What does engine RPM actually measure?

Question

My understanding has always been that the tachometer of an automobile measured the revolutions of the crank shaft. However, i was thinking today that this seems unrealistically fast. Considering a modern engine can easily exceed 6000 RPM under heavy acceleration, if this really is measuring revolutions of the crank shaft that means that each piston goes from its upmost position, all of the way down and all of the way back up 100 times per second. I have a hard time comprehending how that could be possible considering all of the mechanics involved in how an engine works. Is this actually what it measures and is that actually how an engine works? Does the tach actually measure something completely different? Any light you can shed on the subject would be appreciated!

Answer 1

You are not mistaken. Some racecar engines can rev to over 12,000 RPM and THAT IS crankshaft revolutions. And THAT IS 200 revolutions per second.

Answer 2

Engine Rpm

Answer 3

Rpm Engines

Answer 4

This Site Might Help You.



RE:

What does engine RPM actually measure?

My understanding has always been that the tachometer of an automobile measured the revolutions of the crank shaft. However, i was thinking today that this seems unrealistically fast. Considering a modern engine can easily exceed 6000 RPM under heavy acceleration, if this really is measuring...

Answer 5

RPM is the world's simplest measurement. It is how many times per minute that the crankshaft is rotating. This is what the tachometer measures. If you want to know how many times the piston moves up and down completely in one second for a certain RPM, divide the RPM by 60. Frankly, just because you can't comprehend it doesn't mean it doesn't happen. And whether you believe it is happening or not is irrelevant. When you operate your engine at 6,000 RPM, it is happening. Most engines would self-destruct if they were run at 6,000 RPM for very long. But there are engines that routinely operate at much higher RPM. Every part in the engine needs to function in perfect sync in order for the engine to operate. When the engine runs faster, the odds of even one of those parts failing or operating out of sync increases. If one of these parts fails or operates out of sync, the engine fails. As technology improves, engine speed can increase, because lighter, stronger materials, better lubricants, better design and advances in electronics all allow the engine to operate more precisely and at higher RPM before the engine fails.

Answer 6

it is the crankshaft but for the specifics the crankshaft is the center shaft in the engine the pistons are whats used to rotate it by moving up and down. if you look at and engine in a car you will see a big pully right on the front with a belt running over it that is what the crankshaft turns(along several other components) so rpm refers to the number of rotations that it makes in 1 minute

Answer 7

khalid before u get into the math i want u to think about this at 3rd gear if u r 60 kph ur rpm is relatively higher to that at 5th gear however if u see the tyre will rotate at the same rpm as it covers the same amount of the distance if ur tyre dia is 175 mm then at 60 kph it should cover 2x0.175x3.14 m in 1 sec irrespective of the engine rpm.The question on this page is about piston vs crank travel which i hope you can understand better here on this link

http://en.wikipedia.org/wiki/Piston_motion_equations

Answer 8

See I wanted to know about this as well which is how I stumbled upon this little question of yours. I have done some math myself and I agree. It seems unlikely. My calculations told me that a car in direct drive (normal car in 4th or 5th gear) at its top speed revving at close to 6000 rpm (which is where cars typically max out at) would be traveling at well over 300 mph.



Here is the math so someone please tell me I'm wrong.



-Average Tire Diameter 20 inches.

-Multiplied by 3.14 or pie = a circumference of 62.8 inches.

-Multiplied by 6,000 rpm = a distance of 376,800 inches in one minute.

-Multiplied by 60 minutes = a distance of 22,608,000 inches in one hour.

-Divided by 12 inches = a distance of 1,884,000 feet in one hour.

-Divided by 5,280 feet= a distance of 356.bla bla bla miles per hour.



What am I missing?

Answer 9

your right.



Some bikes rev to 15,000 rpm

and i believe formula1 cars can hit 20,000 rpm, 19krpm legally

Answer 10

Revolutions Per Minute of the Crank Shaft. Piston movement is equal to crank movement. Cam shaft movement is not the same as crank movement (it is half)



I believe your 6000 rpm calculations are off a little as I doubt it could run up and down 100 times per second, That would be a warp drive engine on the U.S.S. Enterprise.

Answer 11

It measures how hard you drift