Campbell diagram

[ivymike]

I came up with a question the other day while looking at an NVH-related Campbell diagram, which showed exhaust noise (dB, color contour) vs frequency (Hz, x axis) and engine rpm (rpm, y axis).

* There were clearly visible diagonal lines on the plot, which corresponded to the various harmonic orders.

* There were areas on each order where the response was higher than in other places.

* These high amplitude regions, when viewed from a distance, lined up to make roughly vertical lines. Those vertical lines correspond to resonant frequencies.

* Closer examination of the graph also showed fainter horizontal lines (constant engine speed). In other words, all engine orders had higher amplitudes at one engine speed than at the neighboring speeds.

My question is this - what is the physical significance of the horizontal lines on the plot? The diagonal and vertical lines seem fairly straightforward (vibratory orders, resonant frequencies).

If it influences your answer, bear in mind that as far as I know, the data in the plot were computer generated (simulation results) rather than measurement data.

 

[GregLocock]

Interesting. First guess would be pre-ignition - the higher than expected pressure rise rate excites all resonances. If it is on all cylinders it will still obey the order structure.

You can check that by looking at Campbell's plots of the cylinder pressure.

If it was a real result I'd be looking at non linearities - eg buzzes and so on in the exhaust system.

Oh, how about turbulent flow in the exhaust? is the model that good? Typically if the downpipe is too small then at around max torque you get a really farty note coming into the exhaust (technical term).

Damn I'm rusty at the engine stuff.

Cheers

Greg Locock

 

[ivymike]

thanks for the suggestions.