Low Loaded Diesel Generators 1

[Drivesrock]

Hi all.

Mechanical question.

I know that low loads on large diesel generators is not good for the diesels, apart from not being very efficient. I understand that there is coking-up of the exhaust, increased maintenance and I was told that an engine can seize.

What's going on? - ie what are the problems for a diesel engine with very little load and what sort of load is starting to be good to avoid problems?

I'm referring to 3-4MW diesel-generator sets, marine applications.

Thanks

 

[FreddyNurk]

Typical rule of thumb is 40% of nameplate rating. Having said that, be careful about which nameplate you apply, my current employer applies a number of derates to the engine, which means the actual operating range may be closer to 40% of engine nameplate than expected.

Some manufacturers (of smaller high speed electronically governed engines) tend to state that its less of a problem than before, but I'm not so sure about larger medium and slow speed engines. In this case, better to ask the manufacturer as well.

 

[waross]

Many engines use slightly tapered compression rings on the pistons. The pressure of combustion on the ring works to increase sealing pressure between the ring and the cylinder wall. This also helps less than perfectly fitting rings seat in quickly in a new engine. At low loads, the rings may not seal properly and the engine may start to pump lube oil past the rings and into the exhaust. At low loads there is often not enough heat/temperature to combust the lube oil. The result is wet stacking sometimes called slobbering. Alternating with periods of higher loading the higher temperatures may "coke" the lube oil in the exhaust system. Even greater loading may ignite the lube oil trapped in the exhaust system. I have seen a new engine pump out all its oil into the muffler to the point of shutting down on low oil pressure after less than a day running.
Worst case may be globs of flaming oil blown out the exhaust stack.
Seizing may result from engines producing carbon due to incomplete combustion. The carbon may tend to fill in the piston grooves in the space behind the rings. Then under heavy loading when the engine runs hotter, the piston expands and the space normally available for heat expansion is filled with carbon and the ring is forced outward as the piston expands and jams the cylinder wall.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TheBlacksmith]

In a 2-stroke cycle diesel the lack of ring support in the port area is worsened by the low firing pressures at light load, making the slobber worse. In a turbocharged 4-stroke diesel, low turbocharger speed and the resultant low gas flow at light load allow carbon buildup. Another problem in both types is the buildup of carbon above the top ring, this then grinds or polishes away the cross hatching on the cylinder walls.