opener to remove lids of both tin cans. Save the tops for later
3) Wash out the inside of the cans thoroughly
Note: Any residual contents of the can will cause problems in later steps
Step 5: Drill Holes in the Cans
1) Using a permanent marker, mark the point 2in. from the bottom of each can
2) Using a small 1/4in drill bit, pre-drill each can at their respective marks
3) With the pre-drill to guide the center, use a 3/4in. drill bit to widen both holes
4) Mark a point 1in. from the bottom of only one of the cans
5) Drill a 1/4in. hole at this mark
Step 6: Cut Steel Tube
1) Using a band saw, cut the steel tube to 2.25in. as shown in the image above
2) Carefully sand both ends to assure smoothness
Step 7: Cut Copper Pipe
1) Cut copper pipe to a 6in.
2) bend the pipe into a gentle curve as shown in the image above
Caution: Copper bends easily, but it also crimps or fold. Be careful not to kink the copper.
Note: it is not necessary to have a specific curve at this time. Copper is soft and will allow for fine tuning in the final assembly
Step 8: Create the Pistons
1) Using wire, create a small hook that will fit inside the steel tube
2) Lubricate inside of steel tube with cooking oil
3) On a piece of wax paper, mix 15 minute epoxy as shown in the image above
Warning: carefully follow handling and care instructions on the epoxy.
4) Place epoxy mixture inside steel tube until the epoxy is it is ½in. deep
5) Place hook in the top of the epoxy
6) Allow to harden (30-40 min)
7) Remove the epoxy cylinders. You have created the piston
7) Repeat steps 1-6 to make the second piston
Step 9: Can Assembly
1) Using epoxy, fasten the can with two holes on top of the can with 1 hole. Assure the holes are 90 degrees from each other (see the
2) Use epoxy to secure steel tube in the large hole of the top can
3) Glue the copper tube into the small hole on the top can
Note: The copper tube is not pictured in the image, however it will go into the small hole shown in the figure
4) Use epoxy to re-attach the lid to the top can
5) Set aside to allow all epoxy joints to harden
Step 10: Cut the Heat Sink
1) Using a drill Press bore a 4/5in. hole in the center of the aluminum block. The hole should be 2in. deep.
2) Drill a 1/4in. hole from the left side a 3/4in. from the bottom. Drill until it reaches the larger, bored hole
3) Using a band saw, cut “fins” in the aluminum block to the specifications shown in the image above
4) Insert the copper tubing (previous formed) into the small hole on the side of the block and secure using epoxy
Step 11: Create the Base
1) From a sheet of 1/2in. plywood, use a table saw to cut out the shapes as shown in the diagram
2) Secure the two smaller pieces to the base upright with 3 screws each. The final picture shows what the assembly should look like
These pieces will act as supports for the rest of the engine
3) Drive the nail 1/2in. from the top of piece B so that the head still sticks out a 1/2in.
Step 12: Make the Flywheel
1) Use a band saw to cut a 5in. diameter circle from plywood as shown in the image
Note: The plywood I used for this step was 3/4in. thick, however the same 1/2in. thick plywood from before will also work
2) Drill a 1/4in. hole through the center of the circle
3) Drive a nail 1/2in. from the center so that the head still sticks out a 1/4in.
Step 13: Create Wire Rods
1) Cut a 6in. length of 19 gauge wire
2) Fold the wire in half
3) Using a pliers if necessary, twist the lengths together, leaving 2 “hoops” at each end (see image above)
Note: The original length of 3in. will shorten to 2.75in. after twisting
4) Repeat steps 1-3 for second wire
Step 14: Final Assembly
These steps can sound complicated, please see the image above to help understand the final location of each part.
1) Place wooden circle on the Nail protruding from restraint B
2) Place plastic bowl in front of restraint B
3) Set can assembly (2 cans, steel tube, copper tube, and aluminum block) so that the cans are in front of restraint A, and the aluminum block is at the center of the plastic bowl
4) Connect each twisted wire rod to an epoxy piston, using the hook and loop to attach them
5) If needed, use pliers to bend the hook shut in order to keep the parts from disconnecting
6) Place 1 piston in the steel tube with the wire pointing away from the can
7) Place second piston in the aluminum block with the wire pointing away from the block
8) Attach the free ends of both wires to the nail on the wooden disc (note, my solid works design uses a plastic gear instead of a
Step 15: Testing Your Sterling Engine
1) Fill the bowl with water until it is 1/2in. below the top of the aluminum block
CAUTION: It is imperative that the water level is not too high and enters the aluminum bore. If the piston gets wet the system
2) Place a tea candle inside the hole drilled in the lower can
3) Light the candle and allow 5 min. for the air inside both cans to heat up
4) Gently spin the disc with 1 finger. This will begin the rotation and the system will start to spin on its own
5) If the system does not begin to spin, check to make sure the epoxy pistons are not stuck anywhere and reset the system. The engine’s major disadvantage is that it is very sensitive; If one part is not lined up correctly the whole system will fail. Reposition and try again; it may take some patience but the engine will begin to work when everything is properly aligned.
Step 16: Citation
Sterling engine animation