INTRODUCTION TO THE HMMWV - OD1615 - LESSON 1/TASK 2
passing it back to the tank; and (3) Without a supply pump, the system would
lose its prime whenever the pump is in the nondelivery mode.
The HMMWV uses a cam-driven diaphragm (mechanical) type fuel pump; this type
of fuel pump is currently most popular for an automotive fuel pump.
operation of the pump is as follows:
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. The rocker arm hooks
into an elongated slot in the pull rod. The other end of the pull rod is
attached to the diaphragm. As the camshaft operates the rocker arm, it will
operate the diaphragm against the pull of the diaphragm spring.
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. 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 unseated outlet check valve and through the pump outlet. This
operation is repeated each time the rocker arm operates the diaphragm.
The pressure will build up in the fuel line and the pump chamber as the fuel
pump fills the fuel injection pump.
As the pressure rises to the desired
level in the pump chamber, it will hold the diaphragm down against the
pressure of the diaphragm spring.
When this happens, the rocker arm will
just move up and down in the slotted pull rod with no pumping action, until
the fuel line pressure drops below the desired level. In this way, the fuel
pump regulates fuel line pressure. The operating range of the pump depends
on the tension exerted by the diaphragm spring.
A vent hole is provided under the diaphragm to allow the pressure to change
in the lower chamber as the diaphragm flexes.
The pulsating chamber,
located above the pump chamber, uses a soft diaphragm and a sealed chamber
to cushion the pulsating action inherent in the diaphragm-type pump. An oil
seal is provided to keep crankcase oil from entering the lower chamber and
leaking from the vent hole.