Low-Speed Diesel Incident

Trying to describe an incident with a 8 cylinder low-speed diesel the other day but I’m missing some key details.

Here’s what I recall.

One night we got a high-temp alarm on one cylinder. The relief chief requested we reduce RPM which we did. Next morning I found out that he had the bridge reduce RPMs a couple more times but didn’t solve the issue.

Once the problem was understood we stopped and drifted while the fuel for that cylinder was cut off. We continued until favorable weather when we changed out the piston.

I can’t recall for sure, does that sound like a cracked piston? What high-temp alarm would that give? What are the risks of running with a cracked piston? What risks if any of running with one cylinder cut off (at reduced RPM).?

What’s the worst that can happen, a broken crankshaft? A grenaded torsional damper?

Sounds like one of those questions that should be addressed to the manufacturer if there is no guidance in the manuals.

1 Like

Maybe. It’d be helpful if we knew which high temp alarm it was if you can recall. I’ve seen high scav air temps for a cylinder that had broken rings. That was usually an intermittent alarm as the rings rotated. High scav air temps alarm caused by collapsed rings were usually not usually as intermittent. Both cases were treated in the manner that you described (reduce load, cut the cylinder).

As for running with the cylinder cut out, the manual usually has a section that details when/how to do it and what restrictions apply. The biggest issues will be with vibration and potential for increased stress on the crankshaft.

FWIW, we had an MAN tech ask to visit our ship for lunch during one port stay. He was in port working on a Chinese crewed ship and needed a break from their food. He said that the ship had experienced failures in 5 of the 8 cylinders and came in with only three cylinders.

1 Like

Yes, that’s the one thing I have no idea about, what alarm it was.

My understanding is that there should have been little ambiguity about the fact that it was a cracked piston.

From the relief chief’s experience as 1 A/E on his regular ship how ever he thought that the problem was high-temp from too high of a load and the issue would resolve from taking off a couple turns off.

The problem was not related to high-load and slowing down didn’t solve the issue.

Hearing high-temp on one cylinder I’d think exhaust temp. Maybe bearing temp, but high bearing temp would cause a stronger reaction than just dropping turns and cutting a cylinder.

Scav temp as @Louisd75 mentioned with cracked rings, but a cracked piston itself? Like a damaged piston crown? I’ve never seen a cracked piston, but that doesn’t seem like the kind of thing you’d want to keep running on. I once had a wrist pin loose and rubbing the cylinder wall, but surprisingly there was no indication before it was found on a scheduled piston pull.

If it was high exhaust temp you could have had a bad injector dumping too much fuel into the cylinder, in which case reducing RPM and cutting the fuel to that cylinder on a slow-speed I thought was standard practice. An inline slow-speed should be able to limp by down a cylinder without too much risk as long as LO is maintained, until you get to port and can tear into it.


Most (all?) slow speeds I’ve worked with do have instructions for cutting out a cylinder, some of them even two.

Yes, that’s right, a cracked piston crown.

That’s the issue, we did it at sea, in the North Pacific SW of Kodiak during a weather window with almost no sea or swell between low pressure systems. The job went OK but In hindsight I think we should have continued our transit to Japan and done the job in port.

1 Like

The initial alarm might have been high lube oil temp. Hot gases passing through the piston crown.

It’s my understanding that each cylinder has it’s own temp sensor.

Yeah, sometimes that type of work doesn’t turn out well a la M.V. Green Wave.

i would of suspected injector issue. Since it was still firing I’d think the casecase pressure would be spotted somewhere if it’s blowing past the piston?

1 Like

The chief was not familiar with the ship. He was sailing relief and on his regular ship, where he’d been 1 A/E, evidently it was routine to get high temp alarms (maybe high exhaust temp) and the cure was to take a off a couple turns

That didn’t sound right to me (we didn’t get high-temp alarms as a rule) and we made a couple phone calls ashore and we learned of the possibility of a cracked piston crown.

Chief and I learned this at the same time.

I could see a high temp for piston cooling oil in this case if it were combustion gas getting into the piston. You may even get a low piston cooling oil alarm if the gas was enough to interfere with the flow of oil. That’s not an alarm that I would try to treat by slowing down though; I’d be stopping.

On the newer MAN engines the piston cooling oil flow and temperature alarms are off of the same sensor and each cylinder has one.

1 Like

Different situation

We had a big high pressure system.

The thing was that the pulsing tachometer, sound and feeling from the unusual vibration of the ship and main engine made the relief chief feel uneasy. He felt intuitively that running with one cylinder shut down was much higher risk compared to the risk of changing in good conditions.

It seems now that the additional risk of running on 7 cylinders at reduced RPM was not really a significant factor.


yea, rough call. Those of us in seas where you have to maintain steerage!, My last ship up there had a fuel filter explode underway… no way to shut down and oiling a hot genset nearby was rather nerve wracking. fun times!!

1 Like