Why do we measure engine speed in RPM instead of cycles per second (Hertz)?

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My car idles around 600 RPM which is equal to about 10 cycles per second and redlines at around 6,000 RPM which is 100 cycles per second. This just seems like a more friendly scale to use. I am curious why we use RPM instead of Hertz to measure engine speeds.

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Look at the range between 600-6.000 and 10-100. RPM is more detailed. Also RPM predates Hertz and is better understand by general public.

1. Hertz is a measurement of frequency and can measure various actions. RPM is more specific to the action of an engine.
2. Hertz is usually in cycles per second. While it may be more specific, slight variations in cycles would require decimals to accurately convey changes and be less obvious. Ex: 6 vs 6.2 is less noticeable than 600 vs 620 cycles.
3. Just because.

Note that, in physics, the standard unit for rotational speed is not Hertz but radians per second. If you multiply your engine speed in radians per second by your engine torque in Newton metres then the result is your engine power in Watts, without any messy conversion factors.

There are 2π radians (about 6.3) radians in a revolution and 60 seconds in a minute so 1000 RPM is very roughly 100 radians per second.

Its a convention that emerged during the steam era. Steam engines turned much more slowly than modern internal combustion engines.

Engineers held a watch and counted shaft rotations to determine engine speed; a minute was a convenient period of time for this purpose. Before accurate speed gauges were common, counting turns was the only real way to detect subtle speed differences (e.g. 60 RPM vs 61 RPM).

Even as engine design speeds increased dramatically, no compelling reasons emerged that led engineers to measure engine speeds by units other than that with which they were already familiar.