Exam Question

(a) Operation of a Mechanical Hydraulic Governor
A mechanical, hydraulic governor utilises centrifugal force generated by rotating flyweights to regulate engine speed. The flyweights are mounted on a rotating sleeve whose speed is directly proportional to the engine's speed (often through a gear). As engine speed changes, the centrifugal force acting on the flyweights varies, compressing or expanding a spring. This spring movement acts on a pilot valve, which controls the flow of hydraulic oil to a power piston or servo piston.
Increased Load (Reduced Speed):  When the engine load increases, the engine speed drops. The decrease in centrifugal force allows the spring to push the pilot valve down. This allows pressurised oil to flow under the power piston, forcing it upwards. The upward movement of the power piston increases the fuel supply to the engine, increasing the engine's speed. Simultaneously, the power piston's movement reduces the spring pressure on the pilot valve, causing it to rise and cut off the oil supply, stopping the adjustment.
Decreased Load (Increased Speed):  Conversely, if the load decreases, the engine speed increases. The increased centrifugal force compresses the spring, moving the pilot valve upwards. Oil drains from under the power piston, causing it to move down under spring force, reducing the fuel supply and engine speed. The downward movement of the power piston releases the spring pressure, causing the pilot valve to move down and stop the oil flow.