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")

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

nate

Reply to
Nathan Nagel

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")

Reply to
Bill Putney

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")

Reply to
Bill Putney

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.
Reply to
Steve

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.

Reply to
Steve

I don't own a cranky Porsche. :-)

What happens? And why?

Matt

Reply to
Matthew S. Whiting

What's the redline for this engine?

Matt

Reply to
Matthew S. Whiting

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

Reply to
Matthew S. Whiting

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

Reply to
Matthew S. Whiting

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

Matt

Reply to
Matthew S. Whiting

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

Reply to
Matthew S. Whiting

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")

Reply to
Bill Putney

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")

Reply to
Bill Putney

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")

Reply to
Bill Putney

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:

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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:

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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

Reply to
Matthew S. Whiting

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")

Reply to
Bill Putney

Bill, according to this article fatique isn't an issue and the reason is explained.

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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

Reply to
Matthew S. Whiting

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

Reply to
Nathan Nagel

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

Reply to
Matthew S. Whiting

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