amount of fuel that can be injected at rated load speed is
the hand priming port and resting against the priming
controlled by the torque screw; leaf spring adjustment
bypass spring. During hand priming, the pressure
has no effect.
differential across the transfer pump, caused by the
hand primer, forces the piston down, compressing the
As overload is progressively applied to the engine,
spring until the priming port (fig. 4-44) is uncovered.
the governor continues to hold the metering valve arm
Fuel bypasses the stationary transfer pump to fill the
in contact with the torque screw; therefore, the
system.
metering valve does not move during overload
operation. However, fuel delivery increases as the
When the piston is in the pressure regulating
engine speed drops because the charging ports remain
position, fuel forces the piston up the sleeve until the
in register for a greater length of time because of slower
regulating port or ports (fig. 4-45) are uncovered. Since
engine speeds. This allows more time for the charge of
the pressure on the piston is opposed by the regulating
fuel to pass through the metering valve, which is
spring, the delivery pressure of the transfer pump is
stationary; thus charging the cylinder with a larger
controlled by the spring rate, size, and number of
quantity of fuel.
regulating ports.
As the engine speed continues to drop and reaches a
The torque delivered by an engine increases
speed at which maximum torque is developed, the
progressively with a decreasing rpm caused by an over-
charge of fuel becomes great enough to force the
load. The torque continues to increase until it reaches
plungers outward far enough during each charging
its peak at a certain predetermined engine speed. This
cycle to bring the roller shoes into contact with the leaf
desirable engine feature is called torque backup. The
spring. This prevents further outward movement of the
torque backup, when the engine is overloaded, is
plungers; therefore, it limits the maximum amount of
caused primarily by the following three factors:
fuel that can be injected. From this you can see that the
More time is available for combustion of fuel.
maximum amount of fuel that can be injected during
Volumetric efficiency increases as engine speed
overload operation below peak torque is controlled by
the leaf spring adjustment.
decreases.
In the DC pump, the speed advance mechanism
Engine friction losses (and accessory loads)
provides controlled movement of the cam in the pump
decrease with engine speed.
housing to advance injection at high speeds. The rising
Since volumetric efficiency increases with
fuel pressure from the transfer pump increases flow to
decreasing engine speed, more fuel can be injected and
the power side of the advance piston (fig. 4-46). This
burned at the reduced engine speed, thus increasing the
flow from the transfer pump passes through a cut on the
torque output.
metering valve, through a passage in the hydraulic
head, and then by the check valve in the drilled bottom
To fully explain how torque control is ac-
head locking screw. The check valve provides a
complished when the pump is properly adjusted, it is
necessary to use an example, starting with a condition
where the engine is operating at high idle speed, and
METERING
CAM
VALVE
then progressively load the engine until engine rpm
RING
drops to peak torque speed.
When the engine is operating at high idle speed
with no load, the quantity of fuel delivered is controlled
by governor action on the metering valve. Torque screw
and leaf spring adjustment have no effect under this
condition.
As load is progressively applied, the governor
BOTTOM HEAD
LOCKING SCREW
action on the metering valve continues to control the
CAM
ADVANCE
quantity of fuel delivered until engine rpm drops to
PIN
ADVANCE
rated load speed. At this point, the governor has opened
PISTON
the metering valve enough to bring an arm on the valve
S P E E D A D VA N C E O P E R AT I O N
ASf04046
into contact with the torque screw, which prevents
further opening of the metering valve. The maximum
Figure 4-46.--Fuel flow in speed advance operation.
4-34
