Transparent motor filmed in Super Slo-Mo lets you watch the combustion process


A four-stroke engine is a magnificent thing. A complex machine capable of turning flammable liquid into useful work, it dominated the automotive world for nearly a century. Now thanks to the YouTube channel TROdesigns, you can take a look at what’s going on inside the engine while it is running.

The engine in question is from a 1977 Honda XR75, with the cylinder replaced by a clear acrylic version that allows the combustion process to be visualized. The piston itself is the original part, but deburred and polished to minimize scuffing of the plastic cylinder walls. The compression ratio went from 9.5: 1 to 8.7: 1 by lengthening the stock cylinder to help maximize the time the engine was running before destroying itself. Piston rings have been custom machined from Teflon to reduce friction.

Shot in 4K resolution, the video offers a very detailed look at the combustion process inside the engine. The intake, compression, power and exhaust strokes are all clearly visible, with the valve and piston moving together in turn. Tendrils of flame can be seen inside the cylinder, as well as the puff of exhaust on the last stroke of the cycle. There is even enough detail to see the droplets of the fuel / air charge, the ignition timing and the piston rings doing their best to seal against the cylinder walls.

The video doesn’t just show the end results; skip to 13:00 minutes if you want to see the silver photos of the engine running. Looking from the start, there are a lot of details about the machining of the piston rings, as well as the careful assembly of the engine. It’s also amazing how well this works in practice. After running for a while, the acrylic cylinder is little marked or cloudy and it’s still easy to see what’s going on.

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