It's both intriguing and satisfying watching different processes in slow motion.
And thanks to a high-speed camera, a transparent cylinder, and YouTube channel TROdesigns, we can see what goes on inside a four-stroke engine while its running.
The engine comes from a 1977 Honda XR75. The cylinder has been replaced with a transparent acrylic version, allowing us to watch the combustion process.
The video was filmed in 4k, which gives us a very clear and detailed picture of what exactly goes on inside the engine. The intake, compression, power, and exhaust strokes are all easy to view. Flames inside the cylinder can be seen, as well as the puff of exhaust on the final stroke of the cycle. Details are even clear enough to see the droplets of the fuel/air charge, the spark timing, and the piston rings doing their best to seal against the cylinder walls.
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It has the original piston, albeit polished to minimise scuffing on the plastic cylinder walls, and piston rings were custom machined out of Teflon to reduce friction. The compression ratio was dropped from 9.5:1 to 8.7:1 by lengthening the cylinder from stock to try maximise the time the engine ran before it was destroyed.
The true beauty happens at 13:00, so skip ahead if you're just here for the money shots. But if you do watch from the beginning, you'll see loads of detail from the piston rings, to the the careful assembly of the engine.
It's pretty cool to see how well it all works in practice. After running for quite some time, there's not much marring or clouding of the acrylic cylinder, and it's still clear to see what's happening.