4K | Kontax Beta Stirling Hot Air Engine Explained

Demonstration and explanation of a Beta-type Stirling engine made by Kontax in the UK.

Like other Stirling hot air engines this engine works by taking advantage of the pressure difference between hot air and cooler air. It extracts mechanical work from the pressure difference between expanding the hot air at higher pressure and compressing cooler air at a lower pressure. In essence, the work is extracted by the difference in air pressure.

This particular model engine has several very interesting features that set it apart from other model Stirling engines I've seen. First of all, it's a Beta-type (there are three basic types of Stirling engines, Alpha, Beta and Gamma) Stirling Engine featuring a gland for the displacer piston rod which passes through the power piston. This arrangement allows for an efficient and compact design which lets the power piston and displacer piston operate within the same cylinder, minimizing the amount of dead space which might otherwise reduce the engine's efficiency (admittedly, efficiency is not the most important concern for small model engines like this). Most simpler Stirling engines avoid this arrangement since it obviously requires very high-precision engineering to achieve a perfect fit between the rod and power piston which is not only air-tight but also results in minimal friction. If the fit were too loose, the air would escape rather than work on the power piston, and the engine wouldn't run. On the other hand, If the fit was too tight, the friction between the rod and the gland would be too much for the engine to overcome, and the engine would also stop. This engine passes the test with flying colors and runs like a Swiss clockwork.

This particular engine uses a crank with three takeoff points for implementing the standard 90-degree phase shift between the power piston and the displacer piston, but some other Beta-type Stirling engine designs use the famous and characteristic rhombic drive . An advantage of the rhombic drive is reduced lateral loads on the rods and pistons, at the cost of increased mechanical complexity. The arrangement used on this engine, while simpler, is a very nice solution since the two connection rods for the power piston balance out the forces which are acting on each side of the connecting rod for the displacer piston. Once again, this only works so well because the engine is built to exacting tolerances. Nice!

Another interesting feature is the double flywheel (each balanced with brass counterweights) which provides enough inertia to even out the piston forces for smoother running. Yet another nice touch is the extensive use of ball bearings to reduce friction.

Finally, the engine's power piston is made of graphite which rides inside a glass cylinder. This characteristic arrangement is self-lubricating and ensures very low friction. In fact, one of the Stirling engine's main claims to fame, i.e. it being a closed system with no exhaust, has been one of the engine's main weaknesses: how do you lubricate pistons and cylinders in a closed system? How do you replace lubricating oil that has been worn out? The answer in this engine is: you don't - it's self-lubricating.

In short, I like this engine a lot. It's stylish and elegant in an understated way. It demonstrates interesting engineering principles. And, most importantly, the mechanical workmanship is flawless!

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