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Frequency from diesel engine

Frequency from diesel engine

(OP)
Hi,

I have a 26' box truck from Hinos (Chinese diesel truck). I placed a couple accelerometers on the box to see what frequencies are present. Several low frequencies crop up (1.5, 3.7, 14.7, 35.0 Hz). To calculate the dominant frequency of the diesel engine it would be:

RPM/60 = Hz

Then the number of ignition occurring per crankshaft revolution? for a 4 cylinder, it would be a multiple of 2. 8 cylinder multiple of 4. (ref: https://www.caranddriver.com/features/this-is-why-...)

Just want to make sure what I am doing makes sense to interpret at least one of the frequencies I am observing from the accelerometer. I haven't been able to find an academic source to validate the approach of considering the number of ignition for a 4-stroke.

Thanks!

RE: Frequency from diesel engine

I'd guess the 1.5 hz is the body natural frequency (heave or pitch or roll). The 14.7 might be the wheel hop natural frequency.

RE: Frequency from diesel engine

Idle speed is 600 rpm, so you'll often see 10 Hz if you've got a primary balance issue, at idle.

As you trundle round the streets at 1800 rpm a 4 cylinder will be belting out lots of 60 Hz, as the main forces in a 4 cylinder at twice engine frequency (2E)

Without knowing the conditions you measured the vibration in (obviously it is a state secret) I'm not going to be as brave as Brian and try and guess what your peaks represent.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Frequency from diesel engine

(OP)

Here's the full image, the 1.5 Hz seems to have harmonics as there are multiples (3.7, 6.4, 12.58 Hz...), as you've said, it could be the body natural frequency. There are some published articles about suspension for vehicles being around 1-2 Hz. I think it might be the case for this box truck.

Assuming the engine is a 6 cylinder - 600 RPM x 60= 10 Hz. Multiplying by 3 for the # of ignitions per crankshaft = ~30 Hz? I have a 33 Hz in the plot.

EDIT: Greg, the conditions: driving 43 seconds straight at idle (moving forward at 2-3 MPH). Accelerometer was directly attached to the 26' box of the truck (adhesive)

RE: Frequency from diesel engine

I guess you can independently verify your idle speed like this. Looks like 660 rev/min to me.

Steve

RE: Frequency from diesel engine

(OP)
Greg,

Originally the post was to understand how to calculate the frequency of the engine. Just to make sure I understand your first post:

Engine running at a constant 1500 RPM (4 stroke, 4 cylinder), would have a dominant frequency of 50 Hz and another at 25 Hz (not considering # of ignition), would be seen too?

The graph was just to illustrate all the frequencies observed. I do appreciate feedback on the other frequencies Brian. I have read that the 1-2 Hz could be the tuned suspension. At least it's the case for cars, not sure how applicable it is to diesel trucks. I'll have to read more to see if wheel hop could be the 14 Hz.

RE: Frequency from diesel engine

I'd expect an accelerometer mounted mid span on the wall or ceiling to be responding to acoustic goings on. Intake and exhaust pulses etc even with muffled appliances in place.

RE: Frequency from diesel engine

"Engine running at a constant 1500 RPM (4 stroke, 4 cylinder), would have a dominant frequency of 50 Hz and another at 25 Hz "

I4 engines being their own special brand of misery...

1500 rpm/60= 25 rps. Call this 1E. That's primary balance (2 plane dynamic). Then at 2E, 4E, 6E.... you have combustion forces that are somewhat proportional to throttle opening. If you see .5E,1.5E etc that is usually a combustion problem in one cylinder

Finally there the inertial forces due to the conrod length. These are also 2E 4e etc, but are only speed dependent not throttle.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Frequency from diesel engine

(OP)
Thank you for this information, Greg, it's what I was looking for.

RE: Frequency from diesel engine

a four stroke diesel engine generates frequencies at 0.5x operating speed and multiples (1, 1½, 2, etc)

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