Yanmar 4JH2E Service Manual page 144

Chapter 3 Fuel Injection Equipment
17. Timer Construction and Operation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4JH2-UTE
Servo Valve Timer
Servo valve
Bushing Timer piston
Fig. 45 Servo valve timer construction
Printed in Japin
AOAI029-9109SP
3-79
When pump speed decreases and the timer
spring set force exceeds the pump chamber
pressure. the roller holder assembly is moved
in the direction to retard injection timing.
Additional devices such as the solenoid timer,
cold start device (C.S.D.) and the load timer
etc. are also used with this standard-type timer
to vary the injection timing in the specified
range of engine speeds and loads.
As shown in Fig. 45, through the addition of
some parts (e.g. servo valve). the alteration of
other parts (e.g. timer piston. cover and spring)
and alterations to the fuel oil transfer passage,
the servo valve timer differs from the standard
type timer.
With the servo valve timer. pump chamber
pressure does not act directly on the timer's
high pressure chamber, but flows through the
servo valve before acting on the timer's high
pressure chamber.
The timer spring force does not push the timer
piston, but pushes the servo valve against
pump chamber pressure. The servo valve posi-
tion depends on the balance of these two
opposing forces. and timer characteristics in
turn depend on the servo valve position.
For example, if the timer piston is moved in the
retard direction by fluctuations in the driving
reaction force, the servo valve position will not
change, as the pump chamber pressure does
not change. The servo valve then functions to
compensate for the fluctuations in the driving
reaction force by allowing the supply of pump
chamber pressure to the high pressure side of
the timer piston. The timer piston is therefore
returned to its original position. In other words,
the timer piston position is dependant on the
servo valve position.
From the above, the servo valve timer's
absorbing of the effect of the driving reaction
force on injection timing can be seen.
As the effective pressure area directly acted
upon by the pump chamber pressure de-
creases. and correspondingly the spring con-
stant decreases, an improvement in response
and a decrease in hysteresis can be obtained.