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]

Not nearly enough info. How about a description of the engine, combustion principle, application/mission, & background info (i.e. where this started and how it got to this point)?
A representative cylinder pressure trace and related heat release info would be very informative as well.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[DonMurray]

hemi says -- "Not nearly enough info. How about a description of the engine, combustion principle, application/mission, & background info (i.e. where this started and how it got to this point)?"

Thanks for your post hemi:

I am an engine designer in Southern California. We are in the early manufacturing stage of a new very high pressure multi-fuel compression ignition supercharged direct injection engine. The anticipated applications are wide spread -- transportation, power generation, and industrial applications.

The motivation for this engine is improved fuel economy, lower emissions, reduced manufacturing costs and low weight (high energy density).

My reason to post my question is to advance my insights into potential pre-ignition problems from lubrication oil migrating past the rings into the combustion chamber.

Thanks,
Don ---

 

[Lou Scannon]

If your combustion principle is compression ignition based, pre-ignition is not commonly considered to be an issue. Note that there do exist some estoric variations of compression ignition where this question becomes much more complex, e.g. HCCI.
There is a known issue with conventional diesel engines, when there is a mechanical failure of some kind leading to excessive amounts of lube oil being drawn into the combustion chamber, leading to engine runaway (since there is no external control to prevent the oil getting into the chamber once the failure has occurred, and no way to prevent ignition, since it occurs automatically due to compression. The engine will only stop when the oil supply has been used up, or more likely, it self-destructs or a quick thinking operator brakes it to a stop somehow or starves it for air by blocking the air intake.
Other than the above, assuming competent design and manufacturing of oil control features in the ring pack and intake valve seals, you shouldn't have to worry about lube oil interfering with normal combustion.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[tbuelna]

DonMurray-

With any recip piston engine some lube oil passes by the rings by design. If there were no oil film present on the cylinder wall surface as the compression rings passed by, the compression ring faces and bore surface would rapidly fail due to scuffing. The oil control rings are designed to produce a micro-thin film of oil on the bore surface as they slide up and down, which prevents the compression ring faces from scuffing. If the oil control rings are working properly, the oil film they create on the bore surface is just sufficient to prevent scuffing of the compression rings, but not so thick as to result in the oil film on the bore surface near the upper end to become hot enough to flash and auto-ignite due to compression heating.

What is key to preventing the oil film on the upper bore surface from flashing and auto-igniting is providing sufficient cooling of the cylinder liner such that the bore surface always remains just below the flash temp of the lube oil. However, this can be very difficult to achieve with a very high pressure CI engine.

 

[DonMurray]

Thanks for your thoughts. We should be running up on the dyno in another month or two. I'll let you know how it all goes.

Thanks,
Don ---

 

[mechanic6]

hi there.

am puzzled by your question.
sounds like you are designing something very experimental.
designing a high output Diesel engine from scratch is an awesome task.
forgive my asking but this question is a little basic for such an involved project.
more info will get you a clear cut solution on this board.

I wish you well on your project.

M6

 

[azmio3]

Don,

Try checking the SAE papers published by SWRI in 2012.

I have symphaty to the problems that you may be facing. Check your oil carry over at various load/speed and ensure that it is below 1 g/hr. Conduct also oil consumption test if you have to. Unless you go into ionic liquid based lubricants, it is really difficult to change the engine oil's autoignition temperature.

 

[DonMurray]

Thanks to all of you for your insights.

I am trying to anticipate all potential issues before our first dyno runs. Because our pre-ignition pressure is so high, I was looking for your insights regarding experience with any pre-ignitions stemming from lubrication oil. While our design prevents any friction between the pistons and cylinder walls, we are using lots of oil to augment cooling.

Thanks for all your help,
Don ---

 

[tbuelna]

If your oil control rings are doing their job, the only lube oil that should be present in the working space is the oil film on the cylinder wall surface. Normally this oil film is extremely thin, and because it is in contact with the inner cylinder liner wall that is being aggressively cooled at its outer surface, the oil film is kept cool enough to prevent it from vaporizing, mixing with the intake air and combusting. Do some calculations to determine if the bore surface at the upper end is being cooled sufficiently to keep its temperature within the safe limit of your lube oil.

With high cycle pressure CI engines, it can be very difficult to adequately cool the bore surface at the upper end of the cylinder, due to the very thick liner wall needed to handle the high pressure loads. If the lube oil is flashing off the bore surface due to excessive temperatures, the most obvious symptom will be ring/liner scuffing.

 

[Tmoose]

I recall (I think) having heard that in spark ignition engines GM had determined that preignition can start well before TDC, and maybe even before the compression stroke begins.

over on speedtalk there is a recent thread that has evolved from detonation to about a phenomenon ( in Direct Injection gasoline engines?) resembling pre-ignition by some folks ( well, maybe just one) who has experienced it under circumstances new to me, but I'm pretty much a summer soldier / arm chair athlete. References to research by SWRI has been offered.

Something about accumulation of "stuff" in the relatively cool interstitial spaces between rings over several combustion cycles, and in ring gaps if I'm following it correctly.

 

[DonMurray]

Thanks for your input

 

[tbuelna]

Tmoose-

The accumulation of HC compounds in the crevice volumes around the ring grooves, quench areas between the piston deck and cylinder head, and spaces between the piston crown sides and cylinder wall above the top ring, have presented HC emissions problems for production SI auto engines. But typically these HC mixtures never become hot enough to combust due to the fact that they are in intimate contact with the relatively cool piston, head and liner surfaces.

SI auto engines deal with these HC compounds by burning them in the catalytic converter.

 

[Tmoose]

Hi Tbuelna,

That was and probably still is my take on gases in the far reaches of the combustion chamber of gasoline or diesel engines.

Here is a link to that lengthy impenetrable thread.

http://speedtalk.com/forum/viewtopic.php?f=15&t=38759

notchback is the one working on Low Speed PreIgnition with well monitored engine at SWRI

Nitro2 claims firsthand knowledge of some autoignition events originating in crevices the cool nether regions in extreme race engines.

A few others seem to have real info to offer.

 

[140Airpower]

Don,

The discussion at Speedtalk about preignition in highly supercharged or turbocharged GDI engines at low rpms concerns top ring land or between ring material triggering preignition in cylinders that are fully fueled. Since diesels are "dry" before the injection of fuel and since ignition occurs automatically with the injection, "preignition" is impossible in a normal, healthy engine despite the likely presence (IMHO) of similar ring space material.

 

[140Airpower]

BTW, it appears that low speed pre-ignition, LSPI as it is called in a number of papers, is a serious problem occurring under very high boost leading to very destructive detonation. Since the use of high boost at low speeds to make big low-end torque is an effective strategy to improve low speed responsiveness in "undersized" engines, LSPI is getting a lot of attention.

As I opined above, I think diesels are immune to this problem.

 

[azmio3]

back in 2012, SWRI published a paper linking the engine oil temperature with gasoline knock rather than diesel one. I wont worry much about the effect of engine oil property in diesel engine.

 

[140Airpower]

Hmm, correlating oil temperature with knock is not surprising since in-chamber temperatures are causal to knock and causal to overall engine temperatures, coolant temperatures and oil temperatures. What seems to be implied is that oil temperatures are causal to knock even while controlling everything else. This I would like to read about. The inference that comes to mind is that the amount of oil getting into the chamber is related to oil temperature.

 

[tbuelna]

Lube oil will not combust unless it vaporizes, mixes with sufficient oxygen, and is heated to a high enough temperature. In most piston engines, there are only two ways lube oil can enter the combustion space: leakage past the intake valve guides, or leakage past the piston rings. Neither one is affected by oil temperature.

 

[azmio3]

Tbuelna,

The condition that you described easily available before and during the spark ignition. Even Heywood's book discussed the hydrocarbon molecular structure and how it can affect knock tendency.

Oil can still get into the combustion chamber through the PCV breather and it is bad if the oil separator is not working well. The amount of oil in the blow by may also be caused by oil temperature, engine rpm and windage.