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Old 17-01-2020, 20:28   #1
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How does a diesel shut down

OK, air in the lines is very bad, because air compresses and liquid doesn't. Turning off the key doesn't shut down a diesel engine because you are not killing a spark.



When I pull the cutoff knob, fuel is stopped somewhere alone the line. Where does this happen? Also, the engine is still going to use some fuel after wherever this cutoff happens. Does the line now have a vacuum, and vacuum is ok? What keeps air from filling this vacuum?



Or do I just not understand the fuel system?
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Old 17-01-2020, 20:48   #2
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Re: How does a diesel shut down

You basically got it. Same goes for if you were to shut off the fuel line to the engine. Assuming your lines are all good with zero leaks, the fuel is under vacuum. In these cases you can open the shut off and start the engine again without bleeding.
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Old 17-01-2020, 21:19   #3
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Re: How does a diesel shut down

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Originally Posted by Uturn View Post
........
When I pull the cutoff knob, fuel is stopped somewhere alone the line. Where does this happen? Also, the engine is still going to use some fuel after wherever this cutoff happens. Does the line now have a vacuum, and vacuum is ok? What keeps air from filling this vacuum?
.......
The lift pump sucks fuel from the tank and supplies fuel under positive pressure to the injection pump. The injection pump doesn't suck fuel, it merely pressurises the fuel that is supplied by the lift pump.

The fuel cut-off lever stops the fuel right at the point of delivery into the pressurising piston inside the injection pump. The engine creases to fire immediately the cutoff knob is actuated as it is no longer getting any high pressure fuel being delivered by the injection pump.

It will of course keep turning over due to the momentum of the flywheel etc but it is not firing as there is no fuel being supplied.

There is no vacuum being created when the fuel is cut off as the cut off occurring in the positive pressure line after the lift pump. To repeat, the injection pump isn't sucking any fuel, it is simply pressurising the fuel supplied to it.
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Old 18-01-2020, 01:43   #4
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Re: How does a diesel shut down

The injector has a spring loaded valve just before the nozzle. When the injector pump stops the fuel flow, there is still fuel in the injector tube and injector, it's just not under enough pressure to overcome the spring. When you start the engine, usually a revolution or two is needed to come up to injection pressure. The point of the spring loaded valve is to keep combustion gas and debris out of the injector.
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Old 18-01-2020, 01:44   #5
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Re: How does a diesel shut down

In a mechanical style pump the rack is moved to a position which allows the fuel to bypass the metering section of the pump plungers.

Don't know how they do it on a common rail system maybe just discontinue the opening current to the injectors.
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Old 18-01-2020, 09:02   #6
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Re: How does a diesel shut down

On my Yanmar, the shutoff is actuated by a solenoid. Once, the wire to the solenoid came loose and I could not figure out how to stop the engine - very embarrassing radio call for assistance. I now know that the shutoff can be actuated by hand if you know where it is. (PS I am not a diesels mechanic, in case it's not obvious.)
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Old 18-01-2020, 09:49   #7
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Re: How does a diesel shut down

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Originally Posted by Wotname View Post
The lift pump sucks fuel from the tank and supplies fuel under positive pressure to the injection pump. The injection pump doesn't suck fuel, it merely pressurises the fuel that is supplied by the lift pump.

The fuel cut-off lever stops the fuel right at the point of delivery into the pressurising piston inside the injection pump. The engine creases to fire immediately the cutoff knob is actuated as it is no longer getting any high pressure fuel being delivered by the injection pump.

It will of course keep turning over due to the momentum of the flywheel etc but it is not firing as there is no fuel being supplied.

There is no vacuum being created when the fuel is cut off as the cut off occurring in the positive pressure line after the lift pump. To repeat, the injection pump isn't sucking any fuel, it is simply pressurising the fuel supplied to it.
great explanation Ö.. worth to be read a couple of time
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Old 18-01-2020, 09:55   #8
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Re: How does a diesel shut down

Thanks for the explanation. I understand it now.
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Old 18-01-2020, 09:59   #9
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Re: How does a diesel shut down

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You basically got it. Same goes for if you were to shut off the fuel line to the engine. Assuming your lines are all good with zero leaks, the fuel is under vacuum. In these cases you can open the shut off and start the engine again without bleeding.
That's what I do. I have an inline shutoff valve tapped into my fuel line just before the engine. I shut it off after every sail, open it up before the next sail and engine starts without a hiccup. I guess its a good secondary shutoff option if the engine starts to run away. I have forgotten to open the fuel shutoff a couple of times when going out and just as I clear the slip the engine stops. made for a couple embarrassing moments because it will not re-start without some effort.
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Old 18-01-2020, 10:06   #10
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Re: How does a diesel shut down

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To repeat, the injection pump isn't sucking any fuel, it is simply pressurising the fuel supplied to it.
Understanding this is sort of important, many if not most all injection pumps can in fact suck fuel and the engine run, but in forming a vacuum, itís possible to form a vapor bubble, which isnít air, but works the same. What can happen is over time the injection pump is damaged from cavitation and of course has to be replaced. It wonít happen immediately and can lead some to think that boost pump isnít required.

A watermaker is the same way, you need to feed pressurized water to the suck side of the HP pump to prevent damage. Pretty much any HP liquid pump isnít designed to pull a vacuum.
Even turbine engines are the same, operated for 50 hours with an inop boost pump and your buying a new fuel control on a Pratt & Whitney PT6
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Old 18-01-2020, 10:06   #11
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Re: How does a diesel shut down

A back up is what I was taught in Marine Diesel Repair school about 50 years ago. Cut a piece of plywood 4"X6" with a handle, cover the flat with a piece of rubber,(I used a piece cut from a truck tire inner tube) and glue it to the 4"X6". If an engine failed to stop when the kill button was pushed, you place this over the engine air intake. I still have one of these tethered to my Perkins 4.236 just in case.


As a Firefighter for 30 years we would spray a CO2 extinguisher into the engine air intake. CO2 displaces the oxygen and the engine stops without damage. I remember using this on Greyhound buses and Railway switch engines
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Old 18-01-2020, 10:41   #12
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Re: How does a diesel shut down

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Originally Posted by oldcal46skipper View Post
A back up is what I was taught in Marine Diesel Repair school about 50 years ago. Cut a piece of plywood 4"X6" with a handle, cover the flat with a piece of rubber,(I used a piece cut from a truck tire inner tube) and glue it to the 4"X6". If an engine failed to stop when the kill button was pushed, you place this over the engine air intake. I still have one of these tethered to my Perkins 4.236 just in case.


As a Firefighter for 30 years we would spray a CO2 extinguisher into the engine air intake. CO2 displaces the oxygen and the engine stops without damage. I remember using this on Greyhound buses and Railway switch engines
Too bad they didn't have you on the Deepwater Horizon! On that platform, the diesel generator engines aspirated combustible gasses that surrounded the platform from a well blowout. The engines ran away, and since the generator output voltages are proportional to the engine RPMs, it caused an overvolt event that triggered multiple electrical failures -- igniting the gasses.

It's an especially unforgiveable engineering failure because those engines were supposed to run at a constant 3,000 RPM (50 Hz). It would have been easy to implement an intake-air cutoff if the engines ran significantly over 3,000 RPM. In a diesel runaway caused by aspirated combustible gasses, shutting off the fuel has no effect. A gasoline engine, on the other hand, will shut down when the ignition is shut off.

The movie, by the same name, shows this failure sequence but never explains the failure mechanism.

This could be relevant to anyone who ever has a propane leak aboard. If it gets to your engine intake while the engine is running, your engine may runaway to the point of "dynamic disassembly." Some people have theorized that an unknown number of unexplained vessel losses at sea may have been caused by motoring into subsurface methane releases that are known to occur in some areas of the oceans. I'm skeptical, but it's an interesting idea.
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Old 18-01-2020, 10:55   #13
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Re: How does a diesel shut down

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Originally Posted by er9 View Post
That's what I do. I have an inline shutoff valve tapped into my fuel line just before the engine. I shut it off after every sail, open it up before the next sail and engine starts without a hiccup. I guess its a good secondary shutoff option if the engine starts to run away. I have forgotten to open the fuel shutoff a couple of times when going out and just as I clear the slip the engine stops. made for a couple embarrassing moments because it will not re-start without some effort.
Wotnames explanation is more detailed. I was getting the concept of why and how air isnít introduced and why you donít have to bleed each time you stop. The vacuum created is minimal on the lp side (youíd really need to get it down to start degassing) but as long as itís enough to over power the lift pump the fuel flow stops. If you had a leak air would start entering the system and youíd have trouble restarting. This is why a vacuum gauge on your fuel supply line is great troubleshooting.
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Old 18-01-2020, 11:04   #14
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Re: How does a diesel shut down

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Understanding this is sort of important, many if not most all injection pumps can in fact suck fuel and the engine run, but in forming a vacuum, itís possible to form a vapor bubble, which isnít air, but works the same. What can happen is over time the injection pump is damaged from cavitation and of course has to be replaced. It wonít happen immediately and can lead some to think that boost pump isnít required.

A watermaker is the same way, you need to feed pressurized water to the suck side of the HP pump to prevent damage. Pretty much any HP liquid pump isnít designed to pull a vacuum.
Even turbine engines are the same, operated for 50 hours with an inop boost pump and your buying a new fuel control on a Pratt & Whitney PT6
What you are saying is correct, but I would say not to the extent you are saying.

Sorry for the thread drift.

It doesn't sound like you were S76 rated? They, by design, don't have fuel boost pumps. They are the only heli I'm aware of designed this way. Their theory was a fuel leak is less of a fire hazard as it doesn't squirt fuel around like a flame thrower.

It makes bleeding the fuel system less convenient but the Fuel control units seem to last fine.
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Old 18-01-2020, 12:16   #15
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Re: How does a diesel shut down

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Originally Posted by oldcal46skipper View Post
A back up is what I was taught in Marine Diesel Repair school about 50 years ago. Cut a piece of plywood 4"X6" with a handle, cover the flat with a piece of rubber,(I used a piece cut from a truck tire inner tube) and glue it to the 4"X6". If an engine failed to stop when the kill button was pushed, you place this over the engine air intake. I still have one of these tethered to my Perkins 4.236 just in case.


As a Firefighter for 30 years we would spray a CO2 extinguisher into the engine air intake. CO2 displaces the oxygen and the engine stops without damage. I remember using this on Greyhound buses and Railway switch engines
I wish I'd known this 10 years ago when the turbo charger on my diesel Landcruiser blew an oil seal and the engine started running on the sump oil. It made so much smoke you couldn't see the vehicle and revved flat out. I had to wait for it to run out of oil before it stopped. And all the time I had a CO2 fire extinguisher sitting in the back of the vehicle! Doh
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