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Table II gives some instructive data relating to these
Two cylinders, in tandem, act upon each crank. The normal full
brake horse-power is that which the engines are capable of
maintaining continuously; the 'overload' is 10 percent
greater and should only be imposed during short emergency periods.
The powers as stated are, moreover, for engines working at
sea-level and in a temperate climate; when the plant is installed
above sea-level a deduction of about 3 percent should be made from
the power rating for every 1000 ft. of height. In Table II the
initial temperature of the gaseous mixture before entering the
engine is also assumed as 60 degrees F.; in hot countries some
power loss is incurred by reason of the diminished density of the
mixture when supplied to the engine at a higher temperature; the
loss from this cause may be estimated as 1 percent of the sea-level
rating for every increase of 5° F. above the standard temperature
of 60° F.
The normal full revolution speeds given in Table II are also
maxima values for continuous running; it is well, in order to
minimize wear and tear, to arrange wherever possible to run
ordinarily at from 5 percent to 10 percent below the 'normal
full' values. The flywheels referred to in the table are as
used with engines driving direct-current electrical generators,
air-compressors, etc.; when alternating-current generators are
being driven the necessary steadying rotational inertia is usually
furnished by the generator rotor itself, and in such cases only a
coupling at the crank-shaft end, or a light wheel for barring round
the engine, is necessary. The total 'flywheel effect'
required in parallel running is, however, in general settled by the
suppliers of the alternating-current generators.
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