Before considering the operating principles of various types of governors, you should become familiar with the following terms:

1. SPEED DROOP is the decrease in speed of the engine from a no-load condition to a full-load condition. Speed droop is expressed in rpm or (more commonly) as a percentage of normal or average speed.

2. ISOCHRONOUS GOVERNING is main-taining the speed of the engine truly constant, regardless of the load. This means governing with perfect speed regulation or zero speed droop.

3. HUNTING is the continuous fluctuation (slowing down and speeding up) of the engine speed from the desired speed. Hunting is caused by overcontrol by the governor.

4. STABILITY is the ability of the governor to maintain the desired engine speed without fluctuations or hunting.

5. SENSITIVITY is the change in speed required before the governor will make a corrective movement of the fuel control mechanism and is generally expressed as a percentage of the normal or average speed.

6. PROMPTNESS is the speed of action of the governor. It identifies the time interval required for the governor to move the fuel control mechanism from a no-load position to a full-load position. Promptness depends on the power of the governor; the greater the power, the shorter the time required to overcome the resistance.

Now that you have read the definitions of some of the terms associated with governors, we will proceed with our comparison between mechanical and hydraulic governors. You should refer to views A and B of figure 9-26 and figure 9-27 as you read this information.