Well they normally increase on a 4 cylinder, not decrease. Typical first torsional resonance is going to be more than 300 hz on an automotive unit.

Cheers

Greg Locock


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Thanks Greg. By how much?
This paper seems to indicate they decrease to an extent. http://papers.sae.org/2009-01-1938/

Figure 15 shows what I'd expect to see. The very low frequency stuff in the earlier plots is rigid body modes.

Cheers

Greg Locock


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It really depends what you meanby "angular excitations".If you are tracking 2E (you did say it was 4-cyl, so probably an I4), expect to see a V shaped curve of 2E amplitude vs engine speed. Most that I've measured and/or simulated will decrease to a near-zero 2E amplitude at around 2000-3000 rev/min, then increase from that speed upwards. This is where the two sources of 2E (gas forces, inertia forces) combine in such a way that the 2E amplitude disappears. For the purposes of low-frequency excitations (the kind that affect gear rattle and steering wheel vibration), 2E is dominant. If you know what gear rattle sounds like, you can tune in to hear it at low speed/high load and diminish as speed reaches the bottom of that V. After that, other 2E noise dominates.

- Steve

I'd still be interested in describing why the crank nose TVs are so high at low rpm. I did look at this 25 years ago, but I can't remember the exact outcome.

Cheers

Greg Locock


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Thanks guys.
It is the 2nd order (I presume 2E). That's useful information Steve. Are their any circumstances when it would not increase after 3000rpm?

Here are some plots I did a long time ago (1994). They are from a simulation, but they matched the measurements very well. They both show flywheel speed during a full load slow transient (3rd gear) for a typical 1.8 I4 N/A gasoline in a 90's small hatchback.

The first is amplitude at given orders vs speed, where you can see 2E hitting a minimum.



The second is more interesting. It shows speed variation from mean through each engine cycle and (I think) you can see the 2E shape from gas forces decaying with speed as the 2E from inertia kind of rises out of the valleys. 2E is the dominant shape at each end, but there are two clearly unrelated sources.



You can see the point where the load goes on and also the shuffle it causes too, in the plots vs time. Nostalgia. I kind of miss those days. So much data to analyse. Matlab on hand to dig the story out of it.

- Steve

Very nice Steve!!

Seems to be dominated by 2nd order. I wonder how far the second plot's 2nd order slopes back up at sat 5000 rpm.

typically a deg or so pk-pk from memory

Cheers

Greg Locock


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Thanks Greg!

Let's see if I get thrown off here for hijacking this thread...

I need to know a representative (ballpark) maximum value of repetitive (vibratory) crankshaft angular acceleration (supercharged 3 liter inline four cylinder) at ~10,000 RPM (shooting for 725 HP @ ~9,000 RPM). The only "dampers" will be the vehicle drive off the flywheel end, and supercharger drive off the snout (a significant load- 30 PSI boost). Perhaps one of you can save me blindly buying SAE papers to look for this (I'm not an SAE member)? Or- to "cut to the chase"- are angular accelerations at least a factor of 100 smaller than the 8,500 G's radial acceleration of a point near the circumference of its counterweights?

http://www.pattakon.com/pattakonEduc.htm

Manolis has some nice tools on his site.

Engineering is the art of creating things you need, from things you can get.

Hey guys,
I'd like to revive this thread for a moment. Any data for that same or similar engine above, but for C/S instantaneous torque?
I'm looking for the difference in mean torque in the rpm range and the instantaneous firing pulse torque.
Thanks,