Rhombic Drive Stirling Engine (Opposing Pistons) | Slow motion music video

Slow-motion clip of an opposing-piston Stirling engine with rhombic drive, letting you clearly see the mesmerizing motion of the rhombic drive mechanism.

My camera can't record sound at 180fps so you'll get some relaxing classical music instead.

This particular engine design is unusual to see among model Stirling engines. Stirling engines with a rhombic drive are considered the pinnacle of Stirling engine engineering because they combine high efficiency with very low mechanical stresses (hence longer mechanical lifetime) due to the rhombic drive which reduces unwanted lateral stresses on piston rods etc. The downside of rhombic drive engines is the greater mechanical complexity compared with other Stirling engine designs.

However, this variation of the rhombic drive Stirling Engine is unusual because it uses a rhombic drive yet keeps the worker piston and displacer piston opposed to each other, one on each side of the rhombic gear. Rhombic drive Stirling engines normally keep the working piston and displacer piston inside the same cylinder This results in the added mechanical complexity of fitting the shaft for the displacer piston through the power piston.This isn't trivial because it not only requires a air tight fit (lest the hot air escapes, resulting in poor engine efficiency) but also requires minimal friction to avoid any unnecessary power losses. Designing the engine with the two opposing pistons on each side of the rhombic drive mechanism reduces the mechanical complexity of the engine by doing away with the piston-rod-through-piston problem, but it seems to me that it also must reduce the engine's efficiency due to added dead space when shuttling the hot air between the two cylinders. Nevertheless, it's an interesting design to watch in action.

Lubrication of Stirling engines is always a bit tricky; the printed guide that came with this engine recommends "painting" the piston and piston rods with a pencil. Indeed, given that pencil dust is graphite, and graphite has lubricating properties, that seems like practical advice - much better than using oil which does tend to gum up delicate mechanical parts over time or degrade quickly due to heat.

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