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Željko Nastasic´ —
Gábor Deák Jahn
The Citroën
Technical Guide
There are many car manufacturers, makes, models
and versions on the road today but—as we all
know—none of them compares to Citroën in its
engineering excellence, especially regarding
suspension comfort, roadholding, and stability.
In this book we tried to describe how the various
subsystems work. We never intended to replace
service manuals or similar technical instructions.
Illustrations are schematic, focusing on the
principles of operation rather than on minute
details of implementation.
This guide is not linked to any specific Citroën
model but describes all systems and solutions
used on a large number of cars from the glorious
line of DS, ID, CX, GS, GSA, BX, XM, Xantia, Xsara
and the C5.
Table of Contents
Fuel Injection
 U
The Citroën Guide
Fuel Injection: Electronic Fuel Injection
5
Electronic Fuel Injection
The Otto engine needs a mixture of fuel and air
for its operation. It would be the task of the fuel
supply—carburetor or injection—to provide the
engine with the ideal mixture. Unfortunately,
there is no such thing as an ideal mixture.
up, the mixture can return to normal, but the temperature
of the incoming air still plays a significant role: the cooler
the air, the denser it becomes, and this influences the
lambda ratio as well.
All these requirements are impossible to satisfy with sim-
pler mechanical devices like carburetors. Electronic fuel in-
jection provides a system that can measure the many cir-
cumstances the engine is operating in and decide on the
amount of fuel (in other words, the lambda ratio) entering
the engine. By carefully adjusting the internal rules of this
device, manufacturers can adapt the characteristic of the
fuel injection to the actual requirements: a sporty GTi
would demand rather different settings than a city car; be-
sides, catalytic converters have their own demands that, as
we will later see, upset the applecart quite vehemently.
Earlier, fuel injection systems only knew about fuel, the
ignition was supplied by traditional methods. Later on,
these systems (now called engine management systems)
took on the duty of generating the sparks as well. But even
with this second incarnation, the fuel injection part re-
mained practically the same, thus the following section ap-
plies to both kind of systems.
Perfect combustion, as chemistry calls it, would require air
and fuel in proportion of 14.7 parts to 1 (this is the
stochiometric ratio). While this might be satisfactory for the
scientists, the real-life conditions of a vehicle call for slightly
different characteristics.
We use the ratio of actual mixture to the stochiometric
mixture, called
lambda
(
l
), to describe the composition of
the mixture entering the engine:
l
=1 denotes the chemi-
cally ideal mixture,
l
<1 means rich,
l
>1 is lean.
The best performance would require a slightly rich mix-
ture, with the lambda around 0.9, while fuel economy
would need a slightly lean one, between 1.1 and 1.3. Some
harmful components in exhaust gas would reduce in quan-
tity between lambda values of 1 to 1.2, others below 0.8 or
above 1.4. And if this is not yet enough, a cold engine re-
quires a very rich mixture to keep running. After warming
Fuel injection
The two most important inputs describing the actual oper-
ating condition of the engine, thus determining the fuel de-
mand are the
engine speed
(revolution) and
engine
load.
The engine speed can be measured easily on systems
using traditional ignition: the ignition primary circuit gener-
ates pulses with their frequency proportional to engine
speed (the tachometer uses this same signal to show the
rpm to the driver). When the injection system provides the
ignition as well, it cannot at the same time rely on it, so an
additional sensor is used instead.
The engine load is usually determined by measuring the
quantity of air the engine tries to suck in. There are various
methods of attaining this: earlier systems used a flap which
is deflected by the air flowing through the sensor—the an-
gle of deflection is proportional to the amount of air pass-
ing through
(air flow sensor, AFS).
Later systems used a
pressure sensor measuring the pressure inside the inlet man-
ifold
(manifold absolute pressure, MAP sensor).
Yet an-
other system (although not used on Citroëns) heats a plati-
num wire and lets the incoming air passing around cool it;
by measuring the current needed to keep the wire tempera-
ture at a constant value above the temperature of the in-
coming air, the mass of air can be determined. Some sim-
pler systems do not even measure the amount of air but use
a pre-stored table in their computer to approximate it
based upon the engine speed and the position of the throt-
tle pedal—not that accurate but certainly much cheaper.
Under ideal conditions, these two inputs would already
be enough to control the engine. A large table can be set
up, like the one il-
lustrated here (of
course, this is an il-
lustration only, the
actual values mean
nothing here), and
for any pair of in-
coming engine speed and load values the necessary fuel
amount can be determined. By keeping the pressure of fuel
constant behind the injector valves, the amount of fuel in-
jected depends solely on the time period the injectors are
opened for, hence, the table can contain injector opening
times.
An this is exactly how it is done in modern injection sys-
tems: the controlling microcomputer keeps a lookup table
like this to determine the base pulse width. Earlier systems
were constructed from discrete, analog elements, not like a
small computer; a more or less equivalent circuit made of
various hybrid resistance arrays and semiconductors were
used for the same purpose.
Chip tuning,
by the way, is the simple operation of replac-
ing the said table with another one, yielding different char-
acteristics (usually to gain power, allowing for worse fuel
economy). As the computer stores this table in a program-
mable memory—similar in function to the BIOS in desktop
computers—, replacing it is possible. The earlier systems
with analog circuits cannot be modified that easily.
So, we obtained the base pulse width from the table but
as the operating conditions of automotive engines are
Engine load
0%
5%
…
100%
idle
3
3
…3
850 rpm
4
5
…5
900 rpm
5
6
…7
…
…………
6,000 rpm
9
8
…10
Amount of fuel
injected
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