Final answer:
A pilot valve is used in prime mover governing systems to allow hydraulic oil flow to operate the throttle valve, working on the principle of pressure transmission within a hydraulic system.
Step-by-step explanation:
The type of hydraulic relay commonly used on prime mover governing systems that allows hydraulic oil flow to operate the throttle valve is known as a pilot valve. Hydraulic systems work on the principle of transmitting pressure through a liquid to exert force. For instance, applying a force on a small piston results in a greater force being exerted by a larger piston, as described in Figures 11.16 and 14.16. This concept is essential in systems where controlling fluid power efficiently is necessary, such as in a prime mover governing system. A pilot valve is a precise control device that regulates this flow, serving as a key component to ensure the proper operation of such hydraulic mechanisms.