Diesel engine vibration

[vibrasie]

A vertical acceleration PSD of a diesel engine used as part of a generator shows a second harmonic peak with significantly more energy than the first harmonic. Given the mechanisms at work here is this possible?

If not where else should I look:
(a) that it coincides with a mode of the isolation system?
(b) some magnetic force due to the generator?

Engine runs at 25 Hz and has 4 cylinders. Generator supplies 50 Hz alternating current.

Thank you for your assistance

 

[MikeyP]

That would be the firing frequency - 4 cylinders firing once every two revolutions.

Does the engine have a balancer on it?

M

--
Dr Michael F Platten

 

[ivymike]

yes, second order is very likely to be the firing order for that four cylinder engine.

 

[Tmoose]

inline 4 with crank throws like this?

http://www.engr.colostate.edu/~dga/mech324/figures/F14-03.jpg

The secondary inertial forces are of similar magnitude to a single unbalanced rod and piston rotating at 2X crank speed, and acting along the cylinder centerline. A pair of counter-rotating counterweights rotating at twice crank speed is needed to counter act the secondary force.

Does your engine have them?

 

[SomptingGuy]

I4 engines have 2nd order forces from both firing and inertia. These are not in phase and there is often a speed where they cancel (depending on load, of course). Even with a balancer, you'll still see load-dependent 2nd order.

- Steve

 

[Tmoose]

Secondary inertial forces are "upwards," twice per revolution.

On page 221 of MECHANICAL VIBRATIONS Hartog says of a single cylnder's inertial forces - "In words: the vertical component of the inertia force consists of two parts, a " primary part" equal to the inertia action of the combined reciprocating and rotating masses as if they were moving up and down harmonically with crank-shaft frequency and amplitude r, and a "secondary part" equal to the inertia T action of a mass jyWrec moving up and down with twice the crank-shaft frequency and with the same amplitude r.
The horizontal or lateral component has a primary part only,
viz., that due to the rotating mass."

I think the "firing" forces that sneak out are via torque action/reaction.
So I would not expect firing (torque) to automatically have a vertical component except as a result of mounting scheme.

Goofy 4 cyl engine mount scheme from HArtog attached

 

[ivymike]

on bigger engines (I8, V16, V20) it's common for the block to have important (large magnitude) flexural modes which are excited by the firing events. These can have vertical components. A little four-cylinder would have less to worry about w/r/t block modes.

 

[GregLocock]

2nd order due to L/r ratio is the primary NVH problem with I4 engines, and it gets worse because of course it is firing frequency as well. Typically you'd compare no load and full load spectra to separate those two out. Bear in mind that both of those mechanism generate 4th, 6th 8th etc order as well.

I'd be very surprised to see much first order.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Tmoose]

Hi Greg,

OP said vertical 2X vibration and 4 cylinder, and it was I who leaped to the conclusion inline 4.

You would expect firing frequency to have a vertical (along cylnder axis) component?

Dan T

 

[GregLocock]

Yes, I'd expect firing frequency to show up in all 3 directions. Long time since I've measured it.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[SomptingGuy]

I can't recall every attempting to measure 2E engine motion caused by firing only. It's been quite a while since I last screwed an accelerometer to a block or targeted an inductive probe at the ring gear. But my (unqualified) guess would be that it's more torsional than vertical.

Our issues were always at high-ish speeds and typically when 2E coupled with some other phenomenon, like whole PT bending, or some ancilliary flapping around on the FEAD bracket.

Low speed 2E (in an I4) was normally an issue for gear rattle, both for unsynchronized gears in the transmission and for cam/pump drive systems. Indeed, one I4 diesel engine we analysed had most of its sound power coming from the timing drive at some conditions.


- Steve

 

[GregLocock]

I vaguely remember that inertial and combustion 2E were roughly equal, maxing out at around 10g rms each on a fairly lightly built automotive NA diesel.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Tmoose]

10 g rms at 3600 cpm (2x at 1800 rpm) ~ 14 ips pk or 76 MILS pk-pk.
ya - HOO

50 Hz operation is higher still