The governor is an automatic adjusting device that increases or decreases the fuel supply of the fuel injection pump based on changes in the diesel engine load. This enables the diesel engine to maintain a stable speed.


The governor has been widely used for industrial DC motor speed regulation, industrial conveyor speed regulation, lighting control and regulation, computer power supply cooling, and DC fans, among other applications.

Mode of operation

A governor is utilized to reduce the automatic adjustment of non-periodic speed fluctuations in some machines, keeping the machine speed constant or close to the set value.

Unlike turbines, steam turbines, gas turbines, and internal combustion engines do not automatically adapt their output torque to their own load changes, which results in a loss of stability for the units driven by them when the load changes.

Therefore, this type of unit must be equipped with a governor to establish an adaptive relationship between the load and the energy supply at any time according to conditions like load, ensuring proper operation of the unit.

Theoretical and design issues related to governors are the research content of mechanical dynamics.

There are several types of governors, with the mechanical centrifugal governor being one of the most widely used. However, governors with tachogenerators or other electronic devices as sensors have also been widely used in various industrial sectors.

The governor must meet stability conditions, which are as follows:

① The governor must respond appropriately when the unit speed deviates from the set value, and there must be a repetitive force that acts to return the governor to its initial state.

The centrifugal governor’s spring produces the restoring force, making it a statically stable governor. However, statically stable governors may exhibit dynamic instability during the adjustment process, resulting in an oscillation process when there is excessive adjustment.

A governor that ensures the oscillations decay quickly is called a dynamically stable governor. If not, it is a dynamically unstable governor that cannot guarantee the normal operation of the machine.

② Increasing the damping in the control system is one way to improve dynamic stability.

Damping in the control system, such as friction in the motion pair, results in a certain insensitivity of the governor. This means that when the speed of the controlled shaft deviates slightly from the set value, the governor does not generate a corresponding action.

The typical insensitivity of a mechanical governor is about 1% of its set point.

Governors that are excessively sensitive will also result in unacceptable adjustments due to periodic speed fluctuations during normal operation of the unit.

The governor is responsible for maintaining the speed of the diesel engine stable.

During changes in the load of the diesel engine, its rotational speed will change accordingly.

If the speed is reduced and the governor is not adjusted, the diesel engine will eventually stop.

If the speed increases and the governor does not function, the diesel engine will eventually be unable to withstand excessive centrifugal force and become damaged.

The governor’s role is to maintain the speed of the diesel engine stable. Additionally, the governor can ensure the minimum and maximum speed of the diesel engine are maintained to prevent low-speed operation from exceeding the minimum speed and high-speed operation from exceeding the maximum speed, which could result in mechanical damage.

Main classification

According to its working principle, it can be divided into:

  • Mechanical
  • Pneumatic
  • Hydraulic
  • Mechanical and pneumatic composite
  • Mechanical hydraulic composite
  • Electronic, etc.

The mechanical governor is the most widely used type due to its simple structure, reliable operation, and excellent performance.

In contrast, the hydraulic governor includes a hydraulic amplifying element (known as a hydraulic servo) positioned between the sensing element and the oil quantity adjusting mechanism. This enables the output signal of the sensing element to be transmitted to the oil quantity adjusting mechanism through the amplifying element, thus making it an indirect action type adjustment governor.

The hydraulic amplifying element consists of two main parts: control and execution, and it serves both an amplification and execution function.

1. Hydraulic governor without feedback

Its working principle is as follows:

When the load is reduced, the rotational speed of the drive shaft, driven by the crankshaft, increases. This causes the centrifugal force of the flying ball to increase, pushing the speed lever to the right.

Consequently, the rocker rotates counterclockwise around point A, moving the spool to the right. As a result, pressurized oil enters the right space of the servo cylinder, while the oil in the left space of the cylinder is discharged through the oil hole to the low-pressure oil passage.

Due to the pressure difference, the servo piston drives the fuel pump rack to the left, thus reducing the oil supply. When the speed returns to the original value, the spool valve returns to the center position, and the adjustment process ends.

If the load increases and the speed decreases, the speed control process proceeds in the opposite direction.

Based on the above analysis, the centrifugal force generated by the governor flying ball is solely utilized to push the slide valve, which means that the weight of the flying ball can be smaller.

As a force of the hydraulic servo amplifier, different sizes of servo pistons and varying oil pressures can be chosen to amplify as needed.

In a governor where the sensing element directly drives the spool valve, it can be challenging to return to the original position and close the oil hole accurately, regardless of its direction of movement. As a result, the engine speed becomes unstable and experiences severe fluctuations.

To stabilize the governor, a device is incorporated that counters the slide valve when the servo piston is in motion, moving it to the equilibrium position and decreasing the likelihood of speed fluctuations in the diesel engine. This device is commonly known as a feedback mechanism.

  1. Hydraulic governor with rigid feedback mechanism

The construction of this hydraulic governor is essentially the same as the non-feedback version mentioned above, except for one difference: the upper end A of the lever sense AC is attached to the piston rod of the servo piston, rather than the fixed hinge.

This change alters the relationship between the sensing element, the hydraulic amplifying element, and the oil amount adjusting mechanism in the following way:

When the load is reduced, the engine speed increases, causing the flying ball to move outward and drive the speed lever to the right. At this point, the servo piston has not yet activated, so the upper end point A of the feedback lever AC temporarily acts as a fixed point.

The lever rotates counterclockwise around A, causing the spool to move to the right and open the control hole. High-pressure oil enters the right chamber of the power cylinder, while the left chamber communicates with the low-pressure oil passage.

This high-pressure oil pushes the servo piston to move the fuel injection rod to the left, reducing the fuel supply according to the new load. As the servo piston moves to the left, the lever AC swings to the left at point C, causing the spool connected to point B to move to the left as well, thereby moving the spool in the opposite direction.

A lever device that can interfere with the movement of the spool when the servo piston moves is called a rigid feedback system.

Once the adjustment process is complete, the spool returns to its initial position, shutting the control oil hole and blocking the oil path to the servo cylinder.

At this point, the servo piston comes to a halt, while the fuel pump adjustment lever moves to a new equilibrium position, and the engine operates under the corresponding load.

Hence, the governor produces various steady speeds corresponding to the engine’s different loads.

Because the fuel supply must be altered when the engine load changes, the position of point A shifts with the load.

Point B, which is linked to the spool valve, must remain in its original position under any stable conditions, irrespective of the load.

Therefore, the position of point C must be adjusted correspondingly to point A, resulting in a change in the rotational speed.

If the load is reduced, after the speed control process is complete, the servo piston is positioned to decrease the fuel supply, causing point A to move to the left and point C to move to the right. As a result, the spring comes under more pressure.

The centrifugal force of the flying ball can only be balanced with the spring pressure at a slightly higher speed.

This indicates that, after reducing the load during steady operation, the speed of the diesel engine needs to be slightly higher than the original speed.

Similarly, when the load increases and reaches stable operation, the speed of the diesel engine needs to be slightly lower than the original speed.

To ensure stable working characteristics of the speed regulation process, a hydraulic governor with rigid feedback is used.

However, when the load changes, the engine speed also changes, and the steady rate “t” cannot be zero.

If there is a requirement for load change while maintaining a stable speed regulation process, an elastic feedback system is necessary, which can be achieved by using a hydraulic dispatcher.

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