Stuck diesel fuel injector causing crankshaft to break??

[frankiee]

On a Detroit diesel "Series 92", will a stuck fuel injector cause a crank shaft to completely shear off? The break occurred just 2 inches back from the damper.
The engine had 25 hours use on it from an "engine rebuild".
It was at my work so I do not know all the details of what was done to the engine.
The engine drives a 150 kw generator.
When I got back to work, the generator had a failure. When we took it apart we discovered that the exciter armature had been rubbing against the stator and grounded out. We sent it away suspecting a cracked or bent shaft caused by the crank when it broke.
So my question is, has anybody heard of cranks breaking because of sticking injectors of a diesel?

 

[ivymike]

overfueling could conceivably result in increased crank torsional vibration, but I would think that 25hrs is far too short a time for a high-cycle fatigue failure due to TV, and you'd probably see torched pistons too (which you didn't mention).

Was the crank replaced or just refinished during the rebuild? Replaced with a reman crank? 2" from the damper is probably about where the damper bolts end...

 

[Fabrico]

Crankshafts are pretty stout on the 92's. I would take a very close look at that damper. If it's the same as trucks, it is a viscous unit. Look for ANY sign of fluid seeping out. Shake it and feel if anything moves inside. Look for any dents. If you find anything wrong with the damper it could be your problem. Also check the front pulley for major runout.

Otherwise, someone probably installed a crank that was not fully inspected and just waiting to let go. I highly doubt it was from any problem with the fuel system.

 

[Metalguy]

Any good photos of one of the fractured surfaces?

"When the Eagles Fall Silent
The Parrots Start Jabbering".

W. Churchill

 

[frankiee]

It was on ship and I regret now that I did not borrow a camera to get some pix.
The damper was checked by the mechanic after the fact and he said that it was good. Our chief engineer checked it also and said it was good. The damper was replaced regardless.
I am not on ship for another month and by the time I get back it would be Xmas so I don't think I can get a pic. Sorry.

 

[GregLocock]

How about if the injector was stuck open - could it have hydrauliced the engine? If it was stuck closed - could the TVs have broken the crank in only 25 hours?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ivymike]

To the former - not w/o collateral damage, and the cranknose isn't where I'd expect the crank to break if it did at all (I'd expect a bent rod, busted piston, and ventilated block first).

To the latter - not by my guess. Even if the engine were running in the "worst case" condition continuously, it'd probably take a couple hundred hours or so.

 

[rob768]

A crankshaft does not break due to a cylinder misfiring, since the natural frequency of a crankshaft for torsion is quite high and is not excited by a low order exciting force of a 0.5th order frequency (which is related to misfiring).

For all those doubting that a period of 25 hours isn't enough to cause a fatigue fracture:
I assume the installation is running in the US? at 1800 rpm?
25 hours x 60 minutes x 1800rev/min = 2.700.000 cyles. Tha's the number of rotations. Since a crankshaft usually failes to a higher order frequency, multiply that number by the critical order numner. More than enough to cause a fatigue fracture.

 

[ivymike]

sure, if every single rotating cycle sees several instances of damaging load...

 

[rob768]

A generator usually has a failry constant load and I assumed the number of running hours are related to loaded conditions. In the case of vibration problems, the damaging load is depending only on running speed (operation at or near a critical speed), which is constant in case of generator use.
Still, it was intended to show the number of cylcles you get after "only" a number of hours or days.

 

[ivymike]

In the case of vibration problems, the damaging load is depending only on running speed (operation at or near a critical speed)

That's not true, it's also dependent upon the shape of the cylinder pressure traces (harmonic content and amplitudes), which is related to engine load, injection timing, duration, etc. Engine load will be related to electrical load, and will likely fluctuate.

In the case of misfiring due to electronic problems, it is not uncommon for a "bad" event to happen once every few minutes, rather than once per firing cycle, with cyl. pres. excursions on the order of 2x normal (bust your pressure pickup and hear it outside the test cell type pressures). This usually shows up in busted rings, bent rods, etc., a while before the crank lets go.

In the case of a stuck-closed injector on an I6, I wouldn't really have expected the crank to care. On larger gensets (16-24cyl), a single dead cylinder can have terrible implications for crank vibration (and analysis is often done for a particular application assuming one dead cylinder at a time up and down both banks, with a damper selected to help overcome the worst scenario).

A bad coupling could conceivably be a problem too, but I'd guess this application uses a single-bearing genset with a fairly stiff (torsionally) discs-o-steel flex-plate coupling.

 

[rob768]

Harmonic amplitudes contain the higher orders only. We're talking about a given condition in which one cylinder misfires (constantly), which generates an additional 0.5th order. The resulting torsional vibration is only slightly dependant on actual load, provinding there is a more or less contant base load (and no fluctuations between 0 and maximum).
I'd look for an defective vibration damper, as misfiring conditions are usually part of a torsional vibration calculation and this more or less rules out problems due to misfiring (provided a calculation has been made).

unless it is a common rail engine, injection timing and duration is fairly constant and independant on load.

 

[patprimmer]

As the history on the crank is unknown, it sounds very much to me like the crank could have been cracked before it was installed.



Regards

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[Fabrico]

BTW, 92's are V configuration not inline. To further answer Greg's question, it is not rare to have a mechanic steam clean an engine and get water into it. This was an especially big danger on the pancake (sideways) mounted engines. I've seen at least a dozen engines hydrauliced. In every case but one, it bent a rod or two, or three. When pretzled good enough they take a bite out of the liner or cylinder wall while they're at it. In the one other case the starter gear broke. Never saw it break a crank. Leave it running while cleaning!

 

[GregLocock]

rob768, regular one cylinder misfire creates a harmonic series at every multiple of 1/2 order. Typically the (2*n+1)/2 series is of higher amplitude than the (2*n/2) one, due to the shape of the missing pressure pulse.

Admittedly the higher harmonics are of small amplitude, but they are there.







Cheers

Greg Locock

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[Majik]

Most of the time when we see crank failures like this where the crank snout breaks then it's usually a sideloaded front pulley setup from extra accessories non factory installed such as hydralic pumps or generators bolted to the snout or driven off the side. I'm going to assume this is a commercial generator that's driven from the rear of the engine. Yeah before I'm gonna jump on the cylinder harmonic distruction process.. I'm saying whoever rebuilt it. Didn't chuck it up on a lathe and tap it with a hammer. The fracture was on the unloaded side of the crank so unless there was any abnormal accessories being driven off the crank, I'm guessing previous failure that was undetected during the rebuild. If an injector was stuck open the thing would be pouring out white smoke and sound like hammered crap. A deaf man could feel it through the pavement a cylinder not firing on a 92 series detroit. Eventually the injector would either hydrolock and bend a rod, or break a piston and ventilate the block as mike said. If the crank were to break it would be by one of the main journals not the snout. Snout failures are almost always dead balancer, sideloaded snout or extra accessories not common.

 

[patprimmer]

If in handling, the engine is dropped so that the snout takes the full impact load.



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[patprimmer]

Is it supercharged, if so is the supercharger belt driven off the snout.

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[Fabrico]

It's a 2-Stroke so it is supercharged. But Detroit blowers are gear driven from the big mass of gears that drive everything else.

Could be too that the previous owner of the crank had something incorrect at that end.

Detroit cranks "look" right, wear like diamonds, and are durable as all heck. But I've seen many more of them break for no apparent reason than other brands. Perhaps they are too hard, or perhaps it's because they are a 2-stroke.

 

[frankiee]

I am sorry I can not add more information to the topic then this.
V-8 blown 2 stroke engine that has a 150 kw generator driven by rear of engine.
I do not know the extent of the rebuild done on the engine.
Both generators in the ship were done at the same time. Both engines only drive 150 kw generators and have no accessories attached to them. There are 2 other detroits in the engine room that handle stuff like fire pump and bow thruster etc.
The second engine did experience a leakey front crank seal about 250 hours after the rebuild which we replaced.
I do not know if the engines were even taken out of the ship.
I wonder if someone did not jack up the engine by the damper in effort to get more room from the engine and the bottom of the bilge.(Naw, to crazy)(Although I admit doing it to car engines in my younger day)

I think I will assume that the break did not come from the stuck injector and it did come from some abuse to the front of the crank or a failed damper.
They are old engines and who knows where they have been.
I have only been on this ship part time for 3 years and the history of equiptment is unknown to me.
Thanks for all your input.