CITROEN DS series Technical Manual page 8

U
The Citroën Guide
eration—, the momentum of the car is sufficient to rotate
the engine through the wheels. To save fuel, the injection is
cut off. As soon as the engine speed drops below the limit
or the throttle is opened, the injection is reintroduced—sup-
posedly smoothly and gradually, however, many drivers
complain about some jerkiness.
To avoid prolonged operation at revolutions exceeding
the specification of the engine, the injection is cut off above
a maximum engine speed (6,000-7,000 rpm, depending
on the engine). And finally, to avoid the hazard of fire in a
crash and the fuel squirting from the injection system with
the engine stopped or possibly destroyed, the relay of the in-
jectors is controlled by the ECU, allowing fuel injection only
when the ignition (or the signal of the corresponding sen-
sor) is present.
Who will light our fire?
Models with simpler fuel injection have traditional (elec-
tronic) ignition systems which are practically equivalent to
the solution used on cars with carburetors.
The distributor has two purposes: generating the driv-
ing signal for the ignition system and to distribute the high
voltage to the four cylinders in turn. This two parts inside
the distributor are electrically separate but mechanically
coupled—both are driven by the camshaft to keep them in
sync with the strokes of the engine.
The ignition signal thus starts from the distributor. A
magnetic induction sensor (consisting of a rotating four-
sided magnet and a pick-up coil) sends a pulse to the igni-
tion module at each firing point. This pulse will be switched
to the ignition coil (an autotransformer; auto here does
not mean that it is manufactured for automotive use, auto-
transformers have their primary and secondary coils con-
nected) by a power transistor inside the module. The cur-
rent change in the primary coil induces very high voltage
spikes in the secondary circuit. These spikes then go back to
the HT part of the distributor
which in turn sends them to
the spark plug of the actual
cylinder requiring the spark.
It takes some time for the
spark to ignite the fuel-air mix-
ture inside the combustion
chamber: this means that the
spark has to arrive slightly be-
fore the piston reaches its top
position (top dead center,
TDC), so that it will receive the
downward force of the deto-
nation in the right moment.
However, as the engine speed
increases, so does the speed
of the piston or the distance it
travels during a given period
of time. Therefore, the exact
time of the spark has to be ad-
vanced as the revolution in-
creases. Traditional systems
do this by adding a vacuum
line connecting the inlet mani-
fold to the distributor. As the
Fuel Injection: Electronic Fuel Injection
EMS MULTIPOINT
fuel
pump
fuel
fuel rail
pressure
regulator
vacuum increases with the engine revolution, its sucking
force rotates the inner part of the distributor slightly away
from its original position, causing all its timing devices
switch earlier, as required by the value of the timing ad-
vance.
Clever systems can get away without a distributor: some
CXs have such an ignition setup. This systems has two igni-
tion coils, both serving two spark plugs at the same time.
These two spark plugs belong to cylinders whose pistons
move in unison: one is compressing, the other exhausting.
Although both plugs generate sparks at the same time, the
one in the exhausting cylinder will be wasted.
Two birds with one stone
We made the ignition seem too simple in the previous sec-
tion. While it works as described, there are many factors to
be considered if we want to build a modern ignition sys-
tem. For instance, the timing advance depends not only on
engine speed but on many other factors as well: engine
load, engine temperature and to some extent, the air tem-
perature.
Just like the carburetor was not really good at deciding
the amount of fuel required by the engine, the traditional
ignition is similarly not perfect in estimating the timing ad-
vance and other characteristics of the sparks needed. An
electronic system similar to the one used for fuel injection
shows clear advantages over any earlier system.
And as they use about the same sensors and rely on each
other, what could be more logical than to integrate them
into a common system, elegantly called an engine man-
agement system?
If we compare the schematics of the corresponding EFI
and EMS systems, they look almost the same. There are two
notable differences: the small arrow on the line connecting
the ECU to the distributor has changed its direction and a
new sensor, a crank angle sensor (CAS) has appeared.
ECU
CO pot
OS*
CAS KS
injectors
TS
throttle
ISCV
engine
distri-
butor
CTS
AAV
AFS
idle mixture
8
fuel
exhaust
air
coolant
* not present
in all systems
ATS