Yanmar 4TNV98, Service Manual

Page: 189 / 246

4TNV98 & 4TNE98 Diesel Engine Section 4. Fuel System 187
Structure And Operation Of Timer
The ignition period which occurs in the combustion
process of a diesel engine tends to increase as the
speed becomes higher.
Consequently, the timer is installed under the
injection pump in order to correct the ignition delay
period by shortening the injection timing of the
injection pump.
Figure 7-30
Standard Type Automatic Timer
As shown in
Figure7-30 , the inside of the timer
housing is separated into low and high pressure
sides, the latter directly receiving the pump
chamber pressure. A timer spring having a set force
is installed in the low pressure side. The timer
piston slides horizontally based on the balance
between the spring force and changes in the pump
chamber pressure. The movement of the timer
piston turns the roller holder assembly via the pin.
When pump revolutions increase and the pump
chamber pressure exceeds the set force of the
timer spring, the timer piston moves in the direction
to compress the spring ([B] in
Figure7-30 ). This
turns the roller holder assembly in the reverse
direction of pump revolution via the pin. This
causes the crest of the cam disc to quickly
approach the roller position of the roller holder, to
advance the injection timing. On the contrary, when
pump revolutions decrease and the timer spring
force exceeds the pump chamber pressure, the
timer piston moves in the direction to delay the
timing.