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Low Temperature Stirling Engine

Nigel Taylor

As you may have read some time ago on the editorial page I started this as a project after building my new workshop. The basic design is mine and as you can see is still a little crude in places as it has been a design that has evolved, some parts still need to evolve into their final design and are there just to allow me to run the engine and check it out.

I suppose the first thing to ask is does it run? Well, yes, watch the following video:

This short video shows it running, roughly 60rpm and runs for about 20 minutes on a mug of hot water.

The video shows the engine in it's early days without the cooling fins on the top surface of the displacer cylinder and with steel bolts clamping the displacer together. The steel bolts were particularly poor as they resulted in a very good heat transfer path that I really didn't need.

The solution I came up with for the bolts was to make them thermally insulated. At first I considered making a nylon bolt, but thought this would just not have the look I wanted. After much deliberation I decided that they should have a wooden shaft (in this case walnut) and copper thread and head. As you can see the bolt was bonded together using expoxy. The bolts were assembled with the displacer and bonded so that there was some allowance for tightening of the bolts later. The excess expoxy was later removed with a very sharp knife.

In the above picture you can see that the displacer cylinder wall was constructed from three sheets of 5mm plastic that was bonded together and then machined. The bond lines still show and it would be nice to have made this from one piece, but I did not have any 5 inch diameter perspex that was 15mm thick and so this had to do.

The displacer was made from the pink building insulation foam. This is not that cheap to buy as you have to buy quite large quantities, so best to find a builder doing some renovation and ask if you can have an offcut.

Turning the pink foam is tricky and best done with a sharp tool and taking very small cuts, as you can see in the photo the finished displacer with the connecting rod has a very fine finish, the one next to it was my first trial and I was very heavy handed - the reult is a fluffy finish.

The centre of the displacer is a peice of dowel that was drilled and threaded and then the threaded hole was soaked in super glue and threaded cut again. The dowel was fixed to the foam using pva glue and left a few hours to dry before machining.

As you can see in the above photo there is a clearance of 1 to 2mm around the displacer.

All in all a great model to have a go at making, it is frustrating as it does take some setting up and balancing to get it running. When it runs though it is more or less silent and is very rewarding.

When I say it takes some time to get running I mean this, it is best to use some small pieces of blutack initially and very carefully rotate the flywheel and balance it very carefully. Also, friction needs to be kept to a minimum. This is an engine that runs, don't expect an excess of power.

If you have made a similar engine and would like to share your experience we would love to hear from you - ed.