CITROEN DS series, ID series, CX series, GS series, GSA series Technical Manual

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

Electronic Fuel Injection . . . . . . . . . . . . . . . 5

Diesel engines . . . . . . . . . . . . . . . . . . . 10

Electronic Diesel Control . . . . . . . . . . . . . . 15

Diesel Direct Injection . . . . . . . . . . . . . . . 17

Suspension

A Suspension Primer . . . . . . . . . . . . . . . . 19

Hydropneumatic Suspension . . . . . . . . . . . 20

Hydractive I . . . . . . . . . . . . . . . . . . . . 24

Hydractive II . . . . . . . . . . . . . . . . . . . . 29

Anti-sink system. . . . . . . . . . . . . . . . . . 32

Activa Suspension . . . . . . . . . . . . . . . . . 33

Hydractive 3. . . . . . . . . . . . . . . . . . . . 35

Steering

Power Assisted Steering . . . . . . . . . . . . . . 38

DIRAVI Steering . . . . . . . . . . . . . . . . . . 40

Self-steering Rear . . . . . . . . . . . . . . . . . 42

Brakes

Standard braking system. . . . . . . . . . . . . . 44

Anti-lock Braking System. . . . . . . . . . . . . . 45

Electrical Systems

Multiplex network . . . . . . . . . . . . . . . . . 47

Air Conditioning

Air conditioning . . . . . . . . . . . . . . . . . . 50

Appendix

ORGA number . . . . . . . . . . . . . . . . . . 53

Index

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.

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

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.

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

Amount of fuel

injected

Engine load

0% 5% … 100%

Engine speed

idle 33…3

850 rpm 45…5

900 rpm 56…7

… …………

6,000 rpm 98…10

U

The Citroën Guide Fuel Injection: Electronic Fuel Injection 6

hardly ideal for any reasonable amount of time, several cor

­rections have to be applied. Our air flow meter measures
the volume of the air but we would need to know the mass
of the air to calculate the required lambda ratio—remem

­ber, colder air is denser, thus the same volume contains
more gas, requiring more fuel to provide the same mixture.
To accomplish this, the injection system uses an air temper-
ature sensor (ATS)—although on some systems it mea

­sures not the air but the fuel-air mixture—and lengthens
the injector pulse width according to this input (except for
the case of the airflow meter using a heated wire, this one
takes the air temperature into account automatically, conse

­quently, there is no need for correction).

It is not only the external
circumstances that require
special consideration. While
most of the time an engine
works under partial load, so it
makes sense to spare fuel by
basing on a relatively leaner
mixture across this range of
operation, cold start and
warm-up, modest decelera

-

tion and fully depressed throt

-

tle, idle speed all require dif

-

ferent treatment.

The position of the throttle
pedal is communicated to the
computer by a throttle posi

­tion switch (TS) or throttle
potentiometer (TP). These

devices signal both fully open
and fully closed (idling) throt

­tle positions. When the pedal
is fully depressed, the com

­puter makes the mixture
richer to provide good acceler

­ation performance.

Idle speed is more compli

­cated: the throttle is closed,
so there has to be a bypass to
let the engine receive fuel to
run. In simpler systems this by

­pass is constant (but manu

­ally adjustable to set the cor

­rect idle speed) in a warm en

­gine, providing a fixed
amount of air, although the
computer can decide on a
varying amount of fuel to be
injected. Later systems gener

­ally use a controlling device
changing the cross section of
the bypass, regulating the
amount of air coming
through (these systems often
have no facility to adjust the
idle speed, the computer
knows the correct revolution
and maintains it without any
help from mechanical de

­vices). The controlling device

can either be an idle speed control valve (ISCV) or an
idle control stepper motor (ICSM). The first one can
only open or close the idle bypass, so any regulation must
be done by rapidly opening and closing it by the computer,
the second one can gradually change the bypass, hence
fine tuning is easier and smoother.

Just like the choke on carburetors, there is a complete

subsystem dealing with cold start and warm up, as the re

-

quirements under such circumstances are so different from
the normal operation that they cannot be fulfilled by the
regular control. The ECU monitors the ignition key
switch to learn when the engine is started, then looks for
the input from the coolant temperature sensor (CTS) to

distri­butor

O

S*

engine

ECU

fuel
pump

CTS

MAPATS* TP

throttle

ISCM

fuel

i

njector &
pressure
regulator

ATS*

fuel
exhaust
air
coolant

not present
in all systems

*

EFI MONOPOINT

distri­butor

fuel
exhaust
air
coolant

not present
in all systems

*

OS*

engine

pressure
regulator

ECU

fuel
pump

CTS

AFS ATSTS

AAV

throttle

idle speed idle mixture

fuel

injectors

fuel rail

CSV*

EFI MULTIPOINT

U

The Citroën Guide Fuel Injection: Electronic Fuel Injection 7

see whether this is a cold start or a warm one. If the coolant
fluid is measured cold, a special warm-up sequence will be
started.

The engine needs significantly more fuel, a richer mix

-

ture during this period. This extra fuel is used for two pur

­poses: first, part of the fuel injected is condensed on the
cold walls of the engine, second, to ensure better lubrica­tion, the engine should run at an elevated revolution during
this period.

There are two ways to provide more fuel: through the

usual injectors, making the computer inject more gas than
normal, or by using an additional cold start injector
(CSV)—there is only one such injector even in multipoint
systems. This injector is fed through a temperature-timer
switch, protruding into the coolant just like the CTS, plus it
is heated by its own electric heater. The injector operates as
long as the ignition key is in the starting position but its be

­havior later on is governed by the timer switch. The colder
the engine initially is, the longer it stays closed to let the
cold start injector do its job. In a warm engine (above 40 °C)
it does not close at all.

Without a cold start injector, the computer itself adds
about 50% extra fuel initially and drops this surplus to
about 25% until the end of a 30-second time period.

From that point, the surplus is dictated by the warming
of the engine, communicated by the CTS to the computer.
EFI systems without an idle speed control device often use
an electromechanical auxiliary air valve (AAV). This
valve, which is fully open when the engine is still cold but
will close gradually as it warms up, lets an additional
amount of air measured by the AFS pass through the sys

-

tem. Because it is measured, it tricks the computer into pro

-

viding more fuel. The valve is heated by its own heating ele

-

ment as well as the engine, thus it closes shortly.

The injectors are electrovalves. As with any electromag

­net, there is a small time delay between the arrival of the
control signal and the actual opening of the valve due to
the build-up of electromagnetic fields. The length of this de

-

lay depends heavily on the
voltage the injectors are fed
with. The same pulse width
would result in shorter open

-

ing time, hence less fuel in

­jected if the battery voltage
drops below nominal (which
is often the case on cold morn

­ings). The injection computer
therefore has to sense the bat

­tery voltage and to lengthen
the injector pulse width if nec

­essary.

The final, total pulse width

(also called injector duty cy

­cle) is calculated by summing

up all these values received:
the base pulse width from the
RPM/AFS table lookup, the
various correction factors
based on the temperature sen

­sors, throttle position and the
like, plus finally, the voltage
correction.

As the computer has already calculated the exact

amount of fuel to be injected, there is only one task left: ac

­tually injecting it. There are two possible ways: to inject the
fuel into the common part of the inlet, still before the throt

­tle butterfly, or to inject them close to the inlet valves, indi

­vidually to each cylinder. Depending on the solution cho

­sen, the system will be called monopoint or multipoint.
Monopoint fuel injection requires a single common injec

­tor; the smaller cost and simpler setup makes it more com

­mon on smaller engines (in the case of Citroëns, the 1380
ccm ones). In all cases, the computer actually calculates the
half of the fuel amount required as it will be injected in two
installments, once for each revolution of the engine.

The injectors of the multipoint system can be operated si

­multaneously or individually. Previous Citroëns on the road
today still use simultaneous operation. Individual cylinder in

­jection, however, holds great potential—just to name one,
some of the cylinders of a larger engine can be temporarily
shut off by cutting off their fuel supply if the car is operat

­ing at partial load, saving a considerable amount of fuel—,
so we are sure to meet this sort of fuel injection systems in
the future.

All systems—regardless of the number of injectors—use
a similar fuel supply layout. The fuel is drawn from the tank
by a continuously operating fuel pump, transported via a fil

-

ter to the injectors, then back to the tank. There is a pres

­sure regulator in the circuit as well to keep the pressure of
the fuel at a constant pressure above that in the inlet mani

­fold (this regulator is a separate unit on multipoint systems
while integrated into the injector on monopoint ones). As
the pressure difference between the two sides of the injec

­tors are constant, the amount of fuel injected depends
solely on the opening time of the injectors. The pressure
used in contemporary EFI systems is 3 to 5 bars.

This is practically all there is to it, there are only a couple
of safety and economy features in addition. If the engine
revolution exceeds a certain limit (between 1,200 and
1,500 usually) and the throttle is closed—this is called decel

-

distri­butor

O

S*

engine

ECU

CTS

MAPATS* TP

throttle

ISCM

CAS

fuel
exhaust
air
coolant

not present
in all systems

*

fuel
pump

fuel

i

njector &
pressure
regulator

ATS*

EMS MONOPOINT

U

The Citroën Guide Fuel Injection: Electronic Fuel Injection 8

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 four­sided 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

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.

distri­butor

OS*

engine

pressure
regulator

ECU

fuel
pump

injectors

CTS

AFS ATSTS

AAV

throttle

ISCV

idle mixture

fuel

CO pot

CAS KS

fuel rail

fuel
exhaust
air
coolant

not present
in all systems

*

EMS MULTIPOINT

U

The Citroën Guide Fuel Injection: Electronic Fuel Injection 9

Both changes have to do with the fact that the enhanced
system, whose new task is to generate the ignition signals
as well, cannot at the same time build on them as inputs.
This new sensor—practically a replacement for the induc

­tion magnet in the distributor of earlier systems—informs
the computer of both engine speed and camshaft position.

The flywheel has steel pins set into its periphery. As it ro

­tates, the inductive magnet of the CAS sends pulses to the
computer. Two of the pins are missing and this hole passes
before the sensor just as the first piston reaches its TDC posi

­tion. The missing pins cause a variance in the sensor output
that can be read by the ECU easily.

The rest is the same: the base pulse width is calculated

based on the CAS and AFS/MAP sensors. The correction fac

­tors—air temperature, idle or full load, starting, warming
up, battery voltage—sum up into an additional pulse
width. Besides, the same input signals (AFS, CAS, CTS and
TS/TP) are used for another lookup in a table, yielding the
correct dwell time and timing advance for the ignition. The
dwell period remains practically constant but the duty cycle
varies with the chaging engine speed. The ignition signal is
amplified and sent to a distributor containing only second

­ary HT components: it does not create the ignition signal
only routes the HT current to each spark plug in firing order.

Some systems also have a knock sensor (KS), sensing

the engine vibration associated with pre-ignition (so-called
pinking). If this occurs, the ignition timing is retarded to
avoid engine damage.

Think green

As we saw, fuel injection and engine management systems
are capable of determining the ideal amount of fuel to be in

-

jected, depending on the conditions of operation and sev

­eral other factors in the engine. It is capable of deciding on
lean mixture for general, partial load to save fuel, or on rich
mixture when performance considerations call for this.

Unfortunately, this is not what such systems are used for
today. With the proliferation of catalytic converters, the
only concern of our systems is the welfare of the converter.

Ideal combustion would not generate polluting materi

-

als in the exhaust gas. Fuel is a mixture of various hydrocar

-

bons (C

nHm

), which when burned together with the oxygen

(O

2

) of the air, should transform to carbon-dioxide (CO2)

and water vapor (H

2

O). However, combustion is never ideal,
besides, fuel contains many additives: the exhaust gas, in
addition to the products mentioned, has various byprod

­ucts as well, some of them toxic: carbon-monoxide (CO),
various unburned hydrocarbons (C

nHm

), nitrogen-oxides

(NO

x

) and lead (Pb) in various substances coming from the

anti-knock additives found in the fuel.

The relative amount of these
byproducts depend on the
lambda ratio of the air-fuel mix

-

ture burned. As shown on the di

-

agram, a value between 1.2 and

1.3 would give a relatively low
percentage of toxic byproducts
while, as we can recall, being a
lean mixture would be in the
right direction towards fuel
economy.

By using platinum (Pt) or rhodium (Rh) as a catalyst—a
catalyst is a substance whose presence is required to enable
(or to boost) a chemical transformation while it does not
take part in the process itself, remaining intact—the follow

-

ing processes can be carried out:

2 CO + O

2

W 2 CO2(oxidation)

2 C

2H6

+ 7 O2W 4 CO2+ 6 H2O (oxidation)

2 NO + 2 CO W N

2

+ 2 CO2(reduction)
These precious metals are applied in a very thin layer to the
surface of a porous ceramic body with thousands of holes
to make the surface contacting the exhaust gases much
greater. Actually, a converter does not contain more than 2
or 3 gramms of these metals.

If you compare this diagram
with the previous one, you will
see that the real gain is the
supression of nitrogen-oxides.
CO and C

mHn

will be reduced as
well, although to a much lesser
extent. Nevertheless, the overall
reduction in polluting byprod

­ucts is quite high, amounting
up to 90 percent. Lead sub

­stances are not considered as
lead must not reach the converter anyway, it would clog the
fine pores of the converter in no time. The fuel used in cars
equipped with a catalytic converter has to be completely
free of lead.

But there is something of even greater consequence de

­picted on the diagram: to keep the amount of pollutants
down, the lambda has to be kept inside a very small value
range, practically at l=1 all the time. If the lambda drops
just a fraction below 1, the CO emission rises sharply, while
a small step above 1 skyrockets the NO

x

emission. The main
task of the fuel injection is therefore to ensure that the air­fuel mixture sticks to the stochiometric ratio all the time.
This means higher consumption than the one of a car with
fuel injection without a converter to start with.

There are situations where this lambda cannot be ob

­served. A cold engine will simply stall without a much richer
mixture, thus the cold start mechanism does not obey the
lambda control. The catalytic converter does not work at all
below 250 °C, so this is not a significant compromise (its
normal operating temperature is 400 to 800 °C, above
800 °C is already harmful; unburned fuel getting into the ex

­haust and detonating inside the converter could cause over

­heating, thus ignition and similar problems has to be recti

­fied as soon as possible in catalytic cars).

Dynamic acceleration (full throttle) is also something not
observing the welfare of the converter. Reducing pollution
might be a noble cause but to be able to end an overtaking
is even more important…

The system uses an oxygen sensor (OS, also called
lambda sensor) which measures the oxygen content of the
exhaust gas. It is located between the engine exhaust and
the catalytic converter. Similarly to the converter, it is not
functional below 300 °C, hence it has its own heating ele

-

ment to make it reach its operating temperature faster.

The computer uses the input from this sensor to keep the
mixture injected always as close to l=1 as possible. If the
sensor is still too cold to give accurate input, the computer
can ignore it safely.

%

CH

n

m

CO

NO

x

0.9 1.11.00.99 V

%

CO

CH

nm

NO

x

0.8 1.21.0 1.3

V

U

The Citroën Guide Fuel Injection: Diesel engines 10

Diesel engines

Diesel oil has been a contender to gasoline for
many decades. Earlier diesel engines were not re

­fined enough to win the hearts of many drivers
but recent advances in technology made these en

­gines not only a worthy competitor in all areas
but in some features—fuel economy or low end
torque, to name just two—even exceeding the
characteristics of their gasoline counterparts. And
in addition to the general technological advan

-

tages, Citroën’s diesel engines have a widely ac

­cepted reputation—even among people blaming
the quirkiness of its suspension or other fea

-

tures—of being excellent and robust.

As it is widely known, diesel engines have no ignition to initi

-

ate their internal combustion, they rely on the self-combus

­tion of the diesel oil entering into a cylinder filled with hot
air. Due to this principle of operation, the supply of the fuel
has to comply with much more demanding requirements
than it is necessary in the case of gasoline engines.

Unlike in the gasoline engine, not a mixture but air en

­ters into the cylinders via the inlet valves. During the adia

­batic compression all the energy absorbed is used to in

­crease the temperature of the gas. The small droplets of
fuel will be injected at high velocity near the end of the com

­pression stroke into this heated gas still in motion. As they
start to evaporate, they form a combustible mixture with
the air present which self-ignites at around 800 °C.

This self-ignition, however, is not instantaneous. The lon

­ger the delay between the start of the injection and the ac

­tual ignition (which depends on the chemical quality of the

diesel oil, indicated by the cetane number), the more fuel
will enter the cylinder, leading to harsher combustion, with
the characteristic knocking sound. Only with the careful har

-

monization of all aspects—beginning of injection, the distri

­bution of the amount injected in time, the mixing of the
fuel and air—can the combustion be kept at optimal level.

Small diesel engines suitable for cars were made possible
by a modification to the basic principle, that allowed these
stringent parameters to be considerably relaxed. It includes
a separate swirl chamber connected
to the cylinder via a restrictor orifice.
The air compressed by the piston in
the cylinder enters this chamber
through the orifice, starting to swirl in

­tensively. The fuel will then be injected
into this swirl, and the starting igni

­tion propels the fuel-air mixture still incompletely burned
into the cylinder where it will mix with the air, continue and
finish the combustion process. Using a prechamber results
in smaller ignition delay, softer combustion, with less noise
and physical strain on the engine parts, but introduces
some loss of energy because of the current of air having to
pass between the chambers. Citroën engines of this type
use a tangentially connected spherical prechamber.

As diesel engine evolution continued, better simulation

and modeling techniques became available, which, to

­gether with the improvements in fuel injection technology,
lessened or removed the problems initially solved by the in

­troduction of the prechamber. The direct injection engines
of today have no prechamber, instead, the piston has a spe

­cially formed swirl area embedded in its face.

Mechanical injection

Although the basic principles of fuel injection are similar to
what we have already discussed for gasoline engines, there
are some notable differences. First of all, diesel engines op

-

erate without restricting the amount of air entering the en

­gine: there is no throttle, the only means of regulating the
engine is to vary the amount of fuel injected.

The fuel is injected into the engine, creating a combusti

­ble mixture in the same place it is going to be burned. Be

­cause the forming of this mixture results in its self-combus

­tion, the diesel injection system is, in essence, an ignition
control system. Unlike on the gasoline engine, fuel injection
and ignition cannot be separated in a diesel engine.

The complete mechanical injection system is built into a

single unit which can be divided into five individual—al

­though interconnected—subsystems:

U

a low pressure fuel pump to deliver the fuel for the
rest of the injection system;

U

a high pressure pump and distributor that routes
the fuel to the appropriate cylinders in firing order
(similar in purpose to the distributor on gasoline en

-

gines) and generates the high pressure needed for the
injection as well;

U

a regulator that determines the amount of fuel to be
injected in relation to the engine speed, modified by
additional factors like idle speed, cold starting, full
load, etc.;

U

an injection adjuster to compensate for the higher
engine speed by advancing the start time of the injec

-

tion;

U

a fuel stop valve to cut off the fuel supply when the
ignition has been switched off.

The diesel fuel is drawn—through a filter—from the tank by
the low pressure pump 1 operated by the engine. A pres

-

sure regulating valve 2 ensures that the fuel pressure will
not exceed a preset limit; when the pressure reaches this
value, the valve opens and lets the fuel flow back to the pri

-

mary side of the pump.

The piston 6 of the high pressure part is driven through

a coupling 4 consisting of a cam disc and four cam rollers.
The piston rotates together with the shaft coming from the

U

The Citroën Guide Fuel Injection: Diesel engines 11

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.

As the piston 6 moves to the right, at some point the

side outlets will emerge from under the regulator col­lar 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

engine

fuel from/to tank

low pressure

high pressure

adjusting screws

fuel pump

2

injection
adjuster

high pressure pump
and distributor

fuel stop

valve

regulator

fuel

1

3

4

5

6

f

uel

to tank

7

injectors

8

9

Û

Ü

Ý

accelelator
pedal

Þ

ß

à

á

â

ignition

switch

ã

ä

6

å

idlefull

æ

PHASE 2

PHASE 4PHASE 3

PHASE 1

55

55

66

66

injector

U

The Citroën Guide Fuel Injection: Diesel engines 12

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-by­wire systems or cruise controls to achieve the same.

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…

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

­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
limit is a much stricter task because once soot starts to de

­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.

Timing is of enormous importance in a diesel engine.
During the stroke of combustion, several events take place
in close succession: the fuel injection system starts its deliv

­ery, then the fuel is actually injected (the time elapsed be­tween these two is the injection delay), slightly later the fuel
will self-ignite (this delay is the ignition delay), then the injec

-

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

­nite before the piston passes its TDC position, but with the
increasing engine speed, the distance the piston travels dur

­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 piston­like 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

-

DECREASE/INCREASE

9

Ü

Ý

pedal

Þ

ß

6

å

Û

5

8

STARTING IDLE SPEED

99

ÛÛ

ÜÜ

ÝÝ

pedal

ÞÞ

ßß

66

5

åå

5

8

8

pedal

U

The Citroën Guide Fuel Injection: Diesel engines 13

sure that the amount of fuel injected as well as the timing
provide better combustion and lower pollution when the
engine is started and operated at low temperatures. They
do not have any effect once the engine reaches the normal
operating temperature.

Now that the correct amount of fuel is care

-

fully determined and the necessary high pres

-

sure generated by the pump, it has to be in

-

jected into the swirl chamber. The pressurized
fuel entering the injector through a filter 1
tries to press the piston 2 upwards but a
spring 3 counters this force. As soon as the
pressure exceeds the force of the spring
(which can be adjusted by placing appropri

­ately sized shims behind it), the piston jumps
up and the fuel rushes into the swirl chamber

through the small orifice now opened. After the injection
pump closes its pressure valve at the end of the injection pe

-

riod, the spring 3 pushes the piston 2 back, closing the ori

-

fice until the next injection cycle.

Each swirl chamber has its own glow plug whose only

purpose is to heat up the chamber in cold weather. They
start to glow when the ignition key is turned into the first
position and stay glowing for some time afterwards unless
the starting was unsuccessful.

Turbo

More power requires
more fuel. An efficient
way to boost the perfor­mance is to provide
both more air and fuel
to the engine. The ex

­haust gases rushing out
from the engine waste
a great deal of energy; a
turbocharger 4 spun
by the exhaust flow taps into this source of energy to pro

-

vide added pressure in the air inlet. Diesel engines are partic

­ularly well suited for turbocharging. Gasoline engines may
not have the inlet pressure raised too much because the air
and fuel mixture may subsequently self-ignite when it is not
supposed to, and instead of burning controllably, detonate.
In a diesel such a situation is not possible because the fuel is
injected only when combustion should actually happen in
the first place. As a result, relatively high inlet pressures can
be used, considerably improving the power output of a die

­sel engine, and with proper attention to the subtleties of
the design, engine efficiency and fuel consumption.

On its own, once the amount and pressure in the ex

­haust manifold reaches a level high enough to power it,
with the engine fully loaded, the turbine would spin propor

­tionally to engine speed squared, because both the pres

­sure and the volume of the air pumped into the engine are
increasing.

Because the engine is required to deliver as much torque

as possible at the widest possible range of engine revolu

­tion, the requirements on the turbine are somewhat contra

­dictory. If the turbo is made very small and light, it will spin
up very quickly due to its low mass and inertia, ensuring its
full benefit already at low rpms. However, with a moderate

increase in engine speed, the rotational speed of the tur

-

bine (note the quadratic relationship) would become exces

­sively high. When the turbine blade speed approaches the
speed of sound, a supersonic wave effect occurs that can
abruptly leave it without any load, at which point runaway
would occur, resulting in severe damage to the turbine.

On the other hand, if the turbine was dimensioned so
that even at the highest engine speed it is still operating
within safe limits, it would not be useful at all in the middle
range where the engine is most often used. A compromise
can be achieved using an overpressure valve, the waste

-

gate valve 5. The turbo pressure is constantly monitored

by this valve opening above a set pressure limit, letting the
exhaust escape through a bypass. This avoids turbo run

­away by making the turbo rotational speed proportional to
that of the engine, once the limit pressure is reached. This
way the quick spin-up resulting from the quadratic relation

­ship can be preserved while the turbocharging effect is ex

­tended over a significant percentage of the usable engine
speed range—typically the higher 70-80%. But it comes at
a price: because of the simplicity of such a regulation, the
limit pressure is dictated by the maximum turbine speed,
which is usually calculated for maximum engine speed plus
a safety margin. The maximum pressure is already reached
at lower engine and turbine speeds, where the turbine
could conceivably still provide more pressure because of a
lesser demand for air volume. Although with a simple
wastegate a certain amount of the turbocharging potential
is lost, the increase in power output is still substantial.

Citroën is a pioneer in implementing variable wastegate
limit pressure using a controllable wastegate valve, to tap
into this previously unused turbo potential.

Essentially, a turbocharged diesel engine runs in two dif

­ferent modes: atmospheric pressure or turbo-charged. The
atmospheric pressure mode prevails while the exhaust gas
produced is not yet sufficient to power the turbine (below a
given engine speed and load). Once this limit is crossed and
the turbine starts generating higher than atmospheric pres

­sure, the engine is running in turbocharged mode.

The injection pump regula

-

tor needs to know about the
changes in the inlet pressure,
because those changes mean
differences in the amount of
air entering the engine. And
this also means that the upper
limit of fuel injected needs to
be changed correspondingly.
These injection systems are
tuned for the turbo producing
the rated waste pressure (also
known as full boost). How

­ever, the amount of fuel injected during the atmospheric
mode of the engine—before the turbo kicks in—has to be
reduced in order to avoid crossing the smoke limit. The
turbo pressure drives a limiter in the injection pump: with
the increasing pressure the piston æ moves down. Its vary

­ing diameter forces the lever ç rotate around its pivot,
which then acts as a stop to limit the allowed range of oper

­ation of the regulator lever 9, limiting the amount of fuel to
be injected.

Intercooler

9

Ý

pedal

Þ

ß

turbo
pressure

æ

ç

HP fuel

return

2

1

3

air

exhaust

1

2

3

4

5

U

The Citroën Guide Fuel Injection: Diesel engines 14

Towards a cleaner world

Exhaust Gas Recycling (EGR) systems were used—depend

-

ing on the market—as add-on units. An electronic unit mea

­suring the coolant temperature and the position of the gas
pedal control on the pump (with a potentiometer fitted to
the top of the control lever) controls a valve which lets part
of the exhaust gas get back into the inlet.

Post-glowing is also used as a pollution reducing mecha

­nism. A definite post-glow phase, lasting for up to minutes
is usually controlled by a combination of a timer and the en

­gine coolant temperature: either the timeout of 4 minutes
runs out or the engine reaches 50 °C. An additional mecha

­nism prevents post-glowing if the engine was not actually
started.

U

The Citroën Guide Fuel Injection: Diesel engines 15

Electronic Diesel Control

Just like it is the case with gasoline engines and carburetors,
a mechanical device—even one as complicated as a diesel
injection pump—cannot match the versatility and sensibil

­ity of a microcomputer coupled with various sensors, apply

­ing sophisticated rules to regulate the whole process of fuel
injection.

The only input a mechanical pump can measure is the en

­gine speed. The amount of air entering into the engine, un

­fortunately, is far from being proportional to engine speed,
and the turbo or the intercooler disturbs this relationship
even further. As the injection always has to inject less fuel
than the amount which would already generate smoke, the
mechanical pump—capable only of a crude approximation
of what is actually going on in the engine—wastes a signifi

­cant amount of air, just to be of the safe side.

The satisfactory combustion in diesel engines relies on

the exhaust as well—if this is plugged up, more of the ex

­haust gases stay in the cylinder, allowing less fresh air to en

­ter. A mechanically controlled injection pump has no feed

­back from the engine (except for the engine speed)—it will
simply pump too much fuel into the engine, resulting in
black smoke. An electronically controlled injection pump,
on the other hand, can tell how much air has actually en

­tered by using a sensor (although only the latest systems
use such a sensor).

There are also other factors never considered by a me

­chanical system. The details of the combustion process de

­pend heavily on the chemical characteristics of the fuel. The
ignition delay, as we have already seen, depends on the
cetane number of the diesel oil. In spite of the fact that cor

­rect timing has a paramount influence on the performance
and the low pollutant level of a diesel engine, the mechani

­cal system can have no information about this very impor

­tant input factor. Less essential but still important is the tem

­perature of the incoming air. With measuring all the circum

-

stances and conditions in and around the engine (air, en

­gine and fuel temperatures), the injection system can
achieve better characteristics, lower fuel consumption and
less pollution.

All in all, the electronically controlled injection pump not
only adds precision to the injection process as its gasoline
counterpart does but introduces completely new methods
of regulation; therefore it represents a much larger leap for

­wards than fuel injection in gasoline engines. In spite of
this, it is quite similar to its mechanical predecessor. From
the five subparts, four remain practically the same, only the
regulator is replaced with a simple electromagnetic actua

­tor that changes the position of the same regulator collar 5
as in the mechanical pump, in order to regulate the amount
of fuel to be injected.

The real advantage over the former, mechanical pumps

is that an electronic device, a small microcomputer can han

­dle any complex relationship between the input values and
the required output. With mechanical systems, only simple
correction rules are possible, and as the rules get more com

­plicated, the mechanics quickly becomes unfeasible. In con

­trast to this, the ECU just have to store a set of characteristic
curves digitized into lookup tables, describing the amount
of fuel to be injected using three parameters: engine

speed (measured by a flywheel inductive magnet), cool

­ant temperature (measured by a sensor protruding into

the coolant liquid), air temperature (measured by a sen

­sor in the air inlet).

The newer HDi engines use an air mass sensor using a

heated platinum wire (as that mentioned on page 5). Hav

­ing the exact amount of air to enter the engine, these latest
EDC systems can deliver true closed loop regulation.

A potentiometer attached to the accelerator pedal

sends information about the pedal position to the com

­puter. This signal is used as the main input, conveying the in

-

engine

fuel from/to tank

low pressure

high pressure

adjusting screws

fuel pump

2

injection
adjuster

high pressure pump
and distributor

fuel stop

valve

actuator

fuel

1

4

5

6

fuel
to tank

7

injectors

Û

accelelator
pedal

à

â

ã

ä

6

ECU

temperatures
(air, fuel, coolant)

regulator

position

vehicle speed

air quantity

engine speed
atmospheric

air pressure

ß

8

U

The Citroën Guide Fuel Injection: Diesel engines 16

tentions of the driver. The ECU uses this sensor to learn
about special conditions like idle speed or full load as well.

Air temperature is measured by a sensor in the inlet mani

­fold (but if the air mass is measured by a heated platinum
wire sensor, this already provides the necessary air tempera

­ture correction, thus there is no need for an additional sen

­sor).

The ECU stores the basic engine characteristics, the intrin

­sic relationship between the air intake and the engine
speed (plus the manifold pressure if a turbo is fitted). The
values obtained from this table are corrected according to
the inputs of the various sensors, in order to arrive at a basic
timing and smoke limit value. The actual amount of fuel in

­jected and the accurate timing are a function of these
results and the position of the accelerator pedal.

The final amount of fuel calculated will be used to con

­trol the electric actuator 8 which—by moving a lever Û—
changes the position of the regulating collar 5. To ensure
the necessary precision, the factual position is reported
back to the computer using a potentiometer.

As we have already mentioned, the exact timing of the in

­jection is of utmost importance in a diesel engine. The elec

­tronic system uses a needle movement sensor built into
one of the injectors (the other are assumed to work com

­pletely simultaneously) notifying the computer about the
precise time of the beginning of the injection. Should there
be any time difference between the factual and designated
opening times, the electro-valve ß of the injection ad

­juster à will receive a correction signal until the difference

disappears. If the electro-valve is completely open, the injec

­tion start will be delayed, if it is closed, the start time will be
advanced. To achieve the timing required, the valve is
driven with a modulated pulse signal, with the duty cycle
(on-off ratio) determined by the ECU.

The input from this sensor is also used for compensating
calculations on the amount of fuel injected, and to provide
the on-board computer with the exact amount of fuel used
up so that it can calculate the momentary and average con

-

sumption.

The computer has extensive self-diagnostic functionality.
Many sensors can be substituted with standard input val

-

ues in case of a failure (serious errors will light up the diag

­nostic warning light on the dashboard). Some sensors can
even be simulated using other sensors—for instance, the
role of a failing engine speed sensor might be filled in by
the signal generated from the needle movement sensor.

As there is no standalone ignition in a diesel engine, the

only way to stop it is to cut off the fuel supply. The mechani

­cal default position of the actuator 8 is the position where
no fuel enters the injectors at all; this is where it returns
when the computer receives no more voltage from the bat

­tery, the ignition switch having turned off.

As it has already been mentioned, the inlet pressure is

one of the principal EDC parameters for a turbocharged en

­gine. Later Citroën turbocharged diesels—starting with the

2.5 TD engine of the XM—pioneered variable turbo pres

­sure technology. The wastegate on these turbines has sev

­eral actuators, fed with the turbo pressure through electric
valves. The ECU, based on the relevant engine operation pa

­rameters obtained from the sensors, controls these actua

­tors in various combinations, providing a selection of two
or three different wastegate limit pressures. This lets the sys

-

tem ease the compromise between the turbo pressure and
turbine speed: the pressure is kept at the usual value for
higher engine speeds (limited by the maximum turbine
speed) but is allowed to go higher than that in the middle
rpm ranges, adding a significant amount of torque in the
range where it is most needed.

Green versus Black

Diesel oil, just like gasoline, is a mixture of various hydrocar

-

bons (C

nHm

), and burned together with the oxygen (O2)of

the air, transforms to carbon-dioxide (CO

2

) and water vapor

(H

2

O). However, as the combustion is never ideal, the ex

­haust gas also contains various byproduct gases: carbon­monoxide (CO), various unburned hydrocarbons (C

nHm

), ni

­trogen-oxides (NO

x

). The relatively high lambda value a die

­sel engine is operating with reduces the hydrocarbon and
carbon-monoxide content to 10–15%, and the amount of
nitrogen-oxides to 30–35% of the corresponding figures
measured in gasoline engines without a catalytic converter.
The sulphur content of the fuel—drastically reduced during
the recent decades—is responsible for the emission of sul

­phur-dioxide (SO

2

) and sulphuric acid (H2SO4).

Conversely, these engines emit 10–20 times more par

­ticulates—or black soot—than gasoline engines. These are
unburned or incompletely burned hydrocarbons attached
to large particles of carbon. These substances are mainly al

­dehydes and aromatic hydrocarbons; while the first only
smells bad, the second is highly carcinogenic.

The much higher amount of particulates is due to the dif

­ferent combustion process. The various aspects of mixture
formation, ignition and burning occur simultaneously, they
are not independent but influence each other. The distribu

­tion of fuel is not homogenous inside the cylinder, in zones
where the fuel is richer the combustion only takes place
near the outer perimeter of the tiny fuel droplets, produc

­ing elemental carbon. If this carbon will not be burned later
because of insufficient mixing, local oxygen shortage (large
fuel droplets due to insufficient fuel atomization, caused by
worn injectors) or the combustion stopping in cooler zones
inside the cylinder, it will appear as soot in the exhaust. The
diameter of these small particles is between 0.01 and
10 mm, the majority being under 1 mm. Keeping the
amount of fuel injected below the smoke limit—the
lambda value where the particulate generation starts to rise
extremely—is essential.

Similarly to gasoline engines, the exhaust gas can be

post-processed to reduce the amount of pollutants even fur

-

ther. There are two different devices that can be used:

U

Soot burning filter: as the diesel engine always oper

-

ates with excess air (its lambda is above 1), there is
enough oxygen in the exhaust gas to simply burn the car

-

bon soot present. The burning filter is manufactured
from ceramic materials that can withstand the resulting
high temperatures (up to 1200 °C). As the diesel engine
is very sensitive to excessive back pressure, the filter has
to be able to self-regenerate. This is solved by the addi

-

tion of organic metal substances.

U

Catalytic converter, identical to the simpler ones used
on gasoline engines before the proliferation of three­way, controlled converters. It reduces the carbon-monox

-

ide and hydrocarbon content of the exhaust gas.

U

The Citroën Guide Fuel Injection: Diesel engines 17

Diesel Direct Injection

I think that at this point, soot burning filters will have to be
cut out of the PDF and put in at a similar ecological section
under DI/HDI—since that is the only system that actually
makes soot burning practical, and the only system that im

-

plements it.

Soot burning was experimented with a lot but was never

made practical before HDI due to a too low exhaust temper

­ature. The particle filter would need heating to a very high
temperature and that was deemed to be too dangerous.
Even with cerine additives, essentially, there would have to
be a separate small burner to heat up the filter, which is
again another system that can go wrong. HDI essentially in

­tegrates a burner by alowing post-injection, something
that is simply impossible for injection systems derived from
a classical pump due to teh timing required. I think that for
soot management it is enough to write that the smoke limit
control is vastly improved by the better regulation of the
EDC.

Other things like controlled swirl and multi-valve technol

­ogy, also pioneered by Citroën (XM 2.1 TD!) should be men

­tioned. The catalytic converter section remains unchanged.

And, of course, there should be an "In addition to the pol

­lution management implemented on mechanical injection
systems" sentence somewhere in there, since proper cold
start corrections and EGR are implemented in EDC units by
default.

Suspension

U

The Citroën Guide Suspension: A Suspension Primer 19

A Suspension Primer

From the early days of the automobile—and even
before, in the time of horse-drawn carts—it was
already well known that the body of the car, hous

­ing both the passengers and the load, has to be
decoupled from the unevenness of the road sur

-

face.

This isolation is much more than a question of comfort. The
vertical force of the jolts caused by the repeating bumps
and holes of the road surface are proportional to the square
of the vehicle speed. With the high speeds we drive at to

­day, this would result in unbearable shock to both people
and the mechanical parts of the car. Jolts in the body also
make it more difficult to control the vehicle.

Consequently, there has to be an elastic medium be

­tween the body and the wheels, however, the elasticity and
other features of this suspension medium are governed by
many, mostly contradicting factors.

The softer, more elastic the spring, the less the sus

­pended body will be shaken by various jolts. For the sake of
comfort, we would thus need the softest spring possible.
Unfortunately, too soft a spring will collapse under a given
weight, losing all its elasticity. The elasticity of the spring
would need to be determined as a function of the weight
carried but the weight is never constant: there is a wide
range of possible load requirements for any car. On one
hand, a hard suspension will not be sensitive to load varia

­tions but being hard, will not fulfill its designated purpose,
either. A soft suspension, on the other hand, is comfortable
but its behavior will change significantly on any load varia

­tion. To cope with this contradicting requirements, an elas

­tic medium of decreasing flexibility would be required: such
a spring will become harder as the weight to be carried in

­creases.

When the spring is compressed under the weight of the

load, it’s not only its flexibility that changes. The spring de

­flects, causing the clearance between the car and the road
surface decrease, although a constant clearance would be a
prerequisite of stable handling and roadholding. At first
sight, this pushes us towards harder springs: soft springs
would result in excessive variations of vertical position — un

-

less, of course, we can use some other mechanism to en

­sure a constant ground clearance.

In addition to the static change caused by load varia

­tions, the deflection of the spring is changing constantly
and dynamically when the wheels roll on the road surface.
The body of the vehicle dives, squats, rolls to left and right
as the car goes over slopes, holes and bumps in the road,
corners, accelerates or decelerates.

When a deflected spring is released again, the energy
stored in it will be released but as there is no actual load for
this energy, the elastic element, the mass of the suspension
and the vehicle form an oscillatory system, causing a series
of oscillations to occur instead of the spring simply return

-

ing to its neutral position.

Any vertical jolt would thus cause such oscillations: the
upward ones are transmitted to the car body while the
downward ones make the wheels bounce, losing contact
with and adhesion to the road surface. The first is only dis

-

comforting, but the second is plainly dangerous. In addi

­tion, it’s not only the spring that oscillates; the tires contain
air which is a highly elastic spring medium. Oscillation in it

­self causes unwanted motion but when the corrugation of
the road surface happens to coincide with the period of the
suspension oscillations, it might lead to synchronous reso

­nance, a detrimental situation leading to serious damages
in the suspension elements.

Mass in motion can also be viewed as a source for kinetic
energy; because of this, moving parts of the suspension are
often reduced in weight to decrease this portion of the
stored energy, and this in turn eases the requirements on
the dampers as they have to dissipate less unwanted energy
as heat. This solution, however, often shifts the frequency
of the self-oscillation of the suspension upwards. Unfortu

­nately, occupants are more sensitive to higher frequencies
reducing comfort (mostly adding noise), so this is an area
where compromise is needed.

Conventional suspension systems use a second element,

a shock absorber to dampen these oscillations. The ab

­sorber uses friction to drain some of the energy stored in
the spring in order to decrease the oscillations. Being an ad

­ditional element presents new challenges: the characteris

­tics of both the spring and the absorber have to be
matched carefully to obtain any acceptable results. The ab

­sorber ought to be both soft and hard at the same time: a
soft absorber suppresses the bumps of the road but does
not decrease the oscillations satisfactorily while a hard ab

­sorber reduces the oscillations but lets the passengers feel
the unevenness of the road too much. Due to this contradic

­tion, conventionally suspended cars have no alternative but
to find a compromise between the two, according to the in

­tended purpose of the car: sport versions are harder but of

­fer better roadholding, luxurious models sacrifice roadhold

­ing for increased comfort. This contradiction clearly calls for
a unified component serving both as a spring and an
absorber, harmonizing the requirements.

U

The Citroën Guide Suspension: Hydropneumatic Suspension 20

Hydropneumatic Suspension

As we saw, the ideal suspension would require
elasticity decreasing with the load, constant
ground clearance, shock absorbers integrated
into the suspension—all these beyond the obvi

­ous independent suspension for all wheels. And
this is exactly what Citroën’s unique hydropneu

-

matic suspension offers.

According to the Boyle–Mariotte formula defined in the
17th century, the pressure and the volume of a mass of gas
are inversely proportional at a constant temperature. There

­fore, by keeping the mass of the gas constant and changing
the volume of its container, its pressure can be controlled
(the usual pneumatic suspensions operate on the opposite
principle: air is admitted or withdrawn from the system by
compressors and exhaust valves, modifying its mass while
keeping the volume constant).

The volume changes are controlled by hydraulics, a tech

­nology in widespread use in every branch of the industry. As
liquids are non-compressible, any amount of liquid intro

­duced at one end of a hydraulic line will appear immedi

­ately at the other end (this phenomenon was first formu

­lated by Blaise Pascal). Using this principle, motion can be
transmitted, multiplied or divided (according to the relative
sizes of the operation cylinders), with velocity increased or
decreased (using varying cross sections in the tubing), to
any distance desired, over lines routed freely.

Hydraulics are immensely useful, very efficient, reliable,
simple to use, and—due to their widespread deployment—
relatively cheap. It is no wonder that it is used for many pur

-

poses even in the most conventional vehicles: shock absorb

­ers, brake circuit and power assisted steering being the
most trivial examples; however, Citroën is the only one to
use it for the suspension.

The First Embodiment

The Citroën DS, introduced at the 1955 Paris Motor Show,
was radically different from any of its competitors on the
market at that time: suspension, running gear, steering,
brakes, clutch, body, aerodynamics were all unique, not
only in details but in the main operating principles as well.

The hydropneumatic spring-absorber unit uses an inert
gas, nitrogen (colored blue on the illustrations) as its spring
medium, resulting in very soft springing. The flexibility of
the gas decreases as the increasing load compresses the sus

-

pension pistons, reducing the vol

­ume of the gas and adding to its
pressure. The damping effect is ob

­tained by forcing the fluid (colored
in green) pass through a two-way
restrictor unit between the cylinder
and the sphere. This effect provides
a very sensitive, fast and progres

­sive damping to reduce any un

­wanted oscillations.

There are many great advantages to this hydropneu

­matic suspension. First, by adding or removing fluid from
the suspension units (practically, by adjusting the length of
the hydraulic strut), ground clearance can be kept con

­stant under any load variations. Although this might

not seem very important at first sight, it means that the sus

­pension geometry is also constant—in other words, the
handling of the car does not depend on the load.

The compressed gas has a variable spring effect, becom

­ing harder as the load increases. This compensation for the
increasing load keeps the resonance frequency of the sus

­pension nearly constant. As a consequence, the same excita

­tion in the suspension moves the same amount of fluid
through the dampers regardless of load (which is not the
case with conventional springs). The working range of the

dampers becomes much smaller and this fact makes the
use of a simple damper element very effective.

This basically constant suspension resonance frequency
also contributes to the consistent behaviour independent
of the load. In essence, it ensures that both the road con

-

tact and the feeling transmitted to the driver remains al

-

ways the same. This is something absolutely unique: all con

-

ventional suspensions have an optimum point around aver

-

age load; when carrying more or fewer passengers or load

than this average value, the han

-

dling characteristics change, not sel

-

dom so radically that the car be

-

comes utterly dangerous to drive.

Another advantage is the limited

but very useful anti-dive behav

-

ior: this is essential for efficient

braking with a basically very soft
suspension. The center of mass of

the car moves much less than
usual, hence the braking force is distributed more evenly.
Manufacturers of cars with conventional suspension and
braking only start to add brake force distributors to their ve

­hicles these days. The first DS did have a force distributor
but Citroën later realized that the suspension, with the addi

­tion of a single pipe, can fulfill its role entirely.

The height correction and the constant connection be

­tween the left and right side of the suspension has another
important implication: lower difference in forces on the
wheels. Coupled with variable damping this keeps the
wheels in contact with the road at all times, which in
turn maximizes the tractive forces on the tires—braking
while turning still leaves the vehicle with the grip of all four
wheels: this is essential for security in low adherence condi

­tions, such as ice, snow, rain, mud.

U

The Citroën Guide Suspension: Hydropneumatic Suspension 21

The steady connection between the sides requires an ex

­ternal management of body roll. Ideally, for any vertical
movement of the car body, the two sides of the suspension
should be connected, while for any movement that results
in different displacements of each wheel, they should ide

­ally be separate. This second movement can be viewed as a
rotation around the longitudinal or transversal axis.

For instance, if the front wheels run into a pothole and
the rear wheels go over a bump, the car will rotate around
its transversal axis. The angle of rotation remains relatively
small as the length of the car is its largest dimension; the
higher weights like the engine bay are far from the centre of
mass, resulting in a large inertial torque to counter outside
forces. If all suspension elements of the wheels were con

­nected hydraulically, the vehicle would absorb the bumps
very efficiently (the rear struts compressed by the bump
would deliver fluid into the front struts, resulting in immedi

­ate compensation: the rear would sink, the front would
rise, restoring the horizontal position of the car). Unfortu

­nately, this would also lead to slow transversal (dive and
squat) oscillations, made even worse by acceleration, decel

­eration and varying distribution of weight inside the cabin.

As the inertia of the car body around its transversal axis is
basically sufficient to counter the effect of longitudinal
bumps, the front and rear suspension circuits are sepa

­rated. The active height correction of the system acts as a
further a non-linear stabilizer both countering dive and
squat, and solving weight distribution problems.

On the other hand, if the bumps are transversal—for in

­stance, a pothole under the right wheel and a bump under
the left one—, the car will rotate around its longitudinal
axis. Being much less wide than long, the angle of rotation
will be higher and the inertial torque is considerably lower
to counter this kind of rotation. Completely independent
sides would result in very little damping of roll movements:
the low inertia provided by the body would find the reac

­tion of the suspension too stiff. Hence, the two sides in the
hydropneumatic suspension are interconnected, providing
a push-pull operation of the two sides. The interconnection
has special damping elements which react differently to dif

­ferent fluid movements between the sides: to quick suspen

­sion movements caused by potholes and bumps, or to
slower changes occuring when driving in a curve.

To counter body roll resulting from the second, an addi

­tional element, an anti-roll bar is also needed. The effects
of roll could be eliminated if the center of the roll could be
identical to the center of the mass. As this is not possible,
the opposite approach of moving the center of roll away
from the center of mass could also help overcome body roll
by increasing the opposing torque. This is the role of the
anti-roll bar: similarly to a bike leaning into a curve, it lifts
the inner side of the wheel, using the force on the outer
edge, and this moves the center of roll outwards. In other
words, the wheels and suspension elements do have roll,
the role of the anti-roll bar is to isolate this roll from the
body which should remain, ideally, horizontal. To accom

­plish this, the bar cannot be completely rigid (it has to ab

­sorb the road undulations without transfering them to the
body), a torsion spring is the usual solution.

Such anti-roll bars are used on conventional spring sus

­pension systems as well, however, there are substantial dif

­ferences in the way the bar interacts with the rest of the sus

-

pension on Citroëns. In a spring system, there is a consider

­able amount of interaction, a significant flow of energy in
both directions between the suspension and the bar. The
shock absorbers have to provide the damping for the anti­roll bar, introducing yet another interaction (in the hydrau

­lic setup this is catered for by the damping inside the con

­nection line between the sides).

Consequently, the hydropneumatic suspension has
much less interdependence and compromise between
damping, countering roll, squat and dive. In addition, it can
provide solutions which are simply unfeasible mechanically
in a conventional suspension. Cars with steel springs always
have roll, including diagonal one, induced by undulations
of the road—their anti-roll bar represent a constant me

-

chanical connection between the sides, unable to differenti

­ate between bumps and curves. Citroëns, on the other
hand, have a varying interconnection depending on fluid
movement—this is very easy to accomplish with hydraulics
but extremely complicated with springs.

The only disavantage is that damping occurs further
from the source of the disturbance, and due to the good
conductivity of sound via the hydraulic lines, this results in
slightly more noise. The same effect makes the hydropneu

­matic suspension somewhat noisier than a conventional
one. However, good sound insulation inside the cabin can
help overcome this small annoyance.

This suspension layout reduces the sensitivity to under

­inflated or blown tires and cross-wind. Even with largely un

­even braking forces on the two sides the car will not pull to
either side.

Although the hydropneumatic spring-absorber unit is an
integrated unit from a technical point of view, hydraulics
make it possible to place some hydraulic parts (for instance,
the center spheres on Hydractive systems) in different loca

-

tions, reducing the amount of sprung mass. Conven

­tional springs have a considerable mass of their own while
the mass of the nitrogen in the spheres is practically negligi

­ble. Even adding the mass of the fluid moving around in the
system, the sum remains much below that of a steel spring.
Hydropneumatic struts can be kept relatively small by in

­creasing the operating pressure, which decreases the diam

­eter of the struts. The automatic height correction reduces
the mass further because the basic suspension mechanics
can be simpler, without requiring multilinks and similar
components.

The brakes share the mineral fluid with the suspension.

This fluid boils at a very high temperature, therefore it pro

­vides great resistance to vapor lock. Due to the propor

­tional regulation a hydropneumatical Citroën can keep brak

­ing as long as there is anything left of the brake pad. Even if
the liquid starts to boil, there will be no vapor lock as the
pressure is automatically released and remains proportional
to the braking effort applied by the driver.

This system is often criticized for being overly compli

­cated and prone to error, none of which accusations is true.
The suspension is actually quite simple when considering its
extra services in comparison to a conventional system and
experience shows that the whole system is very reliable. The
perfect functioning of the system relies mainly on the pre

­scribed cleaning of the system and the change of the hy

­draulic fluid—adhering to these simple prescriptions can
make the system very reliable.

U

The Citroën Guide Suspension: Hydropneumatic Suspension 22

Finally, there are no forces in the suspension when the cir

-

cuit is depressurised, allowing very easy and safe servic

-

ing of the relevant suspension and transmission parts.

Modern spring suspension systems are in fact capable of

achieving some of these results. For instance, variable diam

­eter or pitch springs coupled with hydraulic shock absorb­ers (incidentally, with a similar internal geometry as the
damper elements used in Citroën spheres) behave similarly
to these hydropneumatic units. The main difference is that
even if these elements would be practically identical, all
other functionality that comes either for free or at a small
additional cost in Citroën systems—constant height, anti­dive, brake force regulation and so on—, require complex
and expensive additional systems.

The illustration shows the basic layout of the suspension
(differences on models fitted with power steering or ABS
will be described in the corresponding chapters). Most com

-

ponents have an output line to collect leakage (which is in

­tentional to keep the elements lubricated) and return it to
the reservoir—although the outputs are indicated, the lines
themselves are omitted for the sake of clarity. In reality, they
are grouped together and go back to the reservoir.

The high pressure supply subsystem consists of a five-pis

­ton volumetric high pressure pump drawing the mineral
suspension liquid called LHM from the reservoir. The fluid
under pressure is stored in the main accumulator. It is the

task of a pressure regulator—built into the same unit
with the accumulator—to admit fluid into the accumulator
as soon as the pressure drops below the minimum value of
145 bar; as soon as the pressure reaches 170 bar, the regula

­tor closes and the fluid continues its idle circulation from
the pump, immediately back to the reservoir.

On simpler models the out

­put marked with an asterisk
is omitted and it goes to the
return ouput inside the regu

­lator unit instead, as shown
by the dashed line. On mod

­els fitted with power assisted
steering (DIRASS) this inter

­connecting line is missing
and both outputs are used in

­dependently.

The spring below the piston 1 is calibrated so that it will

collapse only when pushed down with a pressure exceed

-

ing the cut-in threshold (145 bar). While the pressure in the
main accumulator remains inferior, the piston stays in the
upper position, allowing the pump to deliver fluid into the
accumulator through the ball valve 5: the unit is switched
on. The piston 2 also remains in the upper position (its
spring is calibrated to the cut-out pressure, 170 bar), letting
the entering fluid fill up the chamber 3 as well. This, in
turn, ensures that the piston 1 stays in the upper position:
the fluid pressure in this chamber plus the force of the
spring counters the downward pressing force even if the
pressure in the accumulator rises well above 145 bar.

The fluid supplied by the pump raises the pressure in the
accumulator; as soon as it reaches 170 bar, its pressing
force will exceed the retaining force of the spring under the
piston 2, forcing it to the lower position. In this moment,
the high pressure line coming from the another piston will
be cut off and the fluid from the chamber 3 can escape
back to the reservoir (yellow in the illustration).

front strut & sphere

front strut & sphere

reservoir

main accumulator &
pressure regulator

H

Ppump

security valve

height
corrector

height
corrector

rear strut & sphere

rear strut & sphere

brake valve

front brake

front brake

rear brake

rear brake

LHM feed
front suspension
rear suspension
front brakes
rear brakes
operational return
leakage return

A typical example: the BX

TRANSITIONON

pump

feed

flow
distr*

rtrn

12 1

2

33

5

44

66

5

OFF

pump

feed

flow
distr*

rtrn

2

1

354

6

U

The Citroën Guide Suspension: Hydropneumatic Suspension 23

With the back pressure now vanished from behind the

piston 1, the pressing force of the accumulator fluid drives
it down at once: the regulator is switched off now. The fluid
supplied by the pump returns back immediately: on PAS­equipped cars, to the flow distributor, on other vehicles,
straight back to the LHM reservoir through the internal con

-

nection (dashed line).

Shortly, as the suspension and braking circuits start to

use up the pressure in the main accumulator, the piston 2
will return to its original position. Once there, the regulator
is ready to start a new cycle.

The characteristic ticking which can be heard in Citroëns

is the sound of the regulator pistons quickly moving one af

-

ter the other, in quick succession: 2 down, 1 down, 2 up.
The opposite tick—1 up, when the regulator is switched on
to replenish the accumulator—is much softer.

The interconnection 6 is normally closed. Opening it lets

all the fluid stored under pressure return back to the LHM
reservoir—this is the way the system is depressurized when
any of the suspension elements need servicing.

The liquid—supplied to the rest of

the system from the main accumula

-

tor—passes through a security valve
whose task is to ensure safety by feed

­ing the brake circuits first. The front
brake circuit is always open but the
other two outputs are blocked by a pis

­ton. If the pressure in the main circuit exceeds 100 bar, the
fluid pushes the piston back against the force of the spring,
opening up the suspension outputs as well. The electrical
switch for the low hydraulic pressure warning lamp on the
dashboard is built into this valve as well. This way, a sudden
failure of the pump or the belt driving it will not leave the
car without sufficient braking power.

The second circuit fed from the security valve is the front sus

-

pension. The fluid goes to the front height corrector.

When the vehicle height is stabilized, the piston inside the
corrector blocks the inlet of fluid, isolating the struts from
the rest of the suspension. Body roll is limited by the damp

­ing effect of the restrictors built into the sphere supports
and by forcing the fluid to run from the left to the right
strut through a connection line. If the movement of the
front anti-roll bar dictates that the front of the vehicle
should be raised, the connecting linkage moves the piston
upward, opening the inlet and letting additional fluid enter
the front struts. When an opposite movement is required,
the piston moves downward, letting the fluid at residual
pressure flow back from the struts to the LHM reservoir.
Both directions of flow are stopped and blocked when the
height corrector piston resumes its middle position.

The mechanical connection between the anti-roll bar
and the height corrector is not a rigid linkage but has some
free play. Just before the height corrector, the connecting
rod coming from the anti-roll bar hooks into a small win

-

dow on the corrector side. Small movements of the control

rod do not change the position of the height corrector, only
those are large enough to exceed this free play. In addition,
the corrector has its internal (albeit low) resistance, besides,
all rods are somewhat elastic, so in the end, all these factors
make the height correction system filter out the higher fre

-

quency components of the suspension movement.

Observing an initial threshold which has to be crossed be

­fore any correction occurs not only reduces the strain and
wear on the correctors but also prevents the system from
developing self-oscillation. A powered system provides am

­plification and any feedback mechanism with a delay—
such as the height correction—could potentially result in os

­cillations. The initial threshold ensures that there is no feed

­back, and consequently, no oscillation when the required
correction is too small.

The next circuit is the rear suspension. Its layout and op

­eration is identical to the front one, having its own height
corrector.

The first circuit, as already mentioned, feeds the front

brakes. The liquid under pressure flows into the brake
compensator valve, operated by the brake pedal. In its

neutral position, the brake circuits are connected to the re

­turn lines to ensure that the brakes are not under pressure.
When the driver pushes on the pedal, this moves the first
piston, closing the return output and opening up the outlet
going to the front brake cylinders.

This piston and a spring behind it pushes the second pis

­ton which works similarly for the rear brakes, although
those are not fed directly from the security valve but receive
their supply from the rear suspension (later brake valves
have three pistons but their method of operation is practi­cally the same). In consequence, the braking force at the
rear depends on the load: the more the back of the car is
loaded, the stronger the rear brakes work. Actually, on a
Citroën mostly used to carry only its driver, without much
load in the trunk, the rear brake pads and disks wear much
slower than those in the front.

The damping elements in the

sphere supports consist of a central
hole which is always open and addi

­tional small holes closed and opened
by a spring as the flow of the hydraulic
liquid dictates. Slower suspension
movements like body roll, squat or
dive result in a slower flow of the liquid and the smaller dy

­namic pressure differences are not sufficient to bend the
spring cover open over the additional holes. The damping
effect is therefore only determined by the diameter of the
center hole.

The abrupt jolts caused by road irregularities, in contrast,
cause faster flow. With the increasing pressure difference
the fluid will open the spring cover and use the additional
holes as well. This increased cross section results in a lower
damping effect.

The additional holes are located in a circle around the
center hole. There are two spring covers, one on each side,
but they do not cover all the holes equally. Half of the holes
(actually, every second one) are slightly enlarged on one
side, the remaining half on the other side. By carefully ad

­justing the size of the holes, the designers could fine tune
the damping factors independently for both directions of
strut travel.

warning
lamp

feed

rear

brakefront

feed

struts

return

feed

struts

struts

return

STABILIZED RAISING LOWERING

U

The Citroën Guide Suspension: Hydractive I 24

Hydractive I

The Hydractive I suspension system appeared
with the XM. Unlike the simpler hydropneumatic
suspension used on the DS, GS/GSA, CX, BX and
some XMs, this one has two modes of operation,
soft and hard. The suspension functions in soft
mode but it will be switched to the hard mode
when the computer deems this necessary for the
sake of roadholding and safety.

To achieve this, the first hydractive system adds two spheres
(one for each axle) and an electric valve to the struts and
spheres of the standard hydropneumatic setup.

During normal driving, the computer keeps the suspen

­sion in soft mode most of the time but—based on the input
provided by many sensors (steering wheel, accelerator
pedal, body movement, road speed and brake), including
the Sport/Comfort switch on the dashboard—the suspen

­sion ECU decides when to switch to hard mode; in other
words, when to deactivate the additional spheres for extra
roadholding and safety.

When the driver selects the Sport setting, the suspension
is switched to hard mode constantly. This setting is not
what any Citroën driver would call comfortable… The suc

-

cessor system, Hydractive II overcomes this limitation.

return return

return

feed from
security valve

return

control from
computer

nitrogen
LHM
moving parts

suspension
control block

height
corrector

strut & sphere strut & sphere

1

2

44

electro-valve

5

rear
suspension

3

The layout of the system (front suspension)

The illustration only depicts the differences to the standard
hydropneumatic layout already presented in the previous
section:

1

A standard Citroën sphere base which fits a sphere
without a damper block. The sphere volume and pres

­sure differ for the front and rear, as well as according to
the model of the car;

2

A hydraulically controlled isolation valve that con

­nects or isolates the sphere from the rest of the suspen

­sion, modifying the string constant of the suspension;

3

A ball and piston valve arrangement that limits fluid
cross-flow between the left and right suspension struts
in case of body roll. This valve is disabled for suspension
height corrections, in order to guarantee that the fluid
pressure in the corner struts remains equalized;

4

Tw o damping elements similar to those used on the
corner spheres, acting as dampers for the center one;

5

An electrically controlled valve driven by the suspen

­sion ECU. In order to reduce heat build-up, the computer
uses pulse width modulation to achieve a constant cur

­rent through the coil. The initial voltage is higher to
make the valve react quicker but it is reduced to a smaller
value once the inductive effects have been overcome,
should the valve stay on for a long enough time. The
valve is capable of being on indefinitely when driven with
this sustained current.

The front and rear suspension circuits are identical and the
same electrovalve serves both subsystems.

Soft, hard, soft, hard…

The default electrical mode of the suspension, when the
electro-valve 5 is not energized, is hard.

???

U

The Citroën Guide Suspension: Hydractive I 25

While the computer keeps the suspension in soft mode,
the electro-valve 5 is energized by the ECU, opening the
feed pressure onto the isolation valve piston 2 and by mov

-

ing it, connecting the center sphere 1 to the rest of the sus

-

pension. The fluid in the suspension has to pass through
two damping elements 4 (one for each strut connection).
When both struts move in unison, the center sphere be

­haves as a standard sphere with a damper hole twice as
large as a single damper element, but when the car starts to
roll, the fluid has to move from one strut to the other, pass

­ing through both damper elements consecutively. In addi

­tion to this double damping, the sphere 1 itself acts as a
damping string, absorbing quick changes in pressure be­tween the two dampers. This dampens the body roll to
some extent even in soft mode.

Whenever the computer feels it necessary to switch to

hard mode, it closes the electro-valve 5, not allowing the
main feed pressure to move the isolation piston 2. The pres

­sure inside the center sphere 1, always higher than that of
the return path under normal operating conditions, will
move the control piston into a position which closes off the
center sphere completely. The remaining pressure in this
sphere remains unknown but as the main circuit pressure
might change while the suspension is in hard mode (due to
either the dynamics of the suspension—acceleration, brak

-

ing, movement due to uneven surface—or the vehicle
height altered by the driver), the computer equalizes the
pressure periodically by enabling the control block to as

-

sume the soft position for a short period of time.

Hard mode serves three reasons. First, it provides higher
resistance to body roll. The cross-flow of LHM from one
strut to the other has to pass through both damper blocks
as in soft mode, but it is additionally limited using the pis

-

ton and ball valve 3, now switched into the hydraulic circuit
between the damper elements instead of the center sphere.
The ball is positioned in the fluid so that any cross-flow
moves the ball and thus limits the flow, dampening the
body roll as well.

Second, it limits dive and squat by helping out the height
correctors. A stiffer suspension damps the vertical motion
and therefore reduces the amount of correction required.

Third, hard mode not only limits the suspension travel be

-

tween the body to the road but between the suspension ele

-

ments and the body. Its aim is to reduce suspension move

­ment at the cost of comfort but to gain safety, limiting the
influence of the body movement to
steering, very important in extreme sit

-

uations like a flat tire.

When the vehicle is making a sharp
left turn, tending to roll to the right,
the right strut will be compressed and
the left one expanded. The fluid is
then forced from the compressed strut to the expanded
one, moving the ball in the valve towards the outlet of the
left strut; as soon as it reaches and covers the outlet orifice,
it closes off any further cross-flow. The corner spheres are
now isolated and has to provide all the damping them

-

selves.

At the same time when the body roll is present, the car
might need to change the ground clearance as well: for in

-

stance, when braking in a curve. The
valve 3 therefore has an additional pis

­ton which lets the LHM flow between
the circuits of the struts and of the
height corrector. If the body has to be
raised, the pressure in the height
correctors will be higher than that in
the suspension. This higher pressure

pushes the piston, which in turn dislodges the ball and the
pressure will raise equally in both struts (without dislodging
the ball, only one of the struts would receive the fluid, result

-

ing in incorrect operation).

If the body has to be lowered, the

higher pressure in the struts will dis

-

lodge the ball again, opening the pis

­ton towards the return line ad the
fluid will escape from both struts, low

­ering the vehicle.

Sensory perceptions

The computer of the suspension system takes its input sig

-

nals from the various sensors and based on a set of rules, dy

-

namically activates the electric valve.

There are eleven inputs to the ECU. First, the Comfort/

Sport switch on the dashboard, enabling the driver to
choose between the two settings. The status light on the in

-

1

2

44

3

strut

strut

SO

FT MODE

suspension pressure

system feed pressu re

security
valve

rear

return

5

control from
computer

height

corrector

1

2

44

3

HARD MODE

suspension pressure

system feed pressu re

residual pressure

strut

strut

security
valve

control from
computer

rear

height

corrector

return

5

strutstrut

height
corrector

strutstrut

height
corrector

strutstrut

height
corrector

U

The Citroën Guide Suspension: Hydractive I 26

strument panel informs about the setting selected (it does
not indicate the mode the suspension is currently in).

The second input comes from a vehicle speed sensor.

This inductive magnet tachogenerator generates 4 pulses
per rotation, that is approximately 5 pulses per meter trav

-

eled (although this depends somewhat on tire size). It is lo

-

cated on the gearbox where the speedometer cable atta

-

ches, or in some versions, on the cable itself. The ECU deter

-

mines the acceleration of the car by evaluating changes in
vehicle speed for the duration of one second.

Another input arrives from the steering wheel angle

and speed sensor, an optoelectronic device consisting of
two infrared light beams, interrupted by a rotating disc
with 28 holes. The ECU senses the quadrature signal
changes of both sensors to effectively increase the resolu

­tion of the sensor (28 pulses per steering wheel revolution)
by a factor of four. This produces one edge change every

3.214 degrees of steering wheel rotation. The direction of
turning can be determined by the sequence of the edge
changes.

To make decisions, the computer needs to know the
straight ahead position of the steering wheel. The sensor
does not have a built-in zero position (as it would not al

-

ways work, due to misalignment and wear in the mechani

-

cal components). The computer uses heuristics instead:

First, the straight line position is assumed if the vehicle
speed is above 30 km/h and the steering wheel position
was not changed (an error margin of up to 4 pulses is al

-

lowed) for the last 90 seconds. Second, we know the maxi

­mum number of pulses in both directions from the center
(lock to lock angle divided by two). If the steering wheel is
found to turn more than this value (an error of up to 4
pulses is accepted here, too), this is a clear indication of an
incorrect center reference: in this case the center position
will be adjusted by the surplus.

The rotational speed of the steering wheel is determined
by measuring the time elapsed between the individual
pulse edges coming from the sensors.

A similar sensor informs the computer about the move

-

ment of the car body. Two infrared beams, the disc hav

-

ing 45 notches, similarly quadrupled by the ECU. Exces

­sively long intervals are considered coming from slow
height changes resulting from the driver selecting a differ

­ent height setting, and are consequently discarded.

The sensor is connected to the front anti-roll bar, to the
right of the height corrector linkage. Due to its location, it is
capable of detecting both squat and dive, and to some ex

­tent, body roll. But as the sensor is mounted off-center, its
sensitivity to roll is about three times less than the sensitivity
to squat and dive. In all directions, it can measure both
movement amplitude and speed of movement, using the
same process as the steering wheel sensor does.

The throttle pedal position sensor is located below

the dashboard, right next to the pedal mechanism, where
the pedal can operate its sprung lever as it moves. The sen

­sor is a potentiometer with an integrated serial resistor in
the wiper’s circuit.

The entire travel of the potentiometer is quantized into
256 steps by the analog-digital converter inside the ECU.
The 5 V reference is supplied by the ECU itself. Due to the
gas pedal initial position and maximum travel, about 160
to 220 steps out of 256 are being actually used.

The brake pressure sensor is a simple pressure acti

­vated switch located on a hydraulic conduit connector
block, right next to the ABS block, at the bottom of the left
front wing, in front of the wheelarch, under the battery.
The switch makes contact at 35 bars of braking pressure.

The door/tailgate open switches are located on the

door frame and in the boot latch. The door switches are all
wired together in parallel and connected to one input line
(and routed to the interior light dimmer and timer as well).
The tailgate switch is connected to the other input line (and
routed to the boot light and the tailgate opened detection
input for the status display on the dashboard, too; the door
open and bonnet open signals for the status display are gen

­erated by a separate set of switches, independent of the
ones used for the suspension).

The usual ignition switch provides a power-on signal,

triggering and internal reset and self diagnostic run in the
ECU. Turning the ignition on and off also triggers internal
events that guarantee proper pressure equalization be

-

tween the center and corner spheres.

The brain behind the suspension

The ECU is a small microcomputer sensing the input signals
coming from the various sensors. A very interesting and im

­portant aspect of the system is that it uses the driver of the
car as a major part of its intelligence, making the operation
very simple but effective. To achieve this, most of the sen

­sors read the controls the driver operates.

The software contains the description of various condi

­tions (status of the input lines and internal timers) govern­ing when to activate-deactivate the electrovalve switching
the suspension to either hard or soft mode. These condi

­tions can be formulated as rules.

Every main input sensor has an associated rule: when the

value collected from the sensor exceeds a specific thresh

­old, the suspension is put into hard mode and the com

­puter starts a timeout counter. For the suspension to return
to soft mode at the end of the timeout period, the thresh

­old must not be exceeded again during this time. If it was ex

­ceeded, the suspension stays in hard mode and the timeout
starts all over again.

There are four additional rules overriding the normal op

­eration—even if the sensor inputs call for a generic rule to
be applied, these four conditions are checked first:

U

the computer puts the suspension into soft mode when
the ignition is turned on or off. This setting prevails until
30 seconds elapse or the vehicle speed exceeds 30 km/h,
whichever comes first;

U

if the computer determines any problem with its own op

-

eration or any of the input or output devices (including
inconsistent values like no body movement but a vehicle
speed above 30 km/h), the suspension will be switched
to hard mode and stay there until the ignition is turned
off or the doors are opened with the vehicle speed below
30 km/h. The ECU does run a self-diagnostic routine
when the ignition is turned on but some sensors cannot
be tested at this time, only during normal use;

U

whenever the suspension stays in hard mode for more
than one minute, the computer switches to soft mode
momentarily to assure the equalization of pressures in
the corner and center spheres. If the circumstances still

U

The Citroën Guide Suspension: Hydractive I 27

call for hard mode, the suspension will revert within
50 ms and restart the one-minute timeout period;

U

below 30 km/h opening the doors or tailgate overrides
any other rules and puts the suspension into soft mode
to equalize the pressures in the spheres.

As already mentioned, the steering wheel sensor is used
to derive two inputs values: steering wheel speed and an

­gle. These values are treated separately with the purpose of
calculating the lateral acceleration of the vehicle (vehicle
speed, steering angle) and the potential change in this ac

­celeration (vehicle speed, steering wheel speed). It is seem

­ingly done this way to save memory which would otherwise
be required for a full three-parameter lookup (based on ve

­hicle speed, steering wheel angle, steering wheel speed).
The steering wheel sensor rules actually give a measure of
potential body roll. Body roll is significantly reduced in hard
mode, consequently, the rules were set up to ensure that
the body roll is minimized when there is potential for it, still
the suspension stays soft to absorb bumps when there is no
body roll caused by the vehicle changing direction.

If the acceleration or deceleration (braking) of the ve

­hicle exceeds 0,3 g (approximately 3 m/s²) while the actual
speed is above 30 km/h, the suspension will be switched to
hard mode and a timeout of 1.2 seconds begin.

The table below shows the thresholds of steering

wheel angle and rotating speed. If any of these values
exceed the threshold for the actual vehicle speed, the sus

­pension will switch to hard mode; it will revert to soft when
the corresponding value drops below the threshold for at
least 1 second if the switching was triggered by the steering
wheel angle and 2 seconds if triggered by the rotational
speed:

Vehicle speed

(km/h)

Steering wheel

angle
(deg)

Vehicle speed

(km/h)

Steering wheel

speed

(deg/s)

< 30 always soft < 30 always soft

31–40 130 31–60 196

41–60 100 61–100 167

61–80 52 101–120 139

81–100 40 121 > 128

101–120 18

121–140 15

141 > 8

The body movement amplitude and speed is derived from
the output of the body movement sensor, although the
two values are used in a different way.

The body movement speed is used as the parameter for

the activation of two types of corrections:

U

Flat tire correction: if the body movement speed ex

­ceeds 300 mm/s, the suspension switches to hard
mode, and all thresholds are modified to 60 mm. The
timeout of the correction will be 0.4 s.

U

Excessive body movement correction: if the body
movement exceeds 60 mm more than three times
within three seconds, the suspension will switch to
hard mode, and all thresholds are modified to 60 mm.
The timeout of this correction will be 0.4 s.

The previous corrections stay enforced until one or more of
the following conditions are satisfied:

U

body movement amplitude remains under the modi

-

fied threshold until the correction timeout elapses;

U

suspension selector is set to the Sport setting;

U

the vehicle accelerates above 159 km/h;

U

the steering wheel angle exceeds the threshold value
dependent on vehicle speed as specified in the follow

-

ing table.

Once any of these conditions are met, the suspension will re

-

vert to normal operation, with thresholds restored accord

­ing to the table. Exceeding any of these thresholds will
force the suspension into hard more. The computer checks
every 0.8 seconds whether the conditions forcing the sus

­pension into hard mode are still present, and if so, the sys

­tem stays in hard mode.

Suspension down > 13 pulses, timeout 1 sec
Suspension up > 9 pulses, timeout 1 sec
Suspension change speed between 30 and 50 ms AND

Durchfederung > 3 pulses, timeout 1 sec

Vehicle

speed

(km/h)

Dive

(mm)

Squat
(mm)

Steering

wh pos

(deg)

Vehicle

speed

(km/h)

Dive

(mm)

Squat
(mm)

Steering

wh pos

(deg)

< 30 — — — < 30 — — —

The values delivered by the throttle pedal sensor are
used with reference to the vehicle speed in order to antici

­pate the vehicle dynamics as a result of acceleration or de

­celeration. The rules for this sensor represent a reaction to
probable vehicle squat (on acceleration) or dive (on deceler

­ation). Both are significantly reduced when the suspension
is in hard mode.

The suspension ECU quantizes the pedal position into

five discrete steps: 0, 30, 40, 50 and 60 percent of the com

­plete pedal travel. The computer measures the time elapsed
as the pedal travels from one step to the next in either direc

­tion. If this time is inside the intervals shown in the table,
the suspension will switch to hard mode. It will revert to
soft if the pedal movement becomes slower for at least the
duration of the timeout specified:

Pedal press
speed (ms)

Timeout

(s)

Pedal release

speed (ms)

Timeout

(s)

< 100 1 < 100 1

101–150 2 101–200 2

The brake pressure sensor detects the pressure in the
front brake hydraulic circuit. Since this is a fixed threshold
sensor, the suspension setting rule is simple: if the vehicle
speed exceeds 30 km/h and the pressure is above 35 bar in
the brake circuit, the suspension switches to hard mode.
The system stays so to prevent excessive dive when brakes
are applied while any of these two conditions are met (the
timeout value is one second).

Without ignition and electrical feed to the suspension
computer, the electro-valve would immediately return to
hard mode. Loading or unloading the car, people getting in
or out would induce pressure differences in the hydraulic
system. These differences would equalize abruptly when
the system is started again, causing the car to jump or sink
vehemently. In order to avoid this, the computer allows an
additional 30 seconds of timeout starting when any of the
doors is opened or closed (as communicated by the door

U

The Citroën Guide Suspension: Hydractive I 28

and tailgate open sensors) , leaving the electro-valve en

-

ergized for the duration of the timeout.

It is important to note that the suspension will switch to
soft mode even with the ignition switch turned off. Early
cars did not have this feature built directly into the com

­puter but used an additional relay and circuits. On those
models, the constantly energized electro-valve can drain
the battery if the doors remain open for a long time.
Starting with the H2 suspension computer (from
ORGA 4860, February 28, 1990) the door sensors are ob

­served by the ECU itself and the operation is enhanced with
a 10-minute timeout period. After this interval, the electro­valves will always return to the hard, non-energized state.

Changing the state of the ignition switch provokes a

transition to soft mode for a maximum of 30 seconds;
reaching a vehicle speed of 30 km/h will cancel this mode
prematurely. When the ignition is turned on, the ECU also
runs a self-test diagnostic sequence lasting three seconds.

When the suspension selector switch is set to the

Sport setting, all sensor inputs except for the vehicle speed
sensor are ignored. Below 30 km/h the car stays in soft
mode and switches to permanent hard mode above this
speed. The suspension status light in the instrument
panel has two functions:

U

when the ignition switch is turned on and the suspen

­sion set to Comfort, it will light up for the duration of
the ECU self test. If the computer detects any mal

­function in the course of this test, the light will flicker
one or more times during this period;

U

when the suspension is set to Sport, the status light
will remain lit to inform the driver of the setting cho­sen. The status light actually lights up or extinguishes
only when the suspension rules have been changed in
response to the mode select switch. This takes a short
while because the internal timeouts are reset and
some of the sensors are recalibrated. Because of this
the light changing state is slightly delayed from the
mode switch changing state.

U

The Citroën Guide Suspension: Hydractive II 29

Hydractive II

The second incarnation of the hydractive suspen

­sion appeared at February 1, 1993 (ORGA 5929).
It was designed to overcome the biggest problem
of the previous system, the very uncomfortable
hard mode.

Switching to Sport does not mean sticking to a hard, un

-

comfortable ride any more. On the Hydractive II, the rela

-

tion between suspension modes and dashboard switch set

­tings became more complicated: in both settings—Normal
(the new name of Comfort) and Sport—the computer can
switch to both hard and soft mode as it finds it necessary,

however, when set to Sport, the suspension becomes more
sensitive and will sooner and more often switch to the hard
mode.

1
2
3
4

Many models were also fitted with an anti-sink system
that locks the system when the car is not running, using yet
another sphere. Its only purpose is to keep the car from sink

­ing when not used, it does not influence the functioning of
the suspension system in any way.

return return

return

feed from

security valve

return

control from
computer

nitrogen
LHM
moving parts

suspension
control block

height
corrector

strut & sphere strut & sphere

1

2

3

5

44

The layout of the system (front suspension)

The center sphere circuits and supports were redesigned:
they now house the electrovalves and the internal conduits
serving the sphere were modified as well; the new control
blocks connect, as previously, to the left and right corner
spheres, the height corrector, and—depending on the con

-

trol signal coming from the suspension computer—the cen

­ter sphere. The elements are practically the same as on
Hydractive I:

1

A sphere base;

2

A hydraulically controlled isolation valve;

3

A ball and piston valve;

4

Tw o damping elements;

5

An electrically controlled valve driven by the suspen

-

sion computer.

The front and rear suspension circuits are identical but hy

­draulically independent. The electro-valves are driven simul

­taneously, in parallel.

Trapped among pistons

The electro-valve 5 is energized when the suspension is in
its soft mode, hence, the default electrical position is hard.
However, due to the indirect coupling between this valve
and the isolation piston 2 inside the control block, the hy

­draulics can stay in either position for extended periods of
time with the electric valve disconnected, depending on the
pressure differences between the strut and the main cir

­cuits. If the main suspension circuit has nominal pressure,
the system stays in hard mode with the electric valve off or
disconnected.

The two modes are practically the same as on the previ

­ous Hydractive system: in soft mode the electro-valve 5
opens the feed pressure onto the isolation piston 2 and by
moving it, connects the center sphere 1 to the rest of the
suspension. In hard mode, the electro-valve 5 closes and

U

The Citroën Guide Suspension: Hydractive II 30

lets the pressure inside the center sphere 1 move the con

­trol piston into a position which closes off the center sphere
completely.

The center sphere 1 is now supplied directly from the

height corrector in soft mode. This simplifies the ball valve
arrangement with respect to Hydractive I.

Higher intelligence

The computer uses the same set of sensors as Hydractive I,
the only difference is the vehicle speed sensor which is a
Hall-effect sensor now. Its resolution have been doubled to
8 pulses generated per rotation, that is approximately 5
pulses per meter traveled (although this depends some

­what on tire size). It is located on the gearbox where the
speedometer cable attaches, or in some versions, on the ca

­ble itself.

The internal algorithm of the computer became more so

­phisticated. While the Hydractive I had only one computer
controlled mode (Sport switched the suspension to con

­stant hard mode above 30 km/h of vehicle speed), the
newer system has two such regimes of operation: in both
Normal and Sport it dynamically activates the electro-valves
of the suspension control blocks whenever it decides that
the driving circumstances call for a firmer suspension. The
difference is in the set of rules the computer uses to evalu

­ate those circumstances: the rules are stricter for the Sport
setting, with most of the thresholds reduced, thus the sus

­pension will switch to hard mode much more readily.

The following table shows the thresholds of steering

wheel angle. If the value observed by the sensor exceeds
the threshold for the actual vehicle speed and the suspen

­sion setting, the suspension will switch to hard mode; it will
revert to soft when the corresponding value drops below
the threshold for at least 1.5 seconds:

Vehicle

speed

(km/h)

Steering wheel

angle (deg)

Vehicle

speed

(km/h)

Steering wheel

angle (deg)

Normal Sport Normal Sport

< 34 — — 90–99 33 22

34–39 174 119 100–119 26 27

40–49 100 67 120–139 23 15

50–59 84 56 140–158 20 13

60–68

68 45 159–179 13 9

69–78 55 37 179 > 10 7

79–89 42 28

There is a similar table for the thresholds of the steering

wheel rotational speed as well:

Vehicle

speed

(km/h)

Steering wheel

speed (deg/s)

Vehicle

speed

(km/h)

Steering wheel

speed (deg/s)

Normal Sport Normal Sport

< 24 — — 79–89 62 41

24–29 535 357 90–99 53 35

30–39 401 267 100–119 42 28

40–49 246 164 120–139 30 20

50–59 178 119 140–158 22 15

60–68 110 73 158 > 20 13

69–78 82 55

The thresholds for body movement are:

Vehicle

speed

(km/h)

Dive

(mm)

Squat
(mm)

Steering

wh pos

(deg)

Vehicle

speed

(km/h)

Dive

(mm)

Squat
(mm)

Steering

wh pos

(deg)

< 10 — — — 100–109 48 48 13

10–33 84 60 — 110–119 48 42 13

34–39 84 60 87 120–129 48 42 11.5

40–49 54 48 50 130–139 42 42 11.5

50–59 54 48 42 140–149 42 42 10

60–68 54 48 34 150–158 42 36 10

69–78 54 48 27.5 159–179 42 36 6.5

79–89 54 48 21 179 > 36 36 5

90–99 48 48 16.5

Note that the thresholds are the same for both Normal and Sport suspension settings

The thresholds of the gas pedal sensor are:

SO

FT MODE

strut

return

control from
computer

suspension pressure

system feed pressu re

strut

security

valve

height
corrector

2

3

5

1

4

4

return

control from
computer

suspension pressure

system feed pressu re

residual pressure

security

valve

height
corrector

1

2

3

4

4

5

strut strut

HARD MODE

U

The Citroën Guide Suspension: Hydractive II 31

V

ehicle

speed

(km/h)

Pedal press rate

(steps/25 ms)

Vehicle

speed

(km/h)

Pedal release rate

(steps/25 ms)

Normal Sport Normal Sport

< 14

2 1.3 < 19 10 6.6

15–49

3 2 20–78 5 3.3

50–99 4 2.6 79–168 6 4

100–134 5 3.3 168 > 7 4.6

135–199

64

199 > 7 4.6

With the improved resolution of the vehicle speed sensor,
the rules formerly referencing to 30 km/h are changed to
24 km/h. Thus, the suspension switches to hard mode if the
brake pressure sensor detects a pressure above 30 bar
and a vehicle speed in excess of 24 km/h.

Similarly, the suspension will switch to soft mode if the

ignition switch is turned on, for a maximum of 30 sec

-

onds, but reaching a vehicle speed of 24 km/h will cancel
this mode prematurely. It will switch to soft also if any door
or the tailgate is opened but the vehicle speed is below
24 km/h. The reason for this is to equalize the pressure be

­tween all three spheres of an axle. Without it, the center
sphere would retain its former pressure and once the vehi

­cle exceeds the speed of 24 km/h, opening it would make
the car jump or drop, depending on the actual pressure.

It is important to note that the suspension will switch to
soft mode even with the ignition switch turned off. Should
the doors remain open with the ignition switch in the off po

­sition, the suspension soft mode will be subjected to a 10­minute timeout period to avoid draining the battery as the
soft mode requires the electric valves to be energized.

U

The Citroën Guide Suspension: Anti-sink system 32

Anti-sink system

Many contemporary Citroëns—including both reg

­ular hydropneumatic and Hydractive Xantiae and
XMs—have an anti-sink system (SC/MAC) fitted,
with the sole apparent purpose of tranquilizing
the average driver by keeping the car from lower

­ing when not used. The system does not interfere
with the normal functioning while in use. It at

-

tempts to minimize leaks inside the system by hav

­ing only one element that can leak, the anti-sink
valve itself.

The layout is rather simple: there is an anti-sink valve fitted
for each axle, between the height corrector and the suspen

-

sion struts (or the hydraulic control block on Hydractive sys

­tems). The valves operate on the pressure differences in the
system, without any electrical control: when there is signifi

­cant pressure in their control circuit, they keep their work cir

­cuit constantly open (in fact, they are analogous to a simple
electrical relay).

Under normal circumstances, the high pressure pump
supplies the pressure regulator and the main accumulator
with fluid. The output from these two feeds the whole sys

­tem with high pressure, going through the security valve
which keeps the brake circuit constantly under pressure, for
obvious reasons of security. If there is enough pressure in
the system, the security valve feeds the rest of the suspen

­sion via the anti-sink valves and the height correctors.

This pressure coming from the security valve appears in
the control circuit of the anti-sink valves. When the car
runs, the valves are constantly open, connecting the height
correctors to the rest of the suspension and brake subsys

­tems—everything functions exactly as in cars not equipped
with this anti-sink system. Even when the engine is turned
off, the valves remain open as long as the feed from the ac

­cumulator remains at a higher pressure than that of the sus

-

pension. But as soon as the leakage in the struts, height reg

­ulators and the brake valve reduces the pressure in the main
accumulator below the suspension pressure, the closing
anti-sink valves isolate the suspension struts from the rest
of the system (see page 29 for a description of the Hydr

­active II control block). It is usually the front valve that
closes first as the front of an unladen car is much heavier
due to the engine and gearbox. Compared to a non-anti­sink car, the leakage is quite drastically reduced. For in

­stance, a standard XM with its suspension in prime condi

­tion takes about 20-30 hours to sink completely, while with
the anti-sink system this would take as much as ten days.

The rear anti-sink valve is connected slightly differently:

in addition to feeding the rear suspension and the brake cir

­cuit, as usual, it connects to an additional anti-sink sphere
as well. The function of this sphere is to maintain pressure
in the braking circuit. As the brake valve is the most leaky el

­ement, it could exhaust the pressure between the piston
and the plunger while the remaining pressure behind the
piston (provided the high pressure and the front suspen

­sion circuits do not leak that much) stays rather high. In this
case the anti-sink valve might open again in error—this ad

­ditional sphere ensures that this will not happen.

The anti-sink system maintains the car height by counter-

acting the internal leakage of the various suspension ele

­ment that would make the pressure escape back to the res

­ervoir. Elements that are in constant motion—height cor

­rectors, for instance—leak past their seals on purpose to lu

­bricate themselves; brake valves, on the other side, start to
leak once they are worn. The anti-sink valves—which move
very rarely, need no intensive lubrication, thus are manufac

­tured with very close tolerances and hardly leak them

­selves—isolate all the struts from the rest of the system to
prevent any possible leakage to reduce the pressure in the
struts, allowing the car to sink.

height
corrector

height
corrector

security
valve

security
valve

anti-sink
valve
(closed)

anti-sink
valve
(open)

reservoir reservoir

REAR
ONLY

REAR
ONLY

rear
brakes

rear
brakes

anti-sink
sphere

anti-sink
sphere

control
block

4

45

3

HYDROPNEUMATIC HYDRACTIVE

U

The Citroën Guide Suspension: Activa Suspension 33

Activa Suspension

The Activa suspension—used only on some Xantia
models—creates mixed feelings. Drivers requiring
sporty handling and roadholding praise it be

­cause this car turns into curves without turning
a hair: it stays completely horizontal and neutral.
However, this comes at the expense of ride
comfort.

The Activa system operates in two distinct steps. The first
one is controlled mechanically by a roll corrector (the com

-

ponent is identical to the height correctors used in the sus

-

pension, see the details on page 23).

The corrector is connected by an L-shaped spring to the
bottom wishbone. When the car takes a sharp left turn, its
front left wheel will be forced down by the body roll caused
by centrifugal force. As the wheel moves down, so does the
end of its wishbone, pulling the linkage to the corrector.
The piston inside the roll corrector moves upwards, open

­ing the pressure feed into the stabilizing cylinders. These
two cylinders are attached to the wheel suspension differ

­ently: in the front, the piston pushes the left wheel upwards
while in the rear, the right wheel will be forced downwards.
This diagonal correction counteracts the roll of the body.

Turning to the other side result in an inverse operation:
the roll corrector opens the connection from the stabilizing
cylinders back to the reservoir. The front left wheel moves
downwards, the rear right one upwards, once again coun

-

tering the effect of body roll.

An additional Activa sphere in the front acts as an extra

accumulator but the rear sphere can be connected or de

-

coupled electrically. Depending on the position of the pis

-

ton inside the electro-valve, the high pressure feed is ei

-

ther allowed to reach the piston 2 inside the control block,
pushing it up and connecting the sphere 1 to the rest of
the circuit (dashed line on the illustration), or the residual
pressure in the sphere moves the piston 2 down, isolating
the sphere 1.

When the Activa sphere is open to the rest of the system,
roll correction is applied through a spring element formed
by the accumulator and the Activa sphere. The supply side
of the stabilizing cylinder pistons have half the area of
the other side, connected to the Activa sphere 1 with the
valve 2 open. Changes in the length of the linkage is there

-

fore not transmitted directly to the roll bar. Upon the influ

­ence of external forces like body roll, the movement of the
piston compresses the gas content in one sphere and at the
same time, expands it in the other.

The stabilizing cylinder works as a spring with asymmetri

­cal characteristics: its effective hardness is smaller around
the corrected position, but it hardens progressively as the
piston is forced out of that position.

The Activa system has two operating modes, depending

on the position of the electro-valve 2. In the first mode roll
correction is always active because the roll corrector is up

­set. The resulting flow of fluid will tend to move the active
linkage upsetting the balance of presssure in the two extra

STRAIGHT-AHEAD

reservoir

pressure feed

roll
corrector

feed pressure
work pressure
leakage return

Activa

sphere

Activa sphere
with control block

front
stabilizing cylinder

electro­valve

ECU

vehicle speed
steering wheel angle
steering wheel speed

anti-roll
bar

rear
stabilizing cylinder

1

2

RIGHT TURN

SHARP RIGHT TURN

U

The Citroën Guide Suspension: Activa Suspension 34

spheres, and making the coercive force be applied through
a spring element which becomes progressively stiffer the
more correction is needed.

The ECU controling the electro-valve uses sensors identi

­cal to the Hydractive system. The values of vehicle speed,
steering wheel rotation angle and speed determine when
the second mode of anti-roll behavior has to be enforced.
Similary to the operation of the suspension computer, the
Activa ECU also uses the driver as the input to determine
the motion of the vehicle body: if the roll is caused by the
unevenness of the road surface, the steering wheel will not
be rotated. In curves, the computer calculates the maxi

­mum potential lateral acceleration (vehicle speed is mea

­sured by its sensor, the turning radius is communicated by
the steering wheel angle sensor, the mass of the car is a
known constant—the centrifugal force can be calculated
from these values) and decides wether the spring element
formed by the two spheres needs to become rigid to make
the system compensate for the body roll.

In this mode the Activa sphere is isolated from the rest of
the system, the fluid line between the roll corrector and the
active linkage is blocked at both ends, making the linkage
completely rigid. Even if the roll collector end is open, the
linkage remains quite rigid (providing for a very hard spring
coupled with high damping); only half of the displacement
escapes from the additional accumulator sphere through a
restrictive regulator.

The additional damping of the Activa sphere is now
switched off, the correction is applied only through the very
hard roll-bar. When the possible range of correction is ex

­hausted (strut linkage extends or contracts as far as it can),
at about 0.6 g lateral acceleration, only the very hard roll­bar remains functional.

The diagrams showing the kinetic characteristics of an

Activa car reveal the details. The first diagram shows the re

­lationship between time and roll angle for a constant lateral
acceleration. It can be observed clearly that the Hydractive
system can only limit roll damping, not roll angle. Note that
the initial slope of both Hydractive curves—the section up
to 0.4–0.6 seconds— is practically the same in both soft
and hard mode. This slope represents the combined
hardness of the roll bar and the associated hydraulic compo

­nents. Yet, the reaction time is longer in the soft mode (0.8
seconds versus 0.6, indicated by the last bend when the
curve turns into a horizontal line). As the corner spheres are
isolated and their combined gas volume is less in hard

mode, the maximum roll angle stabilizes around 2.5 de

-

grees while in soft mode it reaches 3 degrees.

The second diagram depicts the relation between the lat

-

eral acceleration and the roll angle. The hydraulical-mechan

­ical roll bar of the Activa starts the same as the Hydractive
system with minimum lateral acceleration. But, while the
Hydractive stays almost linear—the sharper you turn, the
bigger the body roll angle will be—, the Activa compen

­sates by keeping the body roll angle at a constant below 0.5
degree up to a lateral acceleration of 0.6 g (by providing an
effectively infinitely stiff roll bar setup). But even when the
limits of the roll bar are reached, having contracted or ex

­tended it as far as it can go, the effective roll bar remains
quite stiff: the roll angle will increase only moderately, up to
a maximum of 1 degree.

…

…

deg

s

1.51.00.5 2.0

1

2

3

Activa

Hydractive hard

Hydractive soft

deg

g

0.5 0.8

1

2

3

Activa

Hydractive hard

U

The Citroën Guide Suspension: Hydractive 3 35

Hydractive 3

The new C5 has a new suspension system, doing
away with many solutions used on Citroëns for
several decades, yet offering the same or even
better comfort than before. Recent developments
in electronics and computics made it possible to
delegate many functions previously solved by me

­chanical-hydraulical components to electronic
units.

This third generation suspension system retains the same
basic functioning as the previous systems. It also comes in
two flavors: a simpler Hydractive 3 reminiscent of the origi

-

nal hydropneumatic suspension of the DS–GS–BX–CX and
a slightly more complicated Hydractive 3+, building upon
the former Hydractive I and II (actually, Hydractive 3 is not
hydractive in the sense we used this term before, its only
special activity is to adjust the road clearance depending on
speed and road condition).

Although the basic functioning is practically the same,

the actual layout underwent significant changes. Most im

­portantly, the previously mechanically operated height
correctors became electronically controlled hydraulic units.
And all hydraulic units except for the spheres—which were
redesigned to give unlimited life expectancy—are now
housed in a single unit, the Built-in Hydroelectronic In

­terface (BHI). This compact unit has three main parts:

U

the high pressure for the new synthetic fluid (called LDS,
orange in color) is generated by a five-piston hydraulic
pump 1, driven by an electric motor (rotating at 2,300

rpm) operating independently of the engine, running
only when necessary;

U

the hydraulic units, including an accumulator 2 to even
out the pressure pulsations of the pump, four electro

-

valves 3 and 4 and two hydraulic valves 8 serving the
height regulation and anti-sink behavior, some in-line fil

-

ters 5 and an overpressure valve 7 (taking the role of the
pressure regulator of previous systems).

U

the electronic computer 6, communicating with other
computers across the multiplex network to read the in

­puts of various sensors and to control both the HP pump
motor and the electrovalves.

In contrast to the height correctors of previous systems, op

-

erated mechanically via a linkage coupled to the anti-roll
bars, the new system used electronic sensors to learn the ac

-

tual height of the suspension and electric actuators to mod

-

ify the ground clearance whenever needed. The main advan

-

tage of using them is that the ECU can implement very so

-

phisticated algorithms to derive and apply height correc

-

tion, what were impossible with the mechanically linked
feedback with simple thresholds.

The computer 6 is connected to the CAN multiplex net

-

work, providing access to the messages sent by the BSI and
its fellow computers controlling the engine and the ABS.
The inputs the suspension ECU uses comprise of rear and
front body height, brake pedal, vehicle speed and accelera

-

tion, open-closed status of the doors (including the tail

-

gate), plus the steering steering wheel angle and rotating
speed on the Hydractive 3+.

BHI

front strut & sphere

front strut & sphere

reservoir

anti-sink valves???

rear strut & sphere

rear strut & sphere

LSD feed
front suspension
rear sus pension
operational return
leakage return

1

2

34

34

55

5

555

7

height setting
button

BSI

instrument
panel

doors and
tailgate

Sport switch
(3+ only)

brake

rear
height
sensor

front
height
sensor

6

steering wheel

angle sensor

(3+ only)

inlet valves with non-return valve???

Engine ECU

ABS ECU

U

The Citroën Guide Suspension: Hydractive 3 36

As usual with Citroëns, the driver can select from four

height settings (although the selector is no longer mechani

­cally coupled with the hydraulics, it is a simple electronic
switch sending signals to the computer): high, track (plus
40 mm), normal and low. The selected setting is displayed
on the multifunction screen in the dashboard. The com

­puter also prevents unsuitable settings being selected. The
high option is not available when the car is traveling faster
than 10 km/h and neither track nor low mode can be se

­lected above 40 km/h.

In addition to the manual settings, the system adjusts
the ground clearance automatically. Below 110 km/h on
well surfaced roads the ride height remains standard but as
soon as this speed is exceeded, the vehicle will be lowered
by 15 mm at the front and 11 mm at the rear. This change
lowers the center of gravity, improving stability, lowering
fuel consumption (by reducing drag) and reducing the sen

­sibility to crosswinds. The car resumes the standard ride
height when its speed drops below 90 km/h.

On poorly surfaced roads (the computer learns about
the road quality by monitoring data on vehicle speed,
height and movement of the suspension) the ride height
will be increased. The maximum increase would be 20 mm
but this setting is only used on very poor roads and with the
vehicle traveling below 60 km/h.

The general height of the vehicle (filtering out rapid
movements due to suspension travel) is checked, and if nec

­essary, adjusted every 10 seconds and when any of the
doors is opened or closed (even with the ignition switched
off).

Hydractive 3+

Just like its predecessor, this system also has two modes,
firm and soft. A stiffness regulator—an additional
sphere and a hydraulic control block per axle—isolates or
connects the corner and center spheres. Its functioning is
practically equivalent to the similar control block of the
Hydractive II: the computer controlled electro-valve 4

opens the feed pressure onto the isolation piston 2 and by
moving it, connects the center sphere 1 to the rest of the
suspension, switching the suspension to soft mode. of the
suspension. Closing the electro-valve 4 obstructs the hy

-

draulic supply coming from the BHI; the residual pressure in
the center sphere 1 moves the isolation piston 2 down

-

wards into a position which closes off the center sphere
completely: the suspension switches to hard mode.

The suspension has two settings the driver can choose

from, Normal and Sport. The new stiffness regulators to

­gether with the center spheres are isolated in hard mode
and re-activated in soft mode in response to the various in

­puts received and processed by the suspension ECU. The
functioning of the computer is basically similar to the
Hydractive II ECU: it uses tables and rules to set up thresh

­olds on the value on many sensor inputs to determine when
to switch to hard mode. Just like on its predecessor, the
Sport setting does not mean constant hard mode, just low

­ered, more sensitive thresholds for the switching.

The computer observes the following input parameters:

the height and sport settings specified by the driver (com

-

municated by the BSI); the vehicle speed and the longitu

­dinal-lateral acceleration of the body (communicated
on the CAN), the angle and speed of rotation of the
steering wheel (the type of the sensor depends on

whether the car is equipped with ESP, in this case the sensor
connects to the multiplex network instead of directly to the
suspension ECU), the speed of suspension travel (using
the values of the front and rear height sensors), the open­closed status of the doors (communicated by the BSI) and
the movement of the accelerator pedal or butterfly.

SOFT MODE

strut

return

control from
computer

suspension pressure

system feed pressu re

strut

BHI

2

3

3

4

1

HARD MODE

strut

return

control from
computer

suspension pressure

system feed pressu re

strut

B

HI

2

3

3

4

1

Steering

U

The Citroën Guide Steering: Power Assisted Steering 38

Power Assisted Steering

The PAS steering (DIRASS, Direction Assistée)
used on Citroëns is not radically different from
similar systems on other cars. Naturally, having a
high pressure hydraulic system at disposal influ

-

ences the layout.

The fluid requirements of the various hydraulics subsystems
differ significantly: while the brakes require only a very little
amount of LHM and the suspension somewhat more, the
power steering cannot work without large amounts of min

-

eral fluid provided at a moment’s notice. A flow distribu

-

tor built into the first hydraulic circuit—that of the hydrau

-

lic pump, the main accumulator and the pressure regula

­tor—controls the hydraulic pressure between the steering
circuit and the suspension-brake circuits on PAS cars.

The rest is rather simple. A hydraulic ram cylinder is

mounted on the rack of a traditional rack-and-pinion steer

­ing gear unit. The pressure of the hydraulic fluid supplied to
assist the driver in turning the steering wheel is controlled
by the flow distributor and a control valve. The flow dis

­tributor has the following components:

1

a slide valve to divide the amount of fluid;

2

another slide valve to limit the amount of fluid;

3

a pressure limiting valve to limit the pressure of the
LHM when the steering wheel is turned completely to
lock;

The steering control valve has three important elements:

4

a distributor mounted to the pinion;

5

a rotor fixed on the end of the steering rack;

6

a torsion bar between the distributor and the rotor.

On the main illustration, the power assisted steering system
is shown when it operates with the steering wheel in the
straight-ahead position and the pressure regulator is
switched on. The slide valve 1 inside the flow distributor di

­vides the mineral fluid coming from the high pressure
pump between the main and the steering hydraulic circuits
(the main circuit having priority). Both the distributor 4 and
the rotor 5 are in neutral position—the torsion bar be

­tween the two is not functioning). Both chambers of the
ram cylinder are fed without pressure. All the fluid arriving
through the distributor flows back to the LHM reservoir.

When the pressure regulator switches off while the steer

­ing wheel still is in its straight-ahead position, the pressure
starts to rise until it reaches 170 bar again and disconnects
the feed to the main accumulator. The main slide valve of
the pressure regulator is connected to the second feeding
channel of the flow distributor. All the fluid supplied by the
HP pump now feeds the flow distributor where the slide
valve 2 is responsible for limiting the amount of fluid trans

­ported by the control valve. The whole amount of fluid still
returns to the reservoir.

Now let’s assume the driver starts to steer to the right.

The rotor 5 starts to rotate with reference to the distri­butor 4. The control valve closes the path of the fluid com­ing from the flow distributor which no longer is allowed to
enter the valve. The pressure begins to rise in the circuit be

­tween the control valve and the flow distributor, moving
the slide valve 1, which in turn modifies the ratio of fluid, fa

­voring the PAS circuit. The fluid will enter the right chamber
of the ram cylinder while the left chamber can be emptied

steering rack

steering

wheel

1

pump

right

wheel

LHM feed
high pressure
steering pressure
steering
operational return
leakage return

flow distributor

pressure
regulator

control
valve

suspension
and brakes

reservoir

2

3

4

7

6

5

U

The Citroën Guide Steering: Power Assisted Steering 39

into the reservoir via the rotor of the control
valve. This pressure difference moves the pis

-

ton 7 to the left inside the cylinder, helping
the car to make a right turn. If the steering
wheel stays at the right lock, the pressure
limiting valve 3 inside the flow distributor
maintains a maximum pressure of
140 bar—when the pressure rises above this
value, the fluid pushes the ball of the valve
backwards, sending the excess fluid back to
the reservoir..

When the steering wheel is turned to the

left, the rotor 5 rotates in the opposite direc

­tion. It starts by cutting of the return of fluid
to the main reservoir. The pressure will rise
again in the circuit between the flow distrib

-

utor and the control valve. The rotor allows the LHM to en

­ter both chambers of the steering ram, however, the
pressed area of the left chamber is twice as large as that of
the right chamber, thus the piston will move to the right,
helping the car turn to the left.

The hydraulic assistance is only needed while the driver is
actually turning the steering wheel. When the rotating
force on the steering wheel ceases—the driver has finished
turning the wheel—, the angle difference between the dis

-

tributor 4 and the rotor 5, made possible by the flexibility

of the torsion bar 6 disappears, reverting the system to the
neutral position, stopping the power assistance. When the
driver releases the steering wheel back to the straight­ahead position, an opposite operation will start.

Later XMs and Xantiae omit this distributor and use a

two-section high pressure pump with two independent out

-

puts instead: six pistons provide LHM for the power steer

-

ing, two pistons for the rest of the hydraulics.

TO RIGHTTO LE FT

r

eservoir

control
valve

4

5

f

low distributor

left chamber right chamber

piston

r

eservoir

control
valve

4

5

f

low distributor

left chamber right chamber

piston

6

6

U

The Citroën Guide Steering: DIRAVI Steering 40

DIRAVI Steering

Another gem of engineering, the DIRAVI steering,
made its debut on the SM, excelled in many CXs
and the flagship, V6 XMs (left hand only, the
small amount sold in the UK never justified the ex

-

penses of the conversion to RHD).

The DIRAVI (Direction Rappel Asservi, Steering with Limiting
Counterforce) steering is as unique as the hydropneumatic
suspension—it was never used by any other manufacturer,
although its excellence over conventional power assisted
systems speaks for itself.

As usual, it has some quirks confusing the average driver
during their first meeting. First of all, it is geared very high:
it only took two turns of the steering wheel from lock to
lock (one turn for each side) to steer on the SM. Later mod

­els, the CX and the XM retained this feature although the
number of turns was larger (2.5 and 3.3). The gear ratio
could have been much higher, the engineers themselves in

­sisted on a single turn lock to lock for the SM (which would,
interestingly, void the need for a circular steering wheel
completely). The final solution was a compromise to reduce
the initial strangeness of the steering for the drivers already
accustomed to traditional systems.

Certainly, making the gearing so high is not complicated
in itself but a conventional (even power assisted) system
with such rapid response would be unusable. As the car ob

­viously has to have a similar turning circle as other cars, too
responsive a steering would mean that even the slightest
movement of the steering wheel would induce excessive de

­viation of the car from the straight line. To avoid this, it uses

an opposing force, increasing with the vehicle speed. With
this setup, in spite of the very high gearing, it is very easy to
use it during parking, yet it offers exceptional stability at
high speeds: it actually runs like a train on its rails, requiring
a sensible amount of force on the steering wheel to deviate
it from the straight line. And an additional feature: the steer

-

ing wheel (and the roadwheels, naturally) center them

-

selves even if the car is stationary.

Second, there is no feeling of feedback from the road
through the steering wheel. Other steering systems have a
constant mechanical connection between the steering
wheel and the roadwheels, the DIRASS only adds some
force to the one exerted by the driver. DIRAVI is different:
simply put, the usual path between the steering wheel and
the steering rack is divided into two halves, with a hydraulic
unit in the middle. When the driver turns the steering
wheel, this only operates the gears and valves in the hydrau

-

lic unit. The hydraulic pressure then moves the steering cyl

­inder and the roadwheels. The lower half of the mechanics
works in the opposite direction, as a negative feedback, re

­turning the hydraulic system to the neutral position as soon
as the wheels reach the required direction. The hydraulic cyl

­inder and the wheels become locked, no bump or pothole
can deviate them from their determined direction. Note
that this neutral position is not always the straight-ahead di

­rection, the hydraulics return to neutral whenever the steer

­ing wheel is held at a given angle for any longer period of
time. Letting the steering wheel rotate back or turning it fur

­ther in the previous direction will initiate a new mechanical­hydraulical cycle as described above.

steering rack

steering

wheel

5

7

centering

pressure

regulator

steering

centering

device

6

9

4

3

high pressure

gearbox

8

right

wheel

LHM feed

steering pressure

centering pressure

operational return

leakage return

steering feedback

normal steering

left
wheel

steering
control
unit

RL

2

1

reservoir

Û

adjustment cam

U

The Citroën Guide Steering: DIRAVI Steering 41

Thus, the lower mechanical link, the feedback from the

roadwheels does not extend beyond the hydraulic unit. Ev

-

erything the driver feels is generated artificially. One draw

-

back for uninitiated drivers is the lack of noticeable feed

­back indicating that the wheels are skidding or driving in a
ditch. The driver has to learn to feel the behavior of the car
via other sensory means and this is probably the main rea

­son why anyone not prepared for a period of learning will
immediately dislike DIRAVI. But once accustomed to the sys

­tem, it is more ergonomic and stress-free than any other
steering system.

The DIRAVI system uses four main components:
The steering rack and hydraulic ram cylinder with a

piston inside. The areas on which the pressure acts on the
left and right sides of the piston are different—the left one
is twice as large as the right one—, thus to keep the piston
in neutral position, the right hand side must have twice as
much hydraulic pressure than the left hand side. As this side
is fed from the high pressure of the hydraulic system, a con

­trol unit manipulates the pressure on the other side.

This steering control unit is connected to the steering

column. It has a coupling 4 inside which is very loosely con

­nected, with a significant amount of free play (nearly 30 de

­grees). Under normal circumstances this coupling stays in
the middle, so the free play is irrelevant but it serves as a me

­chanical backup for safety if there would be any failure in
the hydraulic system. In this case, the car can be steered me

­chanically, although much heavier and with a large free
play on the steering wheel.

The main illustration shows the steering system with the
steering wheel in the straight-ahead position. When the
driver rotates the steering wheel, the steering column turns
the gear 1 inside the control unit. The set of levers 9 at

­tached to this wheel transform the relative rotation (relative
to the previous hydraulically stabilized steering wheel posi

­tion) of the steering wheel into a horizontal motion: turn

­ing the steering wheel to the left pulls the slide valve 3, let

­ting the high pressure fluid enter the left chamber of the cyl

­inder. The right chamber is constantly at this same pressure,
however, the area on the left side of the piston is twice as
large as on the other side, thus the resulting higher force
will move the steering rack to the right, turning the road

­wheels to the left.

If the driver rotates the steering wheel to the right, the le

­vers 9 push the slide valve 3, draining the LHM from the
left chamber of the cylinder back to the reservoir. As the
right chamber is still under the constant pressure, the result

­ing force moves the rack to the left, thus the car starts to
turn to the right.

As we have already mentioned, the moving steering rack

rotates the pinion and—through the steering feedback—
the cogwheel 2. The levers linking this gear to the valve 3
now work in the opposite direction, returning the valve to
its neutral position, cutting off the LHM supply to the steer

­ing rack. The roadwheels stay in the angled position corre

-

sponding to the position of the steering wheel; due to the
closed valve 3, the steering gear and the roadwheels are hy

-

draulically locked, resulting in high turning stability.

To make the steering progressively heavier as the speed

of the vehicle increases, the steering centering pressure
regulator—a centrifugal device—is driven by a cable from
the gearbox. Its spinning weights open up a slide valve 8 ad

­mitting some fluid from the high pressure circuit into the
centering device, or closes it to drain the extra fluid back to
the reservoir.

The faster the car runs, the bigger is this hydraulic pres

-

sure sent to the steering wheel centering device. This
consists of an eccentric cam 5 geared to the steering wheel
side of the unit, with a ratio making it turn less than a full
turn while the steering wheel is rotated from lock to lock. A
piston 6 forced down by the mentioned hydraulic pressure
pushes a roller 7 against this cam. Being eccentric, the only
stable position is when the cam is centered. The centering
force can be regulated by changing the hydraulic pressure
behind the piston.

The hydraulic pressure behind the piston 6—being de

­pendent on the vehicle speed—represents the progressive
counter-force needed to make the steering gradually
heavier at highway speeds. In addition, it returns the steer

­ing gear to the neutral, straight-ahead position when the
driver releases the steering wheel. While the wheels of a
DIRASS car return to the center themselves, forcing the rack
and steering wheel as well, on DIRAVI the opposite is true:
the force on the angled wheels is attenuated infinitely, hav

­ing no influence whatsoever on the steering wheel. This ad

­ditional device returns the steering wheel to the center in­stead, just as if you have turned it back yourself.

During the rotation of the steering wheel, the lower pis

­ton was pushed up by the roller 7
and the eccentric cam 5. The fluid
leaves the chamber through the ball
valve now opened. While this piston
moves upwards, it compresses the
spring, which in turn pushes the up

-

per piston slightly up, freeing the cali

-

brated bore Û.

As soon as the driver releases the
steering wheel, the opposite of the
previous operation takes place. The
ball valve will be closed by the enter

­ing fluid, thus the LHM has to go
through the center bore of the upper
piston, leaving via the calibrated
bore Û. Due to this resistance, it car

­ries the upper piston down slightly,
compressing the spring. This downward force pushes the
lower piston together with the roller 7 down, and the
torque exerted on the eccentric cam 5 forces that to rotate
back into its neutral position, returning the complete steer

­ing gear to the straight-ahead position. At the end, the
spring will return the upper piston to its original position in

­side the centering device. The restriction of the bore Û
keeps the steering wheel from returning to the center posi

­tion too fast.

The last component is an adjustment cam allowing the

adjustment of the pinion relative to the disk on the pinion
end of the steering column.

TO RIGHTTO L EFT

R

L

reservoir reservoir

HP

rack

HP

rack

RETURNING

STEERING

U

The Citroën Guide Steering: Self-steering Rear 42

Self-steering Rear

sasasa

Brakes

U

The Citroën Guide Brakes: Standard braking system 44

Standard braking system

dasdsadsa

3) Back to the suspension and brakes for a second. The rear
sinks imperceptibly or not at all when braking—the amount
of LHM that goes into the rear brakes is infinitesimal—prob

­ably on the order of 1–2 ccm. Most of the LHM that is 'lost'
is the leakage of the brake valve. At best the rear end can
sink until the rear corrector starts replenishing the pressure,
and that's normally about 3 cm maximum, typically half of
that. In other words: this scarcely produces any anti-dive be

­haviour. What does produce anti-dive behaviour is the trail

­ing arm geometry of the rear end. Along with the low pro

­file of such suspension, the anti-dive behaviour is it's main
reason for being. When the brakes bite, in effect they want
to fix the wheel to the trailing arm. If the car is moving for

­ward, this automatically wants to move the point where the
trailing arm attaches to the body, down. Voila, the rear end
goes down. Incidentally, this is why HP Cits brake signifi

­cantly worse going backwards, and also tend to lift the rear
end when doing that.

Stop breaking, please…

CX Breaks have a rear brake force limiter to ensure that

When there is no pressure in the rear suspension (the sus

­pension is set to low), the force of the spring 4 keeps the
piston in the neutral position, completely closing the feed
to the rear brakes from the brake compensator valve.

When the suspension is under normal pressure, the

force 1 supplied by the rear suspension fluid exceeds the
counter force 2 provided by the spring. The piston stays in
the open position, letting the fluid pass to the rear brakes.
As soon as the driver starts braking, the force 2 increases by
the additional pressure coming from the front brakes, enter

­ing through the ball valve 3.

As soon as the incoming front brake pressure exceeds
the rear suspension pressure by more than 28 bar (in other
words, the combined force of front pressure and that of the
calibrated spring 4 becomes larger than the rear suspen

­sion pressure), the piston moves again to the left, cutting
out the additional pressure to the rear brakes, which will
then continue to brake with this constant pressure. To avoid
a sudden cut-off of pressure, a ball valve 3 combined with

a damping ??? is used to smoothen the changes. Bypass

???

BRAKINGWITHOUT PRESSURE

rear
brakes

rear
brakes

brake
valve

brake
valve

rear

suspension

rear

suspension

front
brakes

front
brakes

damping
???

damping
???

33

44

2

1

2

1

U

The Citroën Guide Brakes: Anti-lock Braking System 45

Anti-lock Braking System

Models with higher performance level came fitted
with ABS.

The principle of operation is the same as on cars with con

­ventional braking systems but the layout is much simpler as
all we need to control the operating pressure of the brakes
are a few electro-valves.

During breaking, the ABS computer monitors the

changes in the rotational speed of each roadwheel, commu

­nicated by inductive magnetic sensors reading the individ

­ual cogs of a toothed wheel fitted inside the cavity of the
brake discs. The computer does not interfere with the brak

­ing if the vehicle speed (as measured with the same sen

­sors) is below 5 km/h.

If any of the wheels begins to slow at a faster rate than
the others, the ABS reduces the hydraulic pressure fed to
the brake caliper of the wheel in question to avoid the
wheel being locked. Although every wheel has its own sen

­sor, the rear brake calipers receive the same pressure, only
the front ones are fed separately. As soon as road grip is re

­gained, the hydraulic pressure to the brake will be restored.
The computer is capable of cycling the pressure with a fre

­quency of several times a second.

To actually control the pressure, the system uses a three­unit hydraulic block (one block each for the front brakes,
one for both rear brakes). All three units comprise two elec

-

tro-valves, an inlet 1 and a return 2 valve.

During the rising period of normal braking, without the
need for the intervention of the ABS computer, the brakes
operate in phase 1: the inlet valve 1 is open but the return
valve 2 is closed. The braking functions as in a system with

­out ABS: the incoming hydraulic pressure is directly routed
to the brake caliper.

Under constant breaking (phase 2) both valves close to

maintain a steady hydraulic pressure in the brake calipers.

When the ECU senses the need for intervention, the elec

­tro-valves proceed to phase 3: the inlet valve 1 closes
while the return valve 2 closes. Hydraulic pressure will be re

­leased from the brake caliper, reducing the braking force.
To restore the braking effort, the ECU will return to phase 1
in a short while.

The ABS computer has a built-in diagnostic feature,
checking the components both when the ignition is turned
on and during braking. Any failure will be reported by a
warning lamp or a warning message of the board com

-

puter. As you can see from the illustration, the springs in

­side the valves are located in such a way that the mechani­cal default mode is phase 1—the normal braking—for all
three hydraulic blocks. Any failure in the ABS system will
therefore revert it to the usual, non-assisted braking.

Early CXs has a slightly different ABS system. The general
layout is the same, but the hydraulic block only has three
valves, one for each brake circuit, however, they have three
positions. Without energizing current, they route the fluid
coming from the brake accumulator to the brakes. In phase
2, a medium current switches them to isolate the brake cali

­pers, while a larger current opens it completely to let the
pressure escape from the brakes into the return lines.

On XM the hydraulic block has five electro-valves only. I
am not sure how they connect internally, but I suspect that
the valve that closes supply for the front brakes is common
for the left and right wheel ???

security

valve

brake
valve

front brake

front brake

rear brake

rear brake

rear
suspension

ABS
hydraulic
block

ECU

wheel
sensor

return

.

.

.

.

.

.

.

.

.

Brake sphere
(CX only)

PHASE 2PHASE 1 PHASE 3

brake

ECU

12

brake

ECU

12

brake

ECU

12

returnreturnreturnvalve valve valve

CX valve?

Electrical

Systems

U

The Citroën Guide Electrical Systems: Multiplex network 47

Multiplex network

Circuit layouts already universally adopted in com

­puters finally made their way into contemporary
cars. Although their functioning might be fright

-

eningly complex for people used to traditional cir

­cuits, they actually make the cabling very simple
and the addition of component interactions possi

-

ble in ways never experienced before.

Conventionally, cars used individual wires connecting the
various elements—steadily increasing in number—on
board. The huge amount of wires, connectors, wiring har

­nesses were a constant source of connection problems. The
various circuits were largely independent (sharing only the
feed and the ground), although some components had to
interact (for instance, fog lights should work only when the
headlights are switched on), necessitating connections be

­tween the various components (usually using some kind of
a switching logic, relays for simpler tasks and small elec

­tronic modules for more complicated ones).

As various subsystems (engine management, suspen

­sion, ABS, etc.) came from different manufacturers, some
functions were even built in parallel. Several subsystems
might rely on the signal sent by a coolant temperature or a
vehicle speed sensor but it was simpler for the manufactur­ers to fit two or three such sensors into various places, us

­ing every one of them only by their respective subsystem,
than to find ways to share the sensors, introducing intercon

­necting wires and the danger of one failing subsystem to in

­fluence the others.

The multiplex wiring first seen on late XMs and later

used on newer models like the Xsara Picasso or the C5 intro

­duces a radically different concept: just like in the computer
used to read this book, there is a central backbone circuit
called bus which goes around the whole car—actually,
there are four of them, a Controller Area Network
(CAN) and three Vehicle Area Networks (VANs), dealing
with different areas: the CAN is only responsible for the con

­nection between the central unit and the engine, gearbox
and suspension computers, the VANs for the rest of the sys

­tems: the first serves the safety systems like the airbag, the
second the various doors (including the sunroof) and the
anti-theft system, the third everything else: the instrumenta

­tion and the comfort gadgets.

The bus—in contrast to the traditional wiring harnesses
hosting many individual wires running side by side to serve
different components—is a common channel of informa

­tion flow for all components connecting to it. It uses only
two wires which all associated components connect to in
parallel (in addition to this, the devices are connected to the
ground as usual; the two input wires serve as a safety mea

­sure, using them both makes the system resistant to any
outside interference, and the whole system remains func

­tional even if one of the bus wires becomes broken, shorted
to ground or positive feed). There is no special controller or
owner of this bus, each device connecting to it is free to
send or receive messages and commands to the others, at a
rather high speed (approximately ??? messages per sec

­ond).

Suspension ECU

CAN (engine)

VAN 1 (safety)

VAN 2 (d oors)

VAN Comfort

Autobox ECU

Engine ECU

BSI

ABS ECU

fuse box

Airbag ECU

Sterring wheel
and column
switches

Door module

Door module

Sunroof

Anti-theft

Diesel additive

Instrument
panel

Multifunction
display

Radio/CD

Navigation

Aircon

Parkin g
assistance

Buses in the C5

U

The Citroën Guide Electrical Systems: Multiplex network 48

Each message or command is a sequence of a few num

-

bers, specifying:

U

the sender and the intended recipient of the message
(every device connecting to the multiplex bus has its
own address, a unique numerical identifier—for in

­stance, the fuel level sensor has the address 4315, the
instrument panel is 0004);

U

whether the recipient should acknowledge the mes

­sage as it processes it;

U

the actual data the message transmits;

U

some additional values to check the integrity and va

­lidity of the message at the receiving end.

Each major unit sends its own data into the network at pre

-

determined intervals, marking the message with its own ad

-

dress as a sender (some simpler sensors are connected di

­rectly to a computer which sends the messages relating to
their measured values on their behalf). With our example,
the fuel level sensor sends the amount of fuel it measures,
specifying the central unit (BSI) as the intended recipient.
As soon as the BSI sees this message circulating on the net

­work, it processes it by retrieving the data—the value of
fuel level—from the message and comparing it to the previ

­ously known value. As the amount of fuel is not supposed
to change drastically from one moment to the other, it dis

­cards the new value if it differs too much from the previous
one.

If the new value is acceptable, the BSI emits another mes

­sage of its own, addressed to the instrument panel this
time. As the instrument panel receives this second mes

­sage, it extracts the data representing the amount of fuel
left in the tank and turns this signal into the physical rota

­tion of the gauge needle.

All devices are constantly observing the bus for mes

­sages addressed to them, ignoring the ones sent to other re

­cipients (although there are special broadcast messages
sent to all devices, without specifying a single addressee)—
actually, the instrument panel saw the original message
coming from the level sensor as well but ignored it, it only
acted when the second message, sent by the BSI and ad

­dressed specifically to it, arrived.

All components work in a similar way. Some are simple
enough to send a few simple messages (like sensors or
switches) or to receive only a few ones (like electric window
motors). Others are complex subsystems themselves, like
the suspension, observing the input from a large number of
sensors and performing complex operations. But as they
are all connected to a common bus, the possibility of inter

­action is already there. Whether the headlights light up, the
electric windows close and the wiper starts to work in case
of rain, or whether the passenger side external rear view mir

-

ror folds down when engaging reverse gear have all be

­come a simple question of software written for the central
unit. Adding a new feature does not require building a sin

­gle extra wire or connection, just to add a few lines to the
software.

Center of Attention

The four networks all connect to the central unit, the Built­in Systems Interface (BSI). This control unit manages the

flow of information between the networks (many of the

messages generated in one network has to be relayed to
another, just one example is the suspension computer—
connected to CAN—being interested in messages about
the open or closed position of the doors—communicated
on VAN 2).

In addition to that, the BSI offers an interface to the out

­side world as well, a diagnostic socket which can be used to
check, test and configure the whole system.

The multiplex system switches to an energy-saving low

power mode whenever possible.

Air Conditioning

U

The Citroën Guide Air Conditioning: Air conditioning 50

Air conditioning

Once considered pure luxury, air conditioning and
other forms of climate control have became stan

­dard items on today’s car. After all, creating an
acceptable environment for the driver is more
than a mere question of comfort, it contributes to
safety to a great extent.

There were several climate control systems fitted to our
Citroëns, offering various degrees of automation of keep

-

ing the climatic conditions inside the car. The system can be
manual, semi-automatic or automatic. The manual ver

-

sion also came with separate settings for driver and passen

-

ger.

The semi-automatic system is rather similar to the man

­ual one, the visible difference is that the operating knob on
the dashboard is marked in degrees instead of just blue and
red. The direction and recirculation controls are indentical
to the manual system. The automatic climate control looks
radically different, with a controlling panel using buttons
and a digital temperature display.

The AC system in the XM is fairly simple. If it is on, the air
is always cooled to about 8–10 °C on the inlet side (this is
varied between the air intake from outside and
recirculation from the inside) and then if you set a higher
temperature, it's reheated. The heater also always works, its
effect is only regulated by allowing air to flow or not to flow
through it (this is what the flap valve does). The air always
flows through the AC heat exchanger. As a result, the AC
also dries out the air whenever it is on. Once the air passes
out of the temperature regulating flap valve, another flap
valve regulates where it goes inside the cabin. That's really
all there is to it.

The AC system itself is almost self-sufficient. It has a radia

-

tor, compressor, heat exchanger with evaporator, and a con

­denser—and the connecting pipes. The climate control ECU
actually only provides a signal to a relay that switches the
AC system on by operating the electric clutch on the com

­pressor. This same signal switches the radiator fans on to
the low speed. The AC system in turn sends a 'fans to full
speed' signal to the fan controller, when the coolant temper

­ature reaches a trip point (this is handled by a different
switch section in the same pressure sensitive switch that pre

­vents the AC going on without any coolant in the system,
described above).

As far as I know (unless it changed in later versions), the

AC itself (as oposed to climate control) never had an ECU.

The evaporator has an integrated pressure/tempera

-

ture valve, opening up the pressure line to the return line.

After coming through the evaporator, the temperature
of the fluid (more precisely, a mixture of liquid and vapor)
suddenly drops because of the drop in pressure. It enters
the heat exchanger which operates like a radiator, cool

­ing the air and heating itself up. The fluid then goes back to
the drier-radiator-compressor end of the loop. The con

-

densed moisture is collected from the heat exchanger and
let out through to floor of the cabin via a plastic tube.

As the air always enters through the heat exchanger, and
whether it gets cooled at this point, depends only on
whether the compressor is working or not. The tempera

­ture flap only decides which part of the air is going to be
taken before or after the heater radiator. This is how the
temperature is regulated.

When the compressor is on, is to condense the mois

­ture out of the air, and then re-heat it as necessary to the
temperature set on the controls. Since the temperature is
regulated by the temperature flap, it has really nothing to
do with the compressor at all—the only consequence of the
compressor not working (for any reason) is that the system
will obviously not be able to produce a temperature lower
than ambient.

There are four sensors providing input. The first one is at

the entrance of the air, before the heat exchanger, the sec

­ond one after the temperature flap, the third one on the
roof, and the last one in the heat exchanger. They have very
different but sometimes overlapping roles.

The first three collectively influence temperature regula

­tion. In particular, the sensors after the temperature flap
and on the roof determine what the actual temperature is.
The sensors before the heat exchanger and after the temper

­ature flap decide how fast the temperature flap will be
moved to prevent extremely fast changes in temperature in
the cabin. This does not alway work very well, which is why
you get a blast of air when the system is set on auto and
you leave the car in the sun in summer. Both of these param

­eters (temperature and temperature difference) influence
the fan speed.

compressor

condenser

pressure

release

valve

evaporator

receiver

dryer

a

mbient air

cooled air

refrig. fluid, cool

refrig. fluid, warm

U

The Citroën Guide Air Conditioning: Air conditioning 51

The sensor before and in the heat exchanger as well as

the teperature selection, influence the AC part, i.e. the oper

­ation of the compressor. For instance, the compressor will
not operate below a certain external temperature. Also, it
will not operate if the temperature is set to maximum.

When the system is cooling the incoming air, it needs to
have the exchanger at a temperature which is lower than
the ambient air temperature, obviously. As the compressor
either runs or not, it cannot cool just a little bit—it always ei

­ther runs on full or does not run. When it starts, it will start
cooling the heat exchanger. How cold it will get, depends
on how hot the incoming air is and how much air is coming
in. In any case, when it gets significantly colder than the in

­coming air, the moisture from the air will start to condense
on the heat exchanger, which is why there is a collector
underneath it and a drip outlet. If the compressor keeps on
working, while the heat that needs to be taken from the air
is lower from the heat transfer ability of the whole system,
the heat exchanger will continue to progressively get
colder. If nothing is done, it will get well below freezing (it
can go as low as –40 °C given proper fluid, and of course
construction designed for this). What will happen then is
that the condensed water from the air will start freezing on
the heat exchanger fins, and eventually, the whole thing
will become a solid block of ice (usually there will be a crack

­ling noise to acompany the event), preventing actual air
flow. If the condition persists, the pressure in the system
will build up until the valve in the evaporator opens, and by
this time it is possible that the fluid actually gets heated up
enough that the remaining part going through the heat
exchanger will actually melt the ice producing a fog (I've
seen it happen!). All of this will be the lucky turn of events,
asuming the ice has not cracked the heat exchanger and
that there is no fluid leak.

So, obviously, there is a sensor, and that's the fourth one
in this story, which detects the temperature of the heat
exchanger becoming too low. When that happens, the com

­pressor is cut out, until the heat exchanger temperature
rises to an acceptable level. The thermal inertia and differ

-

ent cut out and cut in temperatures insure that the compres

­sor doesn't keep switching on and off too quickly, which
would place an undue strain on the electromagnetic clutch.

The logic in the ECU is done very simply, if the fourth sen

-

sor detects that the heat exchanger is too cold, the compres

­sor will switch off, regardless of the AC switch and tempera

­ture set. The only thing it will do, as I said in the earlier mail,
is that it will switch on for about 1 second whenever the AC
switch is turned on, this is probably some ECU feature. The
compressor will never turn on if the gas pressure is insuffi

­cient, and this part is handled by the pressure switch on the
drier, and has nothing to do with the ECU. In fact, the ECU
only gives the whole system a 'go-ahead'.

Appendix

U

The Citroën Guide Appendix: 53

ORGA number

This number shows the day when your car was actually as

­sembled on the production line. The dealers and parts
stores use this number (often called ORGA or RP number,
the second stands for Replacement Parts) to identify the vari

-

ous parts and components fitted to your car.

On various models, the ORGA number can be found in

different locations. It is on the top of the left hand suspen

­sion turret on Visas, C15s, AXs and CXs (often hidden by
the wiring harness). BXs and XMs have it stamped on the
left hand front door A-pillar, above the courtesy light
switch. On the GSA you find it on the inner right front
wing. Xantiae switched to the other side: the number can
be found on the bulkhead just in front of the right suspen

­sion sphere.

Calculating the production date is very easy using the fol

­lowing table. Locate the largest number in the table still less
than or equal to your organization number. To see an exam

­ple, let's assume the number is 4859. Then the largest num

­ber will be 4832 in the cell February 1990. Just subtract
this number from your organization number to get the day
of the month of the production of your car (in our example,
4859 – 4832=27 yields February 27, 1990).

If you receive the non-existent date zero (this happens
when your organization number is not greater than but
equal to the number in the table), simply take the last day
of the previous month. For instance, for the organization
number 5013 the largest number in the table is 5013 in
the cell August 1990, subtraction results in zero, hence
the production date is July 31, 1990.

Years Months

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

1982 1879 1910 1938 1969 1999 2030 2060 2091 2122 2152 2183 2213

1983 2244 2275 2303 2334 2364 2395 2425 2456 2487 2517 2548 2578

1

984 2609 2640 2669 2700 2730 2761 2791 2822 2853 2883 2914 2944

1985 2975 3006 3034 3065 3095 3126 3156 3187 3218 3248 3279 3309

1986 3340 3371 3399 3430 3460 3491 3521 3552 3583 3613 3644 3674

1987 3705 3736 3764 3795 3825 3856 3886 3917 3948 3978 4009 4039

1988 4070 4101 4130 4161 4191 4222 4252 4283 4314 4344 4375 4405

1989 4436 4467 4495 4526 4556 4587 4617 4648 4679 4709 4740 4770

1

990 4801 4832 4860 4891 4921 4952 4982 5013 5044 5074 5105 5135

1991 5166 5197 5225 5256 5286 5317 5347 5378 5409 5439 5470 5500

1992 5531 5562 5591 5622 5652 5683 5713 5744 5775 5805 5836 5866

1993 5897 5928 5956 5987 6017 6048 6078 6109 6140 6170 6201 6231

1994 6262 6293 6321 6352 6382 6413 6443 6474 6505 6535 6566 6596

1995 6627 6658 6686 6717 6747 6778 6808 6839 6870 6900 6931 6961

1996 6992 7023 7052 7083 7113 7144 7174 7205 7236 7266 7297 7327

1997 7358 7389 7417 7448 7478 7509 7539 7570 7601 7631 7662 7692

1998 7723 7754 7782 7813 7843 7874 7904 7935 7966 7996 8027 8057

1999

8088 8119 8147 8178 8208 8239 8269 8300 8331 8361 8392 8422

2000 8453 8484 8512 8543 8573 8604 8634 8665 8696 8726 8757 8787

2001 8819 8850 8878 8909 8939 8970 9000 9031 9062 9092 9123 9153

2002 9184 9215 9243 9274 9304 9335 9365 9396 9427 9457 9488 9518

Index

U

The Citroën Guide Index: 55

Index

AAV . . . . . . . . . . . . . . . . . . . . . . . . . . 7

ABS. . . . . . . . . . . . . . . . . . . . . . . . 22, 45

computer. . . . . . . . . . . . . . . . . . . . . . 45

hydraulic block . . . . . . . . . . . . . . . . . . . 45

sensor . . . . . . . . . . . . . . . . . . . . . . . 45

AC . . . . . . . . . . . . . . . . . . . . . . . . . . 50

accelerator pedal . . . . . . . . . . . . . . . . . 11, 15

Activa . . . . . . . . . . . . . . . . . . . . . . 33 - 34

computer. . . . . . . . . . . . . . . . . . . . . . 34

electro-valve . . . . . . . . . . . . . . . . . . . . 33

roll corrector . . . . . . . . . . . . . . . . . . . . 33

sphere . . . . . . . . . . . . . . . . . . . . . . . 33

stabilizing cylinders . . . . . . . . . . . . . . . . . 33

AFS . . . . . . . . . . . . . . . . . . . . . . . . . 5, 7

air conditioning . . . . . . . . . . . . . . . . . 50 - 51

air flow sensor . . . . . . . . . . . . . . . . . . . . . 5

air temperature sensor . . . . . . . . . . . . . . . . . 6

anti-dive behavior . . . . . . . . . . . . . . . . . . 20

anti-roll bar. . . . . . . . . . . . . . . . . . . . . . 21

anti-sink . . . . . . . . . . . . . . . . . . . . . . . 32

sphere . . . . . . . . . . . . . . . . . . . . . . . 32

valve . . . . . . . . . . . . . . . . . . . . . . . . 32

ATS . . . . . . . . . . . . . . . . . . . . . . . . . . 6

autotransformer . . . . . . . . . . . . . . . . . . . . 8

auxiliary air valve . . . . . . . . . . . . . . . . . . . . 7

ball and piston valve. . . . . . . . . . . . . . . . 24, 29

BHI . . . . . . . . . . . . . . . . . . . . . . . . . . 35

body movement sensor . . . . . . . . . . . . 26 - 27, 30

brake compensator valve . . . . . . . . . . . . . 23, 44

brake cylinders

front . . . . . . . . . . . . . . . . . . . . . . . . 23

brake force limiter. . . . . . . . . . . . . . . . . . . 44

brake pressure sensor . . . . . . . . . . . . . 26 - 27, 31

broadcast messages . . . . . . . . . . . . . . . . . 48

BSI . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Built-in Hydroelectronic Interface . . . . . . . . . . . 35

Built-in Systems Interface . . . . . . . . . . . . . . . 48

bus . . . . . . . . . . . . . . . . . . . . . . . . . . 47

CAN . . . . . . . . . . . . . . . . . . . . . . . 35, 47

CAS . . . . . . . . . . . . . . . . . . . . . . . . . . 8

catalytic converter

diesel . . . . . . . . . . . . . . . . . . . . . . . . 16

gasoline . . . . . . . . . . . . . . . . . . . . . . . 9

chip tuning . . . . . . . . . . . . . . . . . . . . . . 5

climate control . . . . . . . . . . . . . . . . . . . . 50

cold start injector. . . . . . . . . . . . . . . . . . . . 7

compressor . . . . . . . . . . . . . . . . . . . . . . 50

Controller Area Network . . . . . . . . . . . . . . . 47

coolant temperature sensor . . . . . . . . . . . . . . 6

crank angle sensor . . . . . . . . . . . . . . . . . . . 8

CSV . . . . . . . . . . . . . . . . . . . . . . . . . . 7

CTS . . . . . . . . . . . . . . . . . . . . . . . . . 6 - 7

damping elements . . . . . . . . . . . . . . 23 - 24, 29

DI . . . . . . . . . . . . . . . . . . . . . . . . . . 17

diesel combustion . . . . . . . . . . . . . . . . . . 10

DIRASS . . . . . . . . . . . . . . . . . . . . . . 22, 38

DIRAVI . . . . . . . . . . . . . . . . . . . . . . 40 - 41

direct injection . . . . . . . . . . . . . . . . . . . . 17

distributor

diesel . . . . . . . . . . . . . . . . . . . . . . . . 10

ignition . . . . . . . . . . . . . . . . . . . . . . . 8

door/tailgate open sensor . . . . . . . . . . . 26 - 27, 31

EDC . . . . . . . . . . . . . . . . . . . . . . . . . 15

EFI . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

EGR. . . . . . . . . . . . . . . . . . . . . . . . 14, 17

electronic diesel control . . . . . . . . . . . . . . . . 15

EMS . . . . . . . . . . . . . . . . . . . . . . . . . . 8

engine management system . . . . . . . . . . . . . . 8

engine runaway . . . . . . . . . . . . . . . . . . . 12

evaporator . . . . . . . . . . . . . . . . . . . . . . 50

exhaust gas recycling . . . . . . . . . . . . . . . . . 14

flow distributor . . . . . . . . . . . . . . . . . . . . 38

fuel cut-off

diesel . . . . . . . . . . . . . . . . . . . . . . . . 16

gasoline . . . . . . . . . . . . . . . . . . . . . . . 7

fuel filter

gasoline . . . . . . . . . . . . . . . . . . . . . . . 7

fuel injection

diesel, direct . . . . . . . . . . . . . . . . . . . . 17

diesel, electronic . . . . . . . . . . . . . . . . . . 15

diesel, mechanical . . . . . . . . . . . . . . . . . 10

gasoline, electronic . . . . . . . . . . . . . . . . . 5

fuel pump

diesel . . . . . . . . . . . . . . . . . . . . . . . . 10

gasoline . . . . . . . . . . . . . . . . . . . . . . . 7

fuel stop valve . . . . . . . . . . . . . . . . . . . . 10

fuel supply

gasoline . . . . . . . . . . . . . . . . . . . . . . . 7

fuel tank

gasoline . . . . . . . . . . . . . . . . . . . . . . . 7

glow plug . . . . . . . . . . . . . . . . . . . . . . 13

ground clearance . . . . . . . . . . . . . . . . . . . 20

HDI . . . . . . . . . . . . . . . . . . . . . . . . . . 17

heat exchanger . . . . . . . . . . . . . . . . . . . . 50

heated wire AFS . . . . . . . . . . . . . . . . . . 5, 15

height corrector . . . . . . . . . . . . . . . . 23, 26, 29

front . . . . . . . . . . . . . . . . . . . . . . . . 23

high pressure pump

diesel . . . . . . . . . . . . . . . . . . . . . . . . 10

hydraulics . . . . . . . . . . . . . . . . . . 22, 35, 39

HP brakes. . . . . . . . . . . . . . . . . . . . . . . 44

Hydractive 3 . . . . . . . . . . . . . . . . . . . 35 - 36

Hydractive I. . . . . . . . . . . . . . . . . . . . 24 - 28

Hydractive II . . . . . . . . . . . . . . . . . . . 29 - 31

Hydractive sphere . . . . . . . . . . . . . . . . . 24, 29

Hydractive valve

electric . . . . . . . . . . . . . . . . . . . . . 24, 29

hydraulic . . . . . . . . . . . . . . . . . . . . 24, 29

ICSM. . . . . . . . . . . . . . . . . . . . . . . . . . 6

idle control stepper motor . . . . . . . . . . . . . . . 6

U

The Citroën Guide Index: 56

Idle speed . . . . . . . . . . . . . . . . . . . . . . . 6

idle speed control valve . . . . . . . . . . . . . . . . . 6

ignition coil . . . . . . . . . . . . . . . . . . . . . . 8

ignition delay

diesel . . . . . . . . . . . . . . . . . . . . . . . . 12

gasoline . . . . . . . . . . . . . . . . . . . . . . . 8

ignition key switch . . . . . . . . . . . . . . . . . . . 6

ignition switch . . . . . . . . . . . . . . . . 26, 28, 31

injection adjuster . . . . . . . . . . . . . . . 10, 12, 16

injection delay

diesel . . . . . . . . . . . . . . . . . . . . . . . . 12

gasoline . . . . . . . . . . . . . . . . . . . . . . . 7

injector

diesel . . . . . . . . . . . . . . . . . . . . . . . . 13

gasoline . . . . . . . . . . . . . . . . . . . . . . . 7

injector needle movement sensor . . . . . . . . . . . 16

inlet manifold . . . . . . . . . . . . . . . . . . . . 7 - 8

intercooler . . . . . . . . . . . . . . . . . . . . . . 13

ISCV . . . . . . . . . . . . . . . . . . . . . . . . . . 6

knock sensor . . . . . . . . . . . . . . . . . . . . . . 9

KS . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

lambda ratio . . . . . . . . . . . . . . . . . . . 5, 9, 16

lambda sensor . . . . . . . . . . . . . . . . . . . . . 9

LDS. . . . . . . . . . . . . . . . . . . . . . . . . . 35

lead in gasoline. . . . . . . . . . . . . . . . . . . . . 9

leak returns. . . . . . . . . . . . . . . . . . . . . . 22

main accumulator. . . . . . . . . . . . . . . . . 22, 38

manifold absolute pressure . . . . . . . . . . . . . . . 5

MAP sensor . . . . . . . . . . . . . . . . . . . . . . 5

monopoint EFI/EMS . . . . . . . . . . . . . . . . . . 7

multiplex message . . . . . . . . . . . . . . . . . . 48

multiplex wiring . . . . . . . . . . . . . . . . . . . 47

multipoint EFI/EMS. . . . . . . . . . . . . . . . . . . 7

ORGA number . . . . . . . . . . . . . . . . . . . . 53

OS . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

oxygen sensor . . . . . . . . . . . . . . . . . . . . . 9

particulates . . . . . . . . . . . . . . . . . . . . 12, 16

PAS . . . . . . . . . . . . . . . . . . . . . . . . 22, 38

piston and ball valve . . . . . . . . . . . . . . . . . 25

post-glowing . . . . . . . . . . . . . . . . . . . . . 14

power assisted steering . . . . . . . . . . . . 22, 38 - 39

power steering . . . . . . . . . . . . . . . . . . 22, 40

prechamber . . . . . . . . . . . . . . . . . . . . . 10

pressure regulator

EFI . . . . . . . . . . . . . . . . . . . . . . . . . . 7

hydraulics. . . . . . . . . . . . . . . . . . . . 22, 38

regulator . . . . . . . . . . . . . . . . . . . 10 - 11, 15

reservoir

hydraulics . . . . . . . . . . . . . . . . . . . . . 22

RP number . . . . . . . . . . . . . . . . . . . . . . 53

security valve . . . . . . . . . . . . . . . . . . . . . 23

self-diagnostics . . . . . . . . . . . . . . . . . . . . 16

smoke limit . . . . . . . . . . . . . . . . . . . . 12, 16

spark plug . . . . . . . . . . . . . . . . . . . . . . . 8

steering centering pressure regulator . . . . . . . . . 41

steering control unit . . . . . . . . . . . . . . . . . 41

steering control valve . . . . . . . . . . . . . . . . . 38

steering rack . . . . . . . . . . . . . . . . . . . . . 41

steering ram cylinder . . . . . . . . . . . . . . . 38, 41

steering wheel angle sensor . . . . . . . . . . 26 - 27, 30

steering wheel centering device . . . . . . . . . . . . 41

steering wheel speed sensor. . . . . . . . . . 26 - 27, 30

stiffness regulator . . . . . . . . . . . . . . . . . . 36

suspension ECU. . . . . . . . . . . . . . 24 - 26, 29, 35

suspension mode

hard . . . . . . . . . . . . . . . . . . 25 - 26, 29, 36

soft . . . . . . . . . . . . . . 25 - 26, 28 - 29, 31, 36

suspension resonance frequency . . . . . . . . . . . 20

suspension selector switch . . . . . . . . . . 25, 28 - 29

suspension status light . . . . . . . . . . . . . . . . 28

swirl chamber . . . . . . . . . . . . . . . . . . . . 10

TDC . . . . . . . . . . . . . . . . . . . . . . . . . . 8

temperature-timer switch . . . . . . . . . . . . . . . 7

throttle pedal position sensor . . . . . . . . . 26 - 27, 30

throttle position switch. . . . . . . . . . . . . . . . . 6

throttle potentiometer . . . . . . . . . . . . . . . . . 6

timing advance . . . . . . . . . . . . . . . . . . . . 8

top dead center . . . . . . . . . . . . . . . . . . . . 8

TP . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

TS . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

turbocharger

diesel . . . . . . . . . . . . . . . . . . . . . . . . 13

VAN . . . . . . . . . . . . . . . . . . . . . . . . . 47

variable turbo pressure . . . . . . . . . . . . . . . . 16

Vehicle Area Network . . . . . . . . . . . . . . . . . 47

vehicle speed sensor. . . . . . . . . . . . . . . . 26, 30

wastegate

diesel . . . . . . . . . . . . . . . . . . . . . . . . 13