Function of governors controlling speed of marine diesel engine
Control and safety devices for marine diesel engine :
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
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.
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 . The action of centrifugal force throws the weights outwards; this lifts the vertical spindle and compresses the
spring until an equilibrium situation is reached. The equilibrium
position or set speed may be changed by the speed selector which alters
the spring compression.
As the engine speed increases the weights move outwards and raise
the spindle; a speed decrease will lower the spindle.
The hydraulic unit is connected to this vertical spindle and acts as a
power source to move the engine fuel controls. A piston valve connected
to the vertical spindle supplies or drains oil from the power piston which
moves the fuel controls depending upon the flyweight movement. If the
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 .
The actual arrangement of mechanical engine governors will vary
considerably but most will operate as described above.
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.
Governors and Over-speed Trips
These must be fully operational and regularly tested in accordance with manufacturers’ instructions.
Attention is drawn to the testing of over speed trip and protection devices.
The condition of the linkage coupling the engine’s fuel pump actuating levers and the governor is also to be regularly examined. The governor cannot compensate for either seized fulcrum pins or excessive clearances.
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