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Engine Orders
- Thread starter roseda
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GregLocock
Automotive
Half order is usually from a significant cylinder to cylinder variation. causes include:
Camshaft grind errors
camshaft torsional stiffness problems
crankshaft torsional stiffness problems
block torsional stiffness problems
poor manifolding (this is the usual one), ie one cylinder gets a poor mixture or a poor charge.
camshaft out of balance
I think that's it. All save the last would be expected to get worse under load.
Cheers
Greg Locock
Camshaft grind errors
camshaft torsional stiffness problems
crankshaft torsional stiffness problems
block torsional stiffness problems
poor manifolding (this is the usual one), ie one cylinder gets a poor mixture or a poor charge.
camshaft out of balance
I think that's it. All save the last would be expected to get worse under load.
Cheers
Greg Locock
I was going to get to that eventually... I had a meeting to rush off to.
The slippery part of the question is that it doesn't explicitly mention whether he's interested in factors related to the excitation of the vibration, or factors related to the response.
btw, is it common for torsional vibration to cause significant cylinder-to-cylinder performance differences? And when you say "camshaft out of balance," do you mean "balance" in the inertial sense of the word, or some other way?
The slippery part of the question is that it doesn't explicitly mention whether he's interested in factors related to the excitation of the vibration, or factors related to the response.
btw, is it common for torsional vibration to cause significant cylinder-to-cylinder performance differences? And when you say "camshaft out of balance," do you mean "balance" in the inertial sense of the word, or some other way?
GregLocock
Automotive
"btw, is it common for torsional vibration to cause significant cylinder-to-cylinder performance differences? "
No, not performance differences, that is, the imep from each cylinder is identical. What happens is that the difference in stiffness cause a different delay for the impulse to reach the measuring point and this introduces a half order (and multiples) into the FFT. It is one of the main reasons why stiffer cranks/blocks sound better. The camshaft torsionals may be the same thing, but they also twist under the valve spring load, this produces a repeatable phase error between each cylinder, and ends up causing a similar response.
However, most times 1/2 order comes from regular differences in cyl pressure diagram caused by lousy intake or exhaust manifold design, or possible temperature effects. (why does the 5th cylinder in an I6 usually knock first etc).
Yes, camshaft 2 plane balance would be at half crankshaft speed. I don't know if it is significant, but worth checking.
I've taken a week off from meetings. bliss.
Cheers
Greg Locock
No, not performance differences, that is, the imep from each cylinder is identical. What happens is that the difference in stiffness cause a different delay for the impulse to reach the measuring point and this introduces a half order (and multiples) into the FFT. It is one of the main reasons why stiffer cranks/blocks sound better. The camshaft torsionals may be the same thing, but they also twist under the valve spring load, this produces a repeatable phase error between each cylinder, and ends up causing a similar response.
However, most times 1/2 order comes from regular differences in cyl pressure diagram caused by lousy intake or exhaust manifold design, or possible temperature effects. (why does the 5th cylinder in an I6 usually knock first etc).
Yes, camshaft 2 plane balance would be at half crankshaft speed. I don't know if it is significant, but worth checking.
I've taken a week off from meetings. bliss.
Cheers
Greg Locock
I suppose the question is whether 2-plane camshaft balance would influence the camshaft drive torque at all...
I was going to guess that his question was more along the lines of "why is my 0.5 order response so high," and I was going to suggest that perhaps he was measuring a rolling-mode (rigid body) oscillation of the crankshaft, which could potentially be related to intentionally reduced rotating assembly inertia (a guess about his engine configuration - lightened flywheel, etc, for racing)
I was going to guess that his question was more along the lines of "why is my 0.5 order response so high," and I was going to suggest that perhaps he was measuring a rolling-mode (rigid body) oscillation of the crankshaft, which could potentially be related to intentionally reduced rotating assembly inertia (a guess about his engine configuration - lightened flywheel, etc, for racing)
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I was chiefly interested in what factors might cause a half-order vibration, as I could only think of cam out of balance.
However, I have acquired a waterfall plot from an IRL engine (crank front to crank rear), and it shows a peak in the 0.5 order at around 9000 rpm.
Thanks for your answers so far.
However, I have acquired a waterfall plot from an IRL engine (crank front to crank rear), and it shows a peak in the 0.5 order at around 9000 rpm.
Thanks for your answers so far.
Well, that's probably due to cyl-cyl variation, as Greg said. For a v-8, the fundamental order for a single cylinder's firing pulse is 0.5, so differences between the cyls can excite 0.5 order vibration. The 4th-order excitations from all of the cylinders should add constructively, so expect a big excitation there as well.
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