For engine driven emergency power supplies, which of the following devices maintains a relatively constant...

Governor

For engine driven emergency power supplies, the device that maintains a relatively constant speed of the prime mover throughout the full power output range by varying the fuel input to the prime mover is the governor. A governor is a device that automatically regulates the speed or power of an engine or other prime mover by adjusting the amount of fuel supplied to it. A governor senses the speed of the prime mover and compares it with a desired set point, and then controls the fuel valve to increase or decrease the fuel flow accordingly.By doing so, the governor ensures that the prime mover operates within a narrow range of speed or power, regardless of the load or other factors that may affect its performance12