Page 48 - Introduction to Marine Engineering

Diesel engines 35

is also supplied to an air distributor. This device is usually driven by the
engine camshaft and supplies pilot air to the control cylinders of the
cylinder air start valves. The pilot air is then supplied in the appropriate
sequence for the direction of operation required. The cylinder air start
valves are held closed by springs when not in use and opened by the pilot
air enabling the compressed air direct from the receivers to enter the
engine cylinder. An interlock is shown in the remote operating valve line
which stops the valve opening when the engine turning gear is engaged.
The remote operating valve prevents the return of air which has been
further compressed by the engine into the system.
Lubricating oil from the compressor will under normal operation pass
along the air lines and deposit on them. In the event of a cylinder air
starting valve leaking, hot gases would pass into the air pipes and ignite
the lubricating oil. If starting air is supplied to the engine this would
further feed the fire and could lead to an explosion in the pipelines. In
order to prevent such an occurrence, cylinder starting valves should be
properly maintained and the pipelines regularly drained. Also oil
discharged from compressors should be kept to a minimum, by careful
maintenance.
In an attempt to reduce the effects of an explosion, flame traps, relief
valves and bursting caps or discs are fitted to the pipelines. In addition
an isolating non-return valve (the automatic valve) is fitted to the system.
The loss of cooling water from an air compressor could lead to an
overheated air discharge and possibly an explosion in the pipelines
leading to the air reservoir. A high-temperature alarm or a fusible plug
which will melt is used to guard against this possibility.

Control and safety devices
Governors
The principal control device on any engine is the governor. It governs
or controls the engine speed at some fixed value while power output
changes to meet demand. This is achieved by the governor automatically
adjusting the engine fuel pump settings to meet the desired load at the
set speed. Governors for diesel engines are usually made up of two
systems: a speed sensing arrangement and a hydraulic unit which
operates on the fuel pumps to change the engine power output.

Mechanical governor
A flyweight assembly is used to detect engine speed. Two flyweights are
fitted to a plate or ballhead which rotates about a vertical axis driven by a
gear wheel (Figure 2.22). The action of centrifugal force throws the