PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 1/TASK 2
b. Mechanical-Type. The mechanical-type of fuel pump is generally the
more popular pump used for gasoline engine applications. It is usually more
than adequate and is much cheaper than an electric pump. The electric pump
is more desirable, however, for the following reasons:
(1) The electric pump will supply fuel to the engine immediately
after the ignition key is turned on.
The engine must be tuned by the
starter for a mechanical pump to operate.
(2) The pump, by design, will operate more efficiently if it pushes
the fuel rather than pulling it. An electric pump can be mounted close to
the tank, or in the tank, to take advantage of this characteristic.
(3) The electric pump can be mounted away from heat to reduce the
possibility of vapor lock.
c. Mechanical, Nonpositive-Type (figure 4 on the following page). This
is currently the most popular configuration of an automotive fuel pump.
Operation is as follows:
(1) The rocker arm is moved up and down by the engine camshaft.
rocker arm spring causes the rocker arm to follow the cam lobe.
(2) The rocker arm hooks into an elongated slot in the pull rod.
other end of the pull rod is attached to the diaphragm.
(3) As the camshaft operates the rocker arm, it will operate the
diaphragm against the force of the diaphragm spring.
(4) As the rocker arm pulls the diaphragm down, the inlet check valve
is unseated and fuel is drawn into the pump chamber. The outlet check valve
seals the outlet passage.
(5) As the diaphragm spring pushes the diaphragm back up, the inlet
check valve seals the inlet and the fuel in the pump chamber is pushed
through the outlet check valve and through the pump outlet.
(6) The action is repeated each time the rocker arm operates the