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Old 18-01-2020, 12:23   #16
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Re: How does a diesel shut down

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Originally Posted by RaymondR View Post
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.
Spot on.
Pressurized fuel in a mechanical rack system is bypassed to the tank and mechanical injectors cannot overcome injector spring pressure, and in a electronic fuel rail the opening current is removed and injectors stay closed.
Hand lever or solenoid.
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Old 18-01-2020, 13:01   #17
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Re: How does a diesel shut down

Iíll take this opportunity to try and learn something. Iíve spent the last 25 years with conventional diesels. Yanmar and Westerbeke. I was taught to either cut off fuel or air. The kill pull on Yanmar dumped compression, I.e. air. Turning off the ignition key on the Westerbeke killed the fuel pump solenoid. In the event of a runaway engine, there is uncontrolled fuel, such as oil, entering the cylinders. The ONLY way to stop it is to deprive the engine of air. I have no real idea on the computer controlled engines.
Ok, be kind but please let me know where Iím wrong... if at all. I have no desire to go by fire . On the the bright side, never had a runaway.
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Old 18-01-2020, 13:13   #18
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Re: How does a diesel shut down

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I’ll take this opportunity to try and learn something. I’ve spent the last 25 years with conventional diesels. Yanmar and Westerbeke. I was taught to either cut off fuel or air. The kill pull on Yanmar dumped compression, I.e. air. Turning off the ignition key on the Westerbeke killed the fuel pump solenoid. In the event of a runaway engine, there is uncontrolled fuel, such as oil, entering the cylinders. The ONLY way to stop it is to deprive the engine of air. I have no real idea on the computer controlled engines.
Ok, be kind but please let me know where I’m wrong... if at all. I have no desire to go by fire . On the the bright side, never had a runaway.
On runaways, depriving air is the best option.
The suction is great and the chaos seems scary, and a rag can get sucked in.
If your concerned about that just go thru the procedure, and to get a good piece of material that can block the air intake completely, rubber ball, thick foam, then keep it in close proximity to the air intake.
Yanmars use a pull cable sure enough but it's attached to the fuel rack internally,and starves the fuel, not decompression.
Those decomp. Levers are to allow the engine to spin faster in a cold weather, low battery energy, situation. And can aid a Hydrolocking situation.
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Old 18-01-2020, 13:24   #19
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Re: How does a diesel shut down

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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.
S76 is a civilian aircraft, so no.
However if it uses the GE T-701 engine, (CT7 civilian version) the LP boost pump is part of the engine, so yes some helicopters do suck fuel up to the engine, but itís the LP boost pump thats doing the sucking, just like on our boats.
Many engines like the PT6 for example will have the LP pump on the same PTO shaft the HP pump is mounted on, one sits on top of the other making it look as if they are one pump, but they arenít.

Most aircraft suck fuel to the engine, most do have auxiliary boost pumps for starting though, and some like at least the older OH-58 / (206 civilian) have to turn this Aux boost pump on above 10,000 feet to prevent cavitation as the lower atmospheric pressure can cause it.

So other than for starting there is only one aircraft Iím familiar with that has pressurized fuel supplied to the engine and that is the Thrush H-80 as the Walter / GE engine doesnít have a spare pad to mount a low pressure fuel pump, this is because the engine was designed originally for the Let Airliner and its a high wing, so no pump needed.

Contrast that to automobiles that almost every one of them has a fuel pump in the gas tank and that means pressurized fuel to the engine.

However at least on gas motors in boats I believe the USCG will not allow a pressurized fuel line to the engine, the engine has to suck fuel. On the Mercury Verado for instance the HP fuel pump sits in a tank on the back of the engine, this tank has a float switch that turns on and off the LP pump to keep the tank full ,sort of like a toilet tank. Rube Goldberg contraption but it works and meets the regulation.

Neither here nor there really, just to show that fuel systems are similar and have similar needs.
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Old 18-01-2020, 13:25   #20
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Re: How does a diesel shut down

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......
Yanmars use a pull cable sure enough but it's attached to the fuel rack internally,and starves the fuel, not decompression.
Those decomp. Levers are to allow the engine to spin faster in a cold weather, low battery energy, situation. And can aid a Hydrolocking situation.
SV Cloud Duster
All correct and to complete the picture, the decomp. levers will also stop the engine from running although this isn't their primary function.
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Old 18-01-2020, 13:36   #21
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Re: How does a diesel shut down

My Detroit mechanic always had a clipboard hanging on the side of his toolbox. You should all know what for by now.
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Old 18-01-2020, 13:48   #22
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Re: How does a diesel shut down

Thank you all. Itís refreshing to read an informative thread that remains civil.
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Old 18-01-2020, 13:49   #23
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Re: How does a diesel shut down

I am not technically qualified but just reviewed my Yanmar 4JH-2E service manual. I conclude that the stop function is part of the governor and therefore operates by moving the governor to a position that shuts off the fuel supply. My engine has an optional solenoid instead of the more common mechanical cable that performs the same function of moving the stop lever.

Maybe someone more qualified will jump in here and save us from my technical overreach.
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Old 18-01-2020, 15:00   #24
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Re: How does a diesel shut down

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I am not technically qualified but just reviewed my Yanmar 4JH-2E service manual. I conclude that the stop function is part of the governor and therefore operates by moving the governor to a position that shuts off the fuel supply. My engine has an optional solenoid instead of the more common mechanical cable that performs the same function of moving the stop lever.

Maybe someone more qualified will jump in here and save us from my technical overreach.


The engine kill just turns t fuel rack to a zero fuel position.

What this means is that the injector pump plungers are rotated to a point that allows all of the fuel to bypass the compressing stroke of the plunger.

Coincidentally this is t he same mechanism that regulates the amount of fuel delivered by the plungers. There is a bypass channel cut down the side of the plunger that passes by a bypass port machined into the side of the bore. The rotation of t he plunger lines this bore up with the cut on the plunger earlier or later in the plungers stroke.
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Old 18-01-2020, 15:21   #25
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Re: How does a diesel shut down

Thank you, Sailmonkey! And I guess that is why there is a return line: to return the bypassed fuel to the tank?

I'm impressed that you know this and hope I never really need to.
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Old 18-01-2020, 15:26   #26
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Re: How does a diesel shut down

^^^
He got it, The shut down solenoid pulls the regulating lever on all fuel pumps, yes slot down the side of the fuel plunger the spill port does not get covered thus fuel pressure does not build up.
Had issue recently when the Shut down solenoid and the hand pull didn't work. I shut it down by first closing a fuel supply valve, when the engine started to labor I blocked the air intake with my hand, still didn't stop, but uncovering and covering did.
Found that the split pin that joins the 2 rods inside the engine had failed.



Have come across engines that had emergency shut down by flap other engines have independent fuel rack s/d devices.
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Old 18-01-2020, 16:05   #27
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Re: How does a diesel shut down

To add to the complexity of trying to describe the ways fuel cut off is achieved, it must be remembered that different injection pumps achieve the same outcome (i.e. zero fuel to the injector) in different ways.

For instance, some very early injection pumps on some single cylinder Yanmars (notably the YSE/YSB/SB series), the compression plunger always pumps the full amount of fuel every actuation. To decrease engine rpm, some of the high pressure fuel is bleed off back to the low pressure side. To achieve cut-off, all the fuel on the high pressure side is bleed off back to the low pressure side.
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Old 18-01-2020, 16:13   #28
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Re: How does a diesel shut down

A fellow had his engine runaway at the dock. He called for help and we put plywood over the air intake, as per the book. The engine slowed but kept running...darn. (Presumably enough air was leaking in the sides of the air filter holder.)

My SO had a brainwave and pushed the throttle to maximum. Luckily the engine stalled.

One lesson learned is to make sure there is a separate fuel cock on the line coming from the fuel tank.
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Old 18-01-2020, 16:22   #29
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Re: How does a diesel shut down

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Thank you, Sailmonkey! And I guess that is why there is a return line: to return the bypassed fuel to the tank?

I'm impressed that you know this and hope I never really need to.
Return fuel serves several functions, for one many engines actually self bleed, my old 4JHE is one, the air goes out thru the return fuel. There are limits to how much air it will self bleed of course.


Also a tremendous amount of heat is generated compressing anything, that’s how a Diesel ignites it’s fuel of course, but the Injection pump would also get hot, but is cooled by excess fuel.
Feel your fuel tank after a days motoring, it will most likely be very warm, that’s the heat removed from the injection system cooling it.

I’ve not heard of decompressing an engine to stop it, but of course it will work.
Remember dirt bikes of the 70’s with decompression levers for slowing down going downhill? Or Jake brakes on trucks? Both would stop an engine in a hurry, even a runaway.

A very large number of runaways come from bad fuel pumps dumping fuel into the crankcase from a bad diaphragm, the oil level gets so high the crankshaft beats it into a foam and this foam is sucked into the intake through the breather. Shutting down fuel will not stop this kind of runaway, cutting off the air will, or removing the breather hose. Just do not use your hand to shut off the air, a Diesel is a very effective vacuum pump and will damage your hand.
Piece of wood, hard back book or as was said earlier a clipboard would be excellent, just not a body part.
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Old 18-01-2020, 16:54   #30
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Re: How does a diesel shut down

. Just do not use your hand to shut off the air, a Diesel is a very effective vacuum pump and will damage your hand.
Piece of wood, hard back book or as was said earlier a clipboard would be excellent, just not a body part.[/QUOTE]

Ditto for this advice, especially if the engine is a 8 cylinder turbo. Ask me how I know

Though I must confess I couldn't resist trying again with my hand ( glutton for punishment) on a 8hp naturally aspirated diesel but not at max revs.
It was doable but I'd recommend the decomp lever or manually operate the injection pump rack lever.
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