25-12-2012, 12:58 AM
I've just read a few threads recently where people have been playing with modified pumps and often having failed turbochargers and this happening and people seemingly not quite understanding the dangers of this!
Due to the way Diesels work (as I'm sure you are all aware) they do not have ANY control over the air entering the engine. This means that they are only ever controlled by the amount of fuel that is supplied to the engine by either the Injection pump or an external source... Petrols DO have control over the air entering the engine, which is why you don't see this with them (at least not to the extent of a Diesel).
The danger is that often with people modifying injection pumps you can get to a point where the governor system has been played round with to the point where it actually has no control over the injection quantity and can start running away, now usually you can notice you're on the limit of this when tuning when the revs begin to hang... But sometimes you get to a point where it's WAY too much, the engine enters a positive feedback loop (more revs = more fuel = more revs) and the engine will start taking off on it's own accord, now you'll probably catch it and turn it off quickly enough..
HOWEVER - BE WARNED - TURNING OFF THE KEY DOES NOT MEAN THE ENGINE WILL STOP!!!!!!
If you've ever taken apart a stop solenoid, you'll understand why, the spring is f*cking TINY, once you've got the engine at FULL Injection Quantity, the solenoid won't be able to drop down and kill the engine...
The other (FAR more dangerous sort...) runaway situation is where usually you've got a failed turbocharger, and the oil seal has just destroyed itself on the compressor side and is now pumping pressurized oil into the inlet manifold... Oil DOES combust VERY well in a Diesel engine... To the point where it will literally rev completely out of control...
I know this myself, I had a 6000+rpm runaway in a little village when I popped one of my turbos... No reason it won't go to 7000+rpm and the engine WILL let go after there... Usually if it's not caught within seconds you're going to do IRREVERSIBLE engine damage, as such large quantities of oil can be pumped into the engine in such a short time, almost like driving through a flood... Just hydrolocking the engine...
Now as we know, to kill combustion you've got the fire triangle...
Heat - Well you're going to struggle unless you can whip off the cylinder head and pour water over the precombustion chambers.
Fuel - Unless you can either drain the sump of oil or stop the oil pump... You're stuffed... If you're in the situation after modifying the governor, you can pull off the fuel lines, but it'll have done probably quite a bit of damage by the time it's run out of fuel...
Air - Well this is the most sure fire way really, shove a rag down the air intake, or if you've a CO2 extinguisher, let it off down the air intake... It'll stop the engine after a second or two, who cares if it eats the rag, it'll do less damage than it revving out of control...
There's one other way, that's to stall the engine - grab 5th gear, DROP the clutch and PRAY it's enough to stall the engine out... Often it's picked up enough inertia so that the clutch simply won't stop it if it's old or the engine is producing a lot of power... The only option left is the one above - shove something down the inlet...
Just be prepared as it's a situation that CAN happen, I've probably experienced a run-away 3 or 4 times and it's quite scary when it happens... I've seen engines rev off the clocks on oil...
Just when you start your engine for the first time after fiddling around with the pump, put it in 5th gear, clutch down, right foot on the brakes and start - that way you know if there's any problems you can DROP the clutch and kill the engine very quickly...
Due to the way Diesels work (as I'm sure you are all aware) they do not have ANY control over the air entering the engine. This means that they are only ever controlled by the amount of fuel that is supplied to the engine by either the Injection pump or an external source... Petrols DO have control over the air entering the engine, which is why you don't see this with them (at least not to the extent of a Diesel).
The danger is that often with people modifying injection pumps you can get to a point where the governor system has been played round with to the point where it actually has no control over the injection quantity and can start running away, now usually you can notice you're on the limit of this when tuning when the revs begin to hang... But sometimes you get to a point where it's WAY too much, the engine enters a positive feedback loop (more revs = more fuel = more revs) and the engine will start taking off on it's own accord, now you'll probably catch it and turn it off quickly enough..
HOWEVER - BE WARNED - TURNING OFF THE KEY DOES NOT MEAN THE ENGINE WILL STOP!!!!!!
If you've ever taken apart a stop solenoid, you'll understand why, the spring is f*cking TINY, once you've got the engine at FULL Injection Quantity, the solenoid won't be able to drop down and kill the engine...
The other (FAR more dangerous sort...) runaway situation is where usually you've got a failed turbocharger, and the oil seal has just destroyed itself on the compressor side and is now pumping pressurized oil into the inlet manifold... Oil DOES combust VERY well in a Diesel engine... To the point where it will literally rev completely out of control...
I know this myself, I had a 6000+rpm runaway in a little village when I popped one of my turbos... No reason it won't go to 7000+rpm and the engine WILL let go after there... Usually if it's not caught within seconds you're going to do IRREVERSIBLE engine damage, as such large quantities of oil can be pumped into the engine in such a short time, almost like driving through a flood... Just hydrolocking the engine...
Now as we know, to kill combustion you've got the fire triangle...
Heat - Well you're going to struggle unless you can whip off the cylinder head and pour water over the precombustion chambers.
Fuel - Unless you can either drain the sump of oil or stop the oil pump... You're stuffed... If you're in the situation after modifying the governor, you can pull off the fuel lines, but it'll have done probably quite a bit of damage by the time it's run out of fuel...
Air - Well this is the most sure fire way really, shove a rag down the air intake, or if you've a CO2 extinguisher, let it off down the air intake... It'll stop the engine after a second or two, who cares if it eats the rag, it'll do less damage than it revving out of control...
There's one other way, that's to stall the engine - grab 5th gear, DROP the clutch and PRAY it's enough to stall the engine out... Often it's picked up enough inertia so that the clutch simply won't stop it if it's old or the engine is producing a lot of power... The only option left is the one above - shove something down the inlet...
Just be prepared as it's a situation that CAN happen, I've probably experienced a run-away 3 or 4 times and it's quite scary when it happens... I've seen engines rev off the clocks on oil...
Just when you start your engine for the first time after fiddling around with the pump, put it in 5th gear, clutch down, right foot on the brakes and start - that way you know if there's any problems you can DROP the clutch and kill the engine very quickly...
