An external fuel pump is designed to transport fuel from the fuel tank to the engine’s combustion chamber. Here’s a detailed explanation of how it works, particularly focusing on the most common mechanisms such as impeller and gear-rotor types:
- Location: As the name suggests, external fuel pumps are located outside of the fuel tank, often on the vehicle’s undercarriage. This position allows for easy access for maintenance or replacement, compared to in-tank pumps.
- Impeller Type: One common type of external fuel pump is the impeller, or ‘bladed’ pump. In this design, a set of rotating blades (the impeller) spins inside a housing. As the blades spin, they draw fuel in through an inlet valve and push it outward through centrifugal force. The movement of the blades creates a low-pressure area at the inlet, which helps to pull fuel into the pump, and a high-pressure area at the outlet, which pushes the fuel towards the engine.
- Gear-Rotor Type: Another common design is the gear-rotor pump. In this mechanism, two gears fit closely together inside a housing. As they rotate, fuel is trapped between the teeth of the gears and the outer casing, and it is transported from the inlet side to the outlet side. The rotation creates a vacuum at the pump’s inlet which draws fuel out of the tank, and the meshing of the gears pushes the fuel through the pump and out to the engine.
Regardless of the design, the basic steps of operation are as follows:
- Activation: When the vehicle’s ignition is turned on, the fuel pump is activated, usually by an electrical control unit that signals the pump to begin working.
- Suction: The pump then starts to rotate (whether it’s the impeller or the gears), creating a vacuum that draws fuel out of the tank through a line connected to the pump’s inlet.
- Pressurization: As the fuel is drawn into the pump, it is pressurized by the mechanism of the pump—either by being flung outward by the impeller blades or carried between the meshing gears.
- Delivery: The pressurized fuel is then forced out of the pump through another line, which leads to the engine. Along the way, the fuel may pass through a fuel filter to remove any contaminants.
- Regulation: The pressure of the fuel is regulated to ensure that the engine receives it at the pressure required for optimal combustion. Any excess fuel not used by the engine is typically returned to the fuel tank through a return line.
- Safety Feature: External fuel pumps are equipped with a relief valve that opens if the pressure becomes too high, allowing excess fuel to return to the tank, thus preventing damage to the fuel system.
The fuel pump must provide a consistent supply of fuel under various conditions and engine demands, and it must do so at the pressure and volume that the engine requires. If the fuel pump fails to deliver the necessary fuel with adequate pressure, the engine may experience poor performance or may not run at all.