Diesel Generator - Engine Running Away

If its a fuel injected engine then the common rail feeding the injectors is set to be at a specific pressure, this is maintained by the return valve opening to reduce the pressure and return the unused fuel to the tank.

So if you close the return fuel line to the tank the pressure will now be unregulated but still at a high pressure to keep the injectors operating and the engine running, although the operating efficiency will be dubious.

John.

In my experience, blocking the fuel return line will cause full, high pressure to the common feed rail, regardless of throttle setting. . No bypass= full pressure to injectors= runaway. It will run rich, but run away none-the-less. There should be NO shut-off valves in the fuel return line.

A natural gas, propane or anything else that will burn including dust can cause a run away.

Gordon

Fuel injection for gasoline (or similar fuel) engines has to meter the amount of fuel injected in to the motor at a given ratio of air intake to the engine.

One popular method of metering the fuel (modern electronic fuel injection on passenger cars) is to provide a constant pressure on the common fuel rail and electrically meter the fuel by pulse width modulation of the of the electromechanical injectors.

Another method, used for over half a century, is used on high performance race engines and it is completely mechanical. The fuel is pumped by a volume gear pump (the volume is proportional to the engine RPM) driven of the shaft of the engine and a fuel return valve is mechanicaly proportionaly linked to the air intake throtle valve. This system provides a poorly controled ratio of fuel and air mixture and is completely mechanical.

In either of these methods, if the return fuel valve is closed, the engine will probably run to rich on fuel and stop running.

Therefore no run-away.

Works for gas, which uses an air throttle, but diesel air is always wide open, fuel is the only variable effected by the fhrottle. Diesels run fine on the natuiral gas at sites where a gas line is broken during excavation. The only air throttle on a fre diesels is an air dam to completely shut down in an emergency.

Rich

A SMALL AMOUNT OF FUEL OIL HAD ENTERED THE SCAVENGE AIR COOLER , WHILE THE ENGINE WAS OFF, DUE TO A PREVIOUS MALFUNCTION, BEFORE RESTARTING THE FAULTY INJECTOR WAS REPLACED AND FUEL OIL WAS DRAINED FROM THE AIR AND EXHAUST PASSAGES THE SCAVENGE AIR COOLER WAS ONLY DRAINED, BUT THE COOLER SHOULD HAVE BEEN WASHED WITH WATER. THE FUMES GENERATED WHEN THE COOLER HEATED UP AFTER STARTING THE ENGINE, CAUSED ENGINE TO OVER SPEED . LUCKLY NO DAMAGE TOOK PLACE.

I agree with Guest that any un-metered introduction of a combustible to the intake of a diesel will cause a runaway (due to the absence of a throttle valve in the diesel engine), but I do not see how a blocked fuel return line could cause this malfunction. The fuel return exists because excess fuel is supplied for the purpose of providing cooling and lubrication to the injection pump and the fuel injectors, and if this flow was interrupted then the engine would slow or stop because of a failure of one of the components. Also, a malfunction or maladjustment of the engine governor can lead to a runaway. For some insight into runaway diesels, try http://www.mercedesshop.com/Wikka/RunAwayDiesel. They say it as well as I could.

A diesel engine does not have a throtle control like a gasoline engine.

On a diesel engine, the air intake is not restricted (it intakes 100% air all the time) therfore the speed and power output of a diesel is controled by the amount of fuel injected directly into the cylinder at the top of compression. This is very high pressure and the volume is controled mechanically.

Thus, if the return fuel valve is closed, the diesel engine should not be able to run away but over-pressure on the fuel supply and return lines may get to great and cause a line to blow or leak.

Diesel engine run-away can be caused by fuel vapors or other types of combustible materials that may be sucked into the intake where 100% air is inhailed. This can cause an uncontroled source of fuel to be taken into the engine and then the engine runs away. If this happens, the only way to stop or control the engine is to shut-off or restrict the air intake to the engine or remove the source of the uncontroled fuel intake. Either way, unless you are instantly ready, you probably will loose the battle.

What ever the case, diesel run-away is dangerous!!!

Sooooo be careful.

Hi, I experienced a runaway many years ago on a diesel powered combined emergency fire pump and air compressor on a ship. Because it was always on standby the fuel cock was always open. Over time fuel seeped/dripped and contaminated the sump oil to such an extent that the engine ran away during a test run. It was running on it's own sump oil. We finally managed to stop it by stalling it using the air compressor!!!

I once saw a ford transit blow itself up on , I think it was the M1 near Luton.

If the guy that was running away in fear had taken the keys out, what would cause the running away of the engine? Isn't the fuel pump electrical and therefor would stop the fuel supply alltogether?

All this is still a bit of a mystery to me but then again, I'm no car mechanic. I would like to know the definitive answer though, just out of interest.

The fuel pump is usually part of the fuel distribution system and runs off the engine timing gear drives.

Assuming keys are removed this should shut off diesel to the pump and normally should stop the engine, however if the engine had overheated and had leaky piston oil rings there would be enough oil to burn to cause a runaway engine.

Too much oil caused this one.

My son had his VW diesel engine run away. It was an older diesel engine with over 200,000 Km on it. When he checked the oil on a dark night (on the way to see his girlfriend) he thought it needed another quart. It didn't!

Shortly after adding the extra oil he described it as 'riding a rocket' and after pulling over on the Don Valley in Toronto, fire trucks, police, massive amounts of smoke and dangerously using a rag to block the air intake, the beast was contained.

After a tow to the VW dealer and a change of oil and filter, new air filter, (the other was completely oil soaked) the car was back on the road. Another two years with no problems before he traded cars.

Too much oil is not a good thing!

Brian

My older VW Rabbit Diesel used to run away if I pushed it too hard up long mountain grades. I was sucking lube oil (not overfilled) past the worn rings. It burned nearly a quart in a couple of minutes, so left to itself, I presume that it would have drunk itself dry and burned up the engine.

My solution: no throttle, 5th gear, and let it try to climb the mountain. It stalled quite quickly. I let it cool down, drove to the next service station and topped of the oil. No harm done.

BYW - this was a repeatable experiment!

In my 33 year diesel engine career I've never tried it. But I say 'No'.

If you have a Bosch type scroll metering injector (used in most non electronic systems) blocking the return line would increase the back-pressure of the leak off port of the plunger and barrel design, however, laws of physics would win the pressure fight and the higher leak port back pressure would be insignificant and still allow the critical 'plunger effective stroke pressure bleed off' to take place which is the very basis of the throttle injection volume concept.

Blocking or restricting the return line will have a dramatic effect on fuel temperature and this is the real concern as injectors are cooled by fuel flow.

Your question is theoretical unless you ask it this way: "Does anyone have direct experience with model XYZ diesel engine having an overspeed caused by blocking the fuel return line ?"

Be more specific about the reason you ask such a question. Did you have an overspeed? Are you arguing about an installation where someone wants a valve in the return line? Settling a bet? Just curious? What is the engine make and model and year of manufacture?

Since I've never tried it I'm making an educated guess.

Closing the 'fuel return line' on a mechanically injected diesel WILL NOT cause a 'run-away' condition. It MAY slightly increase the injector delivery/shot due to a slight increase in fuel feed pressure (which is 150 times LOWER than injector delivery pressure. For example, fuel supply pressure = 15 psig, injector opening pressure =2500 psig--is a very common set-up. Closing off the fuel return line MIGHT increase the supply pressure to 25 psig. That will not cause any change in the fuel injected into the combustion chambers. Leakage past the injector plungers is normally routed back via very low pressure connections (slip-on rubber tubes) and those would pop-off.

Closing the fuel return line on a 'common rail' injected diesel also will not cause a 'run-away' condition since fuel delivery is electronically regulated. However, in this case the excess high pressure will probably rupture the 'low pressure' return line. Typical 'common rail' pressure is 25000 psi (25000 psig!), and some of that bleeds off back into the fuel tank via the pressure regulator and return line. Return lines may burst at 50 psig.

the injection if using a rotary impulse pump and poppet injectors should never be able to exceed upper rpm limit but without a clear description of type of generator and fuel system it would be hard to answer accurately exactly what would happen,

What a great signature!!!

WHEN I WAS YOUNGER AND INEXPERIENCED, I MISALIGNED THE FUEL SYSTEM AND DIDN'T OPEN THE RETURN VALVE WHEN I SHIFTED FUEL TANKS, AFTER DISCOVERING MY MISTAKE A COUPLE OF HOURS LATER, I OPENED THE RETURN VALVE. THE RETURNING FUEL AT A HIGH PRESSURE SUCKED THE FUEL LINE DRY AND THE ENGINE SHUT DOWN, SO TO ANSWER YOUR QUESTION, UNLESS THERE WAS ANOTHER PROBLEM, I WOULD SAY NO. THERE ARE OTHER FACTORS THAT WOULD DO THAT BUT I DON'T BELIEVE THE RETURN FUEL VALVE BEING CLOSED WOULD DO THAT. I DID EXPERIENCE A RUNAWAY AFTER A REBUILD OF A DIESEL GENERATOR AND ENDED UP PUTTING SOME PLYWOOD OVER THE AIR INTAKE TO SHUT IT DOWN.

Excuse a daft question, but why have a valve on a fuel return line. Isn't that asking for it to be put to 'off'? If it is ok to switch it off then I'd have thought it'd be ok not to have it in the first place, saving time and money.

Ok, I guess the answer is so you can work on the fuel system without having to drain tonnes of liquid and find somewhere to put it. Still, one day it will be found to be in the wrong position when it matters. Even a solenoid valve wouldn't entriely obviate the risk.

I once witnessed one of our "Diesel Mechanics" Start a 6-53 Detroit with a gas soaked rag. 1) It started.

2) It sucked that rag as far towards that blower as it could.

3) It ran as fast as it could till the gasoline was gone.

4) It became time to rebuild the 6-53.

A year later, same man, same technique. This time a Cat 3208N. This time rag caught fire. But he learned from the 6-53. He held the rag this time. Finally dropped the rag onto a portable radio owned by the fire dept. Then went to emergency room for burnt hand. 3208 recovered quicker. Radio did not survive.

Most of the engines used in Airport Fire trucks do not have emergency shut down provisions. Knowing there is that runaway threat in fuel spills, and gas vapor leak calls, we always have a Co2 extinguisher on one of the responding trucks to shoot at the air intake. Not a great plan. But it is ours.

You told me you would never tell anyone . . . . . . .

Some diesel designs such a Fairbanks Morse Opposed piston engine can run away even with the fuel tripped. The runaway is caused by positive pressure in the crankcase causing lube oil to get into the cylinders causing the diesel to run away. These engines have a blower that sucks on the crankcase to keep a negative pressure and prevent this from happening. When the engine runs away lube oil, it usually destroys itself by overspeeding. An overspeed condition normally trips the fuel racks, which will cut off the fuel. Since the engine is running on lube oil, there is no overspeed protection until damage within the engine occurs. One way to shut it down in this case is to manually cut off the air intake. Some marine engines have this feature.

Love the name. I ask myself that often.

All of you that want to know how to chop off the intake air, Google for "Barber Rig Saver" whose patent may have run out and I believe there are copies, or perhaps Wood Group bought them ???

We, in the diesel game, have been using these for 25 years or so.

The old Detroit Diesels had a non-spring loaded fuel rack unit injector (GM) so when one injector stuck wide open they all stuck wide open . . . . therefore they came out with the famous 'flapper valve' door on the blower that tripped during an overspeed (set at 100%~115% rated). Vacuum would slam it shut, and the blower seals promptly inverted and needed changing. Later they had mechanical seals to prevent this. Later they had a spring loaded rack so if one injector got stuck, only that one was at fuel fuel . . . . but they kept the 'flapper valve' for good measure.

Other 'non-blown' engines (4 strokes) used the "Barber Rig Saver" which is a guillotine valve mounted between the turbo and the after-cooler (or after the air cleaner in naturally aspirated engines) in the external piping. Simple and it worked. It is spring loaded and held open by a latch (you must manually re-set it with a ring spanner (box end wrench). The trip can be by solenoid, air, or hydraulic actuator eventually tied to an overspeed switch . . . either fly-ball type or electronic reading a pulse or magnetic pickup.

Taaah Dahhhh !

Well the origin of this question.

Last week one of our 400kw twin packs came in for service which is basically fuel filters - primary and secondary. Well after the service I went to start this badboy up and Gen B came up great somehow Gen A didnt.

After another start attempt Gen A came up. But it didnt stop accelerating. I watched the frequency climb - 68, 74, 81.. My buddy who was with me, had already run away !

Amongst all the noise and the steadily churning smoke I still was able to think - fuel or air stop one ! I ran over to the 2 water separators and shut off the fuel valves and then exited the Gen Room to join the spectators outside.

Hoping this would shut the beast down..... it didnt. How ? I wondered. Well at this time Engine B was racing his way up too. The noise was unbearable and all the smoke that was bellowing out was making it difficult to see. How could I stand there like these idiots and not even try to prevent losing these 2 machines.

I said what the heck, there has to be a way. I grabbed an adjustable wrench and ran back in there and started shutting everything off - air was out of the question, there were no air shut of walves on these models.However there was fuel so I went at the main fuel valve, auxillary fuel valve and then return fuel valve. For some wierd reason the return fuel was already CLOSED, so I turned it OPEN. The machine died in 5 secs.

I emerged from the Gen room a hero but at the same time a very confused one at that.

You all have made some very good points and I am very gratefull for your help. I just wanted to make sure that it was the fuel return valve.

Hmmmmmmm. What brand and year of engine and what type of injection system? I can almost believe this with a Cummins PT fuel system, but really cannot fathom this with a unit injector or Bosch scroll type. But, I must say I NEVER have tried it as virtually nobody I know would ever dare put a shutoff valve in the return line.

One test is to try it again on a unit running normal and VERY slowly close the return line valve in increments and see if you can get 63 cycles (not more than 63).

How do you know you were at 81 Hz? Digital freq meter? Most analogs stop at 55 ~ 65 low to high. Note that if you really went to 81 Hz there is a good chance you centrifugally damaged the generator poles. Call the generator supplier. They test to 125% and probably can go a bit higher but 81 Hz is screaming.

The black smoke is from not enough air or too much fuel so this is consistent with your theory.

Also, do some math and if based on your stroke length if you went above about 17 meters per second piston speed there is a high chance you have fatigue cracked the wrist pin area, especially double worse with no load. Even 1-2 minutes at 17 m or above.

And next time young man LET IT BLOW. It is NOT worth your life. I've seen rods come out the side (on engines 250 tons) and the unburned fuel cause an explosion when O2 hit it . . . . and the generator fan could have slung off. If you gotta be a hero, where you value iron more than your life, then get on the roof and block the exhaust.

And get rid of return line valves.

Our Gens are equipt with LCD displays for all Gen readings, and yes all out engines for this size are Cummins.

You are not wrong there watdefak, it will also happen with GMs (unit injectors). The reason this occurs is because these engines return nearly 80% of their fuel (used for injector cooling). On ships where you have a number of of fuel tanks and no service or day tanks, you must ensure that the return valve on the tank that is supplying the fuel is open.........OR............they will do somewhat more rpm than they are supposed to do.

If these engines are connected to a propeller does the water drag keep the engines in check, or would they continue to accelerate? What I am trying to ask, is the load on the engines enough to prevent the runaway condition, slow the time to runaway, or would propeller cavitation allow it anyway?

It is the resistance of the propeller in the water that loads the main engine but the engine governor keeps the engine at the throttle setting that has been chosen with of course the approx. 9% droop. e.g. if the engine speed set by the throttle is nominally 1 000rpm with 9% droop the engine speed could vary from 955 rpm to

1 045 rpm without the governor responding. Outside of those ranges the governor would respond automatically keeping the engine within those rev, limits. The only way we alter the governed speed outside those parameters is by altering the throttle setting which alters the loading on the speeder spring on the Governor.

I am afraid that I did not make myself clear. If the main engine were to experience a runaway condition, how would the resistance of the propeller in the water effect the runaway condition?

This would or more correctly should not happen as these engines should be fitted with an over speed trip unit which will either shut off the fuel or the air supply.

If the engine did overspeed, how long it would take to cause serious damage would be dependent on weather conditions and sea state; it would certainly be slower than an engine with no load on it.

In older engines this was the only emergency device fitted to a main propulsion engine; now unfortunately electronically controlled engines shut down if they "sneeze."

Propeller power demand increases with the cube of the shaft speed, so it will not run away and will reach a load equilibrium at dead rack (full fuel), possibly (likely) overload and melt a piston in 2-3 minutes if exhaust temp got near 1100*F.