Max piston speed/acceleration?

[PanzerAce]

So...I would put this in an engine specific sub forum, but I suspect it's something that every one could use, so it's here instead.

 

I'm wondering how people come up with their engine's redline. I see on other forums that people talk about mean piston speed, max piston speed, or sometimes max piston acceleration, and always with a pre-set number as a maximum; but really, what determines how fast you can spin your engine without destroying anything (assuming everything is built right out of quality parts).

[kiwi303]

but really, what determines how fast you can spin your engine without destroying anything (assuming everything is built right out of quality parts).

 

From what I read, mainly the rod bolts. Piston speed up and down the bore involves piston slap as the thrusts on the conrod go to and fro as the rod goes around the circular path of the crank, but that's a matter of accellerated wear, not explosive destruction. Explosive destruction comes from the jerk on the bolts as the piston and rods read the top of the stroke and keep trying to go up against the pull of the bolts, then when a bolt gives way, the thrashing big end causes all sorts of grief.

 

Grumpy, BRAAP and others can probably put it better, but thats what I gather from researches into building motors, and why 9mm bolt L24 rods are favoured in L stroker builds, as being bigger and stronger bolts to stand a bit more abuse. Or ARP rod bolts over chev stock bolts.

[PanzerAce]

I get that stronger bolts are better, but I'm looking for some quantitative explanations, so that I can start doing some math on my long term project (NA 3.6L M104. After the LD28(T))

[woldson]

This list could be a mile long, or, just a simple as, the stocker (fI na) runs out of air in 5500-5800 rpm range, and longevity.

 

As far as how far you can go, some have reached 8000k. There is a harmonic problem in the higher areas, however it was suggested that if you break through and not dwell at that point you will may be OK for a while.

 

Others say that when you get in the range the crank has a tendency to fail at the tail (If memory severs).

 

Either way, you have a long way to go to have effective power up to those types of numbers.

[letitsnow]

So...I would put this in an engine specific sub forum, but I suspect it's something that every one could use, so it's here instead.

 

I'm wondering how people come up with their engine's redline. I see on other forums that people talk about mean piston speed, max piston speed, or sometimes max piston acceleration, and always with a pre-set number as a maximum; but really, what determines how fast you can spin your engine without destroying anything (assuming everything is built right out of quality parts).

 

It depends what you define as quality parts. If you have super-overkill strong parts you can spin it higher than if you have good condition stockers. The're both of 'good quality', but one will be stronger, that's just semantics though. Piston acceleration should be the magic number here because if you recall some basic physics, F=m*a.

 

If you're feeling frisky, find the position of the piston with respect to the crankshaft angle, take the 2nd derivative of the whole thing with respect to time. Do it all in standard metric units(m, s, Hz are the useful ones here) and you should get an an answer that works out to roughly something*cos(something else(t)) m/s^2. The coefficient of the cosine(or sine, depending on where you started) is the max acceleration. You could then find the force, and subsequently weather or not your parts will deform or break.

 

It's always so easy to say it, doing it is a different story. I'd do it, but I've got an english paper due very soon and I havn't finished the book yet.

[grumpyvette]

http://forum.grumpysperformance.com/viewtopic.php?f=53&t=343&p=1170&hilit=+redline#p1170

 

an engines red-line is the rpm limit where the stress is approaching the strength limitations of the weaker components in your engine, in most cases that's the rod bolts, rods, or a valve train control issues, on most engines.

on most American V8 engines that's reached at about 4000 fpm (feet per minute) in piston speed, with high quality forged components and a solid lifter valve train, thats at about 4500fpm,now Id point out Im not about to suggest that at 4100 fpm a stock engine self destructs,because theres a wide variation in components, and different engine designs but stress is cumulative, and those are reasonable limits as a guide, now your engine may exceed that limit hundreds of times or only a few times before problems occur, but exceed the limits and eventually youll over stress components until they fail.

Edited November 14, 2009 by grumpyvette

[MAG58]

There are tons of variables, but I'm going to toss out some references for mean piston speed...

 

low speed diesels

~8.5 m/s for marine and electric power generation application

smedium speed diesels

~11 m/s for trains or trucks

high speed diesel

~14 m/s for automobile engines

medium speed petrol

~16 m/s for automobile engines

high speed petrol

~20-25 m/s for sport automobile engines or motorcycles

competition

Some extreme examples are Nascar Sprint Cup Series and Formula 1 engines with ~25 m/s and Top Fuel engines ~30 m/s

 

 

Though this does not correlate with what IMO kills engines much more quickly which is the accelerations at TDC and BDC. They always say it's not the fall that kills you but the sudden stop, and the faster an engine spins the higher the acceleration. This is where r/s ratio side loading, etc. etc. comes into play in engine design.

[Tony D]

'As high as 8000' huh?

 

As I recall, someone with good knowledge on the subject said if you are breaking cranks on the tail end, look at how you're fastening your flywheel bolts!

[woldson]

I was doing a loose reference to something that I had read that you posted some time back.

 

I can not remember specific of what you had posted.

 

I found your comments clear and concise on the matter.

 

I had also read a long time ago about a fellow that was snaping cranks around the 8k mark, could never find that thread to reference it in the tread that had your comments in it.

 

I do admit, I kinda take your word as gospel....