Fuel pump
A fuel pump is a frequently essential
car parts on a car or other
internal combustion engined device. Many engines do not require any fuel
pump at all, requiring only gravity to feed fuel from the fuel tank
through a line or hose to the engine. But in non-gravity feed designs,
fuel has to be pumped from the fuel tank to the engine and delivered
under low pressure to the carburetor or under high pressure to the fuel
injection system. Often, carbureted engines use low pressure mechanical
pumps that are mounted outside the fuel tank, whereas fuel injected
engines often use electric fuel pumps that are mounted inside the fuel
tank.
Mechanical pump
Prior to the widespread adoption of electronic fuel injection, most
carbureted automobile engines used mechanical fuel pumps to transfer
fuel from the fuel tank into the fuel bowls of the carburetor. Most
mechanical fuel pumps are diaphragm pumps, which are a type of positive
displacement pump. Diaphragm pumps contain a pump chamber whose volume
is increased or decreased by the flexing of a flexible diaphragm,
similar to the action of a piston pump. Specific designs vary, but in
the most common configuration, these pumps are typically bolted onto the
engine block or head, and the engine's camshaft has an extra eccentric
lobe that operates a lever on the pump, either directly or via a
pushrod, by pulling the diaphragm to bottom dead center. The volume
inside the pump chamber increased, causing fuel to be drawn into the
pump from the tank. The return motion of the diaphragm to top dead
center is accomplished by a diaphragm spring, during which the fuel in
the pump chamber is squeezed through the outlet port and into the
carburetor. The pressure at which the fuel is expelled from the pump is
thus limited (and therefore regulated) by the force applied by the
diaphragm spring.
The carburetor typically car parts which contains a float bowl into
which the expelled fuel is pumped. When the fuel level in the float bowl
exceeds a certain level, the inlet valve to the carburetor will close,
preventing the fuel pump from pumping more fuel into the carburetor. Any
remaining fuel inside the pump chamber is trapped, unable to exit
through the inlet port or outlet port. The diaphragm will continue to
allow pressure to the diaphragm, and during the subsequent rotation, the
eccentric will pull the diaphragm back to bottom dead center, where it
will remain until the inlet valve to the carburetor reopens.
The pump creates negative pressure to draw the fuel through the lines.
However, the low pressure between the pump and the fuel tank, in
combination with heat from the engine and/or hot weather, can cause the
fuel to vaporize in the supply line. This condition is different from
vapor lock, where high engine heat on the pressured side of the pump
boils the fuel in the lines, also starving the engine of enough fuel to
run. Mechanical automotive fuel pumps generally do not generate much
more than 10-15 psi, which is more than enough for most carburetors.
Electric fuel pump
In many modern cars the fuel pump is usually electric and located inside
of the fuel tank. The pump creates positive pressure in the fuel lines,
pushing the gasoline to the engine. The higher gasoline pressure raises
the boiling point. Placing the pump in the tank puts the component least
likely to handle gasoline vapor well farthest from the engine, submersed
in cool liquid. Another benefit to placing the pump inside the tank is
that it is less likely to start a fire. Though electrical components
(such as a fuel pump) can spark and ignite fuel vapors, liquid fuel will
not explode and therefore submerging the pump in the tank is one of the
safest places to put it. In most cars, the fuel pump delivers a constant
flow of gasoline to the engine; fuel not used is returned to the tank.
This further reduces the chance of the fuel boiling, since it is never
kept to be controlled via pulse-width modulation of the pump voltage.
This increases the life of the pump, allows a smaller and lighter device
to be used, and reduces electrical load.
Some cars with an electronic control unit have safety logic that will
shut the electric fuel pump off even if the ignition is "on" if there is
no oil pressure, either due to engine bearing damage or a non stalled
engine, e.g. in a car accident. In case of an accident this will also
prevent fuel leaking from any ruptured fuel line. In many other cars
have an additional roll over valve that will shut off the fuel pump in
case the car rolls over. Some Ford cars also have a fuel cut-off switch
that will simply shut power down to the electric fuel pump relay in the
case of a collision.
