Hubert Stierhof's

Seesaw Stirling Engine

new 10/01/99

A few weeks ago, Hubert posted a message on the Stirling Engine society - USA (SESUSA) mailing list as follows: I am developing designs of that kind myself and lately I found an design (using liquid pistons) with only ONE MOVING PART and SELF STARTING in the sunlight. Based on that design I have a modified
do-it-yourself-instruction I can offer to everybody interested in (for free). It can be build in half an afternoon and even if you deletantically build it - it works.

I asked Hubert if he was willing to let us post his plans on this site, and he very generously agreed. so, following is Hubert's text and illustrations for the Seesaw Stirling:


The Seesaw Stirling Engine
How it came to be

Since some years I am trying to develop new Stirling designs for Third World Country pumping and cooling . The intention is to find a design that is simple enough to be build by indigenous craftsman, using cheap material, works reliable and has a power-output that is by far cheaper than a photovoltaic solution. In my opinion a liquid-piston design is simple enough and so this is the realm of my researches.
By thinking about liquid-piston designs and the constrains of having a self-starting engine, I had the idea of using a labile equilibrium instead of a balanced or stabile one. So I came to the following design . After building and testing the first model it came clear to me, that this design is not good for a reasonable power-output because it has no regenerator, but as an easy-to-build-model it is very, very good. So I will describe in the following lines the simplest (beside West's FLUIDYNE) Stirling engine I know with only one moving part.

Detailed design

As you can see in the side view in figure 1 the seesaw engine consists of a case similar to a flat-case-collector/cylinder you might know from other solar LTD Stirling engines. The case is 200mm in length and 100mm wide. Its top plate is an acrylic (glass) sheet (4mm thick) and its back plate a 2mm aluminum sheet (best is a cooling plate for semiconductors with fins ). Inside the case is a displacer, made out of polyurethane foam (polystyrene) will be fine too, for temperatures below 70 degrees Celsius), painted black at its sunny side.

Note: the two illustrations are larger than this page, so click on these pointers for the illustrations:

Side view   Top view

Inside the displacer is a 15mm thick plastic tube (inner diameter about 13.5mm; a plastic tube used for isolation of power wires). The top plate and the back plate (as you can see from figure 1) are in an angle to each other, that allows the displacer inside to turn around the axis in a small angle. The sides of the case are made of polyurethane foam (like the displacer) and about 8mm thick. Because of the low pressure differences inside, this material can be used, but as well (or even better) is acrylic sheet or plastic. The engine works without a regenerator and needs a 2mm gap between the displacer and its side walls to let the air pass by easily.

Top and back plate and the side walls are glued together by acrylic glue (any other glue will do as well as long as it is airtight. At my first test model I even tied it together with thin steel wire like a postal package!).
At the bottom of the case the tube is leading to the outside, going in a (about 90 degrees) curve leading upwards and open at the end. The tube can be made of one, about 400mm long tube, which just has to be
heated in its middle part and bent smoothly until the 90 degrees are achieved. Where the tube exits the case, it is sealed by a diaphragm. I used a thin plastic foil and sealed it with silicon paste, but using a part of a condom, will be better in my opinion. A short distance behind the diaphragm the axis is fixed. Its bearing is made out of a small piece of brass tube (inner diameter 1mm!) fixed to the case (glued) with a needle as the axis. After the brass tubes (to the left and to the right of the 15mm tube) were glued to the case (by leaving the needle inside as a fixing during the time the glue gets hard (dries), the needle is cut in the middle. You now just need to drill on each side of the 15mm tube a tiny whole, stick the half of the needle inside and glue it with a good glue. If you have cut the displacer already you now need to drill a hole of 15mm diameter in it (as you can see by the figure 1) and stick it on to the tube.

Having assembled all the parts, the displacer and the V-tube should be free moving, that means tip over that small angle without friction. So that the blackened surface is in full contact with the acrylic glass at the one extreme position and the back plate in full contact with the other side of the displacer, when tipped back. It is good to fix the engine to a ground plate, so that angle can be adjusted easily (first try a 45 degrees angle;+-20 degrees). Depending on how airtight you have constructed the engine you might need a valve/hole to let out the surplus air when filling the tube with water. The easiest way might be to drill a tiny whole in the side wall and gluing/sealing after filling. Better is to drill a hole for a screw at the lower most point of the case, so you can close it after filling by screwing in the screw and in case you spilled water in the case, you can drain it easily. Now fill the tube half way with water, putting it in an angle so that the displacer almost tips over to the sunny side and heating its acrylic glass side up by the sun (or a 40-100W bulb lamp) would make the engine run
already. BUT... this is a very strong labile equilibrium ! So the last important part to assemble is a spring to work against the labile equilibrium which can be adjusted to almost any position. It must be strong enough to
even turn the labile equilibrium to a stabile equilibrium if water filled !

To achieve this, I barely glued a saw blade from a fretsaw in parallel position to the extern part of the tube fixed at the case. I built a eight-shaped grip from a paper clip gripping around the 15mm tube and on its other side (in a tiny circle) around the saw blade, so that it can be moved up and down the tube and turned to find the optimal working point. Try it with a middle sized saw blade, fixed in a middle position at about 80mm from the axis for your first test run. This is just the easiest design to try the seesaw engine, but of course any shape besides the 200*100mm case (for example a disc) will work as well.

How to run it

When finished the assembling and the tube is half way filled with water up to the water columns are at same level, and the position is found where the displacer almost intents to tip over, the first test run can start. Just put
the engine into the sunlight. After about 20 seconds try to tip the tube. It should start now ! At least for some strokes. It might happen, that the equilibrium is too labile, balanced or stabile, so you have to try another position of your paper-clip-grip. This is indicated by running for some strokes and stopping at the back plate (heating) position or acrylic plate position (cooling) and staying there. So you have to adjust the spring a little bit closer to the axis and/or in a more middle position. You will have to try for some time but supposed your case is really airtight and the axis friction plus the diaphragm friction is the only friction at your engine, there will be a 'region of positions', where your engine will self-start when heated up.

The funny fact you will find out while seeing your engine running is: IT CANNOT SPEED UP! It always keeps the frequency of the water columns inside and only varies the water columns height (tide). The displacer beats against the acrylic glass and at the back plate and when the temperature difference is small the displacer only tips over a little bit without ever touching the plates.

Try some positions and working modes of your engine - its fun !

If you're a keen builder try to connect an automatic valve inlet at the curve of the tube and an outlet at the extern top of the tube AND YOU WILL HAVE A WATER PUMP. Or just put on your desktop in a sunny place. Anyway build it, have fun with it and send your appreciated comments to me !

I am about to further develop this machine (with a modified extern tube, a regenerator and a light-weight design) to a Third World pumping(or cooling) device (one square meter in size). I applied already for a patent on it and perhaps in the next edition of the Stirling Engine Society's Newsletters you will read about this new design. In case you are interested in other liquid piston Stirling engines I can supply you with some other design ideas (like rotary liquid piston engines).

Hubert Stierhof
Bilsteinweg 11
60435 Frankfurt