How to Make a Coffee Cup Stirling Engine

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Jim's Coffee Cup Stirling Engine (Close Up)


This engine is built almost completely from junk that I found in thrift stores around town.  The aluminum sheet was salvaged from a cake pan and a cheap frying pan.  The pressure chamber sidewall is made from an old piece of plastic drain line pipe.  The bearing was taken from a set of roller blades.  The axle shaft was cut from a paint roller handle.  The displacer was cut from a piece of foam poster board. 

The piston and drive cylinder were the only part that was not bought at a junk store.  I went to a local model shop and purchased two pieces of brass tube.  One of them just happened to be the right size to slide inside the other one.  I plugged one end of the smaller diameter tube and fashioned it into a piston.

The engine will run when placed over a large cup of hot water.  

Two helpful files are attached at the end of the article.  There is a .pdf of dimensional drawings, and there is a 3D SketchUp file that you can explore.  Google SketchUp is a free program that lets you create and view 3D image files. 

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Book Offer: How to build a "Heat of the Hand" Stirling engine without a machine shop.

Download the dimensional drawings from the Attachments link at the bottom of this page.

Video #2: First Run  Video #3: Tuned Up

Coffee Cup Stirling Engine Illustrated Instructions:

(Click images to enlarge)

The pressure chamber top plate and bottom plate were cut from old cookware purchased from a thrift store.


The sidewall of the pressure chamber was cut from a piece of 4" plastic drain pipe.


I found two pieces of tubing at a model shop that were sized so that one would slide inside the other.  The inner tube will become the piston.  The outer tube will become the drive cylinder.


The small brass tube is the perfect size to hold the thin music wire.  The music wire will become the displacer push rod.  It will slide through a short section of the brass tube.  The tube will act as the "gland" to keep the pressure chamber sealed as the shaft slides through it.


The displacer is cut from a piece of foam core poster board.  A dot marks the center.


A jig was used to hold the push rod in perfect alignment as it was glued in place.


The pressure chamber is glued together with high temperature silicone adhesive (purchased from an auto parts store).


The gland is made from a short piece of brass tube from the model shop that happens to fit the music wire push rod quite well.  It is held in place with JB Weld epoxy.


The shorter brass tube is sized to slide inside the longer tube.  The short tube will become the piston.


A foil dam was placed in the end of the piston and one end was carefully filled with JB Weld epoxy.


The push rod attachment is made from a small brass tube and a small piece of wood.  The wood will be glued to the inside of the piston.


Here is the piston with the push rod attached to the inside.


The bearing is held in place by the clamping pressure of the bolt.


The main drive axle and crank shaft are made from a piece of music wire and a piece of 1/4" rod cut from an old paint roller handle. This crank section will lift the displacer.


This piece will be screwed to the wooden flywheel and will become the crank shaft that is connected to the piston.  The slots are for screws and make it adjustable.


The main drive axle is ready to accept the wooden flywheel.


Here is the finished engine.  A dime is placed on the flywheel to counter balance the weight of the displacer.  The engine will run when the cup is filled with hot water.  If ice is placed on the top of the pressure chamber the temperature differential is increased and the engine performs even better.

Dimensional drawings with all the critical measurements are available in the links below:

Afterthoughts

The principle behind this design is sound, but there is room for improvement.  I am now aware of some easier methods that will make a good engine.  Here are some improvements that I would make if I were to build this again:

Flywheel

A larger diameter flywheel would have better inertia. I think a couple of CDs would be perfect.  This flywheel is too small and light. 

Bearings

Skip the old ball bearings from a roller skate.  Instead use a small diameter music wire axle in a plastic bearing made from UHMW plastic.  That is the same material I used for the displacer gland on the Paint Can Stirling Engine.  UHMW plastic is naturally slippery and has excellent abrasion resistance.

Displacer Gland

A UHMW plastic displacer gland is a better choice.  There is less friction.

Piston

This piston took a lot of time and effort to get it to move freely in the cylinder.  The process of cutting the tubing causes it to deform just enough to bind and no longer be a perfect fit.  The end result is a good piston with very little drag and a decent seal.  But it would be easier to build a thermoformed diaphragm.  They are cheaper, they offer very little drag, and they don't leak at all.

Conclusion

This was my first original design and there is room for improvement.  Check back in the future and I will post a new design for a coffee cup engine.  The new design will be easy to replicate, it will be made from materials that are easy to obtain, it will not require any machine tools, and it will run well on a coffee cup.

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Jim Larsen,
Apr 19, 2010, 9:26 PM
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CoffeeCupStirlingEngineSketchup.skp
(192k)
Jim Larsen,
Apr 19, 2010, 9:26 PM
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