4-2. PRINCIPLES OF OPERATION (CONT)
d. Fuel Injectors. The fuel injectors are Robert
Bosch, 17 mm closed nozzle, hole-type injectors.
During the injection cycle, high pressure from the
injection pump rises to operating (pop) pressure
which causes the needle valve in the injector to lift.
Fuel is then injected into the cylinder. A shimmed
spring is used to force the needle valve closed as the
injection pressure drops below the pop pressure to
seal off the nozzle after injection.
e. Chassis Mounted Fuel-Water Separator/
Filter. Filtration and separation of water from the
fuel is important for trouble free operation and long
life of the fuel system. Some of the-clearances
between the pump parts are very close. For this
reason the parts can easily be damaged by rust
formation and contaminants. The chassis mounted
separator/filter has two filter elements in a single
glass bowl through which the fuel must flow before
flowing through the outlet. Water and contaminants
settle to the bottom of the sediment bowl and are
removed by removing the drain plug.
f. Engine Mounted Fuel-Water Separator Filter Unit. The engine mounted fuel-water separator/filter unit
has two replaceable elements. One element of the filter has a valve which can be opened regularly to drain the