Blown Engine in '99 300M

We both missed it. Here's the answer to that
question:http://www.mae.wmich.edu/faculty/hathaway/classes/me468/Lecture/Lecture03.ppt
(scroll down to the "Instantaneous Piston Acceleration" section, and
you'll see that the instantaneous accleration increases by the square of
the RPM).

That's a pretty rigorous excercise that guy went thru.

This one has some pretty nice graphs that tell the story visually:
http://www.epi-eng.com/ET-PistnVelAccel.htm. Interesting that the max
acceleration at the bottom end of the stroke is only 1/2 of the max
acceleration at the top of the stroke. I would never have guessed that.

My Google search also turned up these these interesting items:
http://yarchive.net/metal/piston_acceleration.html
http://www.unofficialbmw.com/all/engine/all_redline_rpm_vs_reliability.html

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")

 

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: http://dodgeram.org/tech/dsl/FAQ/engine_op.htm.

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.

Good research, Bill.


Matt

 

Being an engineer is a curse, Bill.


Matt

 

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.

 

Bill Putney wrote:

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.

Matt

 

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.

Matt

 

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.


Matt

 

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")

 

Yep - if everything agreed with common sense or intuition, they wouldn't
need us engineers, and Henry Ford's second car would have been a 300M.

Interesting!

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.)

Best regards,

DJV

 

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?


Matt

 

Mmmmmm....not terribly likely. Pistons generally do not seize tightly or
quickly enough to bend rods at all.

DS

 

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.

C

 

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.

Matt

 

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?

I think another poster suggested something got pulled into the cylinder from
the top of the engine, with the rest of the damage cascading from there.
That certainly seems feasible. Without someone who's both qualified and
interested looking at the remains, I suppose we won't be able to know for
sure.