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Böhm Stirling Engine HB11

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Robert Stirling was a nineteenth-century Scottish inventor. In 1816, he created a closed-cycle air-driven engine as a putative replacement for the ubiquitous steam engines of the period. Steam engines, while cool to look at if you’re sufficiently removed from the devices themselves, are large, heavy, dangerous, maintenance-intensive and tricky to keep running.

Stirling’s engine is a masterwork of sneakiness, and you have to wonder how anyone could have imagined that something like this could possibly work. A classic “alpha” Stirling engine has two cylinders. The end of one cylinder is heated, causing the air inside it to expand. The expanding air pushes a piston along the cylinder, which turns a flywheel. The flywheel pushes the piston in the second cylinder to compress the air behind it. The cold air flows into the first cylinder, where it’s heated, and the cycle starts over.

There’s a cool animation at the Wikipedia page for Stirling engines that illustrates the works.

Because a Stirling engine doesn’t require a boiler and a lot of high-pressure plumbing, it’s a much cheaper and safer alternative to a stationary steam engine. In Stirling’s day, steam boilers were wont to explode… and usually kill some people shortly thereafter. Unfortunately, the metallurgy of the early nineteenth century wasn’t quite up to supporting a functional real-world Stirling engine, which would have had to run at pretty high temperatures.

The first commercial implementation of a Stirling engine was at an iron foundry in Dundee, Scotland. It was replaced after about three years by a conventional steam engine, having suffered multiple failures. To its credit, however, when the Stirling engine failed, nobody died.

Medium-size Stirling engines were used throughout the nineteenth century, although they never quite displaced steam engines.

Stirling engines have enjoyed something of a renaissance in recent years, with a number of green developers devising ways to heat them with solar energy, and use them to drive generators to produce electricity or government grants.

While it may not be entirely obvious from the picture at the beginning of this review, the Böhm HB11 Stirling engine is about four inches high. It’s not likely to be applicable to pumping out a mine shaft unless your mine was dug by moles, nor will it run your Victorian-era textile factory, mill soft timber or perform any of the other tasks which characterized the age of steam. It is, however a blast to see it run… because it arrives unassembled.

The truly enjoyable aspect of this admittedly useless device is that you’ll get to put it together yourself, light it up and see it start turning.

The Wheel

The Böhm HB11 Stirling engine is beautifully made, with most of its parts fabricated from gleaming stainless steel or milled brass. It hails from Germany, from whence originate most of the world’s really nice cars, as well. The kit includes all the parts for the engine, a polished wooden base upon which it can rest, a dedicated alcohol burner to heat the engine and get it moving… and instructions for assembling the machine.

The instructions that accompany the Böhm HB11 Stirling engine arguably constitute the only notable failing of this engaging device. They consist of a single page of text imperfectly translated from German to English, and a clear if somewhat obtuse exploded view of the engine to be assembled.

In fairness, the Böhm HB11 Stirling engine isn’t a Ferrari, and bolting it together isn’t unreasonably complex even if you and the minimalist instructions provided with it don’t immediately connect. With one exception, everything that comprises the engine fits together with bolts, and if you prove to have misunderstood the procedure for assembling it, you can dismantle it and try again.

The one exception is a pair of minute ball bearings that support the engine’s crank shaft. They install with cyanoacrylate glue… that would be crazy glue or super glue for most of us. If you set them up incorrectly, you’ll need to soak the affected parts in acetone for an hour to dissolve the glue before you can try again.

The HB11 Stirling engine kit includes all the metal, plastic and wooden parts required to complete the engine… but it’s missing a few consumables. You’ll need to provide your own glue, some light machine oil… and a mysterious substance referred to in the instructions as “ceramic paste,” which the text maintains is “usually sold for lubricating automotive brake pads.” None of the automotive parts stores I tried knew what I was talking about when I went a’questing for it.

The ceramic paste is used as a lubricant in the secondary cylinder of the engine, to maintain a seal between the piston and the cylinder walls so the engine can keep its vacuum. Any low viscosity silicone lubricant that will survive high temperatures will work. Eze-Slide, for example, is reasonably easy to find, and is a suitable substitute for whatever ceramic paste turns out to be.

It’s worth noting that lubricating the secondary piston is somewhat exacting – you’ll want to apply the thinnest possible layer of lubricant to the piston walls. Excessive lubricant will increase the drag on the piston sufficiently to prevent the engine from turning.

You might also want to shop for some hand tools before you begin assembling the engine. It calls for number 8, 10 and 20 Torx screwdrivers – the former is sufficiently small and unusual to be absent from all but the most robustly stocked tool boxes.

If you proceed through the assembly instructions carefully and methodically… and read everything several times before you start twisting its bolts… the Böhm HB11 Stirling engine will go together in about two hours. The exacting engineering and fairly meticulous manufacturing behind the engine makes it enjoyable to work with. Some of the manufacturing tolerances might prove to be an issue – in our engine, the rocker arm shaft proved to be minutely too large to slide into the bearings it was intended to support, necessitating some adjustment with a scrap of emery paper.

Several of the parts are just this side of microscopic, and will call for the deft application of a pair of tweezers.

When the Stirling engine is complete and affixed to its base, it can be set in motion by filling its attendant burner with isopropyl alcohol, lighting it and staring at it for a minute or two until it reaches operating temperature. Once it gets going, it rips along at speeds that don’t seem as if they should be possible for so diminutive a machine. Its operating speed can be regulated by adjusting the position of its alcohol burner relative to the hot cylinder of the engine.

Watching a former box of parts begin working as a complete engine for the first time is a rush.

While not without its rough edges, and demanding of a modest degree of eye-hand coordination and mechanical skills, the Böhm HB11 Stirling engine is a delightful waste of an afternoon and an engaging desk ornament. Ours came from Lee Valley Tools, which included an additional page of instructions to clarify a few of the ambiguities in the documentation provided with the kit.