Page 50 - Introduction to Marine Engineering

Diesel engines 37

engine speed increases the vertical spindle rises, the piston valve rises
and oil is drained from the power piston which results in a fuel control
movement. This reduces fuel supply to the engine and slows it down. It
is, in effect, a proportional controller (see Chapter 15).
The actual arrangement of mechanical engine governors will vary
considerably but most will operate as described above.

Electric governor
The electric governor uses a combination of electrical and mechanical
components in its operation. The speed sensing device is a small
magnetic pick-up coil. The rectified, or d.c., voltage signal is used in
conjunction with a desired or set speed signal to operate a hydraulic
unit. This unit will then move the fuel controls in the appropriate
direction to control the engine speed.

Cylinder relief valve

The cylinder relief valve is designed to relieve pressures in excess of 10%
to 20% above normal. A spring holds the valve closed and its lifting
pressure is set by an appropriate thickness of packing piece (Figure
2.23). Only a small amount of lift is permitted and the escaping gases are
directed to a safe outlet. The valve and spindle are separate to enable the
valve to correctly seat itself after opening.
The operation of this device indicates a fault in the engine which
should be discovered and corrected. The valve itself should then be
examined at the earliest opportunity.

Crankcase oil mist detector

The presence of an oil mist in the crankcase is the result of oil
vaporisation caused by a hot spot. Explosive conditions can result if a
build up of oil mist is allowed. The oil mist detector uses photoelectric
cells to measure small increases in oil mist density. A motor driven fan
continuously draws samples of crankcase oil mist through a measuring
tube. An increased meter reading and alarm will result if any crankcase
sample contains excessive mist when compared to either clean air or the
other crankcase compartments. The rotary valve which draws the
sample then stops to indicate the suspect crankcase. The comparator
model tests one crankcase mist sample against all the others and once a
cycle against clean air. The level model tests each crankcase in turn
against a reference tube sealed with clean air. The comparator model is
used for crosshead type engines and the level model for trunk piston
engines.