citroen guide.pdf

Željko Nastasic´

Gábor Deák Jahn

The Citroën

Technical Guide

There are many car manufacturers, makes, models

and versions on the road today but—as we all

know—none of them compares to Citroën in its

engineering excellence, especially regarding

suspension comfort, roadholding, and stability.

In this book we tried to describe how the various

subsystems work. We never intended to replace

service manuals or similar technical instructions.

Illustrations are schematic, focusing on the

principles of operation rather than on minute

details of implementation.

This guide is not linked to any specific Citroën

model but describes all systems and solutions

used on a large number of cars from the glorious

line of DS, ID, CX, GS, GSA, BX, XM, Xantia, Xsara

and the C5.

The Citroën Guide U

Table of Contents

Fuel Injection

DIRAVI Steering . . . . . . . . . . . . . . . . . . 44

Self-steering Rear . . . . . . . . . . . . . . . . . 46

Electronic Fuel Injection . . . . . . . . . . . . . . . 6

The HPi engine . . . . . . . . . . . . . . . . . . 11

Diesel engines . . . . . . . . . . . . . . . . . . . 12

Electronic Diesel Control . . . . . . . . . . . . . . 17

Diesel Direct Injection . . . . . . . . . . . . . . . 19

Brakes

Standard braking system. . . . . . . . . . . . . . 48

Anti-lock Braking System. . . . . . . . . . . . . . 50

Suspension

Electrical Systems

A Suspension Primer . . . . . . . . . . . . . . . . 22

Hydropneumatic Suspension . . . . . . . . . . . 23

Hydractive I . . . . . . . . . . . . . . . . . . . . 27

Hydractive II . . . . . . . . . . . . . . . . . . . . 32

Anti-sink system . . . . . . . . . . . . . . . . . . 35

Activa Suspension . . . . . . . . . . . . . . . . . 36

Hydractive 3. . . . . . . . . . . . . . . . . . . . 38

Hydractive Summary . . . . . . . . . . . . . . . 40

Multiplex network . . . . . . . . . . . . . . . . . 54

Air Conditioning

Air conditioning . . . . . . . . . . . . . . . . . . 58

Appendix

Steering

ORGA number . . . . . . . . . . . . . . . . . . 62

Index

Power Assisted Steering . . . . . . . . . . . . . . 42

Jahn.

The authors accept no responsibility or liability for any

problem resulting out of the use of the material contained

herein. In no event shall the authors be liable for any conse-

quential, special, indirect, incidental, punitive or exemplary

damages, costs, expenses or losses.

It is expressly forbidden to sell this book in any way,

shape or form, or to use it for any commercial purpose

whatsoever without the prior written consent of both au-

thors. It is forbidden to re-work or modify the book, to re-

publish it with this copyright statement removed or altered

in any way.

This book has been written for the benefit of the Citroën

community. As such, it will remain free to use, copy and dis-

tribute, in accordance with this copyright statement.

We would like to thank the following people for their valu-

able contribution:

U Adam Reif (HPi diesel engine)

Fuel Injection

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.

warming up, the mixture can return to normal, but the tem-

perature 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 characteristics 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 ig-

nition 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 remained practi-

cally the same, thus the following section applies to both

kind of systems.

Perfect combustion, as chemistry calls it, would require air

and fuel in proportion of 14.7 parts to 1 (this is the so-

called stoechiometric ratio). While this might be satisfac-

tory for the scientists, the real-life conditions of a vehicle

call for slightly different characteristics.

We use the ratio of actual mixture to the stoechiometric

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 call for a slightly lean one, between 1.1 and 1.3.

Some harmful components in exhaust gas would reduce in

quantity 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 en-

gine requires a very rich mixture to keep running. After

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 only

an illustration, the

actual values mean

nothing here), and

for any pair of in-

coming engine speed and load values the necessary fuel

amount can be determined. By keeping the pressure of fuel

constant behind the injector valves, the amount of fuel in-

jected depends solely on the time period the injectors are

opened for, hence, the table can contain injector opening

times.

An this is exactly how it is done in modern injection sys-

tems: the controlling microcomputer keeps a lookup table

like this to determine the base pulse width. Earlier systems

were constructed from discrete, analog elements, not like a

small computer; a more or less equivalent circuit made of

various hybrid resistance arrays and semiconductors were

used for the same purpose.

Chip tuning, by the way, is the simple operation of replac-

ing the said table with another one, yielding different char-

acteristics (usually to gain power, allowing for worse fuel

economy). As the computer stores this table in a program-

mable memory—similar in function to the BIOS in desktop

computers—, replacing it is possible. The earlier systems

with analog circuits cannot be modified that easily.

So, we obtained the base pulse width from the table but

as the operating conditions of automotive engines are

Engine load

…

100%

idle

…3

850 rpm

…5

900 rpm

…7

…

…………

6,000 rpm

…10

The Citroën Guide U

Amount of fuel

injected

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