DETROIT DIESEL 53
Fuel, under pressure, enters the injector at the inlet side through a filter cap and filter (Fig. 2). From the filter, the fuel
passes through a drilled passage into the supply chamber,' that area between the plunger bushing and the spill
deflector, in addition to that area under the injector plunger within the bushing. The plunger operates up and down in the
bushing, the bore of which is open to the fuel supply in the annular chamber by two funnel-shaped ports in the plunger
The motion of the injector rocker arm is transmitted to the plunger by the follower which bears against the follower spring
(Fig. 6). In addition to the reciprocating motion, the plunger can be rotated, during operation, around its axis by the gear
which meshes with the control rack; For metering the fuel, an upper helix and a lower helix are machined in the lower
part of the plunger. The relation of the helices to the two ports changes with the rotation of the plunger.
As the plunger moves downward, under pressure of the injector rocker arm, a portion of that fuel trapped under the
plunger is displaced into the supply chamber through the lower port until the port is closed oil' h, the lower end of the
plunger. A portion of' the fuel trapped below the plunger is then forced up through a central passage in the plunger into
the fuel metering recess and into the supply chamber through the upper port until that port is closed off by the upper helix
of the plunger. With the upper and lower ports both closed off, the remaining fuel under the plunger is subjected to
increased pressure by the continued downward movement of the plunger.
When sufficient pressure is built up, it opens the flat, non-return check valve. The fuel in the check valve cage, spring
cage, tip passages and tip fuel cavity is compressed until the pressure force acting upward on the needle valve is
sufficient to open the valve against the downward force of the valve spring. As soon as the needle valve lifts off of its
seat, the fuel is forced through the small orifices in the spray tip and atomized into the combustion chamber.
When the lower land of the plunger uncovers the lower port in the bushing, the fuel pressure below the plunger is relieved
and the valve spring closes the needle valve, ending injection. A pressure relief passage has been provided in the spring
cage to permit bleed-off of fuel leaking past the needle pilot in the tip assembly.
A check valve, directly below the bushing, prevents leakage from the combustion chamber into the fuel
Fig. 7 Removing Injector from Cylinder Head