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A simple mechanical governor must overcome friction in
the linkages and exert a controlling force. These forces act in
different directions depending upon whether the load is increasing or
decreasing.The effect of this friction is to create a deadband
In hydraulic governors this effect is negated by having oil pressure act as the controlling force
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This simple system has inherent stability due to the the on/off nature
of oil being suppled to the system control due to the control land just
covering the outlet ports. Oversizing the land would create stability
but at the expense of reintroducing a deadband
Servoed system with feedback
An Alternative is to lead the outlet oil to a servo system. The servo piston can be either spring return
or Differential
In the former case the servo is held in the decrease fuel position by
spring pressure, in the latter the servo piston is pushed down by supply
oil pressure. Note that the control side of the servo piston has
greater areas than the supply oil side therefore when control oil is
supplied it is able to lift the piston against it
Should the engine speed fall the flyweights will tend to fall towards
the axis rotation due to pressure from the speeder spring overcoming the
reduced centrifugal force. The pilot valve moves down and the control
land allows oil to flow to the servo piston raising it. When the engine
speed increases the flyweights begin to overcome the speeder spring and
the pilot valve moves up covering the servo supply port
In this design
Setpoint may be varied by use of an adjusting screw altering the compression of the speeder spring. In addition
Feedback is given to increase stability. The term applied to this is
Droop
Droop
Droop is defined as the reduction in speed compared to set speed over full load change x 100 as is expressed as a percentage
When the governor reacts to load change then inertia of the engine
response can lead to overshoot in speed change which can have a
cumulative effect. To prevent this a feedback system is used. In the
case of the governor systems above this has the effect of modifying the
speed set point .
Should the engine speed fall the flyweights will tend to fall towards
the axis rotation due to pressure from the speeder spring overcoming the
reduced centrifugal force. The pilot valve moves down and the control
land allows oil to flow to the servo piston raising it. This increases
the fuel supply to the engine but also reduces the speed set point as
the feed back lever is also raised moving the connection to the speed
spring upwards reducing spring pressure. The Flyweights ar able to raise
the pilot valve closing off the supply of oil to the servo
The engine will now run with some degree of stability. However it will not run at set speed.
Compensation
This allows for the stabiiising effect of droop but maintains original set point speed.
Should the engine speed fall due to the impact of increased load
thecontrol land will fal allowing supply oil pressure to pass through.
As well as forcing up the servo piston via the action on the buffer
spring it also acts on the underside of the compensation land were it
tends to push the pilot valve up against the force of the speeder
spring. The pressure differential across the compenation land bleeds off
via the compensation screw as the engine returns to normal speed
This is
known as
temporary droop
Alternately
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Compensation takes place to provide a further slight fuel change to
return the speed to normal. The centering spring forces the receiving
piston downwards and oil escapes through the adjustable valve.. This
lowers that end of the floating lever until both centering springs are
equally loaded and that end of the floating lever is in its original
position. The pilot valve is open slightly allowing oil to the servo
which gives a further slight increase in fuel. The engine speeds up, the
rotating weights move out and the pilot valve is lifted until it is
closed. The engine now operates with increased load, increased fuel but
at the same original speed. class="noindent">
Complete assembly
Load sharing
Engines share load increase in the inverse ratios of their speed
droop. i.e. the lower the value of droop the greater the share of the
load increase taken
