Monthly Archives: December 2013

Böhm Stirling Engine HB13

I got the finest Christmas gift imaginable: a Böhm Stirling Engine HB13. I will write a few things about it here.

Compared to a Steam Engine
I also own a Wilesco D10. Don’t know if you want to get a Stirling Engine or a Steam Engine?

The Steam Engine is more robust and less sensitive to friction. It is more powerful and can be used to power different toys and accessories.

The Stirling Engine is more clean, more silent, and requires less preparation and cleaning up. The Stirling engine feels more safe as there is much lower pressure involved, and no boiling water.

Building it
I got the build-it-yourself Bausatz. That was a good thing. The website indicates it takes about 2h to assemble. Well, if you are that fast you are really good, but it does not take so much longer. You need a set of torx drivers, and you need to not lose any parts (some parts are very small). If you dont see well, or your hand is not very stable, dont build it yourself.

The tricky part with building it was the little liquids, fluids and grease. My package came with SuperGlue for the Ball bearings – very well – I know how to use it and the package said super glue. Then there was some oil – you can add it later, dont worry while building. Finally there was something called “White ceramic grease” – I suggest you wait with that one too! First build and test drive – then add oil and grease if needed.

The good thing with building it yourself is that if you later feel like cleaning/lubricating/fixing it, you already know all the parts and taking it apart a little will not scare you.

Fuel and fire
The instructions tell you to use 96% ethanol – great if you can purchase it legally. I cant. However, there is denaturated alcohol available for sale – in Sweden it is called Rödsprit – and it works very well.

Now, I had some problems getting my Engine running in the beginning. I suspected friction, too much ceramic grease and too little ceramic grease. In fact, there was a slight draught/wind where I ran the machine and the flame did not heat the heating cylinder perfectly. So my first advice; protect the machine/fire with little boxes/walls to ensure the flame is completely still and effective.

You may need to heat the machine for up to a minute before it can pick up speed. Give it some time before you proceed with checking the friction.

Friction
The Stirling engine does not have much extra power and a little friction can make it fail. If you disconnect the Working Piston / Arbeitskolben you can manually carefully slowly move it up and down. This should be quite hard due to air pressure. If you remove it completely you can let the machine spin freely. It should be quite noiseless and spin easily. If you spin the wheels it should do 3-4 revolutions before coming to a halt (with the Working Piston in place you barely get one revolution). I applied a little oil to the Displace axle / Verdrängerachse as well as to the Crank Shaft (where the connecing rods connect to it).

The amount of Ceramic Grease required on the Working Piston is minimal. I first added enough when building it. I then added far to much when trying to make the machine run better. I finally removed almost all grease I could both from cylinder and piston to make it work again.

Improving it
I find it weird that there is absolutely no insulation between Heating Cylinder and Cooling Cylinder. After all, it is difference in temperature between these two cylinders that make the machine work. I tried to add a thin layer of something between the two cylinders – so far not with very good result. A thin piece of hobby plywood did not maintain pressure/vacuum, and the machine did not run at all. A thin piece of cardboard appeared to work – the machine quickly accelerated, but when reaching top speed it quickly slowed down, to accelerate again. It kept “oscillating” in this way until i shut it down. Perhaps the design has a natural top speed – after all, air is shuffled back and forth, and has to change temperature, and this might require some milliseconds per revolution. I now run the machine with a paper insulation about 0.2mm thick. It works very fine, dont know if it really improves anything though.

I also tried to cool down the cooling cylinder by spraying it with little water. This helps surpisingly little, and is not required. With proper fireing the heating cylinder remains much warmer than the cooling cylinder anyway.

Performance
My machine, when fueled by spirit (denaturated ethanol) runs faster and faster with time. First minutes about 500 rpm. Later above 1000 rpm, and sometimes I have been able to get it up to 2000rpm, after like 20-30 minutes of continous operation.

If I power it with a tea candle (remember, not allowed since it soothens the model) it runs between 400-500 rpm for a while (perhaps up to 10 minutes). After that it comes to a halt.

My conclusion is that the machine essentially runs better at high temperature, but that the tea candle does not produce enough heat to keep the temperature difference up at high temperatures.

Measuring RPM
How do I measure RPM of my Stirling Engine? A device that listens to audio and measures RPM is called “Acoustic Tachometer”. I tried to find one for my iOS device and for my Symbian device with no success (however, there is supposed to be an app for Android simply called RPM).

I ended up programming my own Acoustic Tachometer. It runs in Linux and listens to /dev/dsp, and tells the RPM. It works, but not perfectly. The output has to be interpreted. I may improve it and/or share it some day. Feel free to comment if you are interested – I am happy to share my source code. But I guess the number of people who have a Stirling Engine AND compiles their own linux-binaries is quite small. I’d like to write an iOS version though, but my algorithm has to be better first.