In an electronic fuel-injected engine, the fuel for the injectors must be pressurized before the engine can be started, so an electric fuel pump is necessary.
An electric rotary pump can be located in the tank, or externally, on the underside of the body.
Gasoline enters at one end and leaves at the other, so it is always full. This lubricates the pump motor and keeps it cool. The pumping element is a roller-cell device driven by an electric motor. A rotor disc is mounted eccentrically in the pump housing. It has recesses around its edge, containing metal rollers. As the disc rotates, centrifugal force pushes them outward. This forms a rotating seal, and fuel is carried round in the cavities formed between the rollers. Because of the eccentric mounting of the disc, these cavities expand as they pass the inlet, and contract passing the outlet. This pressurizes the fuel and forces it into the fuel line.
The pump can also be an impeller type, which is less noisy.
For a short time after an engine is switched off, engine temperature keeps rising, and that can cause excess vapor in the fuel lines. This pump stops this, with a non-return valve on its outlet, which maintains the pressure in the fuel line during that short time.
If a fuel blockage occurs further along the fuel line, it can overload the pump motor, and make it overheat. So a high-pressure relief valve inside the pump lets fuel keep circulating.
When the pump is mounted externally, a low-pressure pump can be used to supply fuel to the main pump’s inlet. This low-pressure pump is mounted in the fuel tank. It’s a centrifugal type, and electrical, and it operates in the same way as the other high-pressure pump.
Carburetor systems can also use electric fuel pumps.
Most of these pumps are located outside the tank, though some are inside the tank.
One widely-used pump is the diaphragm type. It has an electrical section, and a mechanical section.
When the ignition is switched on, current magnetizes the solenoid. The magnetic field energizes the armature, which pulls down the diaphragm. This creates an area of low-pressure that draws fuel into the pump.
Pulling down the diaphragm breaks the circuit and stops the current. The solenoid is no longer an electromagnet. The armature is released, and the diaphragm spring forces up the diaphragm, which forces fuel out of the pump and on its way to the carburetor. This action continues, delivering fuel each time.
When the engine needs less fuel, pressure builds up in the fuel lines to the carburetor, and reduces how much fuel is delivered.
Some carburetor-equipped vehicles with electric pumps have a safety switch that prevents the pump continuing to run if the engine stops.
Fuel pumps on carbureted systems can be electrical or mechanical.
The mechanical fuel pump for the carburetor system is usually mounted on the cylinder head or the engine block.
It has a flexible diaphragm. That’s a flexible piece of neoprene rubber separating 2 chambers. This diaphragm is operated by an eccentric on the camshaft. It rotates, making the rocker arm move. This movement is transferred to the diaphragm, pulling it down. That draws fuel into the pumping chamber, above the diaphragm. The diaphragm spring moves the diaphragm up, and this forces fuel from the pumping chamber, out of the pump and into the carburetor.
When the engine needs more fuel, the diaphragm moves through a long stroke to pump a lot of fuel.
When less fuel is needed, pressure builds up in the fuel line to the carburetor, and in the pumping chamber above the diaphragm. The diaphragm spring can’t push the diaphragm up so far, and the pumping stroke is reduced.
Some pumps also have a return line to send excess fuel back to the tank. As the fuel circulates, it cools the fuel pump and lines, and reduces the chance of vapour locks.
