Electric Fuel Pump

Electric

Fuel Pump

An electric fuel pump is used on engines with fuel injection to pump fuel from the gas tank to the injectors. The pump must deliver the fuel under high pressure (typically 30 to 85 psi depending on the application) so the injectors can spray the fuel into the engine. Fuel pressure must be within specifications for the engine to run correctly. Too little pressure can starve the engine for fuel, causing it to run lean, misfire, hesitate or stall. Too much fuel pressure can cause the engine to run rough, waste fuel and pollute.

Electric fuel pumps are usually mounted inside the fuel tank, though some may be mounted outside the tank. Some vehicles may even have two fuel pumps (a transfer pump inside the tank, and a main fuel pump outside). The in-tank location helps muffle the buzzing noise produced by the electric pump motor, and immersing the pump in fuel helps lubricate and cool the pump motor. Driving with the fuel tank less than 1/4 full can shorten pump life by causing it to run hot. It also increases the risk of momentarily starving the pump for fuel when cornering sharply, braking or accelerating. Running out of gas can sometimes damage an electric fuel pump by starving it for cooling and lubrication.

The pump is usually part of the sending unit assembly, that includes a float that sends an electrical signal to the fuel gauge on the instrument panel. If an electric fuel pump needs

to be replaced, it can be replaced as a separate item or as a complete module assembly (which is more expensive but easier and less troublesome).

Electric fuel pumps come in a variety of designs. Some older applications use a positive displacement "roller cell" pump. This type uses rollers mounted on an offset disc that rotates inside a steel ring. Fuel is drawn into the spaces (cells) between the rollers and pushed along from the pump inlet to the outlet. Roller cell pumps typically spin about 3,000 rpm. This type of pump can generate very high pressure, and the flow rate tends to be constant. But the
output comes in pulses, so a muffler is often mounted in the fuel line after the pump to dampen pressure pulses. A roller cell pump may also be mounted outside the fuel tank, and used with a second low pressure supply pump mounted inside the fuel tank.

Another type of positive-displacement pump is the "gerotor" pump. This design is similar to that of an oil pump, and uses an offset rotor to push fuel through the pump. A gerotor pump typically operates at around 4,000 rpm.

Another variation is the roller vane pump. Here, vanes are used instead of rollers to push fuel through the pump.