Think acoustics.

I am trying to verify something I read today. I read that the fundamental frequency of the engine is half the engine speed frequency. Is that correct? For instance, if the engine speed is 1000 RPM, then the corresponding frequency in hertz (for a 2 cylinder engine) is 1000/60 = 16.666 Hertz. And the fundamental frequency would be half of that or 8.3333. Is this correct?

 

 

Well if you want to speak acoustically.....
If I strike an engine block with a ball peen hammer it will resound with a given frequency, this would be its fundamental frequency. If I strike the engine on something other than the engine block it might give off a higher or lower note depending on its mass and physical dimensions. This too would be a fundamental frequency of say a carburetor or valve cover. Now let’s say you have a multi cylinder engine that’s running, each cylinder, pulley and flywheel will resonate the engine with an acoustical vibration depending on rpm. Now, depending on the number of cylinders, firing sequence and engine rpm the vibrational nodes will be set up by either cancelling or adding to the overall engine vibration or resonance if you will. You can use a strobe light to see where the natural resonant shake of the engine is at a given rpm, that will be its fundamental at that rpm.

 

I forgot to add, if you take a non running engine and put it on a large acoustical table driven by a transducer and sweep through frequency ranges from sub-Hertz through ultrasonics you will find a frequency where the engine will vibrate and dance like crazy. This will be the engines natural static resonant frequency.

 

I thought winter was over.

 

Just where did you read it at, and from what did you read it, and why? What difference does it make, are you trying to belittle us by showing us how smart you are?

 

why do i need to know this?

 

To quote Delbert in "O brother where art thou", " In the neighborhood of B boys."

 

I have a recording of an engine running. But do not know what kind of engine it is, is it a motorcycle engine or is it a car engine.

If I display a spectrogram of the recording I see two frequencies, one at 50 Hz and the other at 25 Hz. I initially thought the 25 hertz was the fundamental frequency, but then I found out that the engine speed was 1000 RPM and it was on a six cylinder car. This told me that 50 Hz frequency was caused by the engine speed, but if that is true, what is causing the 25 Hz signal?

 

Again I ask just where, or what, did you read it from? The rest of us might want to read it as well.