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Of course, this is not true for all engines (after all the pre-2014 F1 engines were 4-stroke and were rpm limited to 18,000 rpm). However, from my experience, 2-stroke engines tend to run at a higher rpm than a 4-stroke engine.

Given a 4-stroke engine only has a power stroke on every other revolution of the crankshaft, surely it would make more sense for it to in fact be 4-stroke engines that run at higher rpm, given it could theoretically run at twice the rpm for the same fuel consumption.

I guess I must be missing something here, because I fail to understand why 2-stroke engines tend to run at higher rpm to a 4-stroke engine?

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Valve train design gets more difficult as RPM goes up, because those valves need to be started and stopped faster, without beating the entire valve train apart. So while there may not be any theoretical reasons that a 4-stroke engine can't turn super-fast, there's just fewer things you have to worry about when you're trying to make a 2-stroke engine turn super-fast.

(And big marine diesels turn at 100RPM. And they're 2-strokes. So there's a counter-example for you).

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  • $\begingroup$ The acceleration on the valves as they start and stop is hundreds of times the acceleration due to gravity. As you increase the RPM you need stiffer valve springs (or more powerful hydraulics) to stop valve bounce, and stiffer springs mean higher oil pressures to lubricate the cam followers, etc. Doubling the RPM multiplies the forces and accelerations by 4, not by 2. $\endgroup$ – alephzero Apr 9 '19 at 8:50
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Most 2-stroke engines are small (think: weed-eater, model airplane, etc.), which allows them to spin at absurd speeds because the reciprocating masses are small. But once you scale 2-stroke engines up into the ~100HP range, their wide-open throttle RPM is in the 4500-5500RPM range, which is not that different from the redline limit of a four-stroke engine of similar power rating.

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