specifically advise against extended running at low load/revs. They say if you have to do this, at least run the engine
up to the speed at which maximum power is rated for a short period, several times each hour.
One problem with light-load running is that, because diesel
engines have no throttle, they cannot modulate the amount of air ingested at low loads to match the fuel
This results in very far from optimal combustion, with rapid buildup of carbon in the exhaust
manifold and particularly in the exhaust elbow
, at the zone where the cooling water
is injected, the sudden temperature drop causing the carbon to drop out of the low-velocity exhaust stream.
Carbon, being the one non-metal which conspicuously appears in the galvanic series (in an undesirably noble location), tends to rot
even a stainless elbow
from the inside.
Once there are pinholes, the salt water
seeps in (even in a fresh-water cooled engine
, the water
injected at the elbow is salt
water), and a corrosive atmosphere gradually spreads back towards the exhaust valves as the holes get bigger.
And no good comes of that, together with the increased back pressure and poor breathing, from the elbow becoming increasingly choked up with carbon.
Many sailboat auxiliary engines suffer from some variation of this syndrome, because they often run for short periods and do not reach running temperature (which has similar problems), so the problems arising from charging
duties may not be as obvious from anecdotal info as would otherwise be the case.