pre-ignition triggered by lubricating oil 1

[DonMurray]

thread71-215499

We are looking at a 90 Bar (1323 psi) pre-ignition pressure and a 5200 psi peak cylinder pressure.

I saw a post regarding pre-ignition triggered by lubricating oil getting pas the rings. Our current design has 4 rings -- oil wiper ring plus 3 compression rings.

I am interested in hearing from others that have addressed this issue.

Thanks,
Don ---

 

[Lou Scannon]

Another source of oil in the charge air of boosted engines is failure of the oil sealing system of the compressor/blower.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[tbuelna]

The examples of lube oil entering the combustion chamber from a faulty PCV system or a faulty turbocharger/supercharger are not normal conditions, so they are not relevant to the discussion.

 

[azmio3]

Tbuelna,

I beg to differ. We have a full test data to link high oil carry over from PVC breather to increased knock tendency. Not all engines in the market have oil carry over of less than 1 g per hour

 

[motzingg]

higher oil temps will result in thinner oil, therefore more carryover past the control ring and valve guide. (and turbocharger seals, if present)

not to mention the increased evaporation potential of any lighter hydrocarbons present in the oil.

In two strokes, preignition from oil reducing the effective octane of the fuel, is a major consideration.

 

[140Airpower]

I think the issue of the rate of oil getting past the oil ring being dependent on oil temperature, as normally measured, must consider the fact that the temperature of the oil at the ring is exactly the same as the cylinder wall and ring/ring land temperatures and not the temperature of the oil in the pan or at the temperature sender. So, the rate of oil getting past the ring may correlate better with coolant temp unless oil spray cooling of the piston and walls dominates these temperatures.

 

[tbuelna]

A tiny amount of lube oil is supposed to get past the oil control rings by design. The oil control rings are designed to leave a micro-thin film of lube oil on the cylinder wall during each down stroke. This oil film is just sufficient to provide boundary lubrication for the compression ring contacts, but also thin enough to prevent it from flashing off due to exposure to the heat of intake charge compression or combustion gasses.

 

[BigClive]

Beside the point somewhat - but I have seen very old and worn diesel engines running on their own lubricating that has got past their rings.

 

[Lou Scannon]

And they can only stopped by loading them to the point of stalling, or blocking the air intake with something that can withstand up to 15psi.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[tbuelna]

While it might be possible in theory for a CI engine to continue idling simply from sufficient lube oil finding its way past the oil control rings and into the combustion chamber, in reality it is highly improbable. If the rings are worn enough to allow this level of lube oil blow-by, there would probably not be sufficient compression heating in the engine to produce ignition of the lube oil. Also, the lack of intake throttling in a CI engine would mean there is no negative pressure differential produced in the combustion space that would cause lube oil to be drawn into this volume.

 

[BrianPetersen]

The usual failure route for a diesel engine running away on its own lubricating oil is that the piston rings or turbo seals fail to the extent that there is excessive crankcase blowby (or the oil goes directly into the intake stream via a failed turbocharger), the crank blowby overwhelms the crankcase venting system and goes directly into the intake stream. When it gets to the point of raising the revs, the higher cylinder pressure and higher revs cause more blowby and away it goes until either someone stalls or strangles the engine or a rod goes through the block.

 

[tbuelna]

The seals that usually fail on a diesel engine turbo are those on the turbine side. And this would not result in lube oil entering the combustion spaces. A bit of blow-by into the crankcase due to worn compression rings would also not tend to force lube oil into the combustion spaces with a turbocharged CI diesel engine, since there would be a positive pressure delta between the cylinder volume and the crankcase volume at all times. It would require a low pressure within the intake to cause significant amounts of lube oil to migrate past the piston rings, and this would not happen with a turbocharged four stroke diesel engine.

The only diesel engines I know of that have "run-away" on their own are those that had crude mechanical fuel injection systems, where the fuel delivery was metered by a mechanical control that responded to engine speed.

 

[BrianPetersen]

"Usually" ... perhaps, but not always.

I'm most familiar with the VW diesels. It's not unusual to find oil in the intake system. A small amount doesn't do any harm. A lot of it, is a big problem.

Runaways as a result of an exploded turbo aren't common, but they are not unheard of. On the old mechanical-injection engines, switching off the key cuts off the fuel solenoid, and if the engine is running away on lube oil, that doesn't stop the engine. The problem is NOT failure of the fuel solenoid to shut off fuel delivery, because that's the same solenoid that shuts down the engine normally when you turn off the key every time, and if that didn't work, you would know it (and it would be exceptionally coincidental for the shutoff solenoid to fail at the same moment as the governor failed).

If the turbo explodes and the shaft breaks (it happens) it doesn't matter what condition the seals are in; oil being fed to the turbo is going to pour into the intake ...

 

[braddles90]

There is a record of a diesel engine in an underground machine here in Aus where the turbo impeller failed, the nut that held the impeller on got taken downstream by the airflow and lodged itself into the intake shutoff valve, which prevented the engine from being shut off when it ran away on the lube oil coming from the destroyed turbo. One of those failures where everything lined up to create a dangerous situation - I think the operator ended up loading the engine up in some way to stall it out.

 

[golfpin]

Gentlemen, good day,
I have to support those who have experienced a runaway on the older golf diesels, not pleasant. The order of failure would appear to be worn rings/pistons [bad maintenance] blow by would force engine oil back through the PCV system into the intake system and you had a runaway. As long as there was oil in the sump it would run of its own accord, until it would self destruct.
Golfpin