Hi Paulie, I like tiny engines, too. Have you seen these? At the end, a tiddler is going fairly fast:
http://www.youtube.com/watch?v=dICFVknQ7_Q
Here’s some more:
http://www.stirlingparadise.de/coll_sch ... hager.html. One is a “thumb nail sketch”:
http://www.stirlingparadise.de/coll_sch ... aum_99.jpg.
Those are LTD engines, as developed by the late Ivo Kolin and James Senft. Senft has written several practical books on hot air engines including “An Introduction to the LTD Stirling Engine”. They are listed on
http://www.amazon.com/Miniature-Ringbom ... 0965245535 and I found them all really helpful.
But LTD engines have almost no power because they draw very little heat, so making them small is tricky. Maybe start with a higher dT, like this one (which also shows that you don’t have to use a conventional layout.):
http://www.youtube.com/watch?v=5t5qb0GWnsg.
You ask about displacement ratio. The 150mm LTD model in Senft’s book uses 15, which should be a good starting point. (Note: this is a swept volume ratio not a diameter ratio.) The ratio is independent of size, although it is higher for low dT.
For interest the principle is that air expands by 1% for each 3degC temperature rise from 27degC – eg, by 10% between 27 and 57degC. But that expansion has to do more than just fill the displacement of the power piston. It has also to produce enough pressure to provide the torque – only “half” the expansion is used to move the piston, the rest to give the force. Only the displaced air (swept volume of the displacer) gets heated and, sadly, most of that doesn’t get right up to temperature. Worse, some expansion is wasted through leakage and some through compressing the inevitable dead air. So don’t worry about maths, it doesn’t work anyway - the displacement ratio needs to be much more than calculated. I’ve used far higher than 15:1.
Miniature engines are a challenge! Because the power is so small, friction must be minimal. The bore is best lapped and the piston lapped to it. I find friction is worse than slight leakage, so ease off the piston. Graphite is best for pistons (try old motor brushes). Also, I use light flywheels and thin wire crankshafts to minimize the friction torque in bearings.
An advantage of wire crankshafts is that an extra bend in the crank arm can be re-set to vary the stroke (use a round bend or it will eventually snap).
Another problem with small engines is the limited cooling area. That allows the “cold end” to warm up and stop the engine. So I make the displacement cylinder from wrapped thin film to minimize conduction. I use longer cylinders and thicker displacers than Senft, too, to reduce the temperature gradient (and conduction) and to improve regeneration (in the gap around the displacer). Balsa displacers are less compressible than foam (equivalent to dead air) and take higher temperatures.
It is good for the displacer to rest against the end plates (as in a Ringbohm engine – see Senft), to minimize dead volume, as well as heat up its faces which then help heat the air.
To micro-ize, can you re-arrange the mechanism so that the flywheel lies parallel with the top plate – like closing a powder compact?
Here’s an engine with no flywheel at all:
http://www.youtube.com/watch?v=kkpiq8wwRrc&NR=1. (See it slow and stop as the cold end warms!) Its works:
http://www.youtube.com/watch?v=lljxMZTBIyU .
Good luck with your fascinating project. Keep us up to date.
Jester.
