Which device regulates the idling speed and maximum speed of the engine by controlling the rate of fuel delivery?

• A. Governor
• B. Throttle
• C. Fuel pump
• D. Engine control unit

Answer: (A)

Speed control in a diesel engine is achieved by a governor, which regulates engine speed by adjusting how much fuel is delivered to the injectors. The governor continuously compares the actual RPM to a set target and changes fuel flow to keep idle speed steady and to cap the maximum speed, preventing overspeed. In traditional mechanical governors, centrifugal weights respond to engine speed and move a linkage that increases or decreases fuel delivery. Modern systems may use hydraulic or electronic governors, where sensors and control circuitry (or an engine control unit) fine-tune fuel delivery to meet the speed targets under varying loads. The throttle mainly governs air intake and operator input, but it isn’t the primary mechanism for maintaining a governed speed. The fuel pump supplies fuel, but it doesn’t independently regulate engine speed without the governor’s feedback. The engine control unit can influence ignition timing and fuel quantity, but the device whose primary role is to keep RPM at a set value by adjusting fuel flow is the governor.