An engine-driven fuel pump connects directly to the aircraft engine, is attached to the accessory drive, and is installed to pressurize and distribute oil, fuel, or hydraulic fluid. Mechanical fuel pumps driven by engine power usually serve as the primary pumps, while an electric pump serves as secondary. A centrifugal boost pump located in or below the fuel tank starts the fuel system; the fuel pump then draws fuel through an inlet port so the complete fuel system can transfer fuel from the fuel tanks to the engine.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is an engine-driven pump in an aircraft?
An engine-driven pump (EDP) is a pump attached to an accessory drive, and it connects directly to the aircraft engine through mechanical linkage. Engine-driven fuel pumps deliver the consistent fuel flow the aircraft needs for safe operations, and they work as long as the engine runs. Most engine-driven fuel pumps include a bypass valve, and they work in concert with electric pumps.
An engine-driven pump (EDP) is a pump attached to the accessory drive of a turbojet or turboprop engine. The accessory drive, itself connected by reduction gearing to the main engine shaft, normally links to the high-pressure or N2 shaft. Because of this mechanical linkage, the pump turns whenever the engine turns, fluid delivery during operation. In the hydraulic fuel system the EDP can be installed to pressurize or distribute fuel. If it fails, an electric pump serves as backup.
What is the function of an engine-driven pump in an aircraft?
The function of an engine-driven pump in an aircraft is to convert mechanical energy from the main engine into hydraulic power or fuel-boost energy for aircraft systems. One primary task is to draw fuel from the fuel tanks and increase its pressure so that it delivers at the correct pressure for engine operation. High-wing aircraft with fuel-injected engines, as well as low-wing aeroplanes, need this pump to move fuel because gravity assistance is inadequate without additional pressurisation. The pump increases the fluid's pressure through an impeller or piston mechanism, after which the pressurised fluid is directed to various aircraft systems.
On a turbine engine the same EDP can be installed to pressurise and distribute fuel, oil, or hydraulic fluid. When it pressurises fuel it acts as a high-pressure pump that supplies fuel to the injectors. The resulting burnable spray is atomised inside the combustion chamber at pressures capable of overcoming combustion that exceeds 600 psi in modern engines. When an electric pump fails, the EDP serves as a backup, and conversely, when the EDP fails, the electric pump assumes the load, maintaining an uninterrupted supply.
Beyond fuel, hydraulic versions of the pump convert mechanical energy into hydraulic power that is then consumed by hydraulic loads within the aircraft, like actuators for control surfaces, landing gear, and doors. Boost pumps are sometimes staged with EDPs to provide an additional boost to fuel pressure during takeoff or landing when higher flow and pressure are temporarily required, but they do not replace the continuous duty of the engine-driven unit.
Where is the engine-driven pump located?
The engine-driven pump is mounted on, and driven by, the accessory drive that is normally connected to the N2 (high-pressure) shaft. This accessory drive is housed inside the engine accessory gear box, which itself is located on the aft side of the engine. Because the pump is installed on the aft side of that gear box, it sits immediately behind the engine core, clear of the intake and exhaust paths. A QUAD ring (Quick Attach Detach) secures the pump to the accessory pad, so the unit can be pulled or pushed for rapid replacement while ground service provisions for each system are located in the main wheel wells.
Is a hydraulic pump engine driven?
Yes, on almost every aircraft the principal hydraulic pump is engine-driven. In this application the hydraulic pump is a hydraulic system power element that is driven by an engine to convert mechanical energy into hydraulic energy that moves flight controls, landing gear and brakes. The power source of the hydraulic system is usually a diesel engine. Engines are the prime power source that drives the pump in a hydraulic system for every application, and the hydraulic pump is directly connected to the engine through a hydraulic pump drive, a device that connects the prime mover to the hydraulic pump. A single pump direct drive consists of a flex plate and bell housing plate coupled to one hydraulic pump, while multi-pad pump drives have a gear train that drives the hydraulic pumps at the optimal RPM while running the engine at its optimal RPM. The hydraulic pump drive shaft usually uses a coupling to connect the drive shaft and the pump drive shaft, and pump drive input style is a drive plate input that bolts to the flywheel and housing of the engine. Whatever the exact arrangement, the hydraulic pump draws oil from the hydraulic tank, forms pressure oil and delivers liquid to the pump outlet, creating the flow and pressure needed to operate actuators. Thus, the engine turns the hydraulic pump continuously whenever it is running, making an engine-driven pump the normal source of energy in the hydraulic system.
How does the engine-driven fuel pump work?
When the engine starts, the engine-driven pump is activated to begin pumping fuel from tanks. The pump is either mechanical or electric; if mechanical, an accessory drive connected to the ‘N’ shaft turns an eccentric cam in the accessory case. The cam passes under a pivoted lever and forces one end up while the other end of the lever, linked loosely to a rubber diaphragm forming the floor of the pump chamber, goes down and pulls the diaphragm with it. This creates suction that draws fuel along the fuel pipe into the pump through a one-way valve. A return spring then raises the diaphragm, expelling the fuel through another one-way passage toward the fuel injectors.
On engines fitted with a high-pressure GDI fuel pump, the same shaft motion pushes a plunger up into the pumping chamber. The high-pressure GDI fuel pump runs whenever the engine runs and ensures the engine gets the required fuel pressure it needs to run well. It creates high enough pressure so the fuel fully atomizes, appropriate pressure obtains accurate fuel regulation and satisfactory nozzle spray. The fuel injectors then spray a fine mist into the engine's combustion chamber, igniting it to produce the power needed for flight.
Throughout the cycle, fuel delivered matches what the engine needs at each power setting. A filtering unit located in the lowest part of the system removes water and dirt from the fuel, and an internal bypass valve protects the circuit if the filter blocks. By combining engine-driven motion, one-way valving, and pressure-raising plungers, the engine-driven pump reliably supplies clean, correctly atomized fuel in the quantity the engine demands.
What is the purpose of an engine-driven fuel pump bypass valve?
The primary purpose of an engine-driven fuel pump bypass valve is to allow fuel to flow around the engine-driven pump for emergency operation if that pump fails. When the engine-driven pump becomes damaged or inoperative, the bypass valve prevents it from blocking the fuel flow of another pump in series with it.
During engine starting, the bypass valve allows fuel from the booster pump to reach the engine. A check valve is provided so that boost pump pressure to the system bypasses the engine-driven pump for starting. The boost pump forces fuel through a bypass in the engine-driven pump to the metering device, guaranteeing the engine receives the necessary fuel pressure for ignition.
How is the pulsating electric fuel pump arranged with the engine-driven pump in an aircraft?
The pulsating electric fuel pump is arranged in parallel with the engine-driven pump (EDP). EDP is connected by reduction gearing to the main engine shaft. Pumps installed to supplement engine driven pumps must be installed in parallel with the engine pump. The electric pump provides the initial fuel pressure when starting the engine, serving as the secondary pump. In the event of EDP failure, the electric pump can serve as backup. Conversely if EDP fails electric pump will serve as backup. Low wing aircraft need an electric fuel pump for takeoff and landing. Low wing designs must pump fuel. Boost pumps provide additional boost to fuel pressure during takeoff and landing. A PWM controller changes the speed of the fuel pump to supply engine demand. OEMs control the speed of the pump by using Pulse Width Modulation. OEMs control speed of the pump by using PWM.
What happens to excess fuel discharged by the engine-driven fuel pump?
The excess fuel discharged by the engine-driven fuel pump is returned to the pump interstage. The engine-driven pump delivers more flow than the fuel control assembly can accept. Fuel pump discharge flow in excess of that required by the fuel control assembly is returned from the control to the pump interstage. A pressure relief valve is incorporated in the discharge port of the pump. It opens at approximately 900 psi and is capable of bypassing total flow at 960 psi, so excess fuel is recirculated back to the pump inlet. This same characteristic requires the use of a pressure relief valve for disposing of excess fuel, assuring that pressure is prevented from reaching an excessive level by sending fuel through the pressure relief valve into a vent line.
What is a constant-displacement engine-driven pump in an aircraft?
A constant-displacement engine-driven fuel pump is a mechanical device mounted on the engine accessory gearbox that converts engine rotation into a fixed, predictable flow of fuel. Gear-type constant-displacement pumps are the most common form used on turbine aircraft. Inside the pump, meshed gears trap and carry fuel from inlet to outlet so that, for every revolution of the drive shaft, the same volume of fuel is expelled regardless of system demand. Because displacement is fixed, the output flow rises or falls only with engine speed, making the relationship between rpm and fuel delivery linear and repeatable.
To protect the fuel system from over-pressurization when demand is low or when the engine is at high rpm, gear-type constant-displacement pumps incorporate a pressure relief valve that opens and recirculates excess fuel back to the inlet side, thereby holding system pressure within design limits. The term ‘positive displacement’ applies because internal slippage is negligible. Almost every molecule of fuel swept by the gear teeth reaches the outlet, giving the pump high volumetric efficiency. In contrast, large transport aircraft often use variable-displacement pumps whose internal swash-plate can change stroke length, allowing the unit to modulate flow and maintain near-constant pressure without relief valves, but for many jet engines the simple, rugged gear-type constant-displacement unit remains the preferred choice for reliable, engine-paced fuel delivery.
What is the most common type of engine driven fuel pump?
Vane-type fuel pumps are the most common types of fuel pumps found on reciprocating-engine aircraft. The basic mechanism of a vane-type fuel pump involves a rotary vane installed eccentrically inside the pump, and a pressure relief valve is incorporated to protect the system. A compensated vane pump is used in engine-driven applications to maintain consistent output despite changes in speed or demand.
What is the difference between an engine-driven fuel pump and an auxiliary fuel pump in an aircraft?
The difference between an engine-driven fuel pump and an auxiliary fuel pump is that the engine-driven unit mounts on the accessory case of the engine and is mechanically geared to the engine and turns whenever the engine is running. The auxiliary pump is a secondary, electrically driven unit powered by a self-contained electric motor controlled from the cockpit panel.
The engine-driven pump is the main mover of fuel from the tanks to the carburetor or fuel control unit while the airplane is in normal flight. The auxiliary pump is used primarily for starting, for supplying pressure if the mechanical pump fails, and for maintaining pressure at high altitudes where vapor lock forms. If the primary pump cannot keep up, the electric pump supplements it. In extreme cases the auxiliary pump can take over completely, guaranteeing that the engines continue to receive sufficient fuel.
In higher-power, high-altitude, high-wing designs the auxiliary pump is installed as a mandatory back-up under approved Type Certificates. If it is turned off, the engine-driven pump can still move fuel from the selector valve through the strainer on its own, but the electric unit gives the pilot an independent way to pressurize the system for priming, to suppress vapor formation in hot fuel, and to meet transient flow demands during take-off or landing.
