PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 2/TASK 2
control rack toward the fullfuel position. As the engine speed picks up,
the pressure differential on both sides of the diaphragm and the spring will
achieve equilibrium and the position of the control rod will stabilize. The
same operating principles will apply in reverse to prevent engine overspeed.
As the engine speed increases, the velocity of air through the venturi
increases, causing a corresponding increase in the pressure differential on
both sides of the diaphragm. The increase in pressure differential causes
the diaphragm and the control rod to move against the pressure of the spring
toward the nofuel position. The control rod's position will stabilize when
equilibrium is achieved in the diaphragm unit. When the engine is operating
at wideopen throttle, the pressure differential is about zero and spring
force will move the control rack to the fullfuel position.
(4) For any position of the governor valve between idling and full
load of the engine, the diaphragm finds its relative position. Because any
movement of the diaphragm also is transmitted to the control rod, the amount
of fuel delivery is definitely controlled at all speeds. The diaphragm is
moved in the direction of less fuel delivery as the pressure drop between
the chambers is increased. The spring will move the control rod in the
direction of greater fuel delivery as the pressure drop is decreased.
6.
Timing Device
a. General. A large percentage of fuel injection pumps have timing
devices incorporated in them. Varying the time when fuel injection begins
will improve diesel engine performance and fuel economy, for the same reason
that varying spark timing will improve the performance of a gasoline engine.
b. Description (figure 60 on following page).
(1) The timing device usually consists of an aluminum casting with
mounting flanges at both ends. A bore in the housing guides and supports
the spider assembly. A timing opening, with a cover, is located in the top
of the housing and is used to observe the position of the timing pointer in
relation to the timing mark on the timing device hub during injection pump
timing procedures.
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