Fuel Pump: Description and Operation

Fuel Tank And Pumps:

PURPOSE AND LOCATION
The electric fuel pumps supply fuel to the injectors. The feed pump is located in the fuel tank, the main pump is located under the car beneath the rear seat.
The feed pump delivers the fuel to the main pump. The feed pump's main purpose is to reduce the work load of the main pump, therefore increasing the pumps service life.
The main pump supplies the fuel system with the fuel pressure/amount needed to operate the injection system.
Operation of the pump is controlled by the fuel pump relay (part of [1][2]system relay), which is energized by the fuel injection control unit. The relay will only be energized during cranking (starting) or while the engine is running. This will keep the fuel pumps from running when the engine has stalled in an accident or similar incident. The control unit uses the engine speed signal as an input for fuel pump control.

Fuel Pump (Typical):

CONSTRUCTION
The electric motor actually operates in the fuel in the fuel pump housing. This is possible, because the housing never contains an ignitable mixture. The fuel is used as a lubricant and coolant for the pump motor.
A check valve is incorporated in the fuel outlet and a pressure limiter valve is located on the pressure side of the fuel pump housing with a return passage to the fuel inlet side.
The pump components consist of the following:

For feed pump: The feed pump is of the blade pump type. The pump chamber itself is cylindrical, in which an eccentrically fitted pump rotor disc rotates. This is fitted with metal blades which are held in slot-shaped recesses around the circumference.
In order to maintain a constant pressure in the fuel system under all possible operating conditions, the pump supplies substantially more fuel than the engine would ever need at any engine load.

Roller Pump (Located In Fuel Pump):

For main pump: The main pump is of the roller-cell pump type. The pump chamber itself is cylindrical, in which an eccentrically fitted pump rotor disc rotates. This is fitted with metal rollers which are held in pocket-shaped recesses around the circumference.
In order to maintain a constant pressure in the fuel system under all possible operating conditions, the pump supplies substantially more fuel than the engine would ever need at any engine load.

OPERATION
When the rotor disc or blade disc rotates, the rollers or discs are pressed outwards by centrifugal force and act as a circulating seal. A pumping action is created by the circulating rollers or blades which draw in fuel at the inlet port and force the fuel through the outlet port into the fuel system.
When the pump is shut off, the check valve in the fuel outlet will close, therefore prohibiting the fuel from returning to the fuel tank via the fuel pump. The check valve will therefore maintain a pressure in the fuel lines called the "rest pressure".
For main pump only: The maximum fuel pump pressure (not fuel system pressure) is dependent on the calibration of the pressure limiter. If the fuel pump pressure exceeds a preset limit (for example a clogged fuel filter), the pressure limiter will open a by-pass to the fuel inlet side of the pump.


SPECIFICATIONS
For all fuel pump specifications see SPECIFICATIONS.