Stirling engines :
Novelty or machines for the next millennium?

I have been offered an 80 year old toy hot air engine for the Museum. A little refurbishment and it should be ready to display - working!. However as I like to see museums explain the bits of old iron on show, it was off to find out more about what I thought was just a defunct side branch of history. In fact this subject is very much alive with all sorts of strange developments.

A Stirling engine can be much more efficient than an IC engine, has no valves, tappets, timing chains, or ignition to worry about and can be run from any heat source you can think of. The drawbacks are that it needs time to warm up, has no throttle or governor, needs good cooling and for performance comparable to an IC engine needs high temperatures and pressures which push materials technology to the limit. Ideal applications are where there is ample cooling water and a 'plodding' job to do such as pumping or auxiliary power on a boat. One firm is promoting a 25KW solar powered generator for 3rd world countries which is three times more efficient than the equivalent solar (photovoltaic) panels. This last has not been lost on NASA who are doing a lot of research in the field. They have designed a Stirling engine for use in space stations. In another element, the Swedish navy are getting this year a 60m, 1500t Sterling powered submarine. Reverse the engine and you get a refrigerator that goes down to extremely low temperatures. Some night-vision goggles are cooled by miniature sterling engines in this way.

Since 1816 when The Rev Robert Stirling was granted a patent for a new type of air engine (apparently developed as it had no dangerous boilers that could - and did explode) the hot air engine has remained almost unknown. These machines have the appeal of arms and rods oscillating in strange motions without making more noise, being more dirty, or being more frightening than a sewing machine. DIY kits and novelty engines as well as models expressly for classrooms are available. There are kits and drawings for making simple Stirling engines in a couple of hours in a simple workshop. You can buy engines that run by sitting on a coffee cup or even run off room heat when cooled by ice. I'm hoping to get hold of manufacturers details of as many of these novelties as I can and perhaps we'll have a family of Stirlings working away, each with its own strange motion, novelty or purpose.

From the Museum's point of view, Stirling engines are quiet and easy to run, have many different shapes and mechanisms, and are a good way of explaining heat engines in general. For me the fascinating thing is that although engines 100 years old or so could be shown, every year there are modern developments which makes this subject very much a thing of taday and the future showing how a museum does not just display old history, but showing how old ideas can be revisited and encourage the technologists of tomorrow.

The griff

Heat a gas up and it wants to expand. If this happens in a cylinder then useful work can be extracted as the piston is forced out. Then what? In an IC engine the hot gasses are thrown away with a loud 'pop' but in a Stirling engine the gas is sealed in. This makes it very quiet, but now we have to cool the gas again before we can repeat the cycle. All Sterling engines have a hot bit and a cold bit. The way most Sterling engines get the gas to go from cold to hot and hot to cold again is by using a dis-placer. Imagine a closed cylinder with one end hot and the other cold. If we push all the air up to one end it should get hot or cold depending on which end. By moving a 'displacer' from end to end the gas has to go to the other. After a while the displacer is also hot at one end and cold at the other and might be a sort of 'brillo-pad' that lets the air through and heat up or cool down efficiently. The principles are the same in all engines but there are very many different layouts and variations.