eli5 What’s happening when an engine stalls from low RPM?
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A single piston is relatively simple. It’s a airtight tube, with a bottom that moves like the plunger in a syringe. A valve opens and the piston moves down, and that pulls fuel and air into the piston. Then the piston moves up and compresses this mixture and a spark ignites it. The explosion pushes the piston down, which is where the power comes from, and then it comes back up to push the exhaust gas out and get ready for the next cycle. So that’s the 4 strokes of a 4 stroke engine. I’ll call them inhale, compression, expansion, exhale.
But you may have noticed something… The piston is moving on all of those, but only one of them creates power. It creates a lot of power, but only on one of the strokes. So to solve this you can basically do two things. The first is simple, you use a flywheel, which is basically a big heavy spinning disk, which keeps momentum. So the expansion pushes the flywheel around and the momentum is enough to move the piston through the other 3 strokes to get back to the next explosion.
The second is you can get multiple pistons and attach them to the same shaft out of sync with each other so piston 1 is currently exploding, and that’s moving piston 2 through exhale, piston 3 through inhale, and piston 4 through compression, and then piston 4 will go off moving the others through their strokes, etc.
But the important part of both of these is that the movement of the engine sets up the engine for the next power stroke. So hopefully by now you can see how it would be disruptive to stop the engine in between one power stroke and the next, or otherwise siphon off enough power that there isn’t enough left to make it back to the top. There would be nothing there to go through the other 3 strokes that are essential to setting things up for the next explosion.
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