dampers and piston speeds

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learner

dampers and piston speeds

Post by learner » Sun Feb 08, 2004 9:27 pm

Here is something i've wondered about for years. About 12 years ago i started messing around with stroke and rod lengths on paper by using a compass(to draw circles, not exactly accurate but gave me a decent indication). About three years ago i discovered something. All those years i was plotting piston locations based on the crank pulling the piston down. What i realized is what about the piston being pushed down by cylinder pressure. I was unable to plot instantaneous locations but using a mean pressure, i found that the piston being pushed down by cylinder pressure travels at a different speed than being pulled down by the crank. So, being that no two pistons are at the same location, on the same cycle, at the same time, that means that all 8 pistons are moving at different speeds at the same time. Hence the need for a damper to absorb the speed fluctuations and power impulses.

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Re: dampers and piston speeds

Post by speedtalk » Sun Feb 08, 2004 11:00 pm

learner wrote:i found that the piston being pushed down by cylinder pressure travels at a different speed than being pulled down by the crank.
In theory, the piston should be at the same speed when at the same location no matter if it's going up or down. Are you saying that because of contraction and expansion of components there is a difference?
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learner

Post by learner » Sun Feb 08, 2004 11:37 pm

I was thinking that piston speed may vary based on what stroke it is on. I can see where cylinder pressure or vacuum may affect the speed of the piston, however slight it may be. I can't prove or disprove, nor do i believe it is true or not true. Its just something i have contemplated and i would like to hear yours and others opinions and thoughts.


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learner

Post by learner » Thu Feb 12, 2004 8:30 pm

Hmmm...i was hoping this would start another good thread. C'mon you guys, tell me i'm crazy or something.

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Post by speedtalk » Thu Feb 12, 2004 8:41 pm

Remember, they thought Einstein was crazy. :D
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Post by Lasher... » Thu Feb 12, 2004 9:20 pm

Well I'm a crazy persons idea of a crazy person... So I probably have you beat.... Not that I'm proud of that or anything :wink:

I'm afraid that I prove anything here... but I can certinally see where manifold vac would load the crank differently ( pumping loss ) vs combustion presure vs compression and exhaust ect..... creating different impulses on the crank and needing the dampner to help diminish those impulses.... as the crank whiplashes to some small degree as a result of those impulses.

I can see your reasoning for the different loads and therefore different speeds but I can offer no proof that the piston speeds actually are different. Perhaps in some molecular level the loads may make some difference but for how much time and would it be something measurable ?....

Are you suggesting the material the reciprocating assy are made of ,stretch and compress enough to alter their stock " unloaded " length enough to change the speed ?..... or something where it the length stays static but there is a small amount of bearing clearence reduction as say compressive forces displaces some of the oil wedge between the rod bearing and the crank (under load ) to alter or delay a time event ??.....
IE lets say at an idle with say 20PSI oil pressure you hit the throttle and the crank spins before an increase in oil pressure resists the extra load ?

learner

Post by learner » Thu Feb 12, 2004 10:15 pm

The stretch and compression of materials i considered but at this time is not what i was referring to. I'll try to think of a way to explain it. Don, anyway to update the site to be able to post pictures?


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bsfc9

Post by bsfc9 » Sat Feb 14, 2004 10:03 am

this discussion certainly makes for some good food for thought. i can see how on the induction stroke the piston is being "held back" and on the power stroke it is being "accelerated more" down the bore. how much would one think there is an offsetting effect of one piston on the induction stroke and another on the power stroke, one being held back one being forced down? could one conclude from this the more cylinders in an engine the smoother it would run(less whiplash)? this really shows how complex the dynamic operation of an engine is, they seem to run smooth and predictable but it seems it is really chaos in disguise. now lets throw in valve train loading, harmonics, etc., and a bunch of oil flogging around in there and :shock: . oh yeah and what does all this mean for the balance job we just did that was "perfect"?

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Post by cstraub » Thu Feb 19, 2004 10:00 am

Learner I follow what you are saying. I have always been told that the crank absorbs the energy by deflecting as much as .030". . .well but if its doing that then. . .your right it would have to faster just for a "flash", but then rebound. Hmm, I will have to ponder this.

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