Why 2650rpm max?

[gimpyrobb]

Outside of the FDC the engine is a basic diesel engine. It is the only major change. I never had a chance to research if the MAN multifuel diesel engines have similar failures.

That is wrong. The FDC ONLY adjust power output per fuel's btu content. The piston design of the motor and the angle of the injectors is what makes it a multifuel. You don't need the FDC to run fuels other than diesel.



I don't believe the turbo is a limiting factor, as there is a "D" turbo on my LDS motor.

 

[mktopside]

I think the exact opposite there is much less stress on the whole assembly on the exhaust stroke. The piston is traveling up and the valve is open relieving most of the resistence. Where on the power stroke the the cylinder has just fired causing extreme preasure to be put on the rotating assembly forcing the piston back down with no relief other than the volume of the cylinder inlarging. I agree that piston size has a huge affect on the rods failing just trying to understand your exhaust stroke logic.

It's not the pressure in the cylinder that breaks the rod, it's the force of it being thrust upwards with little resistance, the coming to a complete stop.

If you take a rope and attach a brick to the end and drop it off a building, then two bricks, then three bricks, then four....etc. Eventually the role will break. Crude parallel, but similar to what I'm talking about.

If this were not true, you wouldn't see a correlation between engine failure and high rpm. If you take detonation out of the equation (detonation destroys stuff for sure, but it is a separate issue) each individual power stroke is not all that much more powerful than the one before it, and the forces would be no different at full throttle from idle than at redline. If you break a rod, and the rod bearing is intact (rod bearings will flat spot from "too much power" and from detonation before the rod come close to failure), the cap bolts are intact, and the piston shows no sign of detonation; the rod broke on the exhaust stroke and the root cause is poor mechanical design, or extreme over revving.

I guess what I am trying to say, is that there are numerous other weaker components that will fail on the power stroke long before the rod gets close to failing.

I know nothing about these rods, but I wonder if having a set shot peened might be enough to give you the extra rpm your looking for?

 

[eric_banks32]

It's not the pressure in the cylinder that breaks the rod, it's the force of it being thrust upwards with little resistance, the coming to a complete stop.

If you take a rope and attach a brick to the end and drop it off a building, then two bricks, then three bricks, then four....etc. Eventually the role will break. Crude parallel, but similar to what I'm talking about.

If this were not true, you wouldn't see a correlation between engine failure and high rpm. If you take detonation out of the equation (detonation destroys stuff for sure, but it is a separate issue) each individual power stroke is not all that much more powerful than the one before it, and the forces would be no different at full throttle from idle than at redline. If you break a rod, and the rod bearing is intact (rod bearings will flat spot from "too much power" and from detonation before the rod come close to failure), the cap bolts are intact, and the piston shows no sign of detonation; the rod broke on the exhaust stroke and the root cause is poor mechanical design, or extreme over revving.

I guess what I am trying to say, is that there are numerous other weaker components that will fail on the power stroke long before the rod gets close to failing.

I know nothing about these rods, but I wonder if having a set shot peened might be enough to give you the extra rpm your looking for?

Ok. I see your point now. Thanks for the explanation.

 

[dragonwagon]

Im surprised no one has mentioned balance . Rod/crank and piston assembles are balanced to run at a certain rpm .

Tire dont bounce at 30 but it does at 50 or 60 , blance issue . Im sure it comes into play on there motors as well . Rod strenght and design too . Several factors combined im sure .

Assemebles let go on the exhaust stroke !! How can you tell after a motor has let go as to which stroke it was on ?

I too was a certified mech for many years , non diesel . Havent a clue how your going to know short of high speed film of a failing assemble .

 

[m16ty]

I'm not disagreeing on which stroke the rod breaks on but I've got to ask, how in the world can you tell what stroke it was on when the rod let go? I've never seen a engine throw a rod and stop dead in it's tracks.

The mutifuel is a pretty high rpm engine for it's size and the era in which it was built. Most diesel engines of it's era max out in the low 2k range. I'm sure you could build a mutifuel to do 5k rpm but at the end of the day it's going to be much cheaper and easier just to swap it out with a more modern, more powerful engine.

 

[mckeeranger]

This has nothing to do with the engines in question, but many of the modern top fuel drag engines record enough information to tell when something failed.

For example: You can see the exact crank position when one of the variables changes, such as air consumption, compression pressure, combustion pressure, or exhaust temperature of a particular cylinder.

I know, it's not MV related. I just thought I would throw it out there, since the question of how you could measure such a thing came up.

 

[SCSG-G4]

Well, looks like I need to take one of my LDS-1A pistons to the Post Office and weigh it, then have someone else do the same on an LDT piston.

 

[mktopside]

I'm not disagreeing on which stroke the rod breaks on but I've got to ask, how in the world can you tell what stroke it was on when the rod let go? I've never seen a engine throw a rod and stop dead in it's tracks.

The mutifuel is a pretty high rpm engine for it's size and the era in which it was built. Most diesel engines of it's era max out in the low 2k range. I'm sure you could build a mutifuel to do 5k rpm but at the end of the day it's going to be much cheaper and easier just to swap it out with a more modern, more powerful engine.

By looking at the other end of the rod that isn't still attached to the crankshaft. It's not a sure thing, as stuff often get's smashed around.

I have a bag of pieces somewhere of a "powder cast, pressure forged, fracture split" type rod that shattered into 20-30 pieces when a customer went from 5th to 2nd at 90mph, instead of 5th to 4th. It floated the valves and invited the demons inside. There was no telling what stroke that rod broke on, but high speed camera footage of that would have been a sight to see.

 

[SCSG-G4]

LDS-1A piston and wrist pin weigh 7 pounds, 2.2 ounces on the local Post Office scale. Verified by using some other items (one ounce weight and a 4 pound 9.1 ounce trigger weight) as a cross-check. Now to find out what the LDT piston weighs!

 

[Kohburn]

rod failure modes not from RPM happen on compression stroke:
-pre-ignition or detonation causes rod bend or fracture
-rod bearing failure resulting int he piston impacting the head - end result is failed piston and or rod

Failure modes from high RPM happen at the transition from exhaust to intake stroke:
-rod stretches, later resulting in the piston impacting the heads. Eventually leads to either shattered piston or bent/fractured rod.
-rod cap or bolt is the weak link resulting in fractured rod detaching from the crank.

the RPM failures happen on the exhaust/intake stroke because there is no resustance against the piston, no cusion of air that is being compressed to act as a spring pushing back.

 

[jimk]

Metal is a lot stronger in compression than tension. Power compresses the rod (where it is strong). The piston reversal at TDC puts the rod and rod bolts in tension. As RPM increases the force increases at an exponential rate. The crankshaft rotation after TDC on the intake stroke puts the most stress on rod/rod bolts. This is why/where a lot of rods/rod bolts fail. Tension can also cause the some big ends to go out of round at extreme RPM. The big end can deform making the circle longer and narrower. It takes only .001"deformation to turn a rod bearing into an oil wiper.


The LDS-465-1 has a no-load high speed governor setting of 2850-2900rpm. This is not the same as the red-line. The rated power is listed in the LDS troubleshooting manual at 170-185 at 2600rpm. The output chart, and other text from this publication(avail in SS resources, my hard copy,dated June 68' below) are using 2600rpm as max operating range.

 

[jaxsof]

And nobody seemed to remember that it was built by the lowest bidder! Or that long stroke engines inherently have a lower max RPM. There are marine diesels out there that have a red-line of 60 RPM.

And the cross section of the crown of that piston is way smaller than it is through the rod, its bolts or cap.

The Cummins does have a little more money involved in the R&D than the (White-Hercules) LD's, and judging by the mileage on the military vehicle I've personally had experience with, about a billion more miles to work out the bugs. Uncle Sugar bought the absolute least complicated machines he could, because simple is reliable, and easier to fix. That’s why our 5-tons aren't turbocharged.

I don’t know of any OTR commercial application of the LD's.

If you want to make a race car out of your Deuce, I’m sure you could, but why, for the love of Pete, would you want to be the first to put that kind of money into a machine that even the designers didn’t think was worth it? I have a friend with an MB(early 1942 Ford) Jeep, when rebuilding the engine found the pistons were ounces out of balance. Ran OK for the GI’s to get it, as long as you respected the max speed (or it would THROW A ROD!).

I really know that 50 isn’t a great speed on the Interstate, but if you really want to take your really high, 13000lb truck that was intended to spend 80% of its time off road, fast, look into replacing the whole drive train. Get an engine with at least some R&D time, a transmission that can take a little more torque and is easier to find parts for, and axles that aren’t geared to drag the moon out of orbit. But then it wouldn’t be as much fun, would it?

Just another .

 

[stumps]

I wonder what the results would be to put a rod in a press using some THICK wall pipe to simulate the wrist pin and the crank and spread her till she lets go.

If your intention was to see which failed first, the rod bolts, or the cap, that would be a very interesting test.... but maybe not all that useful.

From looking at the results, the group consensus has been that rod bolt failure is what caused the problem. What we don't know with any certainty is why. There are several reasons a rod bolt could fail:

1) overstress by design (designer fault)
2) weak example of a rod bolt (manufacturer fault)
3) over torque at installation (mechanic fault)
4) reused bolt stressed from repeated torquing at installation.
5) overpower of engine (in excess of design)
6) over speed of engine (in excess of design)

Your destructive test could help tell you which of two rods would have failed first, but it wouldn't tell you whether or not a given sample would have failed in normal operation.

-Chuck

 

[merlot566jka]

great discussion! im writing a response and i will post in a few hrs. i am currently out to sea in the middle east area and its difficult to get time on a computer.

 

[jimk]

Chuck makes many good points. The bolt tension during operation is added to the tension on the assembled bolt. Over-tightening increses this tension even more, as does excessive rpm.

 

[stumps]

One thing I have long thought is it would only take a couple of mechanics, in the rebuilding factories, that were over tightening rod bolts to create a whole lot of time bombs, just waiting [for us to go roaring down the road at 56MPH] to explode.

The same goes, only even more so, for a load of counterfeit rod bolts.

-Chuck

 

[JasonS]

ThThe piston design of the motor and the angle of the injectors is what makes it a multifuel. .

This is not entirely true. The internal combustion engine "bible" by Taylor indicates that high compression ratio alone is sufficient to enable multifuel capability. The MAN combustion is probably just icing on the cake.

 

[merlot566jka]

From Tom Monroe:..." Failure of either on of these [rod bolts] usually means catastrophic engine failure. For rod bolts this typically occures when the rods are stressed in tension to the maximum, or just after the piston goes over TDC during the intake/exhaust overlap period. The crank pin pulls the rod-and-piston assembly down against the inertia of the whole assembly when pressure in the cylider is lowerst, creating maximum tension load on the rod...... You can prevent the above form happening by chosing rod bolts wisely, then being relentless as a Marine Corps DI during inspection. Rod manufactures provide information and support necessary to ensure inspection and installation are done correctly. They know from experience that if a rod failure occures, it will be rod bolt related due to an error made during inspection or installation. Other than minor bending stress that occures when the rod is in tension and compression as the big-end bore distorts to a slight oval shape, the onlu additional load a rod bolt should see is static tension from tightening during installation. this makes it critical that rod-bolts are preloaded as accurately as possible, or about 80% of yield strength. This eliminates cyclical stress, which also eliminates fatigue sress and ultimate bolt failure. so a bolt that is not tightened enough wll break before one that is over-tightened provided the bearing dowesnt spin first. this is caused by bearing crush relieved as the bolts stretch, alllowing the cap to seperate from the rod."

I think jimk hit it on the head!

Chuck, perhaps destructive testing isnt the best idea. I work in the aviation maint field and have access to many NDI (non destructive inspection) lab tests from magnetic to xray. I doubt i could make a metalurgical analysis, but could def gain more knowledge of the stock rod bolts and rods themselvsand i think its backwards...un torqued bolts fail first, over torqued fails over time. but an analysis of the bolts would show how they broke, either over stress or fatuige.

Mike, thanks for the data! I will find a formula to crank out the G-forces that are being had on the rod/pin/crank. from that we wll be able to see if thier is a great difference between lds and ldt pitons as far as weight and acceleration is concerned.

All, yeah its kind of hard to tell what broke first. to me its a chicken and the egg thing. but you can tell what broke from what kind of damage. you can find stresses, burns, impacts, and fatiuge. From there, deductive reasoning can be applied.

to add, i havent seen any pictures of the big end still attatched to the crank pin and the rod actually snapped at any place. the pictures i have seen look like they are all big end, rod cap or bolt failures.

also, i did touch on balance in my first post, it is a fairly important issue, but by the looks of things through discussion, the balance doesnt appear to be the problem.

Thanks everyone for all the good inputs, keep it coming.

and if anyone has a snapped rod and bolt they would like to have analyized, pm me and I will give you the address to my ship and you can send it out!

 

[eric_banks32]

If your intention was to see which failed first, the rod bolts, or the cap, that would be a very interesting test.... but maybe not all that useful.

From looking at the results, the group consensus has been that rod bolt failure is what caused the problem. What we don't know with any certainty is why. There are several reasons a rod bolt could fail:

1) overstress by design (designer fault)
2) weak example of a rod bolt (manufacturer fault)
3) over torque at installation (mechanic fault)
4) reused bolt stressed from repeated torquing at installation.
5) overpower of engine (in excess of design)
6) over speed of engine (in excess of design)

Your destructive test could help tell you which of two rods would have failed first, but it wouldn't tell you whether or not a given sample would have failed in normal operation.

-Chuck

Yes, that is my intention. If it is infact the bolt that breaks, It would be easy enough to address. Some top notch ARP bolts or the like would likely take care of the problem. If it isn't the bolt and the rod is the problem then that leaves few if any realistic options to fix it.

 

[merlot566jka]

its easy and cheap for new rod bolts.

rods...well it would take some investigation to see if a manufacture makes something similar. but i would be willing to bet that any rod /piston manufacturer would build a rod to suit. price may be un-earthly. but quality would be significantly better.