Crack do overspeed

Related Projects. Hi Folks, I recently was asked by a client to investigate a catastrophic overspeed failure on a Dorman V12 diesel engine. The unit provides standby power for a UPS system. The claim is that the fuel rack stuck then went to full fuel, resulting in the engine over-revving, to the point that pistons met valves and seized. The overspeed switch is mounted on the suction side of the turbo blower and was proved to be operational after the rebuild.

The question is why? Why did the overspeed not stop it, the governor not shut the fuel off etc. Has anyone ever experienced this before and has anyone any other ideas as to why the engine should overspeed in this manner?

I do recall a similar frightening experience, many years ago as a trainee in the infernal combustion engine business. I was working next to a big V8 on dyno test when I heard a loud crack and the revs began to build rapidly, the operator pulled the injector pump's control lever to the shut-off position but the revs continued to increase.

He grabbed a couple of 7lb tins of grease from a rack next to him and threw one to me, indicating what he wanted to do as it was way too noisy to shout. We offered the bases of the tins up to the inductiom manifold faces, they fitted well and suffocated the engine before it flew apart. All the big-end bolts had stretched and there were marks on the top of the pistons where the valves were beginning to float and touch. It's over forty years ago now and I've forgotten the details, but we were lucky that the tins bottoms were the right size and survived the suction, albeit well bowed!

It turned out that the rack on the New pump had a faulty heat treatment and had broken at the shut-off lever end, allowing the spring bias on the rack to move all the delivery cams to Max Fuel. Do you think it might be better to move your post to the "Automotive Engineers" forum on this site? How exactly does the overspeed device shut off the engine?

If it's relying on the same governor mechanism then possibly it could not overcome the mechanical failure. If it is an independent fuel shutoff, perhaps there is enough fuel inside the fuel system between the shutoff and the main injection plungers for the engine to blow itself apart. But, there is another possibility. Diesel engines can suffer from a "runaway" condition if they ingest lubricating oil, for example from failed valve stem seals or piston rings or crankcase vent or turbocharger seals.

Look for evidence of excess lubricating oil in the intake system where there shouldn't be any. A similar run away cause a huge explosion and fire a few years back on a gasoline barge when the fumes made it into the induction system. Staying with the theme of unexpected sources of energy going into the system IFR's example is by no means the first time a flammable atmosphere has made a DG run away to destruction , it's probably worth considering briefly the possibility of an electrical fault having caused the generator to motor and backdrive the diesel.

How would a motoring event lead to an overspeed? They lead to all sorts of bad things but an overspeed is definitely a new one to me. Is it possible the blower is faulty? If the overspeed switch is on the suction side, and the blower is kaput, cant the engine still over rev if the switch isnt reading enough vacuum as generated by the turbo? I would guess a good place to start asking questions would be with the motor manufacturer.

Scotty, I think the mechanism for a motoring event would need to have something to do with a multi-pole machine losing some windings - unlikely enough to be able to be eliminated pretty quickly, but maybe worth a moment's thought. The old 2-cycle Detriot diesels or Jimmys as they were known were famous for running away. A part of the design was an air flap in the air intake for emergency shutdown. These machines used a scavenging blower and a lube oil leak in a blower seal would fuel the engine to destruction.

Many contemporary gen sets have the over speed device close the fuel solenoid. Some move the rack to the zero speed position. The old "Jimmys" used a centrifugal trip to release the air flap and cut off the combustion air on gen sets. Truck ad equipment engines had a manual release controlled by the operator. If the engine has been rebuilt and the shutdown switch is working now, it is probably too late to determine the cause of the wreck.

A sticking rack is more probable than a rack going full open. Had a unit with a sticking rack. The set would pick up the load fine. When the electrical load was reduced the rack would not return. The over speed device would close the fuel solenoid to stop the engine. Had the overspeed device failed the engine would probably self destructed. It is obvious that the motor overspeeded. A combination of a rack problem and a failed overspeed device could be the cause.

However, that takes two simultaneous failures. It may happen but the odds favor one failure. That could be an alternate source of fuel that was not under the control of the rack or the fuel shutoff valve. A common source of alternate fuel is a failed seal in a turbo or blower that allows lube oil to enter the air inlet and fuel the engine.

I doubt that you will ever find out, but I also believe that some one or two does have a good idea of the cause. Too late now for much investigation. Bill "Why not the best? The thing that would usually be mounted on the suction side of the blower is an air shutoff valve, biased to the closed position by a serious spring, and held open except in emergency by a mechanical latch that can be triggered manually or by means of a solenoid Some people have told me that overspeed valves that autonomously measure or sense the air flow and trip themselves closed on excess flow exist, but I do not recall ever seeing or touching one.

An air shutoff so positioned could not stop a runaway if the turbo blower case were substantially breached, e. Such a scenario would seem plausible, but good luck proving it from the remains of the engine Thanks Folks, To further explain the engine was undergoing routine maintenance when the failure occurred. The engine was running at rpm normal running speed rpm and as the engineer was checking why it wasnt at normal speed it ran away and failed. It is suspected that the fuel rack stuck and then "unstuck" going to full fuel.

My client has asked me to investigate as an independent engineer with the remit to finding out why and more importantly what he should do to prevent a re-occurrence. The engine was rebuilt and everything checks out correctly ie overspeed switch ok, turbo ok etc. Hence my query Thanks for all your contributions. I just wondered whether you might have been a lawyer. Or worse, some insurance claim investigator. A lawyer or claims adjuster!!

Nope lube oil in the veins, bones made from stainless steel and skin formed from thistlebond patches That overspeed trip setup is, um, not completely right. You can't reliably stop a runaway by cutting off the fuel supply. Have they been using that fuel shutoff solenoid valve as the normal shutdown means also?

If so, bad idea. The fuel rack could have stuck and stayed stuck. At RPM your frequency would have been below 40 Hz. The UPS may have interpreted this as a failure and went to batteries.

Or some protection may have tripped the generator breaker. That could have triggered a load loss. A load loss with a stuck rack would certainly be capable of over speeding an engine. The old Jimmys of the 40's had an air flap and a mechanical centrifugal device to release the air flap.

Not enough airflow from the turbocharger to the engine. Reasons for this could be a torn air pipe, blocked air intake or a leak between the compressor and the engine. Any chipping or remapping work to an engine must be carried out by a professional. Incorrect airflow Poor re-mapping work.

One of the main components that will show signs of Overspeeding is the Compressor Wheel. When looking for the orange peel effect on a Compressor wheel, you may need to adjust the lighting and the angle of the compressor wheel. Extreme cases of Overspeeding, you could see inducer blade damage, partial loss of blades, fatigue fractures or a bust Compressor Wheel. Overspeeding can show the same signs of failure as Insufficient Lubrication.

Take care when analysing all the major components, before diagnosing the failure, if you need help with this, check out our Diagnostic tool on the website. All the information in this video is available in a downloadable PDF on our website. Here, you will find useful tips on how to prevent Overspeeding. Melett Turbo Tip — Interesting Fact — When a turbocharger operates at high altitude, there is less oxygen, resulting in the turbocharger working harder.

This can give the effect of Overspeeding and over time the turbocharger will fail. Enjoyed our Video? Press the like button, leave us a comment below or subscribe to our YouTube Channel to watch further videos from Melett.

When the compressor wheel overspeeds it grows in size. This expansion causes cracks between the grain boundaries of the material. In mild cases the component will return back to its original state like elastic but in most cases these cracks begin to grow and eventually part of the hub can break away. Quite often overspeeding is overlooked as a cause of the turbo failure as the symptoms of other failures can occur as a result of this overspeeding.

Material transfer and discolouration of parts may indicate a lack of lubrication. Scoring to parts could indicate oil contamination, however the particles that have caused the scoring could have broken away from the bearings as a result of the overspeeding and any imbalance caused by this. This imbalance can also cause compressor rub and turbine wheel rub in the housings, which in turn can lead to the shaft snapping and loss to part of the inducer blades. All in all overspeeding causes a lot of damage and is often the primary failure mode!

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