Yanmar 4TNV98, Service Manual

Page: 181 / 246

4TNV98 & 4TNE98 Diesel Engine Section 4. Fuel System 179
Delivery Valve Assembly
Figure 7-20
The delivery valve assembly consists of the delivery
valve and valve seat.
When the pressure of the high-pressure fuel being
fed from the plunger exceeds the force of the
delivery valve spring, the delivery valve opens to
feed the high-pressure fuel to the nozzle and nozzle
holder via the injection pipe. After injection, a
certain degree of remaining pressure exists in the
injection pipe ready for the next injection. The
delivery valve prevents the fuel remaining in the
injection pile from returning to the plunger.
If the remaining pressure is excessively high, cutoff
of the fuel may be adversely affected.
The delivery valve also functions to improve fuel
cutoff of the nozzle by drawing back the fuel in the
pipe by the amount equal to the suck-back stroke of
the piston.
Delivery Valve Holder with Damping
Valve
Figure 7-21
The delivery valve holder with damping valve
functions to prevent cavitation erosion in the pipe at
high speed, unstable fuel injection and secondary
injection.
When high-pressure fuel is fed from the plunger, it
is mainly sent to the nozzle through the outer
periphery of the damping valve since the damping
valve also opens at the same time as the delivery
valve upon completion of each injection cycle.
So, the only fuel path left is the small orifice of the
damping valve. This permits the slow closing of the
delivery valve. Thus, rapid pressure decrease in the
pipe is prevented and transfer of the reflection wave
in the pipe suppressed to ensure stable injection.
NOTE: Cavitation represents the phenomenon
where bubbles are generated in the pipe when the
pressure in the pipe drops. Cavitation erosion
means erosion of the pipe inner wall as a result of
cavitation.