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Project Notes

#185 HB13 Stirling Engine

Building the Böhm Stirling-Technik HB13 Small Bonsai kit.

Here’s a quick video of the first firing of the engine:

First Firing Demo


A Stirling engine is a closed-cycle regenerative heat engine with a permanently gaseous working fluid. They are named after Robert Stirling who invented the first practical example in 1816.

Böhm is a small(?) specialist manufacturer from Germany that’s taken up a side-line in producing Stirling engine kits. When I first discovered them, I couldn’t resist.

This is my first model - the Small Bonsai (HB-13).

I believe it’s strictly an alpha configuration with two power pistons in separate cylinders - one hot and one cold:


Everything went together in a few hours, and it fired up nicely on the first go!

A good read of the Support & FAQ pages prior to construction is highly recommended. There are essential tips not to be found in the main instruction documents.

Some things to note:

  • See ALIGNING PIN ASSEMBLY for details on how to use the supplied jig to attach the connecting rods with pins
  • Step (9) in the instructions does not mention inserting spacers 32 and 33. These need to be inserted as the rocker arms are attached to the side plate
  • Step (12) does not precisely specify the assembly of screw, bushing and conecting rod. The diagram seems wrong for one rod. Played around till I think I have it right (screw -> rod -> bush -> crankshaft for both).

Overall the kit is very well manufactured. All parts arrived and are machined well, but could perhaps do with some better packaging in order to survive being shipped around the world. Just three minor issues:

  • One of the connecting rods (21) arrived bent ~30˚ during shipping. It straightened out without trouble.
  • The heating cylinder (23) had a slight ding. Rough shipping again? It doesn’t affect performance, and I was able to position the imperfection on the lower side, so it is not visible.
  • The screws (42) around the heating cylinder (23) are just a little too snug (screws scrape the cylinder when tightening home). Assembles OK, but an extra micrometer or two would have made assembly perfect.

The only thing that had me worried was whether I could find suitable fuel in Singapore. Fortunately there was not problem - I was able to find a suitable bottle of denatured alchohol at my local Guardian Pharmacy for only $4.30.

So where are the electronics? None involved so far! But some ideas I have in mind:

  • note the power-take-off (PTO) from the fly-wheel … that has possibilities for electricity generation
  • and there’s finding a way to automate operation. I wonder what it would take to replace the alchohol fuel with an electric element?







The Finsihed Kit

Finished Build

Notes on the Fuel

According to the FAQ:

Only use 94% alc methylated alcohol. Never use paraffinic fuels such as oil or tea lights! This leads to extreme contamination and standstill.

Methylated alcohol is also known as methylated spirits - or generically “denatured alcohol”, but without undesirable additives. All the formulations of alcohol can be confusing and misleading, here’s my memory jogger:

  • Denatured alcohol aka methylated alcohol, methylated spirits.
    • typically ethyl alcohol (95%) and methyl alcohol (%5)
    • may have other additives and colourings, especially if marketed for medical applications
    • hobby use: fuel for alcohol burners; solvent for some cleaning applications
  • Rubbing alcohol approximately 70% alcohol but with other additives
    • if isopropyl alcohol, also known as “surgical spirit”
    • alternatively may be denatured alcohol
    • even if high percentage IPA, additives make it unsuitable for hobby use
  • Isopropyl alcohol
    • pure CH3CHOHCH3
    • hobby use: solvent, especially for PCB cleaning


Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.