April 8, 2012

Runaway diesels

VERY LITTLE is ever mentioned about runaway diesel engines, despite the fact that most sailboats have diesel auxiliaries these days. I guess that's because runaways are very rare; but it can be a very frightening experience when it does happen.
It's particularly scary because the engine races out of control and you can't shut it down in the normal way. It will rev up and run out of control until it overheats and seizes or, more likely, starts to disintegrate and throw red-hot pieces of shrapnel at you.

The trouble is that it's not burning diesel fuel. It's firing on its own lubricating oil from leaky seals, an overfilled sump, or badly worn piston rings. It is a very traumatic experience and no amount of fiddling with the throttle or cut-off knob will make any difference, since the injectors are not providing the fuel.

Now you have to remember that a diesel requires three things for combustion: very high compression, fuel, and oxygen.  So one answer to the problem is to cut off the air that supplies the oxygen.  Some engine manufacturers, aware of the possibility of a runaway engine and its potentially disastrous consequences, provide slides or flaps over the air intake that can be brought into action in an emergency.  You can do the same thing by holding a piece of plywood or aluminum sheet against the air intake to close it off, or even a pillow or some substantial piece of cloth — but remember, the suction will be terrific.

The best way I know to stop a runaway is to fire a carbon dioxide fire extinguisher at the air intake. It must be large enough to operate continuously until the engine has come to a dead stop, of course.

It's also possible to stop a runaway by operating a valve lifter, which removes the necessary compression from the cylinders.  But not all engines have valve lifters and in any case most authorities warn you not to try it because the unregulated engine will be spinning so fast by the time you think of it that releasing compression might cause great harm, not only to the engine itself but to you, too.

I should add by way of reassurance that the chances of this happening to you are about the same as your chances of winning the jackpot. But it could happen, and a good sailor should be aware of the possibility. And it would be a good idea to have a carbon dioxide fire extinguisher on board in any case.

Today's Thought
It is questionable if all the mechanical inventions yet made have lightened the day's toil of any human being.
— J. S. Mill, Principles of Political Economy.

Tailpiece
After a hard day's work at a Far East Reconciliation Conference in New York, a group consisting of a South Korean, a Vietnamese, a Burmese, a Japanese, and a Cambodian headed off to a nightclub for relaxation. They were halted by the doorman. "Sorry, gentlemen," he said, "but you can't come in here without a Thai."

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

3 comments:

Aaron Headly said...

That's it. I'm converting to steam.

mgtdOcean said...

My first experience with a diesel. My dad died on his 50ft steel motorsailer. I had to clean up the boat for sale later. He had the wiring looking like a rats nest. Pushing the start button--nothing. So being a EE by training I said to myself "I can fix this". Day later wiring is straightened out, press the cool chrome start button and BAM it runs!!! Let it warm check gears. Days done, OH SHIT turning the key off doesn't stop the motor!!!! This was before the internet and no reference book on hand. An hour later I find the fuel flow cut off. Cool chrome knob 3" from start button.(insert slaping forehead icon)

Rob Morton said...

Hi John,
I have had this experience but on a truck engine. I was in my early teens working in a truck shop and an engine started running on its own oil from a bad turbo seal. I jumped in the cab and was surprised when it didn't shut down and the tach was pegged out. You are right some of the older engines had an air shut off flap and this could save the day. It is surprising the sound a runaway engine can make as well as the damage it can do. The air flap should also be checked if you engine won't start. I won't say how I know that you should do this.
I really have enjoyed your books as well as your blog. Keep it up!
Thanks,
Rob