How to stop a diesel engine

the nut probably was from the governor. An item that DIY people should stay well away from.

Messing with the setting can create an engine that either couldn't rev to do anything or a runaway that cannot be controlled.

I found out some time ago that it's common for the label on an inj. pump to have the max RPM coded into it's ID. Tinkering to make it rev past that can have dire consequences not only for engine but also for the pump.

I'm not a mechanic - just based on what an experienced mechanic told me.
 
Years ago - Ford D800

When I was drivers mate on artics as a teenager - the drivers used to get in trouble with boss for altering the engines.

Co. mechanics were under orders to screw down the engines so they wouldn't 'race'. Drivers used to drill small hole in the air pipe to the governors so that it negated mechanics adjustment ! One D800 I was in went of the speedo clock with that mod. Later driver was called into office to explain !!
 
Interesting. Whilst in the army we shutdown engines of our big trucks during a training exercise with a bare hand. Intuitively expecting the worst we surprisingly felt only a mild suction sensation. Having studied the physics and design of vacuum pumps later in my career, I learnt that this was to be expected on a piston vacuum pump (that what the engine becomes once the hand is fully over the intake). The maximum vacuum it can create is a function of the ratios between the the displacement of ONE piston versus the volume of the air intake system from the filter down to the valve and the volume of the combustion chamber (the later always equalizes to outside pressure through the exhaust valve). Not being design-optimised to create and keep a vacuum anyway, I would gestimate the best vacuum you could achieve is perhaps 0.3 of normal atmospheric pressure. Now knowing how easy it is to hold off 2 bars of over pressure from your car tire valve with your thumb, its easy to see the minimal impact a mild vacuum will have.
The game however changes if you are not able to block off the intake completely at once. The airspeed in the remaining gap then will increase tremendous and could possibly peel off your skin at the edges. Broken bones however are not very likely unless you get your hand into the moving parts of the engine in your panic.
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Been talking to some boaters and have had an interesting chat about one experience. He used Valve lifters to stop a runaway Volvo MD ... caused smashed valves and piston tops.
Engineer told him straight - don't do it. Use "Hand over intake".
 
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Interesting. Whilst in the army we shutdown engines of our big trucks during a training exercise with a bare hand. Intuitively expecting the worst we surprisingly felt only a mild suction sensation. Having studied the physics and design of vacuum pumps later in my career, I learnt that this was to be expected on a piston vacuum pump (that what the engine becomes once the hand is fully over the intake). The maximum vacuum it can create is a function of the ratios between the the displacement of ONE piston versus the volume of the air intake system from the filter down to the valve and the volume of the combustion chamber (the later always equalizes to outside pressure through the exhaust valve). Not being design-optimised to create and keep a vacuum anyway, I would gestimate the best vacuum you could achieve is perhaps 0.3 of normal atmospheric pressure. Now knowing how easy it is to hold off 2 bars of over pressure from your car tire valve with your thumb, its easy to see the minimal impact a mild vacuum will have.
The game however changes if you are not able to block off the intake completely at once. The airspeed in the remaining gap then will increase tremendous and could possibly peel off your skin at the edges. Broken bones however are not very likely unless you get your hand into the moving parts of the engine in your panic.
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Sounds like false assumption that the engine will stop as soon as one ignition fails. Probably the case with an engine that is idling at tickover.
Assuming the engine is over-revving and has enough flywheel to keep turning a few times, the engine will suck down to something like 1/compression ratio, so the majority of a bar. .
The area is very much greater than a tyre valve, so the force is very much greater. Say 4 sq inches for a medium boat engine, so around 50 or 60lb on your skin.
There could indeed be effect due to the inertia of the airstream to increase this.
Next time you are preparing dinner, try applying 60lb force to a bit of dead animal....
Use something disposable, or the decompressor, and try to make sure the situation does not arise!
Of course one would train squaddies to be disposable in this context.
 
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Sounds like false assumption that the engine will stop as soon as one ignition fails. Probably the case with an engine that is idling at tickover.

[/ QUOTE ]Our engine takes a few seconds to come to a stop when the stop control's pulled out. From an idle speed of say 800 rpm then even if it stopped in a couple of seconds that's going to be quite a few "pumping" strokes.
 
There is a common misconception here. Unlike trying to create pressure which can result into theoretical unlimited multiples of the usual atmospheric pressure with the logical destructive consequences, going down has its limit as "-1 bar" or absulute vacuum. Picture the astronauts in their airtight fabric suite. It is not getting any lower than this. Ragardless of how many piston strokes there are it cannot go down further /forums/images/graemlins/grin.gif
 
Why is it called a "decompression" lever, unless it was to "decompress"?

By definition, it means that the lever was designed to be used while the engine is turning. /forums/images/graemlins/confused.gif
 
True but not at breakneck speed. It may work once as an emergency. But there is a good chance something will break. What they are used for mainly is on small engine to allow hand cranking. Set them to decompress, handcrank like a madman and flip them back and hope for the best.
 
Why would the speed make any appreciable difference to the clearances between engine parts?

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Just listen to an old petrol engine as you rev it to 6000 revs and the valves meet the pistons or the tappets dance of the back of the cam lobes!!!!!!! Can even do this on an old diesel.

The biggest problem of stopping on decompress is that if there is carbon build up in the vicinity of the valve there is a danger of the backflow putting it under the valve seat where it gets trapped and starts an air leak.

Another problem is that by decompressing the valve you have taken the closing force from the rockers and so are allowing the pushrods to dance around and at high revs it is possible for them to dislocate in the rocker cup get trapped under the lip and then break. Seen it!!!!

Yes use this method as an absolute last resort but not as your regular method as you are inviting problems. Use the fuel cut off provided.
 
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Another problem is that by decompressing the valve you have taken the closing force from the rockers and so are allowing the pushrods to dance around and at high revs it is possible for them to dislocate in the rocker cup get trapped under the lip and then break. Seen it!!!!


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Now that is a possibility.

I appreciate the reply, I'm just trying to understand the problem rather than blindly accept forum advice. /forums/images/graemlins/smile.gif
 
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