What type of fuel pump can vary the discharge regardless of speed?

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

Answer: (D)

The essential idea is that flow depends on how much fuel the pump moves with each rotation, not just how fast the pump is turning. A pump that can change its displacement per revolution can vary the amount of fuel it delivers at a given speed, so the discharge can be adjusted independently of rpm. In a fixed displacement pump, the volume moved per revolution is constant, so the output changes only with speed; you can’t dial in a different flow without changing the speed. A centrifugal pump’s flow is governed by its speed and the system pressure, and while you can influence it with system valves or speed changes, the pump itself doesn’t vary its discharge through displacement. A gear pump also has a fixed displacement per revolution, so its flow tracks rpm rather than being adjustable at a constant speed. The variable displacement pump, by design, alters the volume moved each cycle, allowing you to set different discharge levels without changing speed, which is exactly what’s asked.