CITROEN DS series Technical Manual page 12

U
The Citroën Guide
STARTING
pedal
ß
Ý
å
Ü
8
Û
5
6
rather weak spring Ü. This will rotate the lever Û, moving
the collar 5 to the left, setting the amount of fuel required
for idling. The accelerator pedal is in the idle position as
well, dictated by the adjustment screw ß. The idle spring Þ
keeps the regulator in equilibrium.
Normally, the amount of
fuel will be regulated by the
position of the pedal as both
springs Ü and Þ are fully
compressed and do not take
an active part in the process.
When the driver pushes on
the pedal, the regulating
spring Ý stretches, both le-
vers 9 and Û rotate and
move the collar 5 to the
right, to allow the maximum
amount of fuel to be in-
jected. As the actual engine
speed catches up, the centrif-
ugal device 8 opens up,
pushing the shaft å to the
right, countering the previ-
ous force, gradually return-
ing the collar 5 towards the no fuel position, until the point
is reached where the amount of fuel injected maintains the
equilibrium. When the driver releases the pedal, the inverse
of this process takes place. During deceleration—pedal at
idle, engine rotated by the momentum of the car—the fuel
is cut off completely.
Without such regulation, if enough fuel is provided to
overcome the engine load, it would continue accelerating
until self-destruction (this is called engine runaway). Speed
regulation is a feedback mechanism comparing the actual
speed of the engine to the one dictated by the gas pedal
and modifies the amount of fuel as necessary. If either the
engine speed changes (because of varying load, going over
a hill, for instance) or the driver modifies the position of the
accelerator pedal, the regulation kicks in, adding more or
less fuel, until a new equilibrium is reached. If the engine is
powerful enough to cope with the load, keeping the pedal
in a constant position means constant cruising speed in a
diesel car; gasoline vehicles need speed regulated fly-by-
wire systems or cruise controls to achieve the same.
Fuel Injection: Diesel engines
IDLE SPEED
pedal
ß
9
Þ Ý
å
8
Û
DECREASE/INCREASE
pedal
ß
Ý
å
8
Û
The excess fuel will finally leave the pump unit through
an overflow valve ã, flowing back to the fuel tank.
Something needs to be corrected...
9
Þ
The chemistry involved in the combustion dictates some pa-
rameters of fuel injection, the most important being the
smoke limit, the maximum amount of fuel injected into a
given amount of air, that results in combustion without re-
Ü
sulting in soot particles. Although gasoline engines also
have this limit, they normally operate with a constant fuel
to air mixture that automatically places the amount of fuel
below this critical limit. Diesel engines, in contrast, operate
with a variable fuel to air mixture, using this very variation
for power regulation. With diesel fuel observing the smoke
5
limit is a much stricter task because once soot starts to de-
6
velop, this changes the character of the combustion itself,
resulting in a sudden and huge increase in the amount of
particulates—a bit like a chain reaction.
Because the maximum amount of fuel injected depends
on how far the lever Û is allowed to rotate counter-clock-
wise, the inability of the pump to inject too much fuel,
thereby crossing the smoke limit, is insured by an end stop
æ for this lever. This very basic means of smoke limit correc-
tion, adjusted for worst case conditions, was developed fur-
ther on turbocharged engines, and still further on electroni-
cally controlled injection systems.
9
Timing is of enormous importance in a diesel engine.
During the stroke of combustion, several events take place
Þ
in close succession: the fuel injection system starts its deliv-
ery, then the fuel is actually injected (the time elapsed be-
tween these two is the injection delay), slightly later the fuel
will self-ignite (this delay is the ignition delay), then the injec-
Ü
tion will stop but the combustion is still raging, first reach-
ing its maximum, then dying away slowly (on the scale of
milliseconds, that is).
Just like in a gasoline engine, the ignition delay remains
constant while the engine speed changes. The fuel has to ig-
5 nite before the piston passes its TDC position, but with the
increasing engine speed, the distance the piston travels dur-
6
ing a given period of time becomes longer. Therefore, the in-
jection has to be advanced in time to catch the piston still in
time. The injection adjuster à feeds on the fuel pressure
provided by the pump 1, proportional to the engine speed.
This will move the piston, which in turn, through the le-
vers, modifies the relative position of the cam rollers to the
cam disc inside the coupling 4, increasing or decreasing
the phase difference between the revolutions of the engine
and the rotating-alternating movement of the distributor
piston 6.
Some engines also have additional minor correction
mechanisms á that modify the idle speed and timing de-
pending on engine temperature, to provide better cold
start performance. The engine temperature is measured in-
directly, through the coolant acting on cylinder and piston-
like elements filled with paraffin. As the paraffin expands or
contracts as the coolant temperature dictates, the trans-
formed mechanical movement, coupled through cables to
two movable end stops for both the lever 9 and the injec-
tion adjuster à, modifies the idle speed and the injection
timing of the engine. Because correct timing depends on
temperature, the corrections, although relatively slight, in-
12