Page 8 - Uninterruptible Power Supplies

Standby Power Generating Sets
6 Chapter One

The load acceptance is closely related to the brake mean effective pres-
sure (BMEP) of the engine. The BMEP is derived from the mechanical
power developed by the engine, its speed, number of cylinders, and the
swept volume per cylinder, from the relationship:

Engine brake power
BMEP
Swept volume of each cylinder Firing strokes per second
(1.2)
It follows that the BMEP is related to the compression ratio and the
degree of turbocharging. ISO 8528-5 includes guide values for step
loading as a function of BMEP.
Subject to the step load limitations, a diesel engine will be ready to
accept load within 10 to 15 seconds of receiving its start signal. To
ensure that the set is ready for immediate starting, it is usual to
include a jacket water heater or heaters which ensure that the bearing
surfaces and cylinder bores do not unduly cool the oil during starting
and initial running. In winter the heat introduced into the engine from
these heaters reduces the heat required to maintain the engine room
temperature.
It is also usual to incorporate a lubricating oil priming system
which ensures that the engine mating surfaces are wet before the
crankshaft is turned for starting. A continual priming cycle is usually
adopted, say a few minutes every hour, to maintain the surfaces in a
condition suitable for cranking. As soon as the engine is up to speed,
the main oil pump takes over the duty and the priming system is shut
down. For large engines the alternative of continuous priming, as
distinct from a continual cycle, can be used but there is a danger
between test runs of oil draining down a valve stem and collecting above
a piston. At the next start this could lead to hydraulic blocking and
engine damage.
The reciprocating masses of an engine lead to vibration of the main
frame which must be isolated from its mountings. Assuming that the
engine and generator are solidly bolted together to form a single mass,
the usual arrangement is for the generating set to be supported by vibra-
tion dampers fixed to a base frame which rests on the engine room floor.
If the engine and generator are separately mounted and connected with
a flexible coupling, a base frame will be required to support the generat-
ing set and this will be supported by vibration dampers either fixed to a
sub-base as described above, or resting directly on prepared mounting
pads at floor level. Some older installations may include engines solidly
bolted to massive concrete blocks independently supported and isolated
from the floor by cushioning material such as cork, but any such instal-
lations will be approaching obsolescence.

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