The purpose of any fuel supply system is to deliver fuel to the engine in a form in which it can mix with air to form a combustible mixture.
The fuel must be finely atomized to mix with the air. This is why injection systems deliver the fuel under pressure.
The EFI system is a circulation system. Fuel is drawn from the tank by a fuel pump, and delivered to solenoid-operated injection valves. Fuel pressure at the valves is maintained by a fuel pressure regulator, and excess fuel flows back to the tank through a return line.
The fuel filter is directional. It is fitted between the pump and injection valves, to stop contaminants reaching other parts of the system.
The fuel pump can be mounted externally on the chassis frame, or submerged in the fuel in the tank. It is electrically operated, and electronically controlled.
It is driven by a permanent-magnet electric motor, a sealed unit integral with the pump. Fuel flows through the pump and around the electric motor when it is running. There is never an ignitable mixture inside the pump housing so there is no danger of explosion.
The pump delivers more fuel than the maximum requirement of the engine, so pressure in the fuel circulation system is maintained at all times. A fuel pressure regulator maintains fuel pressure between the pump and the injectors.
