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I was watching some youtube videos and diesel engine runaway videos started to pop up where apparently the diesel engine runs out of control until it destroys itself.

My Question

  • What is diesel engine runaway?

  • How does diesel engine runaway occur?

  • Can it be prevented?

I've never heard of this condition before.

Here's another video just in case. :)

  • The Wikipedia page on this describes it well. – HandyHowie Dec 20 '15 at 19:52
  • 1
    This happened to me on the M1 Motorway about a year ago. I pulled over to the side of the road, pulled the key from the ignition, then realised that the engine was still revving at the redline. I de-carred and watched horrified from the embankment as the car plumed blue smoke across 10 lanes and nearly caused a pileup before finally choking and spluttering to a halt. – Valorum Nov 10 '16 at 17:47
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Since the diesel has no physical throttle, power is controlled by limiting fuel. Air is always available so any unintended, un-metered fuel will cause the engine to increase rpm and power output. The most common sources are blowby oil and oil leaking from a worn turbocharger, these then pool in the intake manifold. This engine oil gets pulled into the engine during low load high rpm operation. It usually starts when the engine is revved up with no load, once a runaway starts the high air flow rates in the manifold continue to pull oil from the pool.

While engine oil does not burn as well as diesel it will burn well enough to reach redline rpm in a warmed up engine.

Preventative maintenance involves inspection and repair of worn parts with particular attention to piston rings and turbos.

Design prevention should focus on eliminating areas in the intake that allow pooling so that incoming oil is processed a little at a time.

To stop a run away engine plug the intake or open the compression brake valve.

  • Interesting condition. Surprised that there was nothing on the site regarding this. Was also shocked to see a few modern diesels doing this. – DucatiKiller Dec 21 '15 at 15:38
  • Can the oil/fuel pooling occur after extended periods of idling? – DucatiKiller Dec 21 '15 at 22:21
  • @DucatiKiller The lowerer the air velocity in the intake the more time for the oil droplets to settle and condense. So yes, idle is more likely to cause the pooling condition. Good question. – Fred Wilson Dec 21 '15 at 22:50
  • How exactly do you open the compression brake valve manually? Do all diesels have this? – I have no idea what I'm doing Dec 30 '15 at 8:27
  • @IhavenoideawhatI'mdoing A compression brake is a driver operated device that opens the exhaust valve after the compression stroke. This increases the drag produced by the engine during deceleration. – Fred Wilson Dec 30 '15 at 15:22
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If you didn't realize, diesel engines do not have throttle plates (butterflies) like gasoline engines which controls the amount of air going into the system.

The way it is controlled is through the amount of fuel which is injected into the engine. The more fuel, the faster the engine will run. The way this is counteracted is through the use of a governor. The governor can be either mechanical or electronic in nature. Depending on the throttle position, the governor will allow more/less fuel into the system to speed-up/slow-down the RPMs. If something happens to the governor, or it isn't doing it's job, the engine can go into a runaway situation.

If a runaway occurs, the engine will continue to gain speed until it reaches a point where it cannot go faster and will stabilize at that speed. This speed is usually well beyond the redline of the engine. Due to being at this maximum speed, the engine will soon self destruct.

There are two ways you can stop a runaway once it has begun:

  • Stop the intake of air
  • Stop the fuel from entering the cylinder

The easiest of the two, in most cases, is to stop the air from going into the engine. If you saw a video of a runaway which they were able to stop the engine from self destructing, you probably saw them put something solid, like a piece of wood (I saw someone use a clipboard once) in front of the intake to shut off the flow of air. This will shut an engine down very quickly and prevent the runaway from doing any damage.

Runaways are far less common with today's electronic fuel injection, but can still happen. I saw a video of a brand new (at the time ... probably a 2012/2013 model) GMC pickup truck do a runaway where the engine went into thermonuclear meltdown. It got to a point and the engine started slowing down (melting pistons??), then the radiator blew out ... made a huge mess of things. Runaways have been occurring since the advent of the diesel engine. I don't expect to hear the end of them as long as diesel engines work as they do.

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