Diesel Generator Care
Diesel Generators can suffer damage under certain conditions that are sometimes
encountered when used in a generating set, namely internal glazing and carbon buildup
due to prolonged periods of running at low speeds and/or low loads. Such conditions
may occur when an engine is left idling as a 'standby' generating unit, ready to
run up when needed, if the engine powering the set is over-powered for the load
applied to it by the alternator, or if the generator set's maximum output is far
in excess of the normal loads placed on it, causing the diesel unit to be under-loaded.
This is a common problem in generator sets.
For example, a diesel generator set powering the lighting circuit of a building
would be designed to be able to cope with the load of every light in the building
being on. However, this situation rarely occurs, so for the vast majority of its
operating life the diesel engine in the set will not be heavily loaded (maybe as
little as 10% of the maximum load). Ideally diesel engines should run at least around
60-75% of their maximum rated load, and at around 75% of their maximum speed (although
the phasing requirements of engines in generator sets can make these speeds hard
to achieve).
Glazing occurs due to low combustion temperatures and pressures in the engine cylinder.
When an engine is loaded correctly, the load resists the movements of the crankshaft
and piston during combustion. This causes the combustion pressure to rise as the
volume of the cylinder cannot increase directly in line with the increase in pressure
during combustion. Running an engine under low loads low cylinder pressures and
consequent poor piston ring sealing, these rely on the gas pressure to force
them against the oil film on the bores to form the seal. Low initial pressure causes
poor combustion and resultant low combustion pressures and temperatures. This poor
combustion leads to soot formation and unburnt fuel residues which clogs and gums
piston rings. This causes a further drop in sealing efficiency and exacerbates the
initial low pressure.
Glazing occurs when hot combustion gases blow past the poorly-sealing piston rings,
causing the lubricating oil on the cylinder walls to 'flash burn', creating an enamel
like glaze which smooths the bore and removes the effect of the intricate pattern
of honing marks machined into the bore surface. Hard carbon also forms from poor
combustion and this is highly abrasive and scrapes the honing marks on the bores
leading to bore polishing, which then leads to increased oil consumption (blue smoking)
and yet further loss of pressure, since the oil film trapped in the honing marks
maintains the piston seal and pressures. Un-burnt fuel leaks past the piston rings
and contaminates the lubricating oil. At the same time the injectors are being clogged
with soot, causing further deterioration in combustion and black smoking.
3 Phase Generators when improperly loaded also cause severe loads on the Alternator
and if your load is unbalanced on 3 Phases always use a Single Phase Generator in
such a case and common all three phases through a Switch.
Diesel Generators when not serviced at proper intervals recommended by the Generator
Manufacturer will also result in very poor performance.
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