Short Answer:

Hunting of a governor is the continuous or rapid fluctuation of engine speed above and below the mean speed when the governor tries to adjust the fuel supply. It happens because the governor over-corrects or under-corrects the engine speed instead of stabilizing it. Hunting causes the engine to run unevenly and may lead to mechanical vibration or damage.

This problem usually occurs in very sensitive governors or when there is no proper damping system. To avoid hunting, suitable damping mechanisms such as oil dashpots or friction devices are used to stabilize the governor’s movement.

Detailed Explanation :

Hunting of a Governor

The term hunting of a governor refers to the continuous fluctuation of engine speed around the mean speed due to the over-sensitive action of the governor. When the engine load changes, the governor adjusts the fuel supply to maintain a constant speed. However, in some cases, the adjustment is not perfectly balanced — the governor may overreact, causing the engine to alternately speed up and slow down repeatedly. This phenomenon is called hunting.

Hunting represents a state of unstable operation and is undesirable in engines because it leads to irregular running and possible mechanical damage. A properly designed governor should have enough damping to prevent hunting and bring the system quickly to a steady speed after any load change.

Cause of Hunting

Hunting occurs due to a delay or over-response in the governor’s corrective action. When the engine speed changes, the governor balls move and adjust the fuel supply. But if the correction is too much, it reverses the speed deviation and causes another correction in the opposite direction. This repeated over-correction leads to continuous oscillation around the mean speed.

Some main causes are:

Excessive Sensitivity:
If a governor is too sensitive, it reacts even to very small speed changes. This leads to repeated adjustments and oscillations.
Lack of Damping:
Without a proper damping mechanism like a dashpot, the sleeve moves freely and may continue to oscillate even after the engine speed has stabilized.
High Inertia of Moving Parts:
Large inertia of rotating components delays the corrective movement, causing overreaction once it starts moving.
Elastic Connections:
Flexible or elastic linkages between the governor and throttle can cause delayed motion and result in hunting.
Improper Spring Tension:
In spring-loaded governors, incorrect spring stiffness can lead to over or under correction of the speed.

Effect of Hunting

Hunting adversely affects both the performance and life of the engine. Some of its effects include:

Speed Instability:
The engine speed continuously fluctuates and never remains constant.
Increased Fuel Consumption:
Continuous change in throttle position increases fuel wastage.
Mechanical Vibrations:
Rapid changes in engine speed create vibrations, which may cause wear or damage to the mechanical parts.
Uneven Power Output:
The load on the driven machine keeps varying, leading to poor performance and reduced efficiency.
Possible Damage to Governor Mechanism:
Frequent oscillations may lead to wear or breakage of the governor arms, links, or sleeve.

Methods to Prevent Hunting

To control or prevent hunting, certain design and operational measures are used in governors:

Damping Devices:
Damping is the most effective method to prevent hunting. A dashpot filled with oil provides resistance to sleeve movement, helping it settle gradually rather than oscillating.
Frictional Resistance:
Slight friction at the sleeve or linkages prevents unnecessary movements and brings stability.
Reducing Sensitivity:
A governor with moderate sensitivity reacts only to noticeable speed variations, thus avoiding over-correction.
Proper Adjustment of Spring Force:
The stiffness of the spring should be adjusted so that the sleeve movement matches the required correction level.
Avoiding Elastic Connections:
Using rigid connections between governor and throttle prevents delayed actions.
Balanced Design:
The masses of moving parts and the speed of response should be properly balanced for stable control.

Example of Hunting in Operation

Consider an engine driving a generator. If the electrical load suddenly decreases, the engine speed increases. The governor detects this and reduces the fuel supply to bring the speed down. However, if it reduces too much fuel, the speed drops below normal, prompting the governor to again increase fuel supply. This cycle continues repeatedly, resulting in oscillations — this is hunting.

A well-damped governor, however, would slow down the correction process, gradually stabilizing the speed at the desired value.

Importance of Damping in Preventing Hunting

Damping plays a crucial role in stabilizing a governor. Without damping, the sleeve moves freely and oscillates for a long time. With proper damping, the movement becomes smooth, and the sleeve comes to rest quickly after a few small oscillations. Dashpots filled with viscous oil are commonly used for this purpose in mechanical governors like Hartnell or Porter types.

Damping ensures that the governor does not react excessively and maintains a stable engine speed without unnecessary fluctuations. Thus, it is a necessary feature for smooth and efficient governor performance.

Practical Examples

Steam engines: Governors are used to maintain a steady turbine speed. Hunting can cause power fluctuations and vibrations in turbines.
Automobile engines: In cars, hunting can make the engine idle roughly or cause jerks while driving.
Power plants: Generators must maintain a fixed frequency; hunting causes electrical instability and fluctuations in output.

Conclusion

Hunting of a governor is an undesirable condition where the engine speed continuously fluctuates around its mean value due to over-sensitive or poorly damped governor action. It reduces engine efficiency, causes vibrations, and leads to wear of mechanical components. To prevent hunting, damping devices, proper spring tension, and moderate sensitivity are essential. A well-designed governor should respond promptly to load changes but return smoothly to a stable speed without oscillations.