MechanicalType.

Principles of Gasoline And Disesel Fuel Systems
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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, NonpositiveType (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. The

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. The

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

diaphragm.