Introduction

Here is a photo of the raw materials I started with:

For my school science fair, I decided that I wanted to make something that would also help me improve my machining skills. I made a Low Temperature Differential Stirling Engine as designed by James R. Senft in his book “An Introduction to Low Temperature Differential Stirling Engines.” This engine is a closed cycle engine (meaning no air goes in or out) that runs off minor changes in heat, such as the heat of a cup of coffee or a bucket of ice.

I then machined all of the parts (with the exception of the piston and cylinder, and the screws). I used the drill press, the lathe, a sabre saw, table saw, two homemade Hot Wire Nichrome Foam Cutters, a laser cutter, files, a grinder, scissors, bolt cutters, and a soldering iron in this project.

Here is a video of the finished engine running on top of a cup of hot water:

First, using layout fluid and a ruler, I “layed out" the part on a flat piece of aluminum.

I then cut out the outline with a jigsaw and perfected the edges with sandpaper:

Finally, I drilled the holes using my drill press:

The bearing plate is finished!

I put the rough block into the four-jaw chuck on the lathe to square up the sides:

Now with the properly-sized block, I layed out the cuts with the layout fluid (here you see the properly-sized block next to the original block):

I then lined up and taped the bearing plate to the base block, centered the drill bit, then cut a blind hole into the base block. I then used a tap to cut a threaded hole.

Then, I drilled the holes to attach to the top plate by lining up the block on the top plate, taping them together, and drilling through:

Finally, I drilled out the center hole, starting with a small bit and slowly increasing in size until the proper diameter was reached.

The base block was finished!

The bushing was cut from a 1” thick cylinder of aluminum and a steel insert. I put the aluminum into the three-jaw chuck on the lathe and roughed out the shape with the power- drive and cut off the part:

Next, I shaped the steel insert.

Then, I cut it off, and glued it in place. The reason for this insert is that when brass (used for the displacer shaft) rubs against aluminum, it “galls”, or increases in friction and adhesion making it very difficult for the shaft to slide. However, brass and steel do not gall.

Finally, I aligned the part on the top plate and drilled the side holes.

The displacer bushing was finished!

I layed out the circles on 1/4” thick aluminum square with layout fluid and a protractor:

Then, I cut them out with a jigsaw:

I layed out the edge holes and cut them on the drill press. The holes were through-hole (meaning that the bolt went through) on the top and threaded (meaning that the bolt screwed in) on the bottom plate. I first cut with the bit for the inner diameter of the screw-hole on all of the plates, which I had taped together. I cut all the holes, then separated the plates to deal with them individually. Here I am tapping the holes to thread them on the bottom plate.

Next, I cut a circle out of scrap wood with roughly the same dimensions and mounted it on a plate to attach to my lathe:

Next, I cut the base-block holes on the top plate using the base block as a guide. Then I tapped the holes in the top plate. Here I am testing my threaded hole with a screw:

Finally, I cut the center hole, the pressure release hole (and tapped it), and cut the holes to attach the displacer bushing, which I used as a guide:

To clean up the edge of the aluminum circle, I centered and mounted it on the wooden arbor with double-sided duct tape (after I was done I removed the aluminum by heating up the adhesive with a blow torch so it would become looser).

The plates were finished!

To make the ring, I started with a long tube of acrylic from which I chopped a ring on the mitre saw. Next, using a mitre, I cut a circle of wood with a rough outer diameter, nearly the same size as the inner diameter of the tube. This will eventually act as an arbor on which I attach the ring to the lathe head:

I attached this circle to a lathe plate and shaved down the edge of the circle so it would fit snugly inside of the ring.

I fit the ring over the wooden arbor and cleaned up the sides to the proper width.

Next, I cut an indent into the inside of the ring for a rubber rope that insulates the chamber. Note: later I determined that this insulation was leaking. Instead of cutting the indent, I just attached flat insulation on the base of the ring. I found this method creates a better seal.

The ring was now finished!

The piston is an important part of the Stirling engine. It helps regulate the chamber pressure while simultaneously pulling the flywheel, causing the flywheel to rotate.

The piston needs to slide up and down quickly inside a tube, which makes fit essential. Since this was my first engine, I decided to purchase a graphite piston inside a glass tube online from Airpot.

I took one of the brass bolts that I was using to attach the block to the plates and filed it down on one side. I drilled through the flat part of the bolt so I could insert a wire hook to attach the piston to the flywheel later on.

I attached one of the nuts 3/16” from the hole I had cut into the bolt, then silver-soldered it on with a blow-torch.

Finally, I drilled out the center of the graphite slightly so the shaft of the bolt could fit through the graphite.

The piston assembly was finished!

First, I cut a square chunk of foam out of a larger piece. Then I cut a circle of the foam using a nail set into wood. I stuck the foam on the nail and spun it around that point to make the circle.

Then I planed the cylinder into thinner disks using my horizontal foam cutter.

Next using a compass, I marked the circles which would hold the insert air filter material. I cut out those circles using a sharpened/cut-off steel tube.

I also cut air-filter circles using the same tube.

I dabbed a ring of glue onto the inside of each circle and pushed the air filter disks into the holes.

Then, I made a solid brass piece to insert into the foam to connect to the flywheel.

Using an old soldering iron, I melted a circle into the foam to act as a well for the epoxy. (pictured: practicing)

I sunk the brass into the foam and filled the holes with epoxy. I put the foam on top of wax paper so that the bottom of the foam could fill in without sticking to my work-surface. I also put some level scrap pieces on top with the displacer bushing to make sure the brass dried evenly.

The displacer block was finished!

First, I layed out and cut the main shaft using power drive on the lathe (pictured - measuring the depth of the cut):

I added blue layout ink and set my calipers to the correct width. Then, I marked the line around the shaft by rotating the shaft with the calipers pressed on.

I drilled the center hole then cut off the part. The bearing housing was finished.

Then, I created the collar by marking out the width, cutting the center hole, and using a parting tool to cut it off. I used the Allen wrench pictured to hold the collar so it didn’t fly off.

The bearing housing and collar were finished!

I marked all the distances with my calipers and cutoff saw and then I cut the notch down.

I then began the process of hollowing out the center of the part, first gradually stepping up drill bit diameter (large enough to insert my lathe bit) then cut out the portion slowly with autofeed. To assist me, I used an analog gauge to check depth by setting the surface of the part at 0” and slowly moving the carriage inwards to the proper depth.

I cut off the part and added a chamfer. I drilled and tapped a side hole for mounting the hub on the crankshaft:

The part was finished!

First, I cut the rod off on the cutoff saw. Since the rod was so small (and steel!), I decided to grind it down to size on the grinding wheel. I sized the rectangular piece similarly.

Next, I drilled and tapped the holes in the rectangular piece. There are 3 holes: one in the center, for the steel rod to fit into; one on the front but off center to allow for the up and down motion of the displacer to move the shaft; and one to add counterwieghts to the latter.

Finally, I fashioned a small aluminum round which fit into the displacer rod ring on the lathe and screwed everything together. The crankshaft was finished!

First, I designed the to-scale flywheel in the graphics software Inkscape because it exports .dxf files, which are compatible with the lasercutter software.

Next, I sent this file to the laser cutter and cut out the shape.

The flywheel was finished! Later on, I glued the flywheel hub to the flywheel for additional stability.

I started by test fitting that the ring and the two plates fit together by screwing them all together with nylon screws.

Next, I added the base block and bearing plate to the top plate.

I added the chamber ring, the displacer block, and the bottom plate with the nylon screws again. In addition, I glued the bearing housing into position.

Next, I attached the displacer bushing with brass screws. In order to have access to the screwholes for the bushing, the bearing plate and block had to be removed.

I epoxied the chamber in which the piston slides into a ring which fits onto the block. I added the block, tube, and plate back onto the top plate.

I added the crankshaft along with bearings in the bearing housing. I slid the flywheel and flywheel hub over the crankshaft, and tightened everything.

I now made the nylon spacers. I started with a long cylinder on the lathe, cut a hole through and added a notch (which the wire will fit into). Then I cut off the spacer (I held the tip of a pencil near the hole to catch the spacer.) I also cut retaining collars for the wire (pictured).

I estimated a length of steel rod then cut with steel cutters. Using two pairs of pliers, I wrapped the rod around the spacers. I also added a Z-bend in the center of the wire so the length, from end-to-end, was adjustable.

I added a hook at the end so the wire would fit into the piston assembly.

Once I added the wires, voilá! The Low Temperature Differential Stirling Engine was done. Now, it was time to troubleshoot.

First, I checked all the bearings and possible sources of excess friction, but these did not seem to be an issue.

Though, when the flywheel was rotated manually, hissing, coming from the engine, was audible. We pinpointed the issue to the rubber rope glued to the inside of the chamber ring. The rope was too firm, and was not compressed enough against the plates to form a proper seal. The nylon screws could not exert enough force to compress the rope.

To fix this, I decided to scrap the rubber rope and opt for a flat ring of corkboard-like insulation, softer than the rubber. To cut this into the proper shape, I added insulatory glue around the rim of the ring and placed the ring flat-down onto the cork. To ensure a tight seal, I placed a heavy box on top of the ring while the glue was drying.

Once the glue dried, I used an exacto knife to cut off excess cork. With the cork forming a proper seal, the engine was able to cycle air effectively, and it worked!

I had a blast making the Stirling Engine! In fact, I learned how to use a lathe to make this engine.

In making this project, in addition to a lathe, I used a drill press, lasercutter, hot-wire foam cutter, hole tap, mitre saw, sabre saw, (lots of) sandpaper, bolt-cutters, and a hand drill. I followed James R. Senft’s design and measurements and started with raw materials (pictured above.) Below is a video of the engine working off of the heat of a coffee cup. I would recommend this project to anyone looking to improve their machining skills!