Which pump type varies discharge with engine demand?

• A. Fixed displacement pump
• B. Gear pump
• C. Variable displacement pump
• D. Piston pump

Answer: (C)

Discharge varies with engine demand when the pump can change how much fluid it moves with each rotation. A variable displacement pump does just that: by adjusting the pump’s geometry (for example, the swashplate angle in an axial-piston design), it changes the volume per revolution. When the engine needs more fuel, the pump increases displacement so more fuel is delivered each cycle and pressure stays within target. When demand is low, it reduces displacement to lower flow and conserve energy.

Fixed displacement pumps, like typical gear pumps, move a constant volume per revolution, so their output mainly follows engine speed and doesn’t automatically match changing demand. That’s why they don’t vary discharge with engine demand.