We both missed it. Here's the answer to that question:
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down to the "Instantaneous Piston Acceleration" section, andyou'll see that the instantaneous accleration increases by the square ofthe RPM).
That's a pretty rigorous excercise that guy went thru.
This one has some pretty nice graphs that tell the story visually:
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Interesting that the maxacceleration at the bottom end of the stroke is only 1/2 of the maxacceleration at the top of the stroke. I would never have guessed that. My Google search also turned up these these interesting items:
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Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")
Matt - Here's a link from my other post that talks about good design practices relative to mean linear piston speed, but not from peak acceleration and forces issues.
I'm realizing that, unbeknownst to each other, we both have been doing simultaneous searches on this stuff this evening. We're pathetic, man!
8^)
Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")
(scroll down to the "Instantaneous Piston Acceleration" section, and> you'll see that the instantaneous accleration increases by the square of> the RPM).
Yes, very impressive. Now if he'd have thrown in some sample numbers to allow the calculation of the net Fn force we'd have a complete answer to our questions.
Yes, I found the same site and just posted it also! I also just came across this:
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Unfortunately, it isn't discussing a gas engine and I don't think it would be prudent to extrapolate from the Cummins diesel to the 300M's gas engine, but it is interesting to note that Cummins permits extended operation ("several hours") at the redline (governed max engine RPM)of the 5.9l engine. They also permit lugging the engine for up to 1 minute. I still can't believe that this limitation is due to bearing flim strength breakdown as that would occur in less than a minute. I'm betting it is localized heat build-up somewhere, but that is just a guess.
Yep, found this one also!
Yes, I'm aware of the piston speed issues, this is varies dramatically based on engine design and can't be determined from RPM alone. Aircraft engines running at 2500 RPM have piston speeds comparable to small motorcycle engines running at 10,000 RPM. Engine designers take this into account when they establish the red line value. I haven't seen Gordon Jennings name mentioned in a long time. He was my hero back in the 70s when he wrote for, if memory serves, Cycle magazine. Cycle is long since defunct and I think I read some time ago that Gordon had passed away. He wrote many excellent technical articles on Cycle with regard to the way engines and various parts worked. He was very clear in his explanations and used enough calculations and graphics to support his claims. He wrote an excellent article explaining how plain bearings work ... and it is my vague recollection on that which causes me to question the theory about lugging causing the oil film to break down, but I admit my recollection is vague as I read that article probably 20 years ago.
It may not be the dominant force, but its big enough to cause a significant change in the total (tension) force felt by the rod bolts.
Empirically, race engines are more likely to throw rods when the driver lifts to enter a corner than when he drops the hammer to drive out of a corner, also.
I can't find an immediate reference for the hard bearing material statement I made and you questioned, but it *is* a matter of fact that modern automotive engines tend to use aluminum bearings, whereas aircraft engines have stuck with babbit and softer babbit alloys at that. Tri-metal isn't even that common in aircraft engines yet, and although I'm sure that is largely to do with maintaining good embeddability because A/C engines don't use fine oil filtration (better to have a good flow of moderately screened oil than no flow at all due to a clogged fine filter!) but it also does have an adverse affect on lugging operation. Hard bearings with small surface area will score crank journals under lugging and detonation, but softer bearings hold up much better unless they're loaded to the point of deformation.
I think Jake Brakes work by venting compression since you can dump more energy per stroke that way. The maximum "vacuum" you can work against is just 1 atmosphere (~14 psi), but if you vent compression instead you can work against a pressure differential of *many* atmospheres.
Intuition can be very misleading in engineering applications :-) That's the hardest thing I've personally had to learn and re-learn during my career.
Several years ago, I wrote up a little Matlab script that generated plots very similar to those (and ignoring any combustion or pumping pressures). That's when I came to realize just how *bad* the rod ratio in a small-block Chevy really is. I plotted a family of piston acceleration curves as a function of connecting rod length, and by the time you're down to the kind of rod ratios that a Chevy 400 has, that "flattened" lower BDC lobe on the acceleration curve has divided into two sharp peaks before and after BDC, and (IIRC) there are some secondary peaks that begin to show up on either side of TDC as well. The Chevy 350 is right on the hairy edge of a "normal" looking piston acceleration curve, but the 400 is well into the "wierd" zone. All the Chrysler v8s look more sinusoidal than the plot shown on the page you referenced. Just FYI.
I still haven't found a real thorough analysis of this, but I have found a few sites that seem to support this via at least basic calculations. If I get real bored some night I may pull out my dusty engineering texts and give it a whirl myself.
Yes, that is my understanding of how Jake Brakes work. There are some that put a valve in the exhaust stream just past the header, but these are typically not as effective as a real Jacobs brand engine brake that actuates the valves.
I agree which is why I, and Bill also apparently, spent a fair bit of time searching for some analytical data the other evening. I found lots of interesting stuff, but no real complete analysis.
That also happens to be when a piston is likely to crack due to thermal shock (rapid cooling) - not that I'd know that first hand, but my uncle was a Grand National (up until recently known as Winston Cup) driver in the 60's.
Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")
For anyone who's interested, I'm back on the road ($3800 later).
The mechanic didn't have a precise diagnosis as to the "failure mode". Suffice to say there were lots of little pieces of metal all over inside that engine, and hole through the outside of the block to boot. One of the pistons was completely destroyed -- busted into at least a couple of parts and barely recognizable as a piston. Very impressive. Based on the somewhat S-shaped bend in one of the piston rods, my (highly uneducated) theory is that one of the pistons seized on a compression/exhaust stroke. Again, that's little more than a wild guess from someone who doesn't know too much about engines.
Anyway, thanks to those of you who chipped in to the lively discussion. I learned some interesting stuff through your posts. (The tuition was kind of expensive, but this is one of those times when I thank Jesus for the money to pay unexpected bills.)
Interesting. I suppose your theory is possible, but I've never seen a piston seize fast enough to bend a rod. Almost sounds like something got into the combustion chamber. Were all of the valves intact? I've seen engines that swallowed a valve look like what you describe.
If the engine got low on oil, I think one of the bearings would have seized before the piston seized. Was the big end of the pretzel shaped rod welded to the crank journal?
Yup, it's more likely a connecting rod cap popped and the intertia from the running engine did the damage the guy desicribed. There is a lot of momentum in a running engine so a catastrophic failure can result in lots of "follow-up" damage until it comes to a grinding halt.
It would be pretty obvious though if this were the failure mode. He didn't mention a rod being separated from the crank, but maybe he just omitted that inadvertently.
I asked the mechanic about the cap and bolts holding the piston on the piston rod, relating what some of you had discussed regarding the inertial forces under low/no load conditions, but he said that was intact.
I didn't see the rod/crank, so I don't know whether that connection was intact or not -- the mechanic didn't mention it. With the piston being as busted up as it was, it makes me think that the rod and crank must still have been connected. Otherwise where would the force come from?
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