How often do diesels "run away"?

[TnAndy]

I had an 8hp Lombardini diesel on my garden tiller run away once. Cranked it up for the first time one spring, didn't check the oil as I'd changed oil before I put it away in the fall, got about 50' down the drive toward the garden and the engine took off and sounded like a chainsaw at high rev. I had no idea a diesel could even turn that kind of RPM, much less why it was doing it. I jerked the fuel line off....no effect.

What I later determined was tank fuel ( there is no manual shutoff in the line ) must have slipped by a seal in the injector pump over the winter, and seeped into the crankcase. Had I pulled the dipstick and checked, I'm certain I would have found a high reading on the stick. I believe the excess, diluted crankcase oil was sliding by the rings, and firing in the combustion chamber once it warmed up a bit.

SO, it screamed like a banshee for 3-4 minutes, and after pulling the fuel line with no effect, I just backed off and waited for something to fly apart. Nothing did, it finally quit, I changed oil again, and have run it for a couple years now.

Knowing what the deal was now, I'd have dropped the oil bath filter off, and stuff my shirt in the intake....but at the time, I too had never heard the term "run away diesel".

Live and Learn.

 

[BigBoyinMS]

I know this is off-topic, but this reminds me of a question I've had about running diesels on gasoline - will they? In theory, they should not - at least at idle speeds - since the air/fuel ratio is too high. However, I suppose when the injector injects gasoline into the cylinder there is a "flame front" for my lack of better terminology that has the correct ratio and will burn and cause the engine to run. I acquired a tractor a while back which I suspect had gasoline in the tank, or at least a high concentration of it. It seemed to run fine for about 20 mins at which point the pump appartently got hot enough from the engine heat to "vapor lock" causing it to die. Waiting about 10 mins or so and it would fire back up and run again until hot. I dumped the fuel and filled with no. 2 diesel and the problem went away. I noticed the spilled fuel would dry quickly like gasoline when I emptied it.

A small amount of gasoline (say up to 10%) won't usually be a problem. But higher concentrations can do damage.
Gasoline is rated as to how resistant it is to compression ignition.(The Octane number)
Diesel is rated as to how well it will compression ignite. (The Cetane number)
Pure gasoline shouldn't normally ignite in a diesel engine but a high concentration can be ignited by the diesel it's mixed with after the diesel ignites. The problem is that gasoline will ignite at a different time and burn at a different rate which causes "knock" and can actually damage the engine.
Just a guess, but yours was probably mostly diesel but a bit more than 10%.

 

[Ilikeurtractor]

Interesting article on a engine being developed by Chrysler that runs gasoline in a diesel cycle:

Biodiesel Magazine | biodieselmagazine.com

Seems like a lot of problems with running gasoline in a diesel are it can cause problems with the injector pump and components as they rely on the fluid for lubrication. The older Bosch style pumps either use engine oil or thier own oil supply and therefore are a little more tolerant of what one runs through them. In any event, I don't plan on running gasoline in any diesel engine, but am curious as to the probability of it causing damage.

 

[psj12]

The old Army duce and a halfs had a multifuel diesel in them that would burn darn near anything flammable, however gasoline required the addition of oil to help it lubricate the injection pump. Gasoline was considered an emergency fuel only for that engine.