3D render of the Coffee Cup Stirling Engine: a red flywheel on a black A-frame above a silver air chamber with aluminium plates, the white conrods and the orange power cylinder
Free 3D model

Coffee Cup Stirling Engine

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.

  • 19printed parts
  • 12 × 12 × 17 cmassembled (L × W × H)
  • ≈ 0.2 kgfilament in total
  • 118 mmbed size for the base, the chamber ring and the top ring; the flywheel needs 112 mm
Download all files ZIP · 0.3 MB

Free for personal use under CC BY-NC 4.0: print it, remix it and share it with credit to Shapekit. Please don't sell the files or the prints.

Take it apart in 3D · Parts · How to build it

In 3D

Turn it, take it apart

Every part as you print it. Pick colours to see what your filament will look like.

The 3D model (1.6 MB) loads when you get here.

Preview colours

Base, top ring and frame
Chamber ring
Cylinder, gland and clamp
Displacer, conrods and pins
Flywheel and crank

Drag to rotate · scroll or pinch to zoom

In the download

17 parts, ready to print

Every STL is already placed the right way up. Download them all at once, or one by one.

Base ring

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.

Size
118 × 118 × 5 mm
Print
Flat, pockets down. Print it in PETG: it touches the hot plate.
Filament
≈ 19 g
STL 0.2 MB

Chamber ring

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.

Size
118 × 118 × 19 mm
Print
Flat, recess up. PETG.
Filament
≈ 44 g
STL 0.1 MB

Top ring

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.

Size
118 × 118 × 6 mm
Print
Flat, no supports.
Filament
≈ 21 g
STL 0.2 MB

Bearing frame

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.

Size
114 × 90 × 19 mm
Print
Standing on its feet, the way it is used. The bearing seats are teardrops, so no supports.
Filament
≈ 14 g
STL 0.1 MB

Displacer

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.

Size
86 × 86 × 9 mm
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.
Filament
≈ 27 g
STL 61 KB

Rod gland

Glued over the hole in the middle of the top plate. The displacer rod slides through it; a little grease seals the gap.

Size
13 × 13 × 9 mm
Print
Flange down. Run a 3 mm drill bit through the hole by hand so the rod slides without play.
Filament
≈ 1 g
STL 74 KB

Power cylinder

A shallow cup, Ø30 mm inside, glued over the 8 mm hole in the top plate. The diaphragm lies over its rim.

Size
36 × 36 × 7 mm
Print
Floor down.
Filament
≈ 4 g
STL 0.1 MB

Diaphragm clamp

Pushed down over the balloon and the rim of the cup, it holds the diaphragm all round.

Size
40 × 40 × 6 mm
Print
Upside down, lip on the bed.
Filament
≈ 2 g
STL 90 KB

Piston

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.

Size
20 × 20 × 11 mm
Print
Disc down.
Filament
≈ 1 g
STL 98 KB

Clevis

Sits on top of the displacer rod. The displacer conrod hangs in its slot on a 10 mm piece of the steel rod.

Size
16 × 10 × 10 mm
Print
Standing, slot up.
Filament
≈ 2 g
STL 34 KB

Displacer conrod

61.5 mm between the holes, from the displacer crank down to the clevis.

Size
70 × 8 × 3 mm
Print
Flat.
Filament
≈ 1 g
STL 63 KB

Power conrod

73.1 mm between the holes, from the crank pin on the flywheel down to the piston.

Size
81 × 8 × 3 mm
Print
Flat.
Filament
≈ 1 g
STL 63 KB

Displacer crank

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.

Size
25 × 24 × 12 mm
Print
Pin up.
Filament
≈ 2 g
STL 45 KB

Flywheel

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.

Size
112 × 112 × 12 mm
Print
Flat, pin up. 5 walls and 40% infill: the weight helps it run smoothly.
Filament
≈ 21 g
STL 0.1 MB

Spacer × 2

A 1 mm washer between each crank hub and its bearing, so the hub only touches the inner ring.

Size
5 × 5 × 1 mm
Print
Flat.
Filament
≈ 0 g
STL 19 KB

Pin cap × 2

Pushed onto the end of each printed crank pin to keep the conrod on.

Size
7 × 7 × 2 mm
Print
Flat.
Filament
≈ 0 g
STL 19 KB

Drill guide

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.

Size
106 × 106 × 6 mm
Print
Flat, pocket up.
Filament
≈ 32 g
STL 82 KB
Also needed

Parts you buy

  • 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

Slicer settings

0.20 mmlayer height
3walls (5 for the flywheel)
15%gyroid infill (5% for the displacer, 40% for the flywheel)
PETGfor the base, chamber ring and displacer, which get warm; PLA for the rest

For a 0.4 mm nozzle. Filament amounts are estimates for these settings.

Assembly

How to build it

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.

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.

Displacer and rod

Glue the 34 mm rod into the displacer with epoxy so that 27 mm stands out, square to the disc.

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.

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.

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.

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.

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.

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.

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.

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