Piston and liner failure analysis

[Azraelo]

Hello everybody.
This engine is a dedicated, stock, heavy duty engine, running on CNG for city bus applications.
It is 1900 hours old (equals to aprox 39000 Kilometers). Last oil change 2000 Kilometers ago.
Customer refers that engine shutted down 2 times when running at full load. When inspected, oil was being ejected through dipstick hose.
At shop, blowby was very high at idle, no rattling sound. Leakdown test showed only 12% for cyl 6 and 90% for the others five.

When diassembled, the scuffing is only present in cylinder six. The others are in pretty good condition.

Altough ring land and skirt are heavily damaged, crown looks normal.

Cooling nozzles checked, free of obstructions. Oil passages in block checked, no obstructions.
Main and conrod journals in very good condition.

Two rings are "welded" to the piston six. The only one free is the oil ring.

Oil analysis reveals nothing relevant. Only Fe elevated, due to scuffing. Nitration and Oxidation in acceptable limits.
Viscosity is 14.4 (New oil is 14.7), using SAE 15W40.
Oil pump is in good condition. This engine has one methane valve (like "single point injection" technology)just before throttle, so there is not localized rich mixture for this cylinder.
What could be the root cause for this failure?
What caused the oil film to breakdown?
I was thinking this engine ran with low oil, but why the others cylinders were not affected?
If the lubrication relies only in splash from crankshaft, a low oil level could be the cause since oil pan is deeper for cylinder 1 but not for cylinder 6.

Every cylinder has an oil nozzle to cool down the underside of crown piston.
I was also thinking of overheating, but when checking the lower portion of the piston crown (underside of piston), there are not signs of discoloration due to overheating.

Cylinder head and valves looks pretty nice, too.

Please your comments.
Thank you!

 

[TugboatEng]

When a cylinder is splash lubricated, it's not oil whipped up from the oil pan. In fact, there are baffles or windage trays in the pan to keep this from happening if the oil level is close to the crankshaft. Instead, cylinde lubricating oil is slung off the crankshaft from the connecting rod bearings.

I had an engine do this recently (John Deere 4045) it lost cooling water, overheated, and seized. Paint was burned off the cylinder head and turbo. The injector seals were all melted. It filled the oil pan with coolant to the point oil was coming out of the dipstick. #3 showed faint discoloration on the cylinder so we pulled that piston and it had major scuffing. The head gasket was collapsed from the cylinder expanding into it. We slapped a new cylinder in that cylinder and continued in service. I never did get to see if there was damage on the other piston skirts but I believe they are fine. Anyways, the point of the story is that even damage from a whole engine problem like an overheat can be localized to one cylinder. It looks like this one was run after the initial overheat which made the damage much worse.

Overheating of the crown from detonation or high power causes burning of the oil on the underside of the crown. Overheating do to loss of coolant doesn't necessarily. An engine operating much above 250 degrees F is overheating. That's not hot enough to carbonize the oil. I think this damage is from an overheat. A loss of lubrication would be more concentrated on one side due to the asymmetrical nature of thrust on a piston skirt.

 

[romke]

It looks like the engine suddenly seized up due to very local overheating on cylinder 6. That would also explain why the undercrown of the piston is clean. Local overheating might be the result form either lack of cooling or more energy input in cylinder 6. If you look at the cylinder head you will notice that the valves are quite dark for cylinder 6. That could indicate malfunction of the ignition and high speed knock and local overheating as a result.

Assuming that the ignition is done by a sparkplug, I would suspect either the sparkplug being at fault under high load (not always delivering the required spark) or the high voltage cabling to that particular sparlplug.

Have you checked the ignition system?

 

[gruntguru]

Yes, piston #6 has almost certainly overheated and outgrown the bore. On CNG lean mixture or late timing will quickly lead to slow combustion and increased heat input to the piston crown. I have watched it happen a couple of times on a dyno.

- Poor fuel distribution (#6 lean)
- Lean mixture to all cylinders (one cylinder has to be the first to seize)
- Late spark (retard)
- Poor ignition performance on #6
- Less likely to be a lubrication failure but possible

je suis charlie