Mechanical governors

These fall into the oldest class of governors whose history goes back to the invention of the steam engine.

Watt type

The governor assembly is directly driven from the engine. Flyweights are rotated and act to draw the slide up the drive shaft due to centrifugal force pushing them out

Hydraulic governors

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

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 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.


This allows for the stabilising 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


. 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.

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

Electric Governors

Electric governors have become in favour due to their compact size, rapid response and high reliability allied to low maintenance costs.

The main part of the governor is the controller and signal amplifier. This receives a D.C. signal proportional to the engine speed and compares it to a speed set signal. The difference between the measured value (engine speed) and the set value is the offset, this offset value is passed to the output circuit which produces an appropriate output signal . In this case, a signal which raises or lowers the fuel rack by an amount dependent on the degree of offset. This system is inherently stable due to the feedback layout.

For this system the engine speed is measured using an alternator driven off the camshaft- this is a common arrangement. The speed set signal is typically supplied by the bridge control arrangement via the engine management system.

An arrangement for a generator set might replace the camshaft driven alternator with a tapping off the alternator output. The frequency of the alternator output is now the measured value. In addition a load sensing element can be introduced detecting changes in current flow. For increased current, that is an increased electrical load, the governor can act to supply increased fuel before the engine has began to slow.