YachtForums.Com - View Single Post - Will slowing down damage larger diesel engines

Marmot · 03-14-2008, 04:51 PM

There are a few bits of the low-load puzzle missing in this thread.

One, the ratings go from commercial continuous duty “A” rating for vessels operating at 100 percent rated load 100 percent of the time or cycling service at 80 to 100 percent load factors to “E” rating which is for vessels operating at rated load up to 8 percent of the time or up to 30 percent load factor.

If it helps to clear this up a bit, the 3412 is rated at 431 hp @ 1200 rpm for an “A” rating and 1420 hp @ 2300 rpm for the “E” rated version.

If an engine is “broken in” and the rings are properly seated at the beginning of its life there is little risk from operating at low loads for extended periods, particularly in the case of electronically controlled engines. The only way raw fuel can enter the lube oil is through poor injection quality and poor ring sealing. Low load operation does not introduce large quantities of fuel in the cylinder in any event so fuel dilution of lube oil is minimal if the injectors are in good condition.

Blowby will introduce exhaust gases into the crankcase and if you are using a high sulfur fuel and oil temperature is low, condensation can occur and produce sulfuric acid. Where we are now using low(er) sulfur fuels the potential for ring and exhaust path deposits along with increased emissions from too high a TBN may be greater than the risk from sulfuric acid.

Operating the engine at high power for part of each period will heat the oil and reduce the risk of condensation. Either test the oil yourself or send in samples for analysis. Change oil regularly and you won’t go wrong, broke maybe but not wrong.

Generators tend to have issues related to low power operation. Part of the reason for this is that they rarely operate at high loads and many have not been properly run-in when installed. They may see many hours of very low load operation before the boat even leaves the yard and the liners are glazed from the beginning.

Low power operation of mechanically injected engines can lead to poor combustion and soot deposits on turbocharger turbines and fouling of exhaust paths. This can lead to poor scavenging, even poorer combustion and short exhaust valve life.

If the engine is routinely operated across its rated power range there should be little risk of the dreaded low load disease that seems to be scaring so many people recently.

03-14-2008, 04:51 PM	#18
Marmot Senior Member Join Date: May 2007 Location: Fort Lauderdale Posts: 228	There are a few bits of the low-load puzzle missing in this thread. One, the ratings go from commercial continuous duty “A” rating for vessels operating at 100 percent rated load 100 percent of the time or cycling service at 80 to 100 percent load factors to “E” rating which is for vessels operating at rated load up to 8 percent of the time or up to 30 percent load factor. If it helps to clear this up a bit, the 3412 is rated at 431 hp @ 1200 rpm for an “A” rating and 1420 hp @ 2300 rpm for the “E” rated version. If an engine is “broken in” and the rings are properly seated at the beginning of its life there is little risk from operating at low loads for extended periods, particularly in the case of electronically controlled engines. The only way raw fuel can enter the lube oil is through poor injection quality and poor ring sealing. Low load operation does not introduce large quantities of fuel in the cylinder in any event so fuel dilution of lube oil is minimal if the injectors are in good condition. Blowby will introduce exhaust gases into the crankcase and if you are using a high sulfur fuel and oil temperature is low, condensation can occur and produce sulfuric acid. Where we are now using low(er) sulfur fuels the potential for ring and exhaust path deposits along with increased emissions from too high a TBN may be greater than the risk from sulfuric acid. Operating the engine at high power for part of each period will heat the oil and reduce the risk of condensation. Either test the oil yourself or send in samples for analysis. Change oil regularly and you won’t go wrong, broke maybe but not wrong. Generators tend to have issues related to low power operation. Part of the reason for this is that they rarely operate at high loads and many have not been properly run-in when installed. They may see many hours of very low load operation before the boat even leaves the yard and the liners are glazed from the beginning. Low power operation of mechanically injected engines can lead to poor combustion and soot deposits on turbocharger turbines and fouling of exhaust paths. This can lead to poor scavenging, even poorer combustion and short exhaust valve life. If the engine is routinely operated across its rated power range there should be little risk of the dreaded low load disease that seems to be scaring so many people recently.