Figure 4-10.Mechanical positive fuel pump installation.
(fig. 4-10) continues to pump fuel even when the
carburetor bowl is filled; therefore, a method of
bypassing the fuel back to the tank is required. The
NONPOSITIVE type (fig. 4-11) is the one usually
found in a gasoline engine. It delivers fuel to the
carburetor only when it is needed for the requirements
of the engine.
An electric fuel pump, like the mechanical pump,
produces fuel pressure and flow for the fuel-metering
section of a fuel system.
Electric fuel pumps are commonly used in gasoline
fuel systems. They can be located inside the fuel tank as
part of the fuel pickupsending unit. Also, it can be
located in the fuel line between the tank and the engine.
The advantages an electric fuel pump has over the
mechanical fuel pump are as follows:
An electric fuel pump can produce almost instant
fuel pressure. A mechanical pump slowly builds
pressure as the engine is cranked for starting.
Most electric fuel pumps are a rotary type. This
produces a smoother flow of fuel (less pressure
pulsations) than a reciprocating, mechanical pump.
Since most electric pumps are located away from
the engine, they help prevent vapor lock. An electric
fuel pump pressurizes all of the fuel line near the engine
heat. This helps avoid
vapor lock because pressure
makes it more difficult for bubbles to form in the fuel.
Electric rotary fuel pumps include the impeller, the
roller vane, and the sliding vane types. They use a
circular or spinning motion to produce pressure.
An impeller electric fuel pump is a centrifugal
pump, normally located inside the fuel tank. This pump
used a small motor to spin the impeller (fan blade). The
impeller blades cause fuel to fly outward due to
centrifugal force. This produces enough pressure to
move the fuel through the fuel lines.
A roller vane electric fuel pump (fig. 4-12) is a
positive displacement pump (each pump rotation moves
a specific amount of fuel). This pump is located in the
main fuel line. Small rollers and an offset mounted rotor
disc produce fuel pressure in the pump. When the rotor
disc and rollers spin, they pull fuel to one side. The fuel
is then trapped and pushed to a smaller area on the
opposite side of the pump housing. This action
squeezes the fuel between the rollers and the fuel flows
Figure 4-12.Vane-type electric pump.