DIESEL ENGINE EMERGENCY SHUT DOWN SYSTEMS TO INTAKE AIR

[DONKUMPUNEN]

I have worked around diesel engines up to 4000 HP for propulsion on craft and ships for US NAVY as a
QA SPECIALIST during testing and sea trials. We always require the contractor to demonstrate the
emergency pull device that trips a flapper to shut down the engine by cutting off the intake air. On different
occassions we have had complaints from engine contractors about doing this operation. We also test the
overspeed trip device that controls the fuel rack. The engine speeds are 1800 RPM down to 800 RPM.
Whats the opinion on this matter of concern by the engine manufacturers? Also i have heard of diesel
engines overspeeding to the point of major engine damage.Anybody ever see this happen out there?

DON KUMPUNEN

 

[unclesyd]

Yes I have been party to several over speeds on mainly GM 671's in boats. Two of the engines hand-grenaded from the over speed, another one seized up and broke the crank. All three of these were from oil getting into the blower from a failed bearing. I saw another run away due to running on propane where a line to galley had broken and the propane was going into engine room.

I've seen two Fords (I believe they were Perkins) on backhoes run away and hand-grenaded. I never heard the reason for the over speed

When I first started to mess around fishing boats they were a lot of Gray Marine 671's like the one’s used on landing craft, normally a one shot deal, that were notorious for over speeding. Could be that they sometimes spiked the diesel fuel with a little gasoline.

Some of the veterans told me that the navy taught the motorman how to shut the engine down by using his leg and cap. to shut off the air to the blower.

The only people I have ever heard mention not shutting the air off to their engines was Chrysler. I believe at the time their engines were made by Nissan.

 

[russeng]

The major concern with shutting the air off to an operating engine is the possibility of collapsing ducting and pulling seals out of blowers and turbochargers which could lead to overspeeds later. I have seen air shutoffs tested with and without engines running. Normally larger engines are statically tested due to the potential damage that can occur. This is why this is the last resort for securing an engine.

Overpeed trips must be tested with the engine running to ensure they are set properly. Normally a static check is conducted first to ensure all linkage and mechanisms are free and functional. Overspeeds are designed to protect the engine and when PROPERLY tested do no harm.

 

[DieselJunky]

I can understand the complaints of the suppliers.
Besides the damage that could be caused buy the vacuum, the regulator will go to full throttle, thus full fuel injection, because it wants the engine to maintain its RPM's during the choking-procedure. This causes much fuel to be injected without it burning afterward. If you do this on regular basis (for "testing") the exchaust system will be fauled, inculding the turbine. When the engine is stopped the regulator will still be in max. position, so if you forget about this later on the overspeed trip device will be tested at the next start also.
So all in all, why would you stop an engine by choking it? In case of emergency on a (navy) ship, the air will be taken away anyway. By fire, CO2, Halon or even water, you name it. But in most cases even before that time the fuel has already been shut off if you did your routine well, and the engine is already at standstill.As far as I can think of it could be that engines on a navy ship must meet these regulations because of preventing entering Chem/Bio weapens to the engine room. When under attack i assume the air-inlets will be closed, so the engines will start vacuuming the engine room. To prevent this from happaning it could come in handy to be able to directly cut of the air supply to the engine. But now i'm pretty much guessing. I'm a merchant engineer myself.
I would not test it myself, (but it would look pretty cool on paper tho ;-) ) mainly because of the fauling, but i sail engines running on HFO, which causes much more fauling and corrosion in the engine then lighter fuels.

About the overspeed testing, i think this is not so much of a problem. I dont know the regulations in the US navy, but as far as i rememeber ( I have nothing to check it overhere right now) our regulations are: electrical overspeed cut-out on 112% and mechanical on 115%. The engine is running on the testbed at 110% too (according our classification rules), so if you don't run just below the cut-out speed for more than the period to push it though to the trip, I don't see any harm in doing it. You better know it works. You don't kick back a crankshaftcover or counterweight when its coming out. To illustrate this with a story, on a very old ship in our company, an auxiliary engine (abt. 400 kW) had problems and had to testrun. The overspeed didn't work either. They needed about 4 people to start this engine. One of them was an apprentice. Not one of the cleverest as you might understand later on. Various people were doing their things and the apprentice had to control the fuel-rack by hand. I don't know what was explained to him, but after the engine was started he kept pushing the fuel-rack to max. A few seconds later a load crunchy sound and the whole engine was welded or crushed inside, from crackshaft, bearings, liners, cylinders, cyl. heads and valve gears. They were very lucky nothing came out of the crankcase because it had been running well over 115%,m but when an engine is running this speed and it's coming to a standstill in a few tenth of a second, big chance things come loose.
So you better have a working overspeed trip.
Bart