CITROEN DS series Technical Manual page 11

U
The Citroën Guide
fuel from/to tank
low pressure
high pressure
adjusting screws
fuel
engine
injection
adjuster
engine but the coupling adds a horizontal, alternating
movement as well: for each turn, the shaft and the piston 6
performs four push-pull cycles.
It is the pushing movement of this piston 6 that creates
the high pressure and sends the fuel to the injectors. The
fuel, provided by the pump 1 arrives through the fuel stop
electro-valve â, which is constantly open while the ignition
switch is on but cuts the fuel path when it is turned off.
First, the piston 6 is pulled back by the coupling 4,
letting the fuel enter the chamber and the longitudinal bore
inside the piston. As the side outlets are blocked by the regu-
lator collar 5, the fuel stays inside the chamber (phase 1).
In the next phase, the piston rotates and closes the in-
gress of fuel from the stop valve â. On the other side of the
piston, the high pressure outlet opens but as the fuel is not
yet under pressure, it will stay in the chamber.
In phase 3 the piston is energetically pushed by the cam
disc and rollers of the coupling 4, injecting the fuel stored
in the chamber into the output line with a significant force.
PHASE 1
5
6
PHASE 3
5
6
injector
Fuel Injection: Diesel engines
regulator
2 fuel pump
3
1
PHASE 2
5
6
PHASE 4
5
6
ß
full idle
Ý
å
á
8
æ
4
à high pressure pump
and distributor
As the piston 6 moves to the right, at some point the
side outlets will emerge from under the regulator col-
lar 5—the fuel injection into the real output will stop imme-
diately, and the rest of the fuel stored in the chamber will
leave through this path of lesser resistance. This is phase 4,
the end of the injection cycle.
Actually, this operation is repeated four times for each
revolution of the incoming shaft. There are four high pres-
sure outlets radially around the piston, each serving a given
cylinder. As the outlet slot ä of the piston turns around, it al-
lows only one of the outlets to receive the fuel.
The pressure valves 7 serve to drop the pressure in the in-
jector lines once the injection cycle is over. To reduce the cav-
itation caused by the pressure waves generated by the rapid
closing of the injector valves, a ball valve minimizing the
back flow is also used.
The length of phase 3, thus the amount of fuel injected
depends on the position of the collar 5. If it is pushed to the
right, it will cover the side outlets for a longer time, result-
ing in a longer injection phase, and vice versa. If it stays in
the leftmost position, no fuel will be injected at all.
And this is exactly what the regulator part does: it
moves this collar 5 to the left and to the right, as the actual
requirements dictate. The lever 9 attached to the collar is ro-
tated around its pivot by several contributing forces. The
two main inputs are the position of the accelerator pedal
as communicated through a regulator spring Ý and the ac-
tual engine speed, driving a centrifugal device 8 via a pair
of gears 3. The higher the engine speed, the more the
shaft å protrudes to the right, pushing on the lever Û.
When the engine is being started, the centrifugal device
8 and the shaft å are in their neutral position. The starting
lever Û—pushed into its starting position by a spring Ü—
sets the position of the collar 5 to supply the amount of
fuel needed for the starting.
As the engine starts to rotate, a relatively low speed will
already generate a large enough force in the centrifugal de-
vice 8 to push the shaft å and overcome the force of the
Þ
9
fuel
to tank
Ü
fuel stop
valve
ã
â
Û
6
ä
5
7
11
accelelator
pedal
ignition
switch
6
injectors