Runaway TDi

[andy-xr]

Saw one this morning on either the M60 or M61, cant remember where I was, but the smoke and noise was something else! Never driven through that amount of smoke and it properly stank

What's the deal with these? I understand that the engine starts running on oil because the seals have split or something else has gone wrong, is that the only way?

And is it not possible to put both feet on the brake, handbrake up, throw it in fifth and let it stall or something?

Poor guy in the van today, dont really think he was sure what was going on, just looked to be waiting for it to pop

[P-Jay]

I always thought this was a sort of automotive myth, after-all as you said, even given an unlimited fuel supply with a closed throttle it must be reasonably easy to stall it.

[wst]

Yeah, the usual thing to do is stall it (or block the intake). Finding the intake can be kinda tough though - my car breathes through a tiny little pair of slots in the fascia, in a place that's hidden by the bonnet, and I was only looking because I was bored, not because it was trying to spit its guts out at 5000 rpm.

[Jimmyarm]

You are correct, only way it will stop is either

a) stalling it (this is the safest method but requires a manual box)
b) blocking the air intake
C) it runs out of oil
d) the engine smashes itself to pieces

Usual cause is the turbo oil seals going allowing high pressure oil to be squirted into the inlet, as diesel engines RPM are controlled by the amount of fuel being injected it will just rev like crazy until one of the above happen.

I had it happen in my old renault engined V40 pulling out onto a roundabout in 1st gear, fortunately I've seen it before and have knowledge of it so just popped the clutch down, coasted to the side of the roundabout and stalled it.

[TheEnd]

P-Jay said:

I always thought this was a sort of automotive myth, after-all as you said, even given an unlimited fuel supply with a closed throttle it must be reasonably easy to stall it.

Guess what type of engine doesn't have a throttle, so it's always at the equivalent of WOT!

[P-Jay]

TheEnd said:

P-Jay said:

I always thought this was a sort of automotive myth, after-all as you said, even given an unlimited fuel supply with a closed throttle it must be reasonably easy to stall it.

Guess what type of engine doesn't have a throttle, so it's always at the equivalent of WOT!

Seriously?

I have to admit despite not owning a petrol car in 5 years, I haven't clue how diesels work.

[NNH]

P-Jay said:

TheEnd said:

P-Jay said:

I always thought this was a sort of automotive myth, after-all as you said, even given an unlimited fuel supply with a closed throttle it must be reasonably easy to stall it.

Guess what type of engine doesn't have a throttle, so it's always at the equivalent of WOT!

Seriously?

I have to admit despite not owning a petrol car in 5 years, I haven't clue how diesels work.

Yep. That's one reason why turbos work so well on diesels, always lots of airflow.

[ooo000ooo]

wst said:

Yeah, the usual thing to do is stall it (or block the intake). Finding the intake can be kinda tough though - my car breathes through a tiny little pair of slots in the fascia, in a place that's hidden by the bonnet, and I was only looking because I was bored, not because it was trying to spit its guts out at 5000 rpm.

If you blocked the exhaust would it stall?

[PugwasHDJ80]

ooo000ooo said:

wst said:

Yeah, the usual thing to do is stall it (or block the intake). Finding the intake can be kinda tough though - my car breathes through a tiny little pair of slots in the fascia, in a place that's hidden by the bonnet, and I was only looking because I was bored, not because it was trying to spit its guts out at 5000 rpm.

If you blocked the exhaust would it stall?

Theoretically, but i can't imagine what you'd managed to block it with!

The power produced by the exhaust gases is....surprising

although not as surprising as stick your hand over the intak- a big diesel will seriously damage your hand!

[Jimmyarm]

doogz said:

ooo000ooo said:

If you blocked the exhaust would it stall?

If you could, yes.

I'd like to see someone try this, it would be quite hilarious


ETA; Just incase someone reads this and does try blocking the inlet with their hand, don't even get your hand near it. Either use a jumper or towel and let it get sucked in.

TBH if it happens in an automatic car, you are safer letting the engine implode unless you are 100% confident in what you are doing.

[Ozzie Osmond]

P-Jay said:

I have to admit despite not owning a petrol car in 5 years, I haven't clue how diesels work.

Same as a petrol engine except without the need for a spark plug. The mixture just detonates due to compression.

If you don't need a spark plug you don't need an ignition system. So you can't stop the engine by "turning off the ignition". To stop a diesel you need to switch off the fuel supply.

If you switch off the fuel supply but the engine can still draw oil "fuel" from its oil supply it ain't gonna stop..... Hence the need to strangle it (block the air intake) or stall it.

[Silverbullet767]

Jimmyarm said:

doogz said:

ooo000ooo said:

If you blocked the exhaust would it stall?

If you could, yes.

I'd like to see someone try this, it would be quite hilarious

With a banana if possible.

Wait for it, wait for it. BOOM 200mph banana.

[wst]

PugwasHDJ80 said:

ooo000ooo said:

wst said:

Yeah, the usual thing to do is stall it (or block the intake). Finding the intake can be kinda tough though - my car breathes through a tiny little pair of slots in the fascia, in a place that's hidden by the bonnet, and I was only looking because I was bored, not because it was trying to spit its guts out at 5000 rpm.

If you blocked the exhaust would it stall?

Theoretically, but i can't imagine what you'd managed to block it with!

The power produced by the exhaust gases is....surprising

although not as surprising as stick your hand over the intak- a big diesel will seriously damage your hand!

You can see in this LOUD video the first thing the young guy tries to use almost gets digested by the engine, but luckily the older gent grabs a 2by4 and that merely throttles it down until a rag is found to help sort it out!

[P-Jay]

Ozzie Osmond said:

P-Jay said:

I have to admit despite not owning a petrol car in 5 years, I haven't clue how diesels work.

Same as a petrol engine except without the need for a spark plug. The mixture just detonates due to compression.

If you don't need a spark plug you don't need an ignition system. So you can't stop the engine by "turning off the ignition". To stop a diesel you need to switch off the fuel supply.

If you switch off the fuel supply but the engine can still draw oil "fuel" from its oil supply it ain't gonna stop..... Hence the need to strangle it (block the air intake) or stall it.

That's sort of what I expected, but unlike a petrol, the throttle pedal controls the fuel supply rather than air supply?

[Gilhooligan]

doogz said:

wst said:

url=https://www.youtube.com/watch?v=vbiNndfNNKI]You can see in this LOUD video[/url] the first thing the young guy tries to use almost gets digested by the engine, but luckily the older gent grabs a 2by4 and that merely throttles it down until a rag is found to help sort it out!

Ha, calm as you like!

Was that truck turbocharged though? It looks pretty old, I wouldn't have guessed it would have been?

I don't think it needs to be turbocharged, if the piston rings are worn then the engine could consume engine oil 'fuel' that way.

[Gilhooligan]

doogz said:

See, I've heard that before, but never quite understood it. If the rings are that gubbed, or the bores are that worn, that they allow enough oil through to run the engine, surely the compression would be down that far that it wouldn't run?

Neither do I tbh. Maybe there is a 'sweet spot' where there is enough play between the worn piston rings and bore to allow enough oil through but still have enough compression for the engine to run.

[kambites]

Piston rings fail, sump pressurises, oil is blown from the sump up the crankcase breather into the intake manifold, then enters the cylinder through the intake valve.

[BL Fanboy]

Ozzie Osmond said:

Same as a petrol engine except without the need for a spark plug. The mixture just detonates due to compression.

If you don't need a spark plug you don't need an ignition system. So you can't stop the engine by "turning off the ignition". To stop a diesel you need to switch off the fuel supply.

If you switch off the fuel supply but the engine can still draw oil "fuel" from its oil supply it ain't gonna stop..... Hence the need to strangle it (block the air intake) or stall it.

Just to clarify, the diesels not compressed like the fuel air mixture in a petrol engine, its the air that's compressed and gets hot because its compressed - (ever seen how your bicycle pump gets hot when you use it and reciprocating air compressors have cooling fins on them?)

The fuel is squirted in to this hot air and a rather sudden build up of pressure occurs as the diesel fuel starts to burn. This sudden build up in cylinder pressure is what give diesel engines their "knock" sound. The modern stuff uses multiple little and one big squirt of diesel fule per cycle to try and reduce the suddenness of this pressure build up on the piston and hence reduce the noise.

[spaximus]

There are several ways a diesel can run away. The thing it needs is for crank case pressure to build up, this can be due to worn rings, holed piston, faulty turbo. What this does is allows the crank pressure to force the oil up into the breather system. This is usually fed somewhere into the inlet manifold where it starts to get drawn back into the engine and starts to burn. This is uncontrolled so it gets faster and faster and as said there is two ways to stop it, stall it or block the air intake. Having seen a D series 6 cylinder Ford runaway I would never put my hand near the air intake as the power was so high. On this one a guy tried to stall it and it snapped the first motion shaft. It took half a roll of hand towel to stop that one, but the engine and box were toast by the end.

[AJB]

Ozzie Osmond said:

P-Jay said:

I have to admit despite not owning a petrol car in 5 years, I haven't clue how diesels work.

Same as a petrol engine except without the need for a spark plug. The mixture just detonates due to compression.

And without a throttle. In a petrol engine the air:fuel ratio matters a lot, so if you want less power you have to throttle the air intake and inject less petrol. In a diesel it doesn't matter, and can be as lean as you want. So diesels run without a throttle, sucking in as much air as they want, and then simply inject the correct amount of fuel for the desired accelerator position.

This means that once diesels are running on their own oil, they're effectively at wide open throttle with nothing you can do to reduce that other than blocking inlet or exhaust. Even trying to stop it by stalling it could be hard if the clutch isn't up to the job and just slips/cooks.

Incidentally, the fact that diesels run at full air quantity and therefore full air pressure even under very light load is one of the reasons they're a lot more efficient than petrols under light load (eg around town). Under heavier load (eg cruising 70mph+) there's less of an efficiency difference as the petrol is letting in more air and running at higher pressures.