Blown Engine in '99 300M

I've never understood that either. Why is no load as the piston goes
thru TDC any more stress on the conn. rods, crank, and bearings than the
stress at BDC with or without load at the same speed (other than TDC
being tension on the rods, and BDC being compression). And if a plug
misfires at high rpm, does that make more stress than the parts see at
BDC (at the same rpm)?

I've questioned this "no load is harder on an engine at higher rpm"
thing before on this ng, but no one has ever explained it. I want to
learn.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

I wasn't taking a position on that, I was stating that lugging is
definitely bad for an engine.

nate

 

I guess I took the "low rpm" qualifier to mean something a little lower
than 6000 rpm - now I see the context was lugging.

So I guess that question is for Steve.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

Steve - I'm witchew on this except for the no-load qualifier - I think
you've made statements before that no load at high rpm is more damaging
than same rpm with some reasonable load.

I've never understood this since the piston (conn. rods, crank, and
bearings) goes thru the same accelerations and stress levels at BDC as
at TDC during the exhaust cycle or at TDC when a cylinder misfires (or
when a rev limiter cuts in).

Please explain. Thanks.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

55 mph in low gear is "no load."

Redline means different things in different applications, and does NOT
always mean an RPM that can or should be sustained (see last paragraph
below).

There's no myth when it comes to stretching the connecting rod bolts to
the maximum once every crank revolution (no load) as opposed to once
every other revolution (full load) and even then to a lesser degree
because under load there's always residual exhaust pressure helping push
the piston down.

It depends entirely on the engine design and intended application.
"lugging" a modern automobile engine with its relatively small bearing
surface area and hard bearing materials (to save weight and cut
frictional losses) is much more damaging to it than "lugging" an
aircraft engine with its much larger bearing area and relatively softer
bearing materials (to withstand the higher forces of low-RPM high-torque
operation.

One thing that is absolutely certain is that production automotive
engines are NEVER meant to run continuously at redline or at their full
rated power output, they're intended to run up there for short bursts.
That is part of the reason why virtually every time an automobile engine
is converted for use in aircraft, its a dismal failure (witness the
Chevrolet engines that were used to power the Vickers Vimy replica and
all the grief that Ryan Falconer Engines has gone through to get their
Chevrolet-based V12 workable).

When an engine builder sells the same engine for different applications
(eg, large diesels) they often have very different ratings for different
applications also. The engine can achieve a much higher rating for a
relatively short period (say, an 18-wheel truck), but if the application
is a constant load (generator, marine propulsion, etc) then the rating
may be significantly lower. So although the 300M engine has a rating of
255 horsepower at 6500 RPM, that is absolutely NOT a suitable rating for
sustained use.

 

First off, forget about comparing loads at TDC vs BDC. Only TDC stresses
the connecting rod in TENSION, and in turn that is the only time that
excessive stress is placed on the connecting rod BOLTS. The bolts are
always a weaker link than the rod itself, assuming no manufacturing
flaws. At BDC, no forces are actually carried through the rod bolts, the
rod is acting in compression and transfers all forces directly to the
crank. The bolts just act to keep the rod cap from flying off, and thats
a minimal force.

Secondly, compression and combustion pressures reduce or eliminate the
tension on the rods during the power stroke, and pushing exhaust out
helps relieve it on the exhaust stroke when operating at full power. But
when the engine is spinning fast at minimal power, there is far less
compression and combustion pressure and almost no pressure required on
the exhaust stroke, so the connecting rod bolts are the ONLY thing
acting to pull the piston back down the hole on EVERY crank rotation.

This is actually a big consideration in the design of large diesels
where 2-stroke engines (EMD locomotives, Sulzer direct-drive ship
diesels, etc) are common. One of the big advantages of the 2-stroke
design is that the connecting rod bolts almost never see the high
tension loads that they do on the exhaust stroke of a 4-stroke diesel,
since in a 2-stroke there is a combustion on every crank rotation, so
the bottom end of a 2-stroke diesel can be considerably more lightly
built than an equivalent output 4-stroke.

 

I don't own a cranky Porsche.

What happens? And why?

Matt

 

What's the redline for this engine?

Matt

 

Linear acceleration is dv/dt, or the change in velocity over the change
in time. If the change in velocity is twice as much per unit time (as
it would be if the engine was turning twice the RPM), then the
acceleration is twice as high. Since mass isn't changing, the force
imparted on the connecting rod and crank by the piston should also
increase by a factor of two. Maybe I'm not understanding what you are
saying.

Ha, ha, ha... No, I mean I'd read the owner's manual ... and service
manual which I but for all of my vehicles.

I don't know if this is true or not and I doubt you do either. I've
seen engines that were run very hard and lasted a long time. I've seen
engines driven by the proverbial little old lady that didn't last long
at all.

The biggest problem with running engines hard is generally heat
rejection, not mechanical failure due to stress or wearing out.
Running at high RPM under very little load would not cause any
overheating problem.


Matt

 

I sure wish I could remember where I read that. It calculated the
pressure exerted by combustion as well as the inertial forces and
basically, if memory serves, the inertial forces completely overwhelmed
the forces from combustion at high RPM and thus it really didn't change
the load on the crank and rods significantly depending on where you were
under full throttle or at closed throttle.


Matt

 

Why? Extending lugging will cause overheating, and that is certainly
bad. But otherwise, I'm not aware of any ill affects.


Matt

 

It's the same load as in high gear. It takes a certain amount of
horsepower to move a car at 55 MPH. Doesn't matter what gear you are in.

Can you state a manufacturers reference that supports this?

Please provide calculations that support this or a reference to a
creditable technical journal or other source.

Again, anything to substantiate this?

Typically, the failure mode is heat related. I agree that auto engines
aren't designed to run at high power outputs continuously, but it isn't
due to issues of sucking oil out of the valve covers or breaking
internal parts. And, there are now several successful auto engines
running in homebuilt airplanes, the Subaru engines being particularly
successful assuming proper cooling is provided.

I agree, but running at 55 MPH in a car isn't drawing anywhere near 255
horspower from the engine, maybe 25 horsepower.


Matt

 

That would be the early 80's EA81 engine due primarily to the gear
driven cam (vs. single-point failure timing belts).

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

The primary problem with lugging (i.e., developing a lot of torque at
low rpm) is that the pressure of the connecting rod downward on the
bearings during the power stroke is over a longer duration of time
(lower rpm), so the oil cushion between the crank journal and the conn.
rod insert can get totally squeezed out, resulting in the dreaded
metal-to-metal contact under motion and pressure. At higher rpms, the
layer of oil doesn't have time to get squeezed to nothing before the
downward force of the piston disears in time for a new layer of oil to
get pumped in.

In the "good old days" (i.e., before computer control and knock
sensors), the other risk was knocking (pre-ignition) which of course was
hard on several components, including piston, rods, and bearings -
probably not as much of a concern these days with mostly automatic
transmissions and knock sensors, etc. Of course, oils have improved a
lot too over the years.

I vividly remember when I was a teenager (mid sixties), about the time I
was learning to drive, that even the women (mothers) were well aware of
potential damage of "lugging" the engine - of course they didn't know
why, but mechanics and husbands made sure it was common knowledge not to
do it (perhaps it was only my mother that had been coached well by my
father, but somehow, I got the feeling it really was common knowledge
throughout the culture).

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

Even worse if the throttle is suddenly closed where the piston has to be
pulled down against a vacuum in a dead-ended intake. Maybe that's why
Jake brakes work off of pressure on the exhaust rather than vacuum on
the intake?

Actually, though there's something to what you say there, I think the
bigger factor there is that you get two power strokes with the 2-cycle
engine for every power stroke of the 4-cycle engine, so, for the same
power output, you need smaller everything: smaller displacement, lighter
components, lower inertial stresses, etc. for the same power output.

Getting back to the auto engine, from what you're saying, it's not like
there is a one-time catastrophic event if, say, a plug misfires or the
rev limiter cuts ignition - it's more a fatigue issue of the bearing cap
bolts (which eventually becomes catastrophic on the final turn when it
does come apart).

Thanks for the explanation.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

I've not been able to find anything that supports (or denies) this
theory about the oil film collapsing. However, I have found some
interesting other stuff while searching around this evening. Here is
one interesting link: http://www.tfxengine.com/software7.html

It answers the question on one slide about the difference in pressure at
TDC with and without spark plug firing. It appears that the pressure
difference is virtually nil, as the combustion process doesn't really
increase the cylinder pressure much until several degrees after TDC,
unless the cylinder is detonating in which case very large pressures are
present at or very close to TDC. So, it appears that a misfire doesn't
appreciably change the stress on the engine at TDC, but does have an
affect in the 30 or so degrees after TDC. At that point, however, the
inertial force has dropped dramatically so there is much less inertial
force to be countered by the cylinder pressure force.

I found a couple of links relative to acceleration of the piston during
its cycle and this was interesting as well. Maximum acceleration occurs
at TDC as we all probably knew, but the other "maximum" doesn't occur at
BDC as might seem intuitive. It occurs in two places many degrees
before and a little after BDC. This is due to using a finite length
connecting rod to convert rotary motion to linear motion. With an
infinite length connecting rod the maximum acceleration would occur at
BDC as well as TDC. See: http://www.epi-eng.com/ET-PistnVelAccel.htm

I've not yet found a source that shows the relationship between
intertial forces and cylinder pressure to show the net force on the
piston, rod and crank during both WOT and closed throttle operation at
high RPM (or any RPM for that matter). I saw a comment on one site that
said that the highest stress occurs at high RPM with a closed throttle,
but that site gave no explanation or no calculations to support that.
I'm getting more curious by the minute, but haven't yet found definitive
information.


Matt

 

I don't know what the official spec. is (couldn't find it in the '99
FSM), but the tach shows redline at 6500 rpm. Under "tachometer" in the
owner's manual, it simply says "Measures engine revolutions-per-minute
(RPM). The red numbers at the end on the scale show the maximum
permissible RPM's. Ease off on the accelerator before reaching the red
area".

Hmmm - I guess technically you could go above redline before the forced
6600 rpm upshift (as if you can cruise all day at 6499.5 rpm, but the
engine explodes at 6500.5 rpm). 8^)

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

Bill, according to this article fatique isn't an issue and the reason is
explained. http://www.eaa1000.av.org/technicl/rodbolts/rodbolts.htm
This also talks about stress at TDC both at WOT and closed throttle, but
numbers are simply thrown out with no calculations, so I'm not real
confident in them.

Matt

 

It causes much higher forces on the crank and associated bearings over a
longer time period than the same power output at higher RPM. This can
wipe out bearings that aren't designed to withstand such loads. This is
what happened in the example I gave of a Porsche with a roller-bearing
crank... notice that today they use conventional babbited bearings...
the roller bearings gave less friction, but at the cost of the ability
to withstand occasional lugging.

nate

 

One would hope that the Chrysler engineers added in a little something
for a safety factor. To account for lag and inaccuracies in the
tachometers for one thing, unless this has a completely digital tachometer.

I'm still searching, but have yet to find anything that talks about
problems running at slightly under redline for any length of time...


Matt