Do locomotive prime movers have a problem with diesel runaways?

[Perry Babin]

This is a problem with diesels when something like a turbo oil seal fails and the engine begins burning crankcase oil. Does a prime mover have this problem and if it does, does it have a way to shut off the air intake to stop the runaway?

[tree68]

It's been known to happen - there is at least one video on-line of such an incident.

https://www.youtube.com/watch?v=vDIgeN6TsNc

I don't think there's a way to block off the air, at least not that's built in.  

[Overmod]

The video in question isn't an uncontrolled rotational speed excursion, it's excessively rich mixture combined with 'governed' fuel injection, which produces voluminous black smoke (from partial combustion).  As that smoke consists of relatively flammable finely-divided carbon and hydrocarbons, any reasonable source of ignition, like a hotspot in the exhaust manifold, can cause combustion to sustain in the region where the hot plume mixes with air.

 

Turbo seal leak on the exhaust side is like stuck injectors on a 4-stroke: white or blue smoke from volatilizing fuel and oil.  Note that this will not keep the engine turning by itself; timed/governed fuel injection is still involved to keep the engine turning enough to blow the volatilized material.  (See also the blue oil smoke from worn FM OP locomotive engines idling for protracted time, and the fun flame show on some EMD 567/645s that have been idling and then are brought abruptly to a high notch.)  There is a certain self-limiting character to these large-displacement engines that volumetrically limits the uncontrolled runaway speed to whatever the 'mass flow' of the seal leak, blowby, or whatever is feeding the excess combustible.

[bogie_engineer]

EMD two stroke engines have had a mechanical overspeed since the earliest versions as far as I know, definitely since I started there 55 years ago. It actuates a linkage that holds the injector rocker arm in the open position so it no longer fuels the engine. There is manual lever on the front (governor) end of the engine that is used to reset it if activated. It's set to activate at around 1,100 rpm triggered by a flyweight on the camshaft counterweight.

There are EMD engine runaways caused by airbox fires, this was a big problem on 20-645 stationary generator sets deployed in Vietnam during the war. There can be a large accumulation of oil and sludge in the airbox over time that can be set on fire by blowback thru the intake ports in the cylinder liner as it was explained to me by an engine engineer who made several trips there to investigate it. Don't remember how they fixed it then but I also recall that later the engine group in conjunction with the Farr company developed a "slammer" that was actuated by an air cylinder which slid a plate across the air inlet between the filter box and the turbo. Can't recall which RR was having trouble with airbox fires or what model loco it was though.

Dave

[timz]

bogie_engineer

There are EMD engine runaways caused by airbox fires

"Runaway" meaning the engine speeds up to 1100 RPM, and then the governor does what it's supposed to?

[bogie_engineer]

timz

 

"Runaway" meaning the engine speeds up to 1100 RPM, and then the governor does what it's supposed to?

 

 

 

The governor has no control over an airbox fire, the fire feeds hot gas to the turbo thru the cylinder ports to the exhaust valves, which in turn supplies air to the airbox. There is no fuel injected which is the only control the governor has. The overspeed trip may be actuated but it only controls the fuel injector as well, so the engine is free to runaway to whatever speed the fire supports.

 

Dave

[Overmod]

The Woodward governor only holds speed by adjusting fuel feed via the injectors (see for example the 'fuel rack' and how it physically works).  If you consider how the AAR 'notch' control works, the governor is programmed to a particular engine rpm at a given notch, and then works automatically (and very effectively) to hold that speed as load applied to the engine increases and decreases.

The overspeed trip does exactly that: it disconnects the fuel rack which drops back to what would be full injector cutoff (it could have been to 'idle speed' but anything producing engine overspeed is better analyzed without the engine being left rotating).

I am presuming that the airbox fire is volatilizing, and then carbureting in the positive-pressure scavenge air, enough "fuel charge" to have firing occur at the right position relative to TDC, and with adequate compression-ignition pressure for a long enough interval, to spin the engine up to overspeed.  To the extent that mechanical governed injection 'adds fuel' with correct timing, the overspeed trip will remove that timed fuel, so the engine will only turn on the incidental carbureted charge.  Of course by that point either a Roots or centrifugal scavenge-air supply will be producing relatively high pressure, further heated by the fire, so even with prompt rack trip in the absence of a more positive speed limiter the engine might overshoot the trip rpm by a potentially damaging amount even if the injection promptly ceases.

[SD70Dude]

Not sure if it was designed with this in mind, but it sounds like the EMD overspeed trip would do a pretty good job of stopping the stuck rack runaways that 2-stroke Detroits are infamous for.  

The only guaranteed way to stop a true diesel runaway is to shut off its air supply.  Never had the chance to try, but I've heard that spraying CO2 fire extinguishers into the air intake can work.  

Air intake shutoffs are common on engines of all types and sizes that will be operating in environments where there is the potential for fuel gas or combustible chemical leaks (think oil refineries or similar plants).  If a gas leak gets to the engine the 'air' entering the intake may become 'rich' enough to keep the engine running without any additional fuel.