PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 2/TASK 2
pump cavity always is kept full as the fuel flows through. The plunger
moves up in its bore as it is actuated by the camshaft, sealing the ports.
The fuel that is trapped in the cavity is forced out of the pump and to its
(d) The pump plunger has a rectangular slot cut into it that leads
from the top face, down the side, and is finally connecting to a helical
shaped cavity called the bypass helix. In operation, the slot will allow
fuel to pass .to the bypass helix. As the bypass helix passes over the
spill port, it will allow a portion of the fuel charge to bypass back to the
fuel tank rather than be injected into the engine cylinder. The outer pump
sleeve is made to rotate and has gear teeth around its outer diameter. A
horizontal toothed rack meshes with these gear teeth to rotate the sleeve
without any plunger rotation. By moving the rack back and forth, the outer
pump sleeve is rotated, moving the delivery and spill ports in relation to
the bypass helix on the pump plunger. This enables the volume of fuel
injected to the cylinders to be varied by changing the effective length of
the pump stroke (the length of the pump stroke that occurs before the spill
port is uncovered by the bypass helix). The rack extends down the whole row
of injection pumps so that they are all operated simultaneously. The end
result is that the injection pumps can be moved from full to nofuel
delivery by moving the rack back and forth. Rack movement is controlled by
(e) When the plunger begins its pump stroke, it covers both ports.
When this happens, the pressure exerted on the fuel causes the springloaded
delivery valve to lift off of its seat, thereby permitting fuel to discharge
into the tubing that leads to the spray nozzle. At the instant the bypass
helix uncovers the spill port, the fuel begins to bypass. This causes the
pressure in the pump cavity to drop.
High pressure in the delivery line combined with spring pressure causes the
delivery valve to close. When the delivery valve closes, it prevents fuel
from the line from draining back into the pump, which could cause the system
to lose its prime. As the delivery valve seats, it also serves to reduce
pressure in the delivery line. The delivery valve has an accurately lapped
displacement piston incorporated into it to accomplish pressure relief.