firing frequency = numebr cylinder-1 ??? Is this really firing?

[Sofistioelevib]

Good morning to everyone,
some strange spectra today during analisys.

DIESEL TCG 2020 V16 (caterpillar) connected directly on MARELLI generator
Speed: 1500 RPM

I found about 87Hz as exactly 7 times firing frequency (1/2 RPM) till 10 mm/s as below



Lot of time i found some problem with firing but frequency was related exactly to cylinders number but in this case this frequency is like CYLINDERS -1 frequency and this is no sense for me.
Anyone had some experience like this? What suggestions from you in order to inspect better this component and find the origin?

Regards

 

[GregLocock]

You get 2 answers because I can't find out if it is a 2 stroke or a 4 stroke!

4stroke

Nope, firing frequency for a V16 4 stroke at 1500 rpm is 200 hz, which doesn't solve your problem admittedly.

So you've got 3.5 order. Wow, that is odd. Also odd is the absence of ANY firing order vibration (and multiples).


2 stroke

Firing frequency for a V16 2 stroke at 1500 rpm is 100 hz.

So you've got 7th order. Wow, that is odd. Also odd is the absence of much firing order vibration (and multiples).

Discussion

You do seem to have a lot of integer multiples of 1/2 order possibly caused by cylinder to cylinder variation. I always find it hard to figure these things out from single speed data, we always sweep the engine speed.

So I'm going to suggest you have a strong 87 Hz resonance - maybe crank bending, torsional or bending of the block +generator, excited by cylinder to cylinder variation. Maybe the harmonic damper failed.

have you got equivalent data for a good engine? Is this no load or full load?