Modal Chart

[vibrostics]

Please expaine me why the mode seperation chart is important in nvh and on what basis it is defined?

 

[autonoise]

The modal seperation chart is used to ensure the various modes (natural frequencies) of the the various subsystems (powertrain, chassis, body, etc.) are sufficiently seperated, so as to avoid overlapping responses when excited by inputs due to combustion, road vibration, etc. We also overlay the excitation orders of the powertrain to visualize alignment of excitation frequencies with these same resonances. E.G. one would want to ensure the combustion firing frequency at idle does not coincide with an acoustic or structural mode of the body, steering gear, etc.

 

[vibrostics]

Thank you for the answer. I am not very clear on the orders of the powertrain excitations. Since Engine can run at any rpm in its operating range will excite most of systems resonance frequencies. Second thing until what order of engine excitation I can chek for the resonances?

 

[GregLocock]

"Since Engine can run at any rpm in its operating range will excite most of systems resonance frequencies."

Correct

" Second thing until what order of engine excitation I can chek for the resonances?"

Depends on your engine configuration

Typically I'd look at 0.5, 1, 2, firing order and 2*firing order as an absolute minimum. 2 is very dependent on configuration. 1.5 can be important.


Cheers

Greg Locock

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

 

[vibrostics]

Greg,

Does that mean the orders of the engine are assoiciated witht the idle firing speed (only) of the engine to decide on subsystem's resonance frequencies?

Regards,

 

[GregLocock]

No, you have to plot it for the full operating range of the engine, although you'll tend to emphasise idle in your analysis- for instance half order is usually only a big deal at idle.

Cheers

Greg Locock

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

 

[vibrostics]

Coming back to the Mode chart, within the subsystem it self do we need to have pure specific mode shapes at a perticular frequency level or does not matter?

 

[GregLocock]

Mode shape matters. For example, an engine has 6 rigid body modes, but only three of them (typically) are a cause for concern.

Also lower frequency modes of a given subsystem are more 'powerful' than higher frequency modes, in general.

Cheers

Greg Locock

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

 

[vibrostics]

Greag,

What do you mean by powerful? Please expain..

Thanks,

 

[GregLocock]

more easily excited, tend to give moe trouble. That's not a hard and fast rule, just a trend (with some good physical explanation).

What NVH related books do you have?

Cheers

Greg Locock

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

 

[vibrostics]

Please suggest me one. I dont have specific NVH related book.

Thanks,

 

[GregLocock]

Look in the FAQ for this forum, there's a list of books, and a lot of free resources. I'd start with the B&K online stuff.

Cheers

Greg Locock

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