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# Recip defect frequencies2

## Recip defect frequencies

(OP)
I read a piece on aircraft engine vibrations that stated a misfiring engine produces high 1 1/2x vibration.

Then I read a document from a major recip engine manufacturer (not aircraft) which stated that misfire generates 1/2x vibration. It further stated that 1 1/2x and 2 1/2x vibrations were a normal part of combustion and couldn't be alleviated?

Why the difference?

### RE: Recip defect frequencies

Not exactly my area, but I'll throw this out...

Perhaps different arrangements (Flat-6; Inline-4; V8; etc.) exhibit different vibration responses to a misfire.

### RE: Recip defect frequencies

On a 4 stroke engine a regular misfire on one cylinder will generate a harmonic sequence of vibration and noise, at 0.5 order and all integer multiples thereof.

However the particular harmonics are amplified by the structures resonances, and acoustic resonance, so although the forcing at 0.5 is strongest, if there are no resonances at that frequency a higher harmonic may dominate. For instance on an automotive engine you'll see a lot of whatever order is around 300 Hz, as that's a typical crankshaft bending and torsional frequency.

1.5 and 2.5 order would be the dominant firing excitation for a 3 and 5 cylinder engine 4 stroke respectively. So it rather depends on what the engine is, it'd help to see the remarks in context.

Cheers

Greg Locock

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### RE: Recip defect frequencies

(OP)
Thanks very much guys,

Very helpful answer Greg and thanks Cat for that. If I didn't have it already, that would be very helpful. That is actually the document I was referring to in the second part.

### RE: Recip defect frequencies

(OP)
I have a follow-up question:

Does it make any difference (in the discussion about the defect frequencies) with spark engines compared to diesel type?

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