Coffee Cup Stirling Engine - a free 3D model from Shapekit

A Stirling engine that runs on a mug of hot coffee. Set it on the mug, wait a minute for the bottom plate to warm up and give the flywheel a flick. Between the two aluminium plates is a flat chamber of air with a light displacer in it. When the displacer goes up, the air moves to the warm bottom plate, heats up and its pressure rises; when it goes down, the air moves to the cool top plate and the pressure drops. That pushes a balloon diaphragm on top up and down, and the diaphragm turns the flywheel through a crank. The displacer crank runs a quarter turn ahead of the diaphragm's, which is what keeps it going. On a bag of ice it runs the other way round.

Parts (each STL is already placed the right way up for printing)
- stirling-engine-base.stl: Holds the bottom plate from below and stands on the mug or the table. The middle is open, so the plate sits right on the rim of the mug. Six hex pockets underneath take the M3 nuts. Print: Flat, pockets down. Print it in PETG: it touches the hot plate.
- stirling-engine-chamber-ring.stl: The wall of the air chamber: Ø90 mm inside, 17.5 mm between the plates. The top plate drops into the recess on top, the bottom plate sits in the base. A thin smear of petroleum jelly on both faces makes it airtight. Print: Flat, recess up. PETG.
- stirling-engine-top-ring.stl: Clamps the top plate down and carries the bearing frame. The middle stays open, so the plate can give its heat off to the room. The notch in its edge marks the flywheel side. Print: Flat, no supports.
- stirling-engine-bearing-frame.stl: An A-frame with the housing for the two 623 bearings, 87 mm above the top plate. Four M3 × 10 screws hold its feet on the top ring. Print: Standing on its feet, the way it is used. The bearing seats are teardrops, so no supports.
- stirling-engine-displacer.stl: Ø86 × 9 mm, with 2 mm all round it. It moves the air between the warm and the cool plate; it has to be light and airtight, not strong. The displacer rod is glued into the hole in the middle. Print: Flat, in PETG. 2 walls, 4 top and bottom layers and 5% infill: about 17 g instead of the 27 g of a normal print.
- stirling-engine-rod-gland.stl: Glued over the hole in the middle of the top plate. The displacer rod slides through it; a little grease seals the gap. Print: Flange down. Run a 3 mm drill bit through the hole by hand so the rod slides without play.
- stirling-engine-power-cylinder.stl: A shallow cup, Ø30 mm inside, glued over the 8 mm hole in the top plate. The diaphragm lies over its rim. Print: Floor down.
- stirling-engine-diaphragm-clamp.stl: Pushed down over the balloon and the rim of the cup, it holds the diaphragm all round. Print: Upside down, lip on the bed.
- stirling-engine-piston.stl: A Ø20 mm disc glued to the middle of the diaphragm, with the fork for the power conrod. An 8 mm piece of the steel rod is the pin. Print: Disc down.
- stirling-engine-clevis.stl: Sits on top of the displacer rod. The displacer conrod hangs in its slot on a 10 mm piece of the steel rod. Print: Standing, slot up.
- stirling-engine-displacer-conrod.stl: 61.5 mm between the holes, from the displacer crank down to the clevis. Print: Flat.
- stirling-engine-power-conrod.stl: 73.1 mm between the holes, from the crank pin on the flywheel down to the piston. Print: Flat.
- stirling-engine-displacer-crank.stl: On the back end of the crankshaft. Its printed pin is 3.5 mm off centre, and the counterweight takes part of the displacer's weight. Print: Pin up.
- stirling-engine-flywheel.stl: Ø112 mm, with a heavy rim that carries the engine through each turn. The crank pin for the power conrod is printed on its face. Print: Flat, pin up. 5 walls and 40% infill: the weight helps it run smoothly.
- stirling-engine-spacer.stl (print 2): A 1 mm washer between each crank hub and its bearing, so the hub only touches the inner ring. Print: Flat.
- stirling-engine-pin-cap.stl (print 2): Pushed onto the end of each printed crank pin to keep the conrod on. Print: Flat.
- stirling-engine-drill-guide.stl: Not part of the engine: lay it upside down over the top plate and drill through it, Ø4 mm in the middle and Ø8 mm for the port. The holes then sit where the gland and the cylinder need them. Print: Flat, pocket up.

Also needed (not printed)
- Two aluminium discs, Ø100 mm and 1.5 mm thick (1 to 2 mm works): the warm and the cool plate. They have to be metal, because plastic hardly carries heat. Search for an aluminium round plate or blank of 100 mm.
- Two 623ZZ ball bearings (3 × 10 × 4 mm). Wash the grease out with isopropyl alcohol or lighter fluid and add one drop of thin oil: the engine only has a little power, so they must spin freely.
- 75 mm of 3 mm steel rod (silver steel or stainless). Cut it into 20 mm for the crankshaft, 34 mm for the displacer rod and 10 mm and 8 mm for the two pins, and round off the ends.
- A balloon, or the cuff of a nitrile glove, for the diaphragm.
- 6 × M3 × 30 screws with 6 nuts for the chamber, and 4 × M3 × 10 screws for the bearing frame.
- Two-part epoxy, superglue, and a little petroleum jelly or silicone grease.
- A 4 mm and an 8 mm drill bit for the top plate (a step drill makes the 8 mm hole cleanest).

Print settings: 0.4 mm nozzle, 0.20 mm layer height, 3 walls (5 for the flywheel), 15% gyroid infill (5% for the displacer, 40% for the flywheel), PETG for the base, chamber ring and displacer, which get warm; PLA for the rest.

Assembly
1. Print the parts: Print the base, the chamber ring and the displacer in PETG: PLA softens at the temperature of hot coffee. Everything else can be PLA. Nothing needs supports.
2. Drill the top plate: Lay the drill guide upside down over one of the aluminium discs, clamp both to a piece of scrap wood and drill the 4 mm hole in the middle and the 8 mm hole off centre. Deburr both sides. The other disc, the bottom plate, stays whole.
3. Displacer and rod: Glue the 34 mm rod into the displacer with epoxy so that 27 mm stands out, square to the disc.
4. Glue on the gland and the cylinder: Put the rod through the top plate from below and slide the gland over it, so the rod centres the gland on the 4 mm hole while the epoxy sets. Glue the power cylinder over the 8 mm hole. Keep epoxy out of both holes.
5. Close the chamber: Press the six nuts into the base and lay the bottom plate in it. Smear a thin film of petroleum jelly on both faces of the chamber ring and set it on. Lower the displacer and the top plate in, then the top ring. Turn the top plate so the power cylinder points at the notch in the edge of the top ring: that is the flywheel side. Put in the six M3 × 30 screws and tighten them evenly.
6. Fit the diaphragm: Cut a piece of balloon and lay it over the cylinder without stretching it: it must be slack, so the middle moves 3.5 mm up and down without effort. Push the clamp down over it. Glue the piston to the middle of the diaphragm with a drop of superglue, its fork in line with the flywheel.
7. Crankshaft and frame: Press the bearings into the frame and screw the frame onto the top ring with four M3 × 10. Push the 20 mm shaft through the bearings with a spacer on each side, then glue on the flywheel (on the cylinder side) and the displacer crank. With the flywheel's pin straight up, the displacer crank's pin must point to the left, seen from the flywheel side: a quarter turn ahead of it.
8. Hang the conrods: Fit the displacer conrod into the clevis with the 10 mm pin and push the clevis onto the rod as far as it goes; put the other end on the displacer crank and push on a cap. Fit the power conrod into the piston with the 8 mm pin and put it on the flywheel's pin with the other cap. Turn the flywheel slowly: it must turn without catching, and you must not hear the displacer touch at the top or the bottom. If it does, move the clevis a little on the rod and glue it there.
9. Run it: Fill a mug with just-boiled water or hot coffee and set the engine on top. Wait one or two minutes until the bottom plate is warm, then flick the flywheel anticlockwise, seen from the flywheel side. If it slows down and stops, look for friction: the bearings, the rod in the gland and the conrod pins should all move with almost no force.

Safety: Hot water: set the mug on a steady surface, and lift the engine by its rings, not by the flywheel. Small parts: keep them away from young children.

License: Creative Commons Attribution-NonCommercial 4.0 (CC BY-NC 4.0)
https://creativecommons.org/licenses/by-nc/4.0/
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