resonant frequencies

[IceStationZebra]

I have a general question regarding multiple order resonances. In my example I have a 3 cylinder engine running at 3000rpm, which gives me a firing frequency of 75Hz. When I check the sound recordings I have two big peaks at 75Hz and 150Hz, and a smaller peak at 225Hz.

The big question is ---
Do the 150Hz & 225Hz peaks come from some component operating at that frequency (which I haven't found yet!), which then causes other components to resonate...
OR does the 75Hz vibration excite objects that have a natural resonance of 150Hz and 225Hz since they are multiples of 75? (Some 2nd/3rd order vibration node?)

ISZ

 

[GregLocock]

(c) none of the above

Your firing order excitation is not a pure sinewave, it is more similar to a sawtooth. As such it will have a lot of harmonics.

2F and 3F are both fairly strong, although 3F is often weak enough that you need a resonance to make it an issue.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.