Citroën Technical Guide - mycitroen.dkmycitroen.dk/library/Citroen technical guide (GS, CX, BX,...

Željko NastasicGábor Deák Jahn

The CitroënTechnical Guide

The Citroën Guide U2

There are many car manufacturers, makes, modelsand versions on the road today but—as we allknow—none of them compares to Citroën in itsengineering excellence, especially regardingsuspension comfort, roadholding, and stability.

In this book we tried to describe how the varioussubsystems work. We never intended to replaceservice manuals or similar technical instructions.Illustrations are schematic, focusing on theprinciples of operation rather than on minutedetails of implementation.

This guide is not linked to any specific Citroënmodel but describes all systems and solutionsused on a large number of cars from the gloriousline of DS, ID, CX, GS, GSA, BX, XM, Xantia, Xsaraand the C5.

3

Table of ContentsFuel Injection

Electronic Fuel Injection . . . . . . . . . . . . . . . 6The HPi engine . . . . . . . . . . . . . . . . . . 11Diesel engines . . . . . . . . . . . . . . . . . . . 12Electronic Diesel Control . . . . . . . . . . . . . . 17Diesel Direct Injection . . . . . . . . . . . . . . . 19

Suspension

A Suspension Primer . . . . . . . . . . . . . . . . 22Hydropneumatic Suspension . . . . . . . . . . . 23Hydractive I . . . . . . . . . . . . . . . . . . . . 27Hydractive II . . . . . . . . . . . . . . . . . . . . 32Anti-sink system. . . . . . . . . . . . . . . . . . 35Activa Suspension . . . . . . . . . . . . . . . . . 36Hydractive 3. . . . . . . . . . . . . . . . . . . . 38

Steering

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

DIRAVI Steering . . . . . . . . . . . . . . . . . . 44Self-steering Rear . . . . . . . . . . . . . . . . . 46

Brakes

Standard braking system. . . . . . . . . . . . . . 48Anti-lock Braking System. . . . . . . . . . . . . . 50

Electrical Systems

Multiplex network . . . . . . . . . . . . . . . . . 52

Air Conditioning

Air conditioning . . . . . . . . . . . . . . . . . . 56

Appendix

ORGA number . . . . . . . . . . . . . . . . . . 60

Index

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

U Adam Reif (HPi diesel engine)

Fuel Injection


Electronic Fuel InjectionThe Otto engine needs a mixture of fuel and airfor its operation. It would be the task of the fuelsupply—carburetor or injection—to provide theengine with the ideal mixture. Unfortunately,there is no such thing as an ideal mixture.

Perfect combustion, as chemistry calls it, would require airand 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 vehiclecall for slightly different characteristics.

We use the ratio of actual mixture to the stoechiometricmixture, called lambda (l), to describe the composition ofthe 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 economywould call for a slightly lean one, between 1.1 and 1.3.Some harmful components in exhaust gas would reduce inquantity between lambda values of 1 to 1.2, others below0.8 or above 1.4. And if this is not yet enough, a cold en-gine requires a very rich mixture to keep running. After

warming up, the mixture can return to normal, but the tem-perature of the incoming air still plays a significant role: thecooler the air, the denser it becomes, and this influences thelambda 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 theamount of fuel (in other words, the lambda ratio) enteringthe engine. By carefully adjusting the internal rules of thisdevice, manufacturers can adapt the characteristics of thefuel injection to the actual requirements: a sporty GTiwould demand rather different settings than a city car; be-sides, catalytic converters have their own demands that, aswe 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, thesesystems (now called engine management systems) took onthe duty of generating the sparks as well. But even with thissecond incarnation, the fuel injection part remained practi-cally the same, thus the following section applies to bothkind 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 engineload. The engine speed can be measured easily on systemsusing traditional ignition: the ignition primary circuit gener-ates pulses with their frequency proportional to enginespeed (the tachometer uses this same signal to show therpm to the driver). When the injection system provides theignition as well, it cannot at the same time rely on it, so anadditional sensor is used instead.

The engine load is usually determined by measuring thequantity of air the engine tries to suck in. There are variousmethods of attaining this: earlier systems used a flap whichis 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 apressure 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 usea pre-stored table in their computer to approximate itbased 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 alreadybe enough to control the engine. A large table can be set

up, like the one il-lustrated here (ofcourse, this is an il-lustration only, theactual values meannothing here), andfor any pair of in-

coming engine speed and load values the necessary fuelamount can be determined. By keeping the pressure of fuelconstant behind the injector valves, the amount of fuel in-jected depends solely on the time period the injectors areopened for, hence, the table can contain injector openingtimes.

An this is exactly how it is done in modern injection sys-tems: the controlling microcomputer keeps a lookup tablelike this to determine the base pulse width. Earlier systemswere constructed from discrete, analog elements, not like asmall computer; a more or less equivalent circuit made ofvarious hybrid resistance arrays and semiconductors wereused 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 fueleconomy). As the computer stores this table in a program-mable memory—similar in function to the BIOS in desktopcomputers—, replacing it is possible. The earlier systemswith analog circuits cannot be modified that easily.

So, we obtained the base pulse width from the table butas the operating conditions of automotive engines are

Amount of fuelinjected

Engine load

0% 5% … 100%

Eng

ine

spee

d

idle 3 3 … 3

850 rpm 4 5 … 5

900 rpm 5 6 … 7

… … … … …

6,000 rpm 9 8 … 10

Fuel Injection Electronic Fuel Injection7

hardly ideal for any reasonable amount of time, several cor-rections have to be applied. Our air flow meter measuresthe volume of the air but we would need to know the massof the air to calculate the required lambda ratio—remem-ber, colder air is denser, thus the same volume containsmore 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 lengthensthe injector pulse width according to this input (except forthe case of the airflow meter using a heated wire, this onetakes the air temperature into account automatically, conse-quently, there is no need for correction).

It is not only the externalcircumstances that requirespecial consideration. Whilemost of the time an engineworks under partial load, so itmakes sense to spare fuel bybasing on a relatively leanermixture across this range ofoperation, cold start andwarm-up, modest decelera-tion and fully depressed throt-tle, idle speed all require dif-ferent treatment.

The position of the throttlepedal is communicated to thecomputer by a throttle posi-tion switch (TS) or throttlepotentiometer (TP). Thesedevices signal both fully openand fully closed (idling) throt-tle positions. When the pedalis fully depressed, the com-puter makes the mixturericher to provide good acceler-ation performance.

Idle speed is more compli-cated: the throttle is closed,so there has to be a bypass tolet the engine receive fuel torun. In simpler systems this by-pass is constant (but manu-ally adjustable to set the cor-rect idle speed) in a warm en-g ine, prov id ing a f i xedamount of air, although thecomputer can decide on avarying amount of fuel to beinjected. Later systems gener-ally use a controlling devicechanging the cross section ofthe bypass, regulating thea m o u n t o f a i r c o m i n gthrough (these systems oftenhave no facility to adjust theidle speed, the computerknows the correct revolutionand maintains it without anyhelp from mechanical de-vices). The controlling device

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

Just like the choke on carburetors, there is a completesubsystem dealing with cold start and warm up, as the re-quirements under such circumstances are so different fromthe normal operation that they cannot be fulfilled by theregular control. The ECU monitors the ignition keyswitch to learn when the engine is started, then looks forthe input from the coolant temperature sensor (CTS) to

EFI MONOPOINT

distri-butor

OS*

engine

ECU

fuelpump

CTS

MAPATS* TP

throttle

ISCM

fuel

injector &pressureregulator

ATS*

fuelexhaustaircoolant

not presentin all systems

*

distri-butor



*

OS*

engine

pressureregulator

ECU

fuelpump

CTS

AFS ATSTS

AAV

throttle

idle speed idle mixture

fuel

injectorsfuel rail

CSV*

EFI MULTIPOINT


see whether this is a cold start or a warm one. If the coolantfluid is measured cold, a special warm-up sequence will bestarted.

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 thecold walls of the engine, second, to ensure better lubrica-tion, the engine should run at an elevated revolution duringthis period.

There are two ways to provide more fuel: through theusual injectors, making the computer inject more gas thannormal, or by using an additional cold start injector(CSV)—there is only one such injector even in multipointsystems. This injector is fed through a temperature-timerswitch, protruding into the coolant just like the CTS, plus itis heated by its own electric heater. The injector operates aslong as the ignition key is in the starting position but its be-havior later on is governed by the timer switch. The colderthe engine initially is, the longer it stays closed to let thecold 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 addsabout 50% extra fuel initially and drops this surplus toabout 25% until the end of a 30-second time period.

From that point, the surplus is dictated by the warmingof the engine, communicated by the CTS to the computer.EFI systems without an idle speed control device often usean electromechanical auxiliary air valve (AAV). Thisvalve, which is fully open when the engine is still cold butwill close gradually as it warms up, lets an additionalamount 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 thecontrol signal and the actual opening of the valve due tothe build-up of electromagnetic fields. The length of this de-

lay depends heavily on thevoltage the injectors are fedwith. The same pulse widthwould result in shorter open-ing time, hence less fuel in-jected if the battery voltagedrops below nominal (whichis often the case on cold morn-ings). The injection computertherefore has to sense the bat-tery voltage and to lengthenthe injector pulse width if nec-essary.

The final, total pulse width(also called injector duty cy-cle) is calculated by summingup all these values received:the base pulse width from theRPM/AFS table lookup, thevarious correction factorsbased on the temperature sen-sors, throttle position and thelike, plus finally, the voltagecorrection.

As the computer has already calculated the exactamount of fuel to be injected, there is only one task left: ac-tually injecting it. There are two possible ways: to inject thefuel 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 1380ccm ones). In all cases, the computer actually calculates thehalf of the fuel amount required as it will be injected in twoinstallments, once for each revolution of the engine.

The injectors of the multipoint system can be operated si-multaneously or individually. Previous Citroëns on the roadtoday 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 temporarilyshut 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 inthe future.

All systems—regardless of the number of injectors—usea similar fuel supply layout. The fuel is drawn from the tankby a continuously operating fuel pump, transported via afilter to the injectors, then back to the tank. There is a pres-sure regulator in the circuit as well to keep the pressure ofthe fuel at a constant pressure above that in the inlet mani-fold (this regulator is a separate unit on multipoint systemswhile integrated into the injector on monopoint ones). Asthe pressure difference between the two sides of the injec-tors are constant, the amount of fuel injected dependssolely on the opening time of the injectors. The pressureused in contemporary EFI systems is 3 to 5 bars.

This is practically all there is to it, there are only a coupleof safety and economy features in addition. If the enginerevolution exceeds a certain limit (between 1,200 and1,500 usually) and the throttle is closed—this is called decel-

distri-butor

OS*

engine

ECU

CTS

MAPATS* TP

throttle

ISCM

CAS



*

fuelpump

fuel

injector &pressureregulator

ATS*

EMS MONOPOINT


eration—, the momentum of the car is sufficient to rotatethe engine through the wheels. To save fuel, the injection iscut off. As soon as the engine speed drops below the limitor the throttle is opened, the injection is reintroduced—sup-posedly smoothly and gradually, however, many driverscomplain about some jerkiness.

To avoid prolonged operation at revolutions exceedingthe specification of the engine, the injection is cut off abovea maximum engine speed (6,000-7,000 rpm, dependingon the engine). And finally, to avoid the hazard of fire in acrash and the fuel squirting from the injection system withthe engine stopped or possibly destroyed, the relay of the in-jectors is controlled by the ECU, allowing fuel injection onlywhen 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 tothe solution used on cars with carburetors.

The distributor has two purposes: generating the driv-ing signal for the ignition system and to distribute the highvoltage to the four cylinders in turn. This two parts insidethe distributor are electrically separate but mechanicallycoupled—both are driven by the camshaft to keep them insync with the strokes of the engine.

The ignition signal thus starts from the distributor. Amagnetic induction sensor (consisting of a rotating four-sided magnet and a pick-up coil) sends a pulse to the igni-tion module at each firing point. This pulse will be switchedto the ignition coil (an autotransformer; auto here doesnot 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 voltagespikes in the secondary circuit. These spikes then go back tothe HT part of the distributorwhich in turn sends them tothe spark plug of the actualcylinder requiring the spark.

It takes some time for thespark to ignite the fuel-air mix-ture inside the combustionchamber: this means that thespark has to arrive slightly be-fore the piston reaches its topposition (top dead center,TDC), so that it will receive thedownward force of the deto-nation in the right moment.However, as the engine speedincreases, so does the speedof the piston or the distance ittravels during a given periodof time. Therefore, the exacttime of the spark has to be ad-vanced as the revolution in-creases. Traditional systemsdo this by adding a vacuumline connecting the inlet mani-fold to the distributor. As the

vacuum increases with the engine revolution, its suckingforce rotates the inner part of the distributor slightly awayfrom its original position, causing all its timing devicesswitch earlier, as required by the value of the timing ad-vance.

Clever systems can get away without a distributor: someCXs 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 pistonsmove in unison: one is compressing, the other exhausting.Although both plugs generate sparks at the same time, theone 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 tobe considered if we want to build a modern ignition sys-tem. For instance, the timing advance depends not only onengine speed but on many other factors as well: engineload, engine temperature and to some extent, the air tem-perature.

Just like the carburetor was not really good at decidingthe amount of fuel required by the engine, the traditionalignition is similarly not perfect in estimating the timing ad-vance and other characteristics of the sparks needed. Anelectronic system similar to the one used for fuel injectionshows clear advantages over any earlier system.

And as they use about the same sensors and rely on eachother, what could be more logical than to integrate theminto a common system, elegantly called an engine man-agement system?

If we compare the schematics of the corresponding EFIand EMS systems, they look almost the same. There are twonotable differences: the small arrow on the line connectingthe ECU to the distributor has changed its direction and anew sensor, a crank angle sensor (CAS) has appeared.

distri-butor

OS*

engine

pressureregulator

ECU

fuelpump

injectors

CTS

AFS ATSTS

AAV

throttle

ISCV idle mixture

fuel

CO pot

CAS KS

fuel rail



*

EMS MULTIPOINT


Both changes have to do with the fact that the enhancedsystem, whose new task is to generate the ignition signalsas 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—informsthe 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 thecomputer. Two of the pins are missing and this hole passesbefore the sensor just as the first piston reaches its TDC posi-tion. The missing pins cause a variance in the sensor outputthat can be read by the ECU easily.

The rest is the same: the base pulse width is calculatedbased on the CAS and AFS/MAP sensors. The correction fac-tors—air temperature, idle or full load, starting, warmingup, battery voltage—sum up into an additional pulsewidth. Besides, the same input signals (AFS, CAS, CTS andTS/TP) are used for another lookup in a table, yielding thecorrect dwell time and timing advance for the ignition. Thedwell period remains practically constant but the duty cyclevaries with the chaging engine speed. The ignition signal isamplified and sent to a distributor containing only second-ary HT components: it does not create the ignition signalonly routes the HT current to each spark plug in firing order.

Some systems also have a knock sensor (KS), sensingthe engine vibration associated with pre-ignition (so-calledpinking). If this occurs, the ignition timing is retarded toavoid engine damage.

Think green

As we saw, fuel injection and engine management systemsare 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 onlean mixture for general, partial load to save fuel, or on richmixture when performance considerations call for this.

Unfortunately, this is not what such systems are used fortoday. With the proliferation of catalytic converters, theonly 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 (CnHm), which when burned together with the oxygen(O2) of the air, should transform to carbon-dioxide (CO2)and water vapor (H2O). However, combustion is never ideal,besides, fuel contains many additives: the exhaust gas, inaddition to the products mentioned, has various byprod-ucts as well, some of them toxic: carbon-monoxide (CO),various unburned hydrocarbons (CnHm), nitrogen-oxides(NOx) and lead (Pb) in various substances coming from theanti-knock additives found in the fuel.

The relative amount of thesebyproducts depend on thelambda ratio of the air-fuel mix-ture burned. As shown on the di-agram, a value between 1.2 and1.3 would give a relatively lowpercentage of toxic byproductswhile, as we can recall, being alean mixture would be in theright direction towards fueleconomy.

By using platinum (Pt) or rhodium (Rh) as a catalyst—acatalyst is a substance whose presence is required to enable(or to boost) a chemical transformation while it does nottake part in the process itself, remaining intact—the follow-ing processes can be carried out:

2 CO + O2W 2 CO2 (oxidation)2 C2H6 + 7 O2W 4 CO2 + 6 H2O (oxidation)2 NO + 2 COW N2 + 2 CO2 (reduction)

These precious metals are applied in a very thin layer to thesurface of a porous ceramic body with thousands of holesto make the surface contacting the exhaust gases muchgreater. Actually, a converter does not contain more than 2or 3 gramms of these metals.

If you compare this diagramwith the previous one, you willsee that the real gain is thesupression of nitrogen-oxides.CO and CmHn will be reduced aswell, although to a much lesserextent. Nevertheless, the overallreduction in polluting byprod-ucts is quite high, amountingup to 90 percent. Lead sub-stances are not considered aslead must not reach the converter anyway, it would clog thefine pores of the converter in no time. The fuel used in carsequipped with a catalytic converter has to be completelyfree of lead.

But there is something of even greater consequence de-picted on the diagram: to keep the amount of pollutantsdown, the lambda has to be kept inside a very small valuerange, practically at l=1 all the time. If the lambda dropsjust a fraction below 1, the CO emission rises sharply, whilea small step above 1 skyrockets the NOx emission. The maintask of the fuel injection is therefore to ensure that the air-fuel mixture sticks to the stoechiometric ratio all the time.This means higher consumption than the one of a car withfuel 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 richermixture, thus the cold start mechanism does not obey thelambda control. The catalytic converter does not work at allbelow 250 °C, so this is not a significant compromise (itsnormal operating temperature is 400 to 800 °C, above800 °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 notobserving the welfare of the converter. Reducing pollutionmight be a noble cause but to be able to end an overtakingis even more important…

The system uses an oxygen sensor (OS, also calledlambda sensor) which measures the oxygen content of theexhaust gas. It is located between the engine exhaust andthe catalytic converter. Similarly to the converter, it is notfunctional 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 themixture injected always as close to l=1 as possible. If thesensor is still too cold to give accurate input, the computercan ignore it safely.

%

C Hn m

CO

NOx

0.9 1.11.00.99 V

%

CO

C Hn m

NOx

0.8 1.21.0 1.3 V


The HPi engine


Diesel enginesDiesel oil has been a contender to gasoline formany decades. Earlier diesel engines were not re-fined enough to win the hearts of many driversbut recent advances in technology made these en-gines not only worthy competitors in all areas butin some features—fuel economy or low endtorque, to name just two—even exceeding thecharacteristics of their gasoline counterparts. Andin addition to the general technological advan-tages, Citroën’s diesel engines have a widely ac-cepted reputation—even among people blamingthe 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 hotair. Due to this principle of operation, the supply of the fuelhas to comply with much more demanding requirementsthan 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 offuel will be injected at high velocity near the end of the com-pression stroke into this heated gas still in motion. As theystart to evaporate, they form a combustible mixture withthe 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 fuelwill enter the cylinder, leading to harsher combustion, withthe 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 thefuel and air—can the combustion be kept at optimal level.

Small diesel engines suitable for cars were made possibleby a modification to the basic principle, that allowed thesestringent parameters to be considerably relaxed. It includesa separate swirl chamber connectedto the cylinder via a restrictor orifice.The air compressed by the piston inthe cylinder enters this chamberthrough the orifice, starting to swirl in-tensively. The fuel will then be injectedinto this swirl, and the starting igni-tion propels the fuel-air mixture still incompletely burnedinto the cylinder where it will mix with the air, continue andfinish the combustion process. Using a prechamber resultsin smaller ignition delay, softer combustion, with less noiseand physical strain on the engine parts, but introducessome loss of energy because of the current of air having topass between the chambers. Citroën engines of this typeuse a tangentially connected spherical prechamber.

As diesel engine evolution continued, better simulationand 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 enginesof 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 towhat we have already discussed for gasoline engines, thereare 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 theengine 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 ignitioncontrol system. Unlike on the gasoline engine, fuel injectionand ignition cannot be separated in a diesel engine.

The complete mechanical injection system is built into asingle 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 theinjection as well;

U a regulator that determines the amount of fuel to beinjected in relation to the engine speed, modified byadditional factors like idle speed, cold starting, fullload, etc.;

U an injection adjuster to compensate for the higherengine speed by advancing the start time of the injec-tion;

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

The diesel fuel is drawn—through a filter—from the tank bythe low pressure pump1 operated by the engine. A pres-sure regulating valve 2 ensures that the fuel pressure willnot exceed a preset limit; when the pressure reaches thisvalue, the valve opens and lets the fuel flow back to the pri-mary side of the pump.

The piston6 of the high pressure part is driven througha coupling 4 consisting of a cam disc and four cam rollers.The piston rotates together with the shaft coming from the

Fuel Injection Diesel engines13

engine but the coupling adds a horizontal, alternatingmovement as well: for each turn, the shaft and the piston6performs four push-pull cycles.

It is the pushing movement of this piston 6 that createsthe high pressure and sends the fuel to the injectors. Thefuel, provided by the pump 1 arrives through the fuel stopelectro-valve â, which is constantly open while the ignitionswitch 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 boreinside 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 valveâ. On the other side of thepiston, the high pressure outlet opens but as the fuel is notyet under pressure, it will stay in the chamber.

In phase 3 the piston is energetically pushed by the camdisc and rollers of the coupling 4, injecting the fuel storedin the chamber into the output line with a significant force.

As the piston 6 moves to the right, at some point theside outlets will emerge from under the regulator col-lar5—the fuel injection into the real output will stop imme-diately, and the rest of the fuel stored in the chamber willleave through this path of lesser resistance. This is phase 4,the end of the injection cycle.

Actually, this operation is repeated four times for eachrevolution of the incoming shaft. There are four high pres-sure outlets radially around the piston, each serving a givencylinder. As the outlet slotä of the piston turns around, it al-lows only one of the outlets to receive the fuel.

The pressure valves7 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 rapidclosing of the injector valves, a ball valve minimizing theback flow is also used.

The length of phase 3, thus the amount of fuel injecteddepends on the position of the collar5. If it is pushed to theright, it will cover the side outlets for a longer time, result-ing in a longer injection phase, and vice versa. If it stays inthe leftmost position, no fuel will be injected at all.

And this is exactly what the regulator part does: itmoves this collar5 to the left and to the right, as the actualrequirements dictate. The lever9 attached to the collar is ro-tated around its pivot by several contributing forces. Thetwo main inputs are the position of the accelerator pedalas communicated through a regulator spring Ý and the ac-tual engine speed, driving a centrifugal device8 via a pairof gears 3. The higher the engine speed, the more theshaft å protrudes to the right, pushing on the lever Û.

When the engine is being started, the centrifugal device8 and the shaft å are in their neutral position. The startinglever Û—pushed into its starting position by a spring Ü—sets the position of the collar 5 to supply the amount offuel needed for the starting.

As the engine starts to rotate, a relatively low speed willalready 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

injectionadjuster

high pressure pumpand distributor

fuel stopvalve

regulator

fuel

fuelto tank

injectors

accelelatorpedal

ignitionswitch

idlefull

2

1

3

8

4

à

Û

5 ä7

6 6

ãæ

â

á

å9

Ü

ÞÝ

ß

injector

PHASE 1 PHASE 2

PHASE 3 PHASE 4

5 6 5 6

6565


rather weak spring Ü. This will rotate the lever Û, movingthe collar 5 to the left, setting the amount of fuel requiredfor idling. The accelerator pedal is in the idle position aswell, dictated by the adjustment screw ß. The idle spring Þkeeps the regulator in equilibrium.

Normally, the amount offuel will be regulated by theposition of the pedal as bothsprings Ü and Þ are fullycompressed and do not takean active part in the process.When the driver pushes onthe pedal, the regulatingspring Ý stretches, both le-vers 9 and Û rotate andmove the collar 5 to theright, to allow the maximumamount of fuel to be in-jected. As the actual enginespeed catches up, the centrif-ugal device 8 opens up,pushing the shaft å to theright, countering the previ-ous force, gradually return-ing the collar5 towards the no fuel position, until the pointis reached where the amount of fuel injected maintains theequilibrium. When the driver releases the pedal, the inverseof this process takes place. During deceleration—pedal atidle, engine rotated by the momentum of the car—the fuelis cut off completely.

Without such regulation, if enough fuel is provided toovercome the engine load, it would continue acceleratinguntil self-destruction (this is called engine runaway). Speedregulation is a feedback mechanism comparing the actualspeed of the engine to the one dictated by the gas pedaland modifies the amount of fuel as necessary. If either theengine speed changes (because of varying load, going overa hill, for instance) or the driver modifies the position of theaccelerator pedal, the regulation kicks in, adding more orless fuel, until a new equilibrium is reached. If the engine ispowerful enough to cope with the load, keeping the pedalin a constant position means constant cruising speed in adiesel car; gasoline vehicles need speed regulated fly-by-wire systems or cruise controls to achieve the same.

The excess fuel will finally leave the pump unit throughan 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 thesmoke limit, the maximum amount of fuel injected into agiven amount of air, that results in combustion without re-sulting in soot particles. Although gasoline engines alsohave this limit, they normally operate with a constant fuelto air mixture that automatically places the amount of fuelbelow this critical limit. Diesel engines, in contrast, operatewith a variable fuel to air mixture, using this very variationfor power regulation. With diesel fuel observing the smokelimit 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 ofparticulates—a bit like a chain reaction.

Because the maximum amount of fuel injected dependson 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 stopæ 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 placein close succession: the fuel injection system starts its deliv-ery, then the fuel is actually injected (the time elapsed be-tween these two is the injection delay), slightly later the fuelwill 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 ofmilliseconds, that is).

Just like in a gasoline engine, the ignition delay remainsconstant while the engine speed changes. The fuel has to ig-nite before the piston passes its TDC position, but with theincreasing 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 intime. The injection adjuster à feeds on the fuel pressureprovided by the pump1, 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 thecam disc inside the coupling 4, increasing or decreasingthe phase difference between the revolutions of the engineand the rotating-alternating movement of the distributorpiston 6.

Some engines also have additional minor correctionmechanisms á that modify the idle speed and timing de-pending on engine temperature, to provide better coldstart performance. The engine temperature is measured in-directly, through the coolant acting on cylinder and piston-like elements filled with paraffin. As the paraffin expands orcontracts as the coolant temperature dictates, the trans-formed mechanical movement, coupled through cables totwo movable end stops for both the lever 9 and the injec-tion adjuster à, modifies the idle speed and the injectiontiming of the engine. Because correct timing depends ontemperature, the corrections, although relatively slight, in-

9

Ý

pedal

Þ

ß

å

Û8

INCREASEDECREASE

Ü

65

STARTING

9 9

Û Û

Ý Ý

pedal

Þ Þ

ß ß

å å

8 8

pedal

IDLE SPEED

6 655

Ü Ü


sure that the amount of fuel injected as well as the timingprovide better combustion and lower pollution when theengine is started and operated at low temperatures. Theydo not have any effect once the engine reaches the normaloperating 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 pressurizedfuel entering the injector through a filter 1tries to press the piston 2 upwards but aspring 3 counters this force. As soon as thepressure exceeds the force of the spring(which can be adjusted by placing appropri-ately sized shims behind it), the piston jumpsup and the fuel rushes into the swirl chamber

through the small orifice now opened. After the injectionpump closes its pressure valve at the end of the injection pe-riod, the spring3 pushes the piston2 back, closing the ori-fice until the next injection cycle.

Each swirl chamber has its own glow plug whose onlypurpose is to heat up the chamber in cold weather. Theystart to glow when the ignition key is turned into the firstposition and stay glowing for some time afterwards unlessthe starting was unsuccessful.

Turbo

More power requiresmore fuel. An efficientway to boost the perfor-mance is to provideboth more air and fuelto the engine. The ex-haust gases rushing outfrom the engine wastea great deal of energy; aturbocharger 4 spunby 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 maynot have the inlet pressure raised too much because the airand fuel mixture may subsequently self-ignite when it is notsupposed to, and instead of burning controllably, detonate.In a diesel such a situation is not possible because the fuel isinjected only when combustion should actually happen inthe first place. As a result, relatively high inlet pressures canbe used, considerably improving the power output of a die-sel engine, and with proper attention to the subtleties ofthe 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 areincreasing.

Because the engine is required to deliver as much torqueas 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 spinup very quickly due to its low mass and inertia, ensuring itsfull 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 thespeed of sound, a supersonic wave effect occurs that canabruptly leave it without any load, at which point runawaywould occur, resulting in severe damage to the turbine.

On the other hand, if the turbine was dimensioned sothat even at the highest engine speed it is still operatingwithin safe limits, it would not be useful at all in the middlerange where the engine is most often used. A compromisecan be achieved using an overpressure valve, the waste-gate valve 5. The turbo pressure is constantly monitoredby this valve opening above a set pressure limit, letting theexhaust escape through a bypass. This avoids turbo run-away by making the turbo rotational speed proportional tothat of the engine, once the limit pressure is reached. Thisway 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 enginespeed range—typically the higher 70-80%. But it comes ata price: because of the simplicity of such a regulation, thelimit pressure is dictated by the maximum turbine speed,which is usually calculated for maximum engine speed plusa safety margin. The maximum pressure is already reachedat lower engine and turbine speeds, where the turbinecould conceivably still provide more pressure because of alesser demand for air volume. Although with a simplewastegate a certain amount of the turbocharging potentialis lost, the increase in power output is still substantial.

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

Essentially, a turbocharged diesel engine runs in two dif-ferent modes: atmospheric pressure or turbo-charged. Theatmospheric pressure mode prevails while the exhaust gasproduced is not yet sufficient to power the turbine (below agiven engine speed and load). Once this limit is crossed andthe turbine starts generating higher than atmospheric pres-sure, the engine is running in turbocharged mode.

The injection pump regula-tor needs to know about thechanges in the inlet pressure,because those changes meandifferences in the amount ofair entering the engine. Andthis also means that the upperlimit of fuel injected needs tobe changed correspondingly.These injection systems aretuned for the turbo producingthe rated waste pressure (alsoknown as full boost). How-

ever, the amount of fuel injected during the atmosphericmode of the engine—before the turbo kicks in—has to bereduced in order to avoid crossing the smoke limit. Theturbo pressure drives a limiter in the injection pump: withthe 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 lever9, limiting the amount of fuel tobe injected.

Intercooler

9

Ý

pedal

Þ

ß

turbopressure

æç

HP fuelreturn

1

3

2

air

exhaust

5

1

2 3

4


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 gaspedal control on the pump (with a potentiometer fitted tothe top of the control lever) controls a valve which lets partof 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 minutesis usually controlled by a combination of a timer and the en-gine coolant temperature: either the timeout of 4 minutesruns out or the engine reaches 50 °C. An additional mecha-nism prevents post-glowing if the engine was not actuallystarted.


Electronic Diesel Control

Just like it is the case with gasoline engines and carburetors,a mechanical device—even one as complicated as a dieselinjection 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 fuelinjection.

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 relationshipeven further. As the injection always has to inject less fuelthan the amount which would already generate smoke, themechanical pump—capable only of a crude approximationof 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 onthe 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 willsimply pump too much fuel into the engine, resulting inblack 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 systemsuse 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. Theignition delay, as we have already seen, depends on thecetane number of the diesel oil. In spite of the fact that cor-rect timing has a paramount influence on the performanceand 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 canachieve better characteristics, lower fuel consumption andless pollution.

All in all, the electronically controlled injection pump notonly adds precision to the injection process as its gasolinecounterpart does but introduces completely new methodsof regulation; therefore it represents a much larger leap for-wards than fuel injection in gasoline engines. In spite ofthis, it is quite similar to its mechanical predecessor. Fromthe five subparts, four remain practically the same, only theregulator is replaced with a simple electromagnetic actua-tor that changes the position of the same regulator collar5as in the mechanical pump, in order to regulate the amountof fuel to be injected.

The real advantage over the former, mechanical pumpsis that an electronic device, a small microcomputer can han-dle any complex relationship between the input values andthe required output. With mechanical systems, only simplecorrection 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 characteristiccurves digitized into lookup tables, describing the amountof fuel to be injected using three parameters: enginespeed (measured by a flywheel inductive magnet), cool-ant temperature (measured by a sensor protruding intothe coolant liquid), air temperature (measured by a sen-sor in the air inlet).

The newer HDi engines use an air mass sensor using aheated platinum wire (as that mentioned on page 6). Hav-ing the exact amount of air to enter the engine, these latestEDC systems can deliver true closed loop regulation.

A potentiometer attached to the accelerator pedalsends 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

injectionadjuster

high pressure pumpand distributor

fuel stopvalve

actuator

fuel

fuelto tank

injectors

accelelatorpedal

ECU

temperatures(air, fuel, coolant)

regulatorposition

vehicle speed

air quantity

engine speedatmosphericair pressure

2

1 4

à ß

5

Û

8 ã

â

ä7

6 6


tentions of the driver. The ECU uses this sensor to learnabout 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 platinumwire 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 enginespeed (plus the manifold pressure if a turbo is fitted). Thevalues obtained from this table are corrected according tothe inputs of the various sensors, in order to arrive at a basictiming and smoke limit value. The actual amount of fuel in-jected and the accurate timing are a function of theseresults 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 ensurethe necessary precision, the factual position is reportedback 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 intoone of the injectors (the other are assumed to work com-pletely simultaneously) notifying the computer about theprecise time of the beginning of the injection. Should therebe any time difference between the factual and designatedopening times, the electro-valve ß of the injection ad-juster à will receive a correction signal until the differencedisappears. If the electro-valve is completely open, the injec-tion start will be delayed, if it is closed, the start time will beadvanced. To achieve the timing required, the valve isdriven 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 compensatingcalculations on the amount of fuel injected, and to providethe on-board computer with the exact amount of fuel usedup 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 caneven be simulated using other sensors—for instance, therole of a failing engine speed sensor might be filled in bythe signal generated from the needle movement sensor.

As there is no standalone ignition in a diesel engine, theonly way to stop it is to cut off the fuel supply. The mechani-cal default position of the actuator 8 is the position whereno fuel enters the injectors at all; this is where it returnswhen 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 isone of the principal EDC parameters for a turbocharged en-gine. Later Citroën turbocharged diesels—starting with the2.5 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 electricvalves. 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 twoor three different wastegate limit pressures. This lets the sys-

tem ease the compromise between the turbo pressure andturbine speed: the pressure is kept at the usual value forhigher engine speeds (limited by the maximum turbinespeed) but is allowed to go higher than that in the middlerpm ranges, adding a significant amount of torque in therange where it is most needed.

Green versus Black

Diesel oil, just like gasoline, is a mixture of various hydrocar-bons (CnHm), and burned together with the oxygen (O2) ofthe air, transforms to carbon-dioxide (CO2) and water vapor(H2O). However, as the combustion is never ideal, the ex-haust gas also contains various byproduct gases: carbon-monoxide (CO), various unburned hydrocarbons (CnHm), ni-trogen-oxides (NOx). The relatively high lambda value a die-sel engine is operating with reduces the hydrocarbon andcarbon-monoxide content to 10–15%, and the amount ofnitrogen-oxides to 30–35% of the corresponding figuresmeasured in gasoline engines without a catalytic converter.The sulphur content of the fuel—drastically reduced duringthe recent decades—is responsible for the emission of sul-phur-dioxide (SO2) and sulphuric acid (H2SO4).

Conversely, these engines emit 10–20 times more par-ticulates—or black soot—than gasoline engines. These areunburned or incompletely burned hydrocarbons attachedto large particles of carbon. These substances are mainly al-dehydes and aromatic hydrocarbons; while the first onlysmells bad, the second is highly carcinogenic.

The much higher amount of particulates is due to the dif-ferent combustion process. The various aspects of mixtureformation, ignition and burning occur simultaneously, theyare not independent but influence each other. The distribu-tion of fuel is not homogenous inside the cylinder, in zoneswhere the fuel is richer the combustion only takes placenear the outer perimeter of the tiny fuel droplets, produc-ing elemental carbon. If this carbon will not be burned laterbecause of insufficient mixing, local oxygen shortage (largefuel droplets due to insufficient fuel atomization, caused byworn injectors) or the combustion stopping in cooler zonesinside the cylinder, it will appear as soot in the exhaust. Thediameter of these small particles is between 0.01 and10 mm, the majority being under 1 mm. Keeping theamount of fuel injected below the smoke limit—thelambda value where the particulate generation starts to riseextremely—is essential.

Similarly to gasoline engines, the exhaust gas can bepost-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 isenough oxygen in the exhaust gas to simply burn the car-bon soot present. The burning filter is manufacturedfrom ceramic materials that can withstand the resultinghigh temperatures (up to 1200 °C). As the diesel engineis very sensitive to excessive back pressure, the filter hasto be able to self-regenerate. This is solved by the addi-tion of organic metal substances.

U Catalytic converter, identical to the simpler ones usedon gasoline engines before the proliferation of three-way, controlled converters. It reduces the carbon-monox-ide and hydrocarbon content of the exhaust gas.


Diesel Direct Injection

I think that at this point, soot burning filters will have to becut out of the PDF and put in at a similar ecological sectionunder DI/HDI—since that is the only system that actuallymakes soot burning practical, and the only system that im-plements it.

Soot burning was experimented with a lot but was nevermade practical before HDI due to a too low exhaust temper-ature. The particle filter would need heating to a very hightemperature and that was deemed to be too dangerous.Even with cerine additives, essentially, there would have tobe a separate small burner to heat up the filter, which isagain another system that can go wrong. HDI essentially in-tegrates a burner by alowing post-injection, somethingthat is simply impossible for injection systems derived froma classical pump due to teh timing required. I think that forsoot management it is enough to write that the smoke limitcontrol is vastly improved by the better regulation of theEDC.

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 injectionsystems" sentence somewhere in there, since proper coldstart corrections and EGR are implemented in EDC units bydefault.

Suspension


A Suspension PrimerFrom the early days of the automobile—and evenbefore, in the time of horse-drawn carts—it wasalready well known that the body of the car, hous-ing both the passengers and the load, has to bedecoupled from the unevenness of the road sur-face.

This isolation is much more than a question of comfort. Thevertical force of the jolts caused by the repeating bumpsand holes of the road surface are proportional to the squareof the vehicle speed. With the high speeds we drive at to-day, this would result in unbearable shock to both peopleand the mechanical parts of the car. Jolts in the body alsomake 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 andother features of this suspension medium are governed bymany, mostly contradicting factors.

The softer, more elastic the spring, the less the sus-pended body will be shaken by various jolts. For the sake ofcomfort, we would thus need the softest spring possible.Unfortunately, too soft a spring will collapse under a givenweight, losing all its elasticity. The elasticity of the springwould need to be determined as a function of the weightcarried but the weight is never constant: there is a widerange of possible load requirements for any car. On onehand, 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 comfortablebut 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: sucha spring will become harder as the weight to be carried in-creases.

When the spring is compressed under the weight of theload, it’s not only its flexibility that changes. The spring de-flects, causing the clearance between the car and the roadsurface decrease, although a constant clearance would be aprerequisite of stable handling and roadholding. At firstsight, this pushes us towards harder springs: soft springswould 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 constantlyand dynamically when the wheels roll on the road surface.The body of the vehicle dives, squats, rolls to left and rightas the car goes over slopes, holes and bumps in the road,corners, accelerates or decelerates.

When a deflected spring is released again, the energystored in it will be released but as there is no actual load forthis energy, the elastic element, the mass of the suspensionand the vehicle form an oscillatory system, causing a seriesof oscillations to occur instead of the spring simply return-ing to its neutral position.

Any vertical jolt would thus cause such oscillations: theupward ones are transmitted to the car body while thedownward ones make the wheels bounce, losing contactwith 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 containair which is a highly elastic spring medium. Oscillation in it-self causes unwanted motion but when the corrugation ofthe road surface happens to coincide with the period of thesuspension oscillations, it might lead to synchronous reso-nance, a detrimental situation leading to serious damagesin the suspension elements.

Mass in motion can also be viewed as a source for kineticenergy; because of this, moving parts of the suspension areoften reduced in weight to decrease this portion of thestored energy, and this in turn eases the requirements onthe dampers as they have to dissipate less unwanted energyas heat. This solution, however, often shifts the frequencyof the self-oscillation of the suspension upwards. Unfortu-nately, occupants are more sensitive to higher frequenciesreducing comfort (mostly adding noise), so this is an areawhere 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 inthe 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 bematched carefully to obtain any acceptable results. The ab-sorber ought to be both soft and hard at the same time: asoft absorber suppresses the bumps of the road but doesnot decrease the oscillations satisfactorily while a hard ab-sorber reduces the oscillations but lets the passengers feelthe unevenness of the road too much. Due to this contradic-tion, conventionally suspended cars have no alternative butto 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 fora unified component serving both as a spring and anabsorber, harmonizing the requirements.

Suspension Hydropneumatic Suspension23

Hydropneumatic SuspensionAs we saw, the ideal suspension would requireelasticity decreasing with the load, constantground clearance, shock absorbers integratedinto the suspension—all these beyond the obvi-ous independent suspension for all wheels. Andthis is exactly what Citroën’s unique hydropneu-matic suspension offers.

According to the Boyle–Mariotte formula defined in the17th century, the pressure and the volume of a mass of gasare inversely proportional at a constant temperature. There-fore, by keeping the mass of the gas constant and changingthe volume of its container, its pressure can be controlled(the usual pneumatic suspensions operate on the oppositeprinciple: air is admitted or withdrawn from the system bycompressors and exhaust valves, modifying its mass whilekeeping the volume constant).

The volume changes are controlled by hydraulics, a tech-nology in widespread use in every branch of the industry. Asliquids 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 betransmitted, multiplied or divided (according to the relativesizes of the operation cylinders), with velocity increased ordecreased (using varying cross sections in the tubing), toany 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 themost trivial examples; however, Citroën is the only one touse 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 themarket at that time: suspension, running gear, steering,brakes, clutch, body, aerodynamics were all unique, notonly in details but in the main operating principles as well.

The hydropneumatic spring-absorber unit uses an inertgas, nitrogen (colored blue on the illustrations) as its springmedium, resulting in very soft springing. The flexibility ofthe gas decreases as the increasing load compresses the sus-pension pistons, reducing the vol-ume of the gas and adding to itspressure. The damping effect is ob-tained by forcing the fluid (coloredin green) pass through a two-wayrestrictor unit between the cylinderand the sphere. This effect providesa 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 fromthe suspension units (practically, by adjusting the length ofthe hydraulic strut), ground clearance can be kept con-stant under any load variations. Although this mightnot seem very important at first sight, it means that the sus-pension geometry is also constant—in other words, thehandling 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 theincreasing 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 fluidthrough the dampers regardless of load (which is not thecase with conventional springs). The working range of the

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

This basically constant suspension resonance frequencyalso contributes to the consistent behaviour independentof 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 limitedbut very useful anti-dive behav-ior: this is essential for efficientbraking with a basically very softsuspension. The center of mass ofthe car moves much less than

usual, hence the braking force is distributed more evenly.Manufacturers of cars with conventional suspension andbraking only start to add brake force distributors to their ve-hicles these days. The first DS did have a force distributorbut 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 anotherimportant implication: lower difference in forces on thewheels. Coupled with variable damping this keeps thewheels in contact with the road at all times, which inturn maximizes the tractive forces on the tires—brakingwhile turning still leaves the vehicle with the grip of all fourwheels: this is essential for security in low adherence condi-tions, such as ice, snow, rain, mud.

ID

DS GS

GSA


The steady connection between the sides requires an ex-ternal management of body roll. Ideally, for any verticalmovement of the car body, the two sides of the suspensionshould be connected, while for any movement that resultsin different displacements of each wheel, they should ide-ally be separate. This second movement can be viewed as arotation around the longitudinal or transversal axis.

For instance, if the front wheels run into a pothole andthe rear wheels go over a bump, the car will rotate aroundits transversal axis. The angle of rotation remains relativelysmall as the length of the car is its largest dimension; thehigher weights like the engine bay are far from the centre ofmass, resulting in a large inertial torque to counter outsideforces. If all suspension elements of the wheels were con-nected hydraulically, the vehicle would absorb the bumpsvery efficiently (the rear struts compressed by the bumpwould deliver fluid into the front struts, resulting in immedi-ate compensation: the rear would sink, the front wouldrise, restoring the horizontal position of the car). Unfortu-nately, this would also lead to slow transversal (dive andsquat) 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 isbasically sufficient to counter the effect of longitudinalbumps, the front and rear suspension circuits are sepa-rated. The active height correction of the system acts as afurther a non-linear stabilizer both countering dive andsquat, 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 underthe left one—, the car will rotate around its longitudinalaxis. Being much less wide than long, the angle of rotationwill be higher and the inertial torque is considerably lowerto counter this kind of rotation. Completely independentsides 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 thehydropneumatic suspension are interconnected, providinga push-pull operation of the two sides. The interconnectionhas 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 toslower 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 effectsof roll could be eliminated if the center of the roll could beidentical to the center of the mass. As this is not possible,the opposite approach of moving the center of roll awayfrom the center of mass could also help overcome body rollby increasing the opposing torque. This is the role of theanti-roll bar: similarly to a bike leaning into a curve, it liftsthe inner side of the wheel, using the force on the outeredge, and this moves the center of roll outwards. In otherwords, the wheels and suspension elements do have roll,the role of the anti-roll bar is to isolate this roll from thebody 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 thebody), 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 inboth directions between the suspension and the bar. Theshock absorbers have to provide the damping for the anti-roll 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 hasmuch less interdependence and compromise betweendamping, countering roll, squat and dive. In addition, it canprovide solutions which are simply unfeasible mechanicallyin a conventional suspension. Cars with steel springs alwayshave roll, including diagonal one, induced by undulationsof 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 otherhand, have a varying interconnection depending on fluidmovement—this is very easy to accomplish with hydraulicsbut extremely complicated with springs.

The only disavantage is that damping occurs furtherfrom the source of the disturbance, and due to the goodconductivity of sound via the hydraulic lines, this results inslightly more noise. The same effect makes the hydropneu-matic suspension somewhat noisier than a conventionalone. However, good sound insulation inside the cabin canhelp 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 toeither side.

Although the hydropneumatic spring-absorber unit is anintegrated unit from a technical point of view, hydraulicsmake 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 whilethe mass of the nitrogen in the spheres is practically negligi-ble. Even adding the mass of the fluid moving around in thesystem, 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 reducesthe mass further because the basic suspension mechanicscan be simpler, without requiring multilinks and similarcomponents.

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 ifthe liquid starts to boil, there will be no vapor lock as thepressure is automatically released and remains proportionalto 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 itsextra services in comparison to a conventional system andexperience shows that the whole system is very reliable. Theperfect 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 canmake the system very reliable.

Suspension Hydropneumatic Suspension25

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 ofachieving some of these results. For instance, variable diam-eter or pitch springs coupled with hydraulic shock absorb-ers (incidentally, with a similar internal geometry as thedamper elements used in Citroën spheres) behave similarlyto these hydropneumatic units. The main difference is thateven if these elements would be practically identical, allother functionality that comes either for free or at a smalladditional cost in Citroën systems—constant height, anti-dive, brake force regulation and so on—, require complexand expensive additional systems.

The illustration shows the basic layout of the suspension(differences on models fitted with power steering or ABSwill 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 tothe reservoir—although the outputs are indicated, the linesthemselves are omitted for the sake of clarity. In reality, theyare 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 mineralsuspension liquid called LHM from the reservoir. The fluidunder pressure is stored in the main accumulator. It is the

task of a pressure regulator—built into the same unitwith the accumulator—to admit fluid into the accumulatoras soon as the pressure drops below the minimum value of145 bar; as soon as the pressure reaches 170 bar, the regula-tor closes and the fluid continues its idle circulation fromthe pump, immediately back to the reservoir.

On simpler models the out-put marked with an asteriskis omitted and it goes to thereturn ouput inside the regu-lator unit instead, as shownby the dashed line. On mod-els fitted with power assistedsteering (DIRASS) this inter-connecting line is missingand both outputs are used in-dependently.

The spring below the piston 1 is calibrated so that it willcollapse only when pushed down with a pressure exceed-ing the cut-in threshold (145 bar). While the pressure in themain accumulator remains inferior, the piston stays in theupper position, allowing the pump to deliver fluid into theaccumulator through the ball valve 5: the unit is switchedon. The piston 2 also remains in the upper position (itsspring is calibrated to the cut-out pressure, 170 bar), lettingthe entering fluid fill up the chamber 3 as well. This, inturn, ensures that the piston 1 stays in the upper position:the fluid pressure in this chamber plus the force of thespring counters the downward pressing force even if thepressure in the accumulator rises well above 145 bar.

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

front strut & sphere


reservoir

main accumulator &pressure regulator

HP pump

security valve

heightcorrector

heightcorrector

rear strut & sphere

rear strut & sphere

brake valve

front brake

front brake

rear brake

rear brake

LHM feedfront suspensionrear suspensionfront brakesrear brakesoperational returnleakage return

A typical example: the BX

TRANSITION

pumpfeed

flowdistr* rtrn

ON

51 2

6

3 4

51 2

6

3 4

OFF

pumpfeed

flowdistr*

rtrn

51 2

6

3 4BX

CX


With the back pressure now vanished from behind thepiston 1, the pressing force of the accumulator fluid drivesit down at once: the regulator is switched off now. The fluidsupplied by the pump returns back immediately: on PAS-equipped 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 touse up the pressure in the main accumulator, the piston 2will return to its original position. Once there, the regulatoris ready to start a new cycle.

The characteristic ticking which can be heard in Citroënsis 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 onto replenish the accumulator—is much softer.

The interconnection6 is normally closed. Opening it letsall the fluid stored under pressure return back to the LHMreservoir—this is the way the system is depressurized whenany of the suspension elements need servicing.

The liquid—supplied to the rest ofthe system from the main accumula-tor—passes through a security valvewhose task is to ensure safety by feed-ing the brake circuits first. The frontbrake circuit is always open but theother two outputs are blocked by a pis-ton. If the pressure in the main circuit exceeds 100 bar, thefluid pushes the piston back against the force of the spring,opening up the suspension outputs as well. The electricalswitch for the low hydraulic pressure warning lamp on thedashboard is built into this valve as well. This way, a suddenfailure of the pump or the belt driving it will not leave thecar 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 thecorrector blocks the inlet of fluid, isolating the struts fromthe rest of the suspension. Body roll is limited by the damp-ing effect of the restrictors built into the sphere supportsand by forcing the fluid to run from the left to the rightstrut through a connection line. If the movement of thefront anti-roll bar dictates that the front of the vehicleshould be raised, the connecting linkage moves the pistonupward, opening the inlet and letting additional fluid enterthe front struts. When an opposite movement is required,the piston moves downward, letting the fluid at residualpressure flow back from the struts to the LHM reservoir.Both directions of flow are stopped and blocked when theheight corrector piston resumes its middle position.

The mechanical connection between the anti-roll barand the height corrector is not a rigid linkage but has somefree play. Just before the height corrector, the connectingrod 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, onlythose 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 factorsmake 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 andwear on the correctors but also prevents the system fromdeveloping 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 requiredcorrection is too small.

The next circuit is the rear suspension. Its layout and op-eration is identical to the front one, having its own heightcorrector.

The first circuit, as already mentioned, feeds the frontbrakes. The liquid under pressure flows into the brakecompensator valve, operated by the brake pedal. In itsneutral 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 firstpiston, closing the return output and opening up the outletgoing to the front brake cylinders.

This piston and a spring behind it pushes the second pis-ton which works similarly for the rear brakes, althoughthose are not fed directly from the security valve but receivetheir supply from the rear suspension (later brake valveshave three pistons but their method of operation is practi-cally the same). In consequence, the braking force at therear depends on the load: the more the back of the car isloaded, the stronger the rear brakes work. Actually, on aCitroën mostly used to carry only its driver, without muchload in the trunk, the rear brake pads and disks wear muchslower than those in the front.

The damping elements in thesphere supports consist of a centralhole which is always open and addi-tional small holes closed and openedby a spring as the flow of the hydraulicliquid dictates. Slower suspensionmovements like body roll, squat ordive result in a slower flow of the liquid and the smaller dy-namic pressure differences are not sufficient to bend thespring cover open over the additional holes. The dampingeffect is therefore only determined by the diameter of thecenter hole.

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

The additional holes are located in a circle around thecenter 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 oneside, the remaining half on the other side. By carefully ad-justing the size of the holes, the designers could fine tunethe damping factors independently for both directions ofstrut travel.

warninglamp

feedrear

brakefront

RAISING LOWERINGSTABILIZED

feedstrutsreturn

feedstruts struts

return

Suspension Hydractive I27

Hydractive IThe Hydractive I suspension system appearedwith the XM. Unlike the simpler hydropneumaticsuspension used on the DS, GS/GSA, CX, BX andsome XMs, this one has two modes of operation,soft and hard. The suspension functions in softmode but it will be switched to the hard modewhen the computer deems this necessary for thesake 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 andspheres 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 inputprovided by many sensors (steering wheel, acceleratorpedal, body movement, road speed and brake), includingthe Sport/Comfort switch on the dashboard—the suspen-sion ECU decides when to switch to hard mode; in otherwords, when to deactivate the additional spheres for extraroadholding and safety.

When the driver selects the Sport setting, the suspensionis switched to hard mode constantly. This setting is notwhat any Citroën driver would call comfortable… The suc-cessor system, Hydractive II overcomes this limitation.

return return

return

feed fromsecurity valve return

control fromcomputer nitrogen

LHMmoving parts

suspensioncontrol block

heightcorrector

strut & sphere strut & sphere

electro-valve

rearsuspension

3

5

42

4

1

The layout of the system (front suspension)

The illustration only depicts the differences to the standardhydropneumatic layout already presented in the previoussection: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 tothe 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 fluidcross-flow between the left and right suspension strutsin case of body roll. This valve is disabled for suspensionheight corrections, in order to guarantee that the fluidpressure in the corner struts remains equalized;

4 Two damping elements similar to those used on thecorner 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 computeruses pulse width modulation to achieve a constant cur-rent through the coil. The initial voltage is higher tomake the valve react quicker but it is reduced to a smallervalue once the inductive effects have been overcome,should the valve stay on for a long enough time. Thevalve is capable of being on indefinitely when driven withthis sustained current.

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

Soft, hard, soft, hard…

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

XM


While the computer keeps the suspension in soft mode,the electro-valve 5 is energized by the ECU, opening thefeed 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 throughtwo 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 aslarge as a single damper element, but when the car starts toroll, 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 adamping string, absorbing quick changes in pressure be-tween the two dampers. This dampens the body roll tosome extent even in soft mode.

Whenever the computer feels it necessary to switch to hardmode, it closes the electro-valve 5, not allowing the mainfeed pressure to move the isolation piston 2. The pressureinside the center sphere1, always higher than that of the re-turn path under normal operating conditions, will move thecontrol piston into a position which closes off the centersphere completely. The remaining pressure in this sphere re-mains unknown but as the main circuit pressure mightchange while the suspension is in hard mode (due to eitherthe dynamics of the suspension—acceleration, braking,

movement due to uneven surface—or the vehicle height al-tered by the driver), the computer equalizes the pressure pe-riodically by enabling the control block to assume the softposition for a short period of time.

Hard mode serves three reasons. First, it provides higherresistance to body roll. The cross-flow of LHM from onestrut to the other has to pass through both damper blocksas in soft mode, but it is additionally limited using the pis-ton and ball valve3, now switched into the hydraulic circuitbetween the damper elements instead of the center sphere.The ball is positioned in the fluid so that any cross-flowmoves the ball and thus limits the flow, dampening thebody roll as well.

Second, it limits dive and squat by helping out the heightcorrectors. A stiffer suspension damps the vertical motionand 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 theinfluence of the body movement tosteering, very important in extreme sit-uations like a flat tire.

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

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

stance, when braking in a curve. Thevalve3 therefore has an additional pis-ton which lets the LHM flow betweenthe circuits of the struts and of theheight corrector. If the body has to beraised, the pressure in the heightcorrectors will be higher than that inthe suspension. This higher pressure

pushes the piston, which in turn dislodges the ball and thepressure will raise equally in both struts (without dislodgingthe ball, only one of the struts would receive the fluid, result-ing in incorrect operation).

If the body has to be lowered, thehigher pressure in the struts will dis-lodge the ball again, opening the pis-ton towards the return line ad thefluid 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 tochoose between the two settings. The status light on the in-

strut strut

suspension pressure

system feed pressure

securityvalve

rear

return

control fromcomputer

heightcorrector

SOFT MODE

5

3

42

4

1

HARD MODE

suspension pressure


residual pressure

strut strut

securityvalve


rear

heightcorrector

return

5

4

1

24

3

strutstrut

heightcorrector

strutstrut

heightcorrector

strutstrut

heightcorrector


strument panel informs about the setting selected (it doesnot indicate the mode the suspension is currently in).

The second input comes from a vehicle speed sensor.This inductive magnet tachogenerator generates 4 pulsesper 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 invehicle speed for the duration of one second.

Another input arrives from the steering wheel angleand speed sensor, an optoelectronic device consisting oftwo infrared light beams, interrupted by a rotating discwith 28 holes. The ECU senses the quadrature signalchanges 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 every3.214 degrees of steering wheel rotation. The direction ofturning can be determined by the sequence of the edgechanges.

To make decisions, the computer needs to know thestraight ahead position of the steering wheel. The sensordoes 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 vehiclespeed is above 30 km/h and the steering wheel positionwas 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 isfound to turn more than this value (an error of up to 4pulses is accepted here, too), this is a clear indication of anincorrect center reference: in this case the center positionwill be adjusted by the surplus.

The rotational speed of the steering wheel is determinedby measuring the time elapsed between the individualpulse 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 slowheight 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 theright of the height corrector linkage. Due to its location, it iscapable of detecting both squat and dive, and to some ex-tent, body roll. But as the sensor is mounted off-center, itssensitivity to roll is about three times less than the sensitivityto squat and dive. In all directions, it can measure bothmovement amplitude and speed of movement, using thesame process as the steering wheel sensor does.

The throttle pedal position sensor is located belowthe dashboard, right next to the pedal mechanism, wherethe pedal can operate its sprung lever as it moves. The sen-sor is a potentiometer with an integrated serial resistor inthe wiper’s circuit.

The entire travel of the potentiometer is quantized into256 steps by the analog-digital converter inside the ECU.The 5 V reference is supplied by the ECU itself. Due to thegas pedal initial position and maximum travel, about 160to 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 connectorblock, right next to the ABS block, at the bottom of the leftfront 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 thedoor frame and in the boot latch. The door switches are allwired 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 (androuted to the boot light and the tailgate opened detectioninput for the status display on the dashboard, too; the dooropen and bonnet open signals for the status display are gen-erated by a separate set of switches, independent of theones used for the suspension).

The usual ignition switch provides a power-on signal,triggering and internal reset and self diagnostic run in theECU. Turning the ignition on and off also triggers internalevents 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 signalscoming from the various sensors. A very interesting and im-portant aspect of the system is that it uses the driver of thecar as a major part of its intelligence, making the operationvery 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) govern-ing when to activate-deactivate the electrovalve switchingthe suspension to either hard or soft mode. These condi-tions can be formulated as rules.

Every main input sensor has an associated rule: when thevalue 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 returnto 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 timeoutstarts all over again.

There are four additional rules overriding the normal op-eration—even if the sensor inputs call for a generic rule tobe 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 until30 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 (includinginconsistent values like no body movement but a vehiclespeed above 30 km/h), the suspension will be switchedto hard mode and stay there until the ignition is turnedoff or the doors are opened with the vehicle speed below30 km/h. The ECU does run a self-diagnostic routinewhen the ignition is turned on but some sensors cannotbe tested at this time, only during normal use;

U whenever the suspension stays in hard mode for morethan one minute, the computer switches to soft modemomentarily to assure the equalization of pressures inthe corner and center spheres. If the circumstances still


call for hard mode, the suspension will revert within50 ms and restart the one-minute timeout period;

U below 30 km/h opening the doors or tailgate overridesany other rules and puts the suspension into soft modeto equalize the pressures in the spheres.

As already mentioned, the steering wheel sensor is usedto derive two inputs values: steering wheel speed and an-gle. These values are treated separately with the purpose ofcalculating the lateral acceleration of the vehicle (vehiclespeed, 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 otherwisebe 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 ofpotential body roll. Body roll is significantly reduced in hardmode, consequently, the rules were set up to ensure thatthe body roll is minimized when there is potential for it, stillthe suspension stays soft to absorb bumps when there is nobody 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 actualspeed is above 30 km/h, the suspension will be switched tohard mode and a timeout of 1.2 seconds begin.

The table below shows the thresholds of steeringwheel angle and rotating speed. If any of these valuesexceed the threshold for the actual vehicle speed, the sus-pension will switch to hard mode; it will revert to soft whenthe corresponding value drops below the threshold for atleast 1 second if the switching was triggered by the steeringwheel angle and 2 seconds if triggered by the rotationalspeed:

Vehicle speed(km/h)

Steering wheelangle(deg)

Vehicle speed(km/h)

Steering wheelspeed(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 fromthe output of the body movement sensor, although thetwo values are used in a different way.

The body movement speed is used as the parameter forthe activation of two types of corrections:U Flat tire correction: if the body movement speed ex-

ceeds 300 mm/s, the suspension switches to hardmode, and all thresholds are modified to 60 mm. Thetimeout of the correction will be 0.4 s.

U Excessive body movement correction: if the bodymovement exceeds 60 mm more than three timeswithin three seconds, the suspension will switch tohard 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 ofthe 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 willforce the suspension into hard more. The computer checksevery 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 secSuspension up > 9 pulses, timeout 1 secSuspension change speed between 30 and 50 ms AND

Durchfederung > 3 pulses, timeout 1 sec

Vehiclespeed(km/h)

Dive(mm)

Squat(mm)

Steeringwh pos(deg)

Vehiclespeed(km/h)

Dive(mm)

Squat(mm)

Steeringwh pos(deg)

< 30 — — — < 30 — — —

The values delivered by the throttle pedal sensor areused 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 toprobable vehicle squat (on acceleration) or dive (on deceler-ation). Both are significantly reduced when the suspensionis in hard mode.

The suspension ECU quantizes the pedal position intofive discrete steps: 0, 30, 40, 50 and 60 percent of the com-plete pedal travel. The computer measures the time elapsedas 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 tosoft if the pedal movement becomes slower for at least theduration of the timeout specified:

Pedal pressspeed (ms)

Timeout(s)

Pedal releasespeed (ms)

Timeout(s)

< 100 1 < 100 1

101–150 2 101–200 2

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

Without ignition and electrical feed to the suspensioncomputer, the electro-valve would immediately return tohard mode. Loading or unloading the car, people getting inor out would induce pressure differences in the hydraulicsystem. These differences would equalize abruptly whenthe system is started again, causing the car to jump or sinkvehemently. In order to avoid this, the computer allows anadditional 30 seconds of timeout starting when any of thedoors is opened or closed (as communicated by the door


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 tosoft mode even with the ignition switch turned off. Earlycars did not have this feature built directly into the com-puter but used an additional relay and circuits. On thosemodels, the constantly energized electro-valve can drainthe battery if the doors remain open for a long time.Starting with the H2 suspension computer (fromORGA 4860, February 28, 1990) the door sensors are ob-served by the ECU itself and the operation is enhanced witha 10-minute timeout period. After this interval, the electro-valves will always return to the hard, non-energized state.

Changing the state of the ignition switch provokes atransition to soft mode for a maximum of 30 seconds;reaching a vehicle speed of 30 km/h will cancel this modeprematurely. When the ignition is turned on, the ECU alsoruns a self-test diagnostic sequence lasting three seconds.

When the suspension selector switch is set to theSport setting, all sensor inputs except for the vehicle speedsensor are ignored. Below 30 km/h the car stays in softmode and switches to permanent hard mode above thisspeed. The suspension status light in the instrumentpanel 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 ofthe ECU self test. If the computer detects any mal-function in the course of this test, the light will flickerone or more times during this period;

U when the suspension is set to Sport, the status lightwill remain lit to inform the driver of the setting cho-sen. The status light actually lights up or extinguishesonly when the suspension rules have been changed inresponse to the mode select switch. This takes a shortwhile because the internal timeouts are reset andsome of the sensors are recalibrated. Because of thisthe light changing state is slightly delayed from themode switch changing state.


Hydractive IIThe second incarnation of the hydractive suspen-sion appeared at February 1, 1993 (ORGA 5929).It was designed to overcome the biggest problemof the previous system, the very uncomfortablehard 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 canswitch to both hard and soft mode as it finds it necessary,

however, when set to Sport, the suspension becomes moresensitive and will sooner and more often switch to the hardmode.

1234

Many models were also fitted with an anti-sink systemthat locks the system when the car is not running, using yetanother sphere. Its only purpose is to keep the car from sink-ing when not used, it does not influence the functioning ofthe suspension system in any way.

return return

return

feed fromsecurity valve

returncontrol fromcomputer nitrogen

LHMmoving parts

suspensioncontrol block

heightcorrector

strut & sphere strut & sphere

5

4 4

3

2

1

The layout of the system (front suspension)

The center sphere circuits and supports were redesigned:they now house the electrovalves and the internal conduitsserving the sphere were modified as well; the new controlblocks connect, as previously, to the left and right cornerspheres, 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 onHydractive I:1 A sphere base;2 A hydraulically controlled isolation valve;3 A ball and piston valve;4 Two 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 inits soft mode, hence, the default electrical position is hard.However, due to the indirect coupling between this valveand the isolation piston 2 inside the control block, the hy-draulics can stay in either position for extended periods oftime with the electric valve disconnected, depending on thepressure 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 ordisconnected.

The two modes are practically the same as on the previ-ous Hydractive system: in soft mode the electro-valve 5opens the feed pressure onto the isolation piston 2 and bymoving it, connects the center sphere 1 to the rest of thesuspension. In hard mode, the electro-valve 5 closes and

XM

Xan

Suspension Hydractive II33

lets the pressure inside the center sphere 1 move the con-trol piston into a position which closes off the center spherecompletely.

The center sphere 1 is now supplied directly from theheight corrector in soft mode. This simplifies the ball valvearrangement 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 aHall-effect sensor now. Its resolution have been doubled to8 pulses generated per rotation, that is approximately 5pulses per meter traveled (although this depends some-what on tire size). It is located on the gearbox where thespeedometer 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 computercontrolled mode (Sport switched the suspension to con-stant hard mode above 30 km/h of vehicle speed), thenewer system has two such regimes of operation: in bothNormal and Sport it dynamically activates the electro-valvesof the suspension control blocks whenever it decides thatthe driving circumstances call for a firmer suspension. Thedifference is in the set of rules the computer uses to evalu-ate those circumstances: the rules are stricter for the Sportsetting, 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 steeringwheel angle. If the value observed by the sensor exceedsthe threshold for the actual vehicle speed and the suspen-sion setting, the suspension will switch to hard mode; it willrevert to soft when the corresponding value drops belowthe threshold for at least 1.5 seconds:

Vehiclespeed(km/h)

Steering wheelangle (deg)

Vehiclespeed(km/h)

Steering wheelangle (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 steeringwheel rotational speed as well:

Vehiclespeed(km/h)

Steering wheelspeed (deg/s)

Vehiclespeed(km/h)

Steering wheelspeed (deg/s)


< 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:

Vehiclespeed(km/h)

Dive(mm)

Squat(mm)

Steeringwh pos(deg)

Vehiclespeed(km/h)

Dive(mm)

Squat(mm)

Steeringwh 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:

strut

return


suspension pressure


strut

securityvalve

heightcorrector

SOFT MODE

53

4

24

1

return


suspension pressure


residual pressuresecurity

valveheightcorrector

strut strut

HARD MODE

53

4

2

1

4


Vehiclespeed(km/h)

Pedal press rate(steps/25 ms)

Vehiclespeed(km/h)

Pedal release rate(steps/25 ms)


< 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 6 4

199 > 7 4.6

With the improved resolution of the vehicle speed sensor,the rules formerly referencing to 30 km/h are changed to24 km/h. Thus, the suspension switches to hard mode if thebrake pressure sensor detects a pressure above 30 barand a vehicle speed in excess of 24 km/h.

Similarly, the suspension will switch to soft mode if theignition switch is turned on, for a maximum of 30 sec-onds, but reaching a vehicle speed of 24 km/h will cancelthis mode prematurely. It will switch to soft also if any dooror the tailgate is opened but the vehicle speed is below24 km/h. The reason for this is to equalize the pressure be-tween all three spheres of an axle. Without it, the centersphere would retain its former pressure and once the vehi-cle exceeds the speed of 24 km/h, opening it would makethe car jump or drop, depending on the actual pressure.

It is important to note that the suspension will switch tosoft mode even with the ignition switch turned off. Shouldthe doors remain open with the ignition switch in the off po-sition, the suspension soft mode will be subjected to a 10-minute timeout period to avoid draining the battery as thesoft mode requires the electric valves to be energized.

Suspension Anti-sink system35

Anti-sink systemMany contemporary Citroëns—including both reg-ular hydropneumatic and Hydractive Xantiae andXMs—have an anti-sink system (SC/MAC) fitted, tokeep the car from lowering when not used. Thesystem does not interfere with the normal func-tioning while in use. It attempts to minimize leaksinside the system by having only one elementthat can leak, the anti-sink valve itself.

The introduction of this anti-sink valve coincided with theappearance 6+2 piston high pressure pump. As the suspen-sion is fed from the smaller, two-piston side of the pump,pumping the car up from the low position would require alot of time (although its performance is perfectly sufficientfor the normal operation once the car is already running).

To avoid this scenario, the anti-sink valves fitted for eachaxle between the height corrector system and the suspen-sion struts (or the hydraulic control block on Hydractive sys-tems) keeps the car body from lowering when the engine isswitched off. The valves operate on the pressure differencesin the system, without any electrical control: when there issignificant pressure in their control circuit, they keep theirwork circuit constantly open.

Under normal circumstances, the high pressure pumpsupplies the pressure regulator and the main accumulatorwith fluid. The output from these two feeds the whole sys-tem with high pressure, going through the security valvewhich keeps the brake circuit constantly under pressure, forobvious reasons of security. If there is enough pressure inthe 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 inthe control circuit of the anti-sink valves. When the carruns, the valves are constantly open, connecting the heightcorrectors 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 turnedoff, 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 mainaccumulator below the suspension pressure, the closinganti-sink valves isolate the suspension struts from the restof the system. It is usually the front valve that closes first asthe front of an unladen car is much heavier due to the en-gine and gearbox. Compared to a non-anti-sink car, theleakage is quite drastically reduced. For instance, a standardXM with its suspension in prime condition takes about 20-30 hours to sink completely, while with the anti-sink systemthis 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 sphereas well. The function of this sphere is to maintain pressurein the braking circuit. As the brake valve is the most leaky el-ement, it could exhaust the pressure between the pistonand the plunger while the remaining pressure behind thepiston (provided the high pressure and the front suspen-sion circuits do not leak that much) stays rather high. In thiscase the anti-sink valve might open again in error—this ad-ditional sphere ensures that this will not happen.

This system maintains the car height by counteractingthe internal leakage of the various suspension element thatwould make the pressure escape back to the reservoir. Ele-ments that are in constant motion—height correctors, forinstance—leak past their seals on purpose to lubricatethemselves. The anti-sink valves—which move very rarely,need no intensive lubrication, thus are manufactured withvery close tolerances and hardly leak themselves—isolate allthe struts from the rest of the system to prevent any possi-ble leakage to reduce the pressure in the struts, allowingthe car to sink.

heightcorrector

heightcorrector

securityvalve

securityvalve

anti-sinkvalve(closed)

anti-sinkvalve(open)

reservoir reservoirREARONLY

REARONLY

rearbrakes

rearbrakes

anti-sinksphere

anti-sinksphere

controlblock

HYDROPNEUMATIC HYDRACTIVE

4

43

5

XM

Xan


Activa SuspensionThe Activa suspension—used only on some Xantiamodels—creates mixed feelings. Drivers requiringsporty handling and roadholding praise it be-cause this car turns into curves without turninga hair: it stays completely horizontal and neutral.However, this comes at the expense of ridecomfort.

The Activa system operates in two distinct steps. The firstone 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 26).

The corrector is connected by an L-shaped spring to thebottom wishbone. When the car takes a sharp left turn, itsfront left wheel will be forced down by the body roll causedby centrifugal force. As the wheel moves down, so does theend 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. Thesetwo cylinders are attached to the wheel suspension differ-ently: in the front, the piston pushes the left wheel upwardswhile 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 stabilizingcylinders back to the reservoir. The front left wheel movesdownwards, the rear right one upwards, once again coun-tering the effect of body roll.

An additional Activa sphere in the front acts as an extraaccumulator 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 ofthe circuit (dashed line on the illustration), or the residualpressure in the sphere moves the piston 2 down, isolatingthe sphere 1.

When the Activa sphere is open to the rest of the system,roll correction is applied through a spring element formedby the accumulator and the Activa sphere. The supply sideof the stabilizing cylinder pistons have half the area ofthe other side, connected to the Activa sphere 1 with thevalve 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 thepiston compresses the gas content in one sphere and at thesame time, expands it in the other.

The stabilizing cylinder works as a spring with asymmetri-cal characteristics: its effective hardness is smaller aroundthe corrected position, but it hardens progressively as thepiston is forced out of that position.

The Activa system has two operating modes, dependingon the position of the electro-valve 2. In the first mode rollcorrection is always active because the roll corrector is up-set. The resulting flow of fluid will tend to move the activelinkage upsetting the balance of presssure in the two extra

reservoir

pressure feed

rollcorrector

feed pressurework pressureleakage return

Activasphere

Activa spherewith control block

frontstabilizing cylinder

electro-valve

ECU

vehicle speedsteering wheel anglesteering wheel speed

anti-rollbar

rearstabilizing cylinder

STRAIGHT-AHEAD

RIGHT TURN

SHARP RIGHT TURN

2

1

Xan

Suspension Activa Suspension37

spheres, and making the coercive force be applied througha spring element which becomes progressively stiffer themore 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 whenthe second mode of anti-roll behavior has to be enforced.Similary to the operation of the suspension computer, theActiva ECU also uses the driver as the input to determinethe motion of the vehicle body: if the roll is caused by theunevenness of the road surface, the steering wheel will notbe 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 bythe steering wheel angle sensor, the mass of the car is aknown constant—the centrifugal force can be calculatedfrom these values) and decides wether the spring elementformed by the two spheres needs to become rigid to makethe system compensate for the body roll.

In this mode the Activa sphere is isolated from the rest ofthe system, the fluid line between the roll corrector and theactive linkage is blocked at both ends, making the linkagecompletely rigid. Even if the roll collector end is open, thelinkage remains quite rigid (providing for a very hard springcoupled with high damping); only half of the displacementescapes from the additional accumulator sphere through arestrictive regulator.

The additional damping of the Activa sphere is nowswitched off, the correction is applied only through the veryhard 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 roll-bar remains functional.

The diagrams showing the kinetic characteristics of anActiva car reveal the details. The first diagram shows the re-lationship between time and roll angle for a constant lateralacceleration. It can be observed clearly that the Hydractivesystem can only limit roll damping, not roll angle. Note thatthe initial slope of both Hydractive curves—the section upto 0.4–0.6 seconds— is practically the same in both softand hard mode. This slope represents the combinedhardness of the roll bar and the associated hydraulic compo-nents. Yet, the reaction time is longer in the soft mode (0.8seconds versus 0.6, indicated by the last bend when thecurve turns into a horizontal line). As the corner spheres areisolated 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 Hydractivesystem with minimum lateral acceleration. But, while theHydractive stays almost linear—the sharper you turn, thebigger the body roll angle will be—, the Activa compen-sates by keeping the body roll angle at a constant below 0.5degree up to a lateral acceleration of 0.6 g (by providing aneffectively infinitely stiff roll bar setup). But even when thelimits of the roll bar are reached, having contracted or ex-tended it as far as it can go, the effective roll bar remainsquite stiff: the roll angle will increase only moderately, up toa maximum of 1 degree.

……

deg

s1.51.00.5 2.0

1

2

3

Activa

Hydractive hard

Hydractive soft

deg

g0.5 0.8

1

2

3

Activa

Hydractive hard


Hydractive 3The new C5 has a new suspension system, doingaway with many solutions used on Citroëns forseveral decades, yet offering the same or evenbetter comfort than before. Recent developmentsin electronics and computics made it possible todelegate many functions previously solved by me-chanical-hydraulical components to electronicunits.

This third generation suspension system retains the samebasic functioning as the previous systems. It also comes intwo flavors: a simpler Hydractive 3 reminiscent of the origi-nal hydropneumatic suspension of the DS–GS–BX–CX anda slightly more complicated Hydractive 3+, building uponthe former Hydractive I and II (actually, Hydractive 3 is nothydractive in the sense we used this term before, its onlyspecial activity is to adjust the road clearance depending onspeed and road condition).

Although the basic functioning is practically the same,the actual layout underwent significant changes. Most im-portantly, the previously mechanically operated heightcorrectors became electronically controlled hydraulic units.And all hydraulic units except for the spheres—which wereredesigned to give unlimited life expectancy—are nowhoused 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 hydraulicpump 1, driven by an electric motor (rotating at 2,300

rpm) operating independently of the engine, runningonly when necessary;

U the hydraulic units, including an accumulator 2 to evenout the pressure pulsations of the pump, four electro-valves 3 and 4 and two hydraulic valves 8 serving theheight regulation and anti-sink behavior, some in-line fil-ters5 and an overpressure valve7 (taking the role of thepressure regulator of previous systems).

U the electronic computer 6, communicating with othercomputers across the multiplex network to read the in-puts of various sensors and to control both the HP pumpmotor and the electrovalves.

In contrast to the height correctors of previous systems, op-erated mechanically via a linkage coupled to the anti-rollbars, 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 linkedfeedback with simple thresholds.

The computer 6 is connected to the CAN multiplex net-work, providing access to the messages sent by the BSI andits fellow computers controlling the engine and the ABS.The inputs the suspension ECU uses comprise of rear andfront 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 rotatingspeed on the Hydractive 3+.

BHI



reservoiranti-sink valves???

rear strut & sphere

rear strut & sphere

LSD feedfront suspensionrear suspensionoperational returnleakage return

height settingbutton

BSI

instrumentpanel

doors andtailgate

Sport switch(3+ only)

brake

rearheightsensor

frontheightsensor

steering wheelangle sensor

(3+ only)

inlet valves with non-return valve???

Engine ECU ABS ECU

7

2

1

5555

3 4 4 35

6

5

C5

Suspension Hydractive 339

As usual with Citroëns, the driver can select from fourheight settings (although the selector is no longer mechani-cally coupled with the hydraulics, it is a simple electronicswitch sending signals to the computer): high, track (plus40 mm), normal and low. The selected setting is displayedon the multifunction screen in the dashboard. The com-puter also prevents unsuitable settings being selected. Thehigh option is not available when the car is traveling fasterthan 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 adjuststhe ground clearance automatically. Below 110 km/h onwell surfaced roads the ride height remains standard but assoon as this speed is exceeded, the vehicle will be loweredby 15 mm at the front and 11 mm at the rear. This changelowers the center of gravity, improving stability, loweringfuel consumption (by reducing drag) and reducing the sen-sibility to crosswinds. The car resumes the standard rideheight when its speed drops below 90 km/h.

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

The general height of the vehicle (filtering out rapidmovements due to suspension travel) is checked, and if nec-essary, adjusted every 10 seconds and when any of thedoors is opened or closed (even with the ignition switchedoff).

Hydractive 3+

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

opens the feed pressure onto the isolation piston 2 and bymoving it, connects the center sphere 1 to the rest of thesuspension, switching the suspension to soft mode. of thesuspension. Closing the electro-valve 4 obstructs the hy-draulic supply coming from the BHI; the residual pressure inthe center sphere 1 moves the isolation piston 2 down-wards into a position which closes off the center spherecompletely: the suspension switches to hard mode.

The suspension has two settings the driver can choosefrom, Normal and Sport. The new stiffness regulators to-gether with the center spheres are isolated in hard modeand re-activated in soft mode in response to the various in-puts received and processed by the suspension ECU. Thefunctioning of the computer is basically similar to theHydractive II ECU: it uses tables and rules to set up thresh-olds on the value on many sensor inputs to determine whento switch to hard mode. Just like on its predecessor, theSport 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 (communicatedon the CAN), the angle and speed of rotation of thesteering wheel (the type of the sensor depends onwhether the car is equipped with ESP, in this case the sensorconnects to the multiplex network instead of directly to thesuspension ECU), the speed of suspension travel (usingthe values of the front and rear height sensors), the open-closed status of the doors (communicated by the BSI) andthe movement of the accelerator pedal or butterfly.

strut

return


suspension pressure


strut

BHI

SOFT MODE

4

3

32

1

strut

return


suspension pressure


strut

BHI

HARD MODE

4

3

32

1

Steering


Power Assisted SteeringThe PAS steering (DIRASS, Direction Assistée)used on Citroëns is not radically different fromsimilar systems on other cars. Naturally, having ahigh pressure hydraulic system at disposal influ-ences the layout.

The fluid requirements of the various hydraulics subsystemsdiffer significantly: while the brakes require only a very littleamount of LHM and the suspension somewhat more, thepower 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 steeringcircuit and the suspension-brake circuits on PAS cars.

The rest is rather simple. A hydraulic ram cylinder ismounted on the rack of a traditional rack-and-pinion steer-ing gear unit. The pressure of the hydraulic fluid supplied toassist the driver in turning the steering wheel is controlledby 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 tolock;

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 systemis shown when it operates with the steering wheel in thestraight-ahead position and the pressure regulator isswitched on. The slide valve 1 inside the flow distributor di-vides the mineral fluid coming from the high pressurepump between the main and the steering hydraulic circuits(the main circuit having priority). Both the distributor4 andthe rotor 5 are in neutral position—the torsion bar be-tween the two is not functioning). Both chambers of theram cylinder are fed without pressure. All the fluid arrivingthrough 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 pressurestarts to rise until it reaches 170 bar again and disconnectsthe feed to the main accumulator. The main slide valve ofthe pressure regulator is connected to the second feedingchannel of the flow distributor. All the fluid supplied by theHP pump now feeds the flow distributor where the slidevalve 2 is responsible for limiting the amount of fluid trans-ported by the control valve. The whole amount of fluid stillreturns 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 distri-butor 4. The control valve closes the path of the fluid com-ing from the flow distributor which no longer is allowed toenter the valve. The pressure begins to rise in the circuit be-tween the control valve and the flow distributor, movingthe slide valve1, which in turn modifies the ratio of fluid, fa-voring the PAS circuit. The fluid will enter the right chamberof the ram cylinder while the left chamber can be emptied

steering rack

steeringwheel

pump

rightwheel

LHM feedhigh pressuresteering pressuresteeringoperational returnleakage return

flow distributor

pressureregulator

controlvalve

suspensionand brakes

reservoir

1

3

2

4

6 5

7

BX

Steering Power Assisted Steering43

into the reservoir via the rotor of the controlvalve. This pressure difference moves the pis-ton 7 to the left inside the cylinder, helpingthe car to make a right turn. If the steeringwheel stays at the right lock, the pressurelimiting valve 3 inside the flow distributormainta ins a max imum pressure of140 bar—when the pressure rises above thisvalue, the fluid pushes the ball of the valvebackwards, sending the excess fluid back tothe reservoir..

When the steering wheel is turned to theleft, the rotor5 rotates in the opposite direc-tion. It starts by cutting of the return of fluidto the main reservoir. The pressure will riseagain 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, thepressed area of the left chamber is twice as large as that ofthe 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 isactually turning the steering wheel. When the rotatingforce on the steering wheel ceases—the driver has finishedturning 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 theneutral position, stopping the power assistance. When thedriver releases the steering wheel back to the straight-ahead position, an opposite operation will start.

Later XMs and Xantiae omit this distributor and use atwo-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 LEFT

reservoir

controlvalve

flow distributor

left chamber right chamberpiston

reservoir

controlvalve

flow distributor

left chamber right chamberpiston

TO RIGHT

4

65

4

6 5

XM


DIRAVI SteeringAnother gem of engineering, the DIRAVI steering,made its debut on the SM, excelled in many CXsand the flagship, V6 XMs (left hand only, thesmall amount sold in the UK never justified the ex-penses of the conversion to RHD).

The DIRAVI (Direction Rappel Asservi, Steering with LimitingCounterforce) steering is as unique as the hydropneumaticsuspension—it was never used by any other manufacturer,although its excellence over conventional power assistedsystems speaks for itself.

As usual, it has some quirks confusing the average driverduring their first meeting. First of all, it is geared very high:it only took two turns of the steering wheel from lock tolock (one turn for each side) to steer on the SM. Later mod-els, the CX and the XM retained this feature although thenumber of turns was larger (2.5 and 3.3). The gear ratiocould 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 wheelcompletely). The final solution was a compromise to reducethe initial strangeness of the steering for the drivers alreadyaccustomed to traditional systems.

Certainly, making the gearing so high is not complicatedin itself but a conventional (even power assisted) systemwith such rapid a response would be unusable. As the carobviously has to have a similar turning circle as other cars,too responsive a steering would mean that even the slight-est movement of the steering wheel would induce excessivedeviation 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 veryeasy to use it during parking, yet it offers exceptional stabil-ity at high speeds: it actually runs like a train on its rails, re-quiring a sensible amount of force on the steering wheel todeviate it from the straight line. And an additional feature:the steering wheel (and the roadwheels, naturally) centerthemselves even if the car is stationary.

Second, there is no feeling of feedback from the roadthrough the steering wheel. Other steering systems have aconstant mechanical connection between the steeringwheel and the roadwheels, the DIRASS only adds someforce to the one exerted by the driver. DIRAVI is different:simply put, the usual path between the steering wheel andthe steering rack is divided into two halves, with a hydraulicunit in the middle. When the driver turns the steeringwheel, 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 mechanicsworks in the opposite direction, as a negative feedback, re-turning the hydraulic system to the neutral position as soonas the wheels reach the required direction. The hydraulic cyl-inder and the wheels become locked, no bump or potholecan deviate them from their determined direction. Notethat 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 oftime. Letting the steering wheel rotate back or turning it fur-ther in the previous direction will initiate a new mechanical-hydraulical cycle as described above.

steering rack

steeringwheel

centeringpressure

regulator

steeringcentering

device

high pressure

gearbox

rightwheel

LHM feed

steering pressure

centering pressure

operational return

leakage return

steering feedback

normal steering

leftwheel

steeringcontrolunit

R L

reservoir

adjustment cam

29

1

3

5

4

7

Û6

8

CX

SM

Steering DIRAVI Steering45

Thus, the lower mechanical link, the feedback from theroadwheels 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 aditch. The driver has to learn to feel the behavior of the carvia other sensory means and this is probably the main rea-son why anyone not prepared for a period of learning willimmediately dislike DIRAVI. But once accustomed to the sys-tem, it is more ergonomic and stress-free than any othersteering 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 theleft and right sides of the piston are different—the left oneis twice as large as the right one—, thus to keep the pistonin neutral position, the right hand side must have twice asmuch hydraulic pressure than the left hand side. As this sideis 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 steeringcolumn. It has a coupling4 inside which is very loosely con-nected, with a significant amount of free play (nearly 30 de-grees). Under normal circumstances this coupling stays inthe middle, so the free play is irrelevant but it serves as a me-chanical backup for safety if there would be any failure inthe hydraulic system. In this case, the car can be steered me-chanically, although much heavier and with a large freeplay on the steering wheel.

The main illustration shows the steering system with thesteering wheel in the straight-ahead position. When thedriver rotates the steering wheel, the steering column turnsthe gear 1 inside the control unit. The set of levers 9 at-tached to this wheel transform the relative rotation (relativeto 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 aslarge as on the other side, thus the resulting higher forcewill 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 theleft chamber of the cylinder back to the reservoir. As theright chamber is still under the constant pressure, the result-ing force moves the rack to the left, thus the car starts toturn to the right.

As we have already mentioned, the moving steering rackrotates the pinion and—through the steering feedback—the cogwheel 2. The levers linking this gear to the valve 3now work in the opposite direction, returning the valve toits 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 theclosed valve3, the steering gear and the roadwheels are hy-draulically locked, resulting in high turning stability.

To make the steering progressively heavier as the speedof the vehicle increases, the steering centering pressureregulator—a centrifugal device—is driven by a cable fromthe gearbox. Its spinning weights open up a slide valve8 ad-mitting some fluid from the high pressure circuit into thecentering device, or closes it to drain the extra fluid back tothe reservoir.

The faster the car runs, the bigger is this hydraulic pres-sure sent to the steering wheel centering device. Thisconsists of an eccentric cam5 geared to the steering wheelside of the unit, with a ratio making it turn less than a fullturn while the steering wheel is rotated from lock to lock. Apiston 6 forced down by the mentioned hydraulic pressurepushes a roller7 against this cam. Being eccentric, the onlystable position is when the cam is centered. The centeringforce can be regulated by changing the hydraulic pressurebehind the piston.

The hydraulic pressure behind the piston 6—being de-pendent on the vehicle speed—represents the progressivecounter-force needed to make the steering graduallyheavier at highway speeds. In addition, it returns the steer-ing gear to the neutral, straight-ahead position when thedriver releases the steering wheel. While the wheels of aDIRASS car return to the center themselves, forcing the rackand 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 in-stead, 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 7and the eccentric cam 5. The fluidleaves the chamber through the ballvalve now opened. While this pistonmoves upwards, it compresses thespring, which in turn pushes the up-per piston slightly up, freeing the cali-brated bore Û.

As soon as the driver releases thesteering wheel, the opposite of theprevious operation takes place. Theball valve will be closed by the enter-ing fluid, thus the LHM has to gothrough the center bore of the upperpiston, leaving via the calibratedbore Û. Due to this resistance, it car-ries the upper piston down slightly,compressing the spring. This downward force pushes thelower piston together with the roller 7 down, and thetorque exerted on the eccentric cam 5 forces that to rotateback into its neutral position, returning the complete steer-ing gear to the straight-ahead position. At the end, thespring will return the upper piston to its original position in-side the centering device. The restriction of the bore Ûkeeps the steering wheel from returning to the center posi-tion too fast.

The last component is an adjustment cam allowing theadjustment of the pinion relative to the disk on the pinionend of the steering column.

RL

reservoir reservoir

HPrack

HPrack

TO LEFT TO RIGHT

STEERING

RETURNING

XM


Self-steering Rearsasasa

Brakes


Standard braking systemThe brake compensator valve is the most compli-cated part of the hydropneumatic suspension.This component is not only responsible for the op-eration of the brakes but forms a central part ofthe whole system.

With no pressure applied on the pedal, the brake valve con-nects both the front and the rear brake circuits to the opera-tional return, ensuring that the brake pads retract from thedisks. Note that both braking pistons (4 and 5) have inter-nal conduits (drawn as Y-shaped lines) offering a passagefor the brake pressure to escape into the return line:

At the same time, the pressure coming from the rear sus-pension exerts pressure on the right hand side of the pis-ton1. As soon as this pressure exceeds a given (albeit quitelow) level, the piston will be forced to the left. The cup 2will be forced to the left as well which, in turn, will com-press the spring 3. As soon as the piston 1 starts to move,the lip on its the face will move away from the edge of thehousing, effectively increasing the area the hydraulic liquidcan exert its pressure upon. This setup introduces some hys-teresis in order to prevent unwanted oscillations in the sys-tem: although the piston1 and the cup2moves as soon asthe pressure exceeds the threshold level, it stays in that posi-tion even if the pressure drops a little:

The reason for this is to lower the counter force on pis-ton4, forming the valve for the rear brakes. This makes the

rear brakes extremely sensitive at the very beginning of brak-ing, minimize their delay in order to allow the anti-dive be-havior of the rear trailing arm suspension to take effect.

As soon as the driver starts to brake by pressing on thebrake pedal, which in turn presses the front piston 5 be-hind the rubber boot at the end of the unit, the feed fromthe main accumulator becomes connected to the frontbrake circuit:

In addition, the high pressure liquid will also pass throughthe channel in the middle of the front piston 5 and entersthe chamber between the front 5 and rear 4 pistons. Thepressure of the LHM will push the rear piston 4 to the left,too, which in turn activates the rear brakes:

The presence of pressure in the rear suspension has keptthe piston1 and cup2 pushed back to the left (against theforce of the spring3) until now. But the increasing pressurein the rear brake circuit, being present on the left side of thepiston 1 as well, will exert a counter-force, forcing the pis-ton 1 to the right. Via the passage in the rear piston 4, thepressurized LHM of the rear suspension enters the chamberbetween that and the piston 1 (all these pistons havelipped faces to prevent unwanted oscillations by introduc-ing hysteresis action, as described above).

We have discussed piston1 as a single component so farbut in reality, it is made of two parts. We introduce part 6here so that we can refer to its right side (forming a cham-ber with the left side of the rear piston 4). In the following

BX

returnrearsuspension

mainaccu

brakepedal

frontbrakes

rearbrakes

bleeding

3

3

2

1 4 5


mainaccu

brakepedal

frontbrakes

rearbrakes

bleeding

32

3

1 4 5


mainaccu

brakepedal

frontbrakes

rearbrakes

bleeding

3

3

2

1 4 5


mainaccu

brakepedal

frontbrakes

rearbrakes

bleeding

3

3

2

1 4 5

Brakes Standard braking system49

formulae, piston 1 will only denote the part in direct touchwith the pressure coming from the rear suspension. Thus,the force on the left side of the piston 1 comes from boththe spring3 and the braking pressure (blue) exerted on theleft side of piston 1:

F = F +p Aleft brake left3 1

As we can see from the cross section of the valve, the com-bined areas of 6 and 1 right are equal to that of 1 left:

( )F = F +p A + Aleft brake right right3 1 6

Similarly, the force on the right comes from the suspensionpressure (green) on the right side of piston 1 as well as thebraking pressure (blue) on the right side of piston 6 (arriv-ing via the channel in the rear piston 4):

F = p A +p Aright rear right brake right1 6

Brake force regulation is performed by the movement of pis-tons 16 until an equilibrium of forces is obtained on theirtwo sides, and the consequent influence on pistons 4 and5. Limiting of rear brake pressure with respect to rear sus-pension pressure is insured by the force generated from thepressure in the rear brake circuit being assisted by thespring 3. Because of this spring, that pressure is alwayssmaller by an amount proportional to the spring tension:

F = Fleft right

F +p A +p A = p A +pbrake right brake right rear right brake3 1 6 1 A right6

p A = p A – Fbrake right rear right1 1 3

p = p –F

Abrake rearright

3

1

The balance of forces depend on the spring, as well as thesuspension and brake pressures. Limiting occurs wheneverthe rise in rear brake pressure causes the pistons 16 moveto the right:

As the LHM trapped between the rear 4 and front 5 pis-tons is not compressible, the second one will also move tothe right, providing a counter-force to the pressure of thedriver’s foot. As this movement will partially close the pas-sage between the main pressure feed and the front brakes,the pressure in the front circuit is also forced to reach anequilibrium between the force applied to the brake pedaland that of pushing the piston 5 to the right.

The springs between the pistons 6, 4 and 5 are not tak-ing part in this process. They are small, soft springs, aiming

only to center the rear piston 4 between the other two buttheir effect is otherwise negligible.

From this description, we can draw a few conclusions.First of all, the maximum rear brake pressure can never ex-ceed that of the rear suspension. Consequently, if there isno rear suspension pressure, the rear wheels do not brake.

When the driver starts to brake, the rear brakes will biteimmediately as the suspension pressure overpowers thespring 3, pushing the pistons 16 to the left and allowingthe rear piston 4 to open quickly and completely. This pis-ton will also balance the forces in the front and rear brakecircuit, so, during this initial braking period, the rear brakesoperate just like the front ones. As the rear brake pressurebuilds up, it will always be dynamically limited with respectto the rear suspension pressure, as described by the for-mula above.

Stop breaking, please…

The brake compensator valve used on early CXs was slightlydifferent from the one described above, withouth the built-in brake force limiting effect of the piston 1 and the cup 2.These CX Break models had a separate rear brake forcelimiter to provide this functionality.

When there is no pres-sure in the rear suspension(for instance, the suspen-sion is set to low), the forceof the spring 4 keeps thepiston in the neutral posi-tion, completely closingthe feed to the rear brakesfrom the brake compensa-tor valve.

When the suspension isunder normal pressure, theforce 1 supplied by therear suspension fluid ex-ceeds the counter force 2provided by the spring. Thepiston stays in the open po-sition, letting the fluid passto the rear brakes. As soonas the driver starts braking,the force2 increases by theadditional pressure comingfrom the front brakes, viathe LHM entering throughthe ball valve 3.

As soon as the incoming front brake pressure exceedsthe rear suspension pressure by more than 28 bar (in otherwords, the coymbined force of front pressure and that ofthe calibrated spring 4 becomes larger than the rear sus-pension pressure), the piston moves again to the left, cut-ting out the additional pressure to the rear brakes, whichwill then continue to brake with this constant pressure.

To avoid a sudden cut-off of pressure, the ball valve 3 iscombined with a damping element 5 to smoothen thechanges. This element is a coil made of hydraulic piping,closed at the far end. The elasticity of the pipe and the coilacts as a small hydraulic spring capable of lowering the reso-nance of the chamber, hindering unwanted oscillations.

rearbrakes

brakevalve

rearsuspension

frontbrakes

21

WITHOUTPRESSURE

rearbrakes

brakevalve

rearsuspension

frontbrakes

21

BRAKING

5

4

3

5

4

3


mainaccu

brakepedal

frontbrakes

rearbrakes

bleeding

54612

3

3

CX


Anti-lock Braking SystemModels with higher performance level came fittedwith ABS.

The principle of operation is the same as on cars with con-ventional braking systems but the layout is much simpler asall we need to control the operating pressure of the brakesare a few electro-valves.

During breaking, the ABS computer monitors thechanges 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 thebrake 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 thanthe others, the ABS reduces the hydraulic pressure fed tothe brake caliper of the wheel in question to avoid thewheel being locked. Although every wheel has its own sen-sor, the rear brake calipers receive the same pressure, onlythe 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 three-unit 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 theneed for the intervention of the ABS computer, the brakesoperate in phase 1: the inlet valve 1 is open but the returnvalve 2 is closed. The braking functions as in a system with-out ABS: the incoming hydraulic pressure is directly routedto the brake caliper.

Under constant breaking (phase 2) both valves close tomaintain 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 closeswhile the return valve2 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 1in a short while.

The ABS computer has a built-in diagnostic feature,checking the components both when the ignition is turnedon and during braking. Any failure will be reported by awarning 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 mechani-cal default mode is phase 1—the normal braking—for allthree hydraulic blocks. Any failure in the ABS system willtherefore revert it to the usual, non-assisted braking.

Early CXs has a slightly different ABS system. The generallayout is the same, but the hydraulic block only has threevalves, one for each brake circuit, however, they have threepositions. Without energizing current, they route the fluidcoming from the brake accumulator to the brakes. In phase2, a medium current switches them to isolate the brake cali-pers, while a larger current opens it completely to let thepressure escape from the brakes into the return lines.

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

securityvalvebrake

valve

front brake

front brake

rear brake

rear brake

rearsuspension

ABShydraulicblock

ECU

wheelsensor

return

... ... ...

Brake sphere(CX only)

brake

ECU

brake

ECU

brake

ECU

returnreturnreturnvalve valve valve

CX valve?

PHASE 1 PHASE 2 PHASE 3

1 2 1 2 1 2

BXCX

XM

ElectricalSystems


Multiplex networkCircuit layouts already universally adopted in com-puters finally made their way into contemporarycars. Although their functioning might be fright-eningly complex for people used to traditional cir-cuits, they actually make the cabling very simpleand the addition of component interactions possi-ble in ways never experienced before.

Conventionally, cars used individual wires connecting thevarious elements—steadily increasing in number—onboard. The huge amount of wires, connectors, wiring har-nesses were a constant source of connection problems. Thevarious circuits were largely independent (sharing only thefeed and the ground), although some components had tointeract (for instance, fog lights should work only when theheadlights are switched on), necessitating connections be-tween the various components (usually using some kind ofa 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, somefunctions were even built in parallel. Several subsystemsmight rely on the signal sent by a coolant temperature or avehicle speed sensor but it was simpler for the manufactur-ers 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 laterused on newer models like the Xsara Picasso or the C5 intro-duces a radically different concept: just like in the computerused to read this book, there is a central backbone circuitcalled bus which goes around the whole car—actually,there are four of them, a Controller Area Network(CAN) and three Vehicle Area Networks (VANs), dealingwith different areas: the CAN is only responsible for the con-nection between the central unit and the engine, gearboxand suspension computers, the VANs for the rest of the sys-tems: the first serves the safety systems like the airbag, thesecond the various doors (including the sunroof) and theanti-theft system, the third everything else: the instrumenta-tion and the comfort gadgets.

The bus—in contrast to the traditional wiring harnesseshosting many individual wires running side by side to servedifferent components—is a common channel of informa-tion flow for all components connecting to it. It uses onlytwo wires which all associated components connect to inparallel (in addition to this, the devices are connected to theground as usual; the two input wires serve as a safety mea-sure, using them both makes the system resistant to anyoutside interference, and the whole system remains func-tional even if one of the bus wires becomes broken, shortedto ground or positive feed). There is no special controller orowner of this bus, each device connecting to it is free tosend or receive messages and commands to the others, at arather high speed (approximately ??? messages per sec-ond).

Suspension ECU

CAN (engine)

VAN 1 (safety)

VAN 2 (doors)

VAN Comfort

Autobox ECU

Engine ECUBSI

ABS ECU

fuse box

Airbag ECU

Sterring wheeland columnswitches

Door module

Door module

Sunroof

Anti-theft

Diesel additive

Instrumentpanel

Multifunctiondisplay

Radio/CD

Navigation

Aircon

Parkingassistance

Buses in the C5

XM

C5

Xs

Electrical Systems Multiplex network53

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 itsown address, a unique numerical identifier—for in-stance, the fuel level sensor has the address 4315, theinstrument 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 totheir 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 offuel level—from the message and comparing it to the previ-ously known value. As the amount of fuel is not supposedto change drastically from one moment to the other, it dis-cards the new value if it differs too much from the previousone.

If the new value is acceptable, the BSI emits another mes-sage of its own, addressed to the instrument panel thistime. As the instrument panel receives this second mes-sage, it extracts the data representing the amount of fuelleft 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 messagessent to all devices, without specifying a single addressee)—actually, the instrument panel saw the original messagecoming from the level sensor as well but ignored it, it onlyacted when the second message, sent by the BSI and ad-dressed specifically to it, arrived.

All components work in a similar way. Some are simpleenough to send a few simple messages (like sensors orswitches) or to receive only a few ones (like electric windowmotors). Others are complex subsystems themselves, likethe suspension, observing the input from a large number ofsensors and performing complex operations. But as theyare all connected to a common bus, the possibility of inter-action is already there. Whether the headlights light up, theelectric windows close and the wiper starts to work in caseof rain, or whether the passenger side external rear view mir-ror folds down when engaging reverse gear have all be-come simple questions of software written for the centralunit. Adding a new feature does not require building a sin-gle extra wire or connection, just to add a few lines to thesoftware.

Center of Attention

The four networks all connect to the central unit, the Built-in Systems Interface (BSI). This control unit manages theflow of information between the networks (many of the

messages generated in one network has to be relayed toanother, just one example is the suspension computer—connected to CAN—being interested in messages aboutthe open or closed position of the doors—communicatedon 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 tocheck, test and configure the whole system.

The multiplex system switches to an energy-saving lowpower mode whenever possible.

Air Conditioning


Air conditioningOnce considered pure luxury, air conditioning andother forms of climate control have became stan-dard items on today’s car. After all, creating anacceptable environment for the driver is morethan a mere question of comfort, it contributes tosafety to a great extent.

There were several climate control systems fitted to ourCitroëns, offering various degrees of automation of keep-ing the climatic conditions inside the car. The system can bemanual, 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 onthe dashboard is marked in degrees instead of just blue andred. The direction and recirculation controls are indenticalto the manual system. The automatic climate control looksradically different, with a controlling panel using buttonsand a digital temperature display.

The AC system in the XM is fairly simple. If it is on, the airis always cooled to about 8–10 °C on the inlet side (this isvaried between the air intake from outside andrecirculation from the inside) and then if you set a highertemperature, it's reheated. The heater also always works, itseffect is only regulated by allowing air to flow or not to flowthrough it (this is what the flap valve does). The air alwaysflows through the AC heat exchanger. As a result, the ACalso dries out the air whenever it is on. Once the air passesout of the temperature regulating flap valve, another flapvalve regulates where it goes inside the cabin. That's reallyall 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 ECUactually only provides a signal to a relay that switches theAC system on by operating the electric clutch on the com-pressor. This same signal switches the radiator fans on tothe low speed. The AC system in turn sends a 'fans to fullspeed' signal to the fan controller, when the coolant temper-ature reaches a trip point (this is handled by a differentswitch 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), theAC 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 temperatureof the fluid (more precisely, a mixture of liquid and vapor)suddenly drops because of the drop in pressure. It entersthe heat exchanger which operates like a radiator, cool-ing the air and heating itself up. The fluid then goes back tothe drier-radiator-compressor end of the loop. The con-

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

As the air always enters through the heat exchanger, andwhether it gets cooled at this point, depends only onwhether the compressor is working or not. The tempera-ture flap only decides which part of the air is going to betaken before or after the heater radiator. This is how thetemperature 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 thetemperature set on the controls. Since the temperature isregulated by the temperature flap, it has really nothing todo with the compressor at all—the only consequence of thecompressor not working (for any reason) is that the systemwill obviously not be able to produce a temperature lowerthan ambient.

There are four sensors providing input. The first one is atthe entrance of the air, before the heat exchanger, the sec-ond one after the temperature flap, the third one on theroof, and the last one in the heat exchanger. They have verydifferent but sometimes overlapping roles.

The first three collectively influence temperature regula-tion. In particular, the sensors after the temperature flapand 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 bemoved to prevent extremely fast changes in temperature inthe cabin. This does not alway work very well, which is whyyou get a blast of air when the system is set on auto andyou leave the car in the sun in summer. Both of these param-eters (temperature and temperature difference) influencethe fan speed.

compressor

condenser

pressurerelease

valve

evaporator

receiverdryer

ambient air

cooled air

refrig. fluid, cool

refrig. fluid, warm

XM

Air Conditioning Air conditioning57

The sensor before and in the heat exchanger as well asthe teperature selection, influence the AC part, i.e. the oper-ation of the compressor. For instance, the compressor willnot operate below a certain external temperature. Also, itwill not operate if the temperature is set to maximum.

When the system is cooling the incoming air, it needs tohave the exchanger at a temperature which is lower thanthe ambient air temperature, obviously. As the compressoreither 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 startcooling the heat exchanger. How cold it will get, dependson how hot the incoming air is and how much air is comingin. In any case, when it gets significantly colder than the in-coming air, the moisture from the air will start to condenseon the heat exchanger, which is why there is a collectorunderneath it and a drip outlet. If the compressor keeps onworking, while the heat that needs to be taken from the airis lower from the heat transfer ability of the whole system,the heat exchanger will continue to progressively getcolder. If nothing is done, it will get well below freezing (itcan go as low as –40 °C given proper fluid, and of courseconstruction designed for this). What will happen then isthat the condensed water from the air will start freezing onthe heat exchanger fins, and eventually, the whole thingwill become a solid block of ice (usually there will be a crack-ling noise to acompany the event), preventing actual airflow. If the condition persists, the pressure in the systemwill build up until the valve in the evaporator opens, and bythis time it is possible that the fluid actually gets heated upenough that the remaining part going through the heatexchanger will actually melt the ice producing a fog (I'veseen it happen!). All of this will be the lucky turn of events,asuming the ice has not cracked the heat exchanger andthat there is no fluid leak.

So, obviously, there is a sensor, and that's the fourth onein this story, which detects the temperature of the heatexchanger becoming too low. When that happens, the com-pressor is cut out, until the heat exchanger temperaturerises 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, whichwould 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 ACswitch is turned on, this is probably some ECU feature. Thecompressor will never turn on if the gas pressure is insuffi-cient, and this part is handled by the pressure switch on thedrier, and has nothing to do with the ECU. In fact, the ECUonly gives the whole system a 'go-ahead'.

Appendix


ORGA number

This number shows the day when your car was actually as-sembled on the production line. The dealers and partsstores 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 indifferent locations. It is on the top of the left hand suspen-sion turret on Visas, C15s, AXs and CXs (often hidden bythe wiring harness). BXs and XMs have it stamped on theleft hand front door A-pillar, above the courtesy lightswitch. On the GSA you find it on the inner right frontwing. Xantiae switched to the other side: the number canbe found on the bulkhead just in front of the right suspen-sion sphere. C5 ?????????????

Calculating the production date is very easy using the fol-lowing table. Locate the largest number in the table still lessthan 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 subtractthis number from your organization number to get the dayof 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 happenswhen your organization number is not greater than butequal to the number in the table), simply take the last dayof the previous month. For instance, for the organizationnumber 5013 the largest number in the table is 5013 inthe cell August 1990, subtraction results in zero, hencethe 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

1984 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

1990 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


IndexAAV . . . . . . . . . . . . . . . . . . . . . . . . . . 8ABS. . . . . . . . . . . . . . . . . . . . . . . . 25, 50

computer. . . . . . . . . . . . . . . . . . . . . . 50hydraulic block . . . . . . . . . . . . . . . . . . . 50sensor . . . . . . . . . . . . . . . . . . . . . . . 50

AC . . . . . . . . . . . . . . . . . . . . . . . . . . 56accelerator pedal . . . . . . . . . . . . . . . . . 13, 17Activa . . . . . . . . . . . . . . . . . . . . . . . 36-37

computer. . . . . . . . . . . . . . . . . . . . . . 37electro-valve . . . . . . . . . . . . . . . . . . . . 36roll corrector . . . . . . . . . . . . . . . . . . . . 36sphere . . . . . . . . . . . . . . . . . . . . . . . 36stabilizing cylinders . . . . . . . . . . . . . . . . . 36

AFS . . . . . . . . . . . . . . . . . . . . . . . . . 6, 8air conditioning . . . . . . . . . . . . . . . . . . 56-57air flow sensor . . . . . . . . . . . . . . . . . . . . . 6air temperature sensor . . . . . . . . . . . . . . . . . 7anti-dive behavior . . . . . . . . . . . . . . . . . . 23anti-roll bar. . . . . . . . . . . . . . . . . . . . . . 24anti-sink . . . . . . . . . . . . . . . . . . . . . . . 35

sphere . . . . . . . . . . . . . . . . . . . . . . . 35valve . . . . . . . . . . . . . . . . . . . . . . . . 35

ATS . . . . . . . . . . . . . . . . . . . . . . . . . . 7autotransformer . . . . . . . . . . . . . . . . . . . . 9auxiliary air valve . . . . . . . . . . . . . . . . . . . . 8ball and piston valve. . . . . . . . . . . . . . . . 27, 32BHI . . . . . . . . . . . . . . . . . . . . . . . . . . 38body movement sensor . . . . . . . . . . . . 29-30, 33brake

compensator valve. . . . . . . . . . . . . . 26, 48-49force limiter . . . . . . . . . . . . . . . . . . . . 49front cylinders . . . . . . . . . . . . . . . . . . . 26pressure sensor . . . . . . . . . . . . . . . 29-30, 34

broadcast messages . . . . . . . . . . . . . . . . . 53BSI . . . . . . . . . . . . . . . . . . . . . . . . . . 53Built-in Hydroelectronic Interface . . . . . . . . . . . 38Built-in Systems Interface . . . . . . . . . . . . . . . 53bus . . . . . . . . . . . . . . . . . . . . . . . . . . 52CAN . . . . . . . . . . . . . . . . . . . . . . . 38, 52CAS . . . . . . . . . . . . . . . . . . . . . . . . . . 9catalytic converter

diesel . . . . . . . . . . . . . . . . . . . . . . . . 18gasoline . . . . . . . . . . . . . . . . . . . . . . 10

chip tuning . . . . . . . . . . . . . . . . . . . . . . 6climate control . . . . . . . . . . . . . . . . . . . . 56cold start injector. . . . . . . . . . . . . . . . . . . . 8compressor . . . . . . . . . . . . . . . . . . . . . . 56Controller Area Network . . . . . . . . . . . . . . . 52coolant temperature sensor . . . . . . . . . . . . . . 7crank angle sensor . . . . . . . . . . . . . . . . . . . 9CSV . . . . . . . . . . . . . . . . . . . . . . . . . . 8CTS . . . . . . . . . . . . . . . . . . . . . . . . . 7-8damping elements. . . . . . . . . . . . . . . 26-27, 32DI . . . . . . . . . . . . . . . . . . . . . . . . . . 19diesel combustion . . . . . . . . . . . . . . . . . . 12

DIRASS . . . . . . . . . . . . . . . . . . . . . . 25, 42DIRAVI . . . . . . . . . . . . . . . . . . . . . . 44-45direct injection . . . . . . . . . . . . . . . . . . . . 19distributor

diesel . . . . . . . . . . . . . . . . . . . . . . . . 12ignition . . . . . . . . . . . . . . . . . . . . . . . 9

door/tailgate open sensor . . . . . . . . . . . 29-30, 34EDC . . . . . . . . . . . . . . . . . . . . . . . . . 17EFI . . . . . . . . . . . . . . . . . . . . . . . . . . . 6EGR. . . . . . . . . . . . . . . . . . . . . . . . 16, 19electronic diesel control . . . . . . . . . . . . . . . . 17EMS . . . . . . . . . . . . . . . . . . . . . . . . . . 9engine management system . . . . . . . . . . . . . . 9engine runaway . . . . . . . . . . . . . . . . . . . 14evaporator . . . . . . . . . . . . . . . . . . . . . . 56exhaust gas recycling . . . . . . . . . . . . . . . . . 16flow distributor . . . . . . . . . . . . . . . . . . . . 42fuel cut-off

diesel . . . . . . . . . . . . . . . . . . . . . . . . 18gasoline . . . . . . . . . . . . . . . . . . . . . . . 8

fuel filtergasoline . . . . . . . . . . . . . . . . . . . . . . . 8

fuel injectiondiesel, direct . . . . . . . . . . . . . . . . . . . . 19diesel, electronic . . . . . . . . . . . . . . . . . . 17diesel, mechanical . . . . . . . . . . . . . . . . . 12gasoline, electronic . . . . . . . . . . . . . . . . . 6

fuel pumpdiesel . . . . . . . . . . . . . . . . . . . . . . . . 12gasoline . . . . . . . . . . . . . . . . . . . . . . . 8

fuel stop valve . . . . . . . . . . . . . . . . . . . . 12fuel supply

gasoline . . . . . . . . . . . . . . . . . . . . . . . 8fuel tank

gasoline . . . . . . . . . . . . . . . . . . . . . . . 8glow plug . . . . . . . . . . . . . . . . . . . . . . 15ground clearance . . . . . . . . . . . . . . . . . . . 23HDI . . . . . . . . . . . . . . . . . . . . . . . . . . 19heat exchanger . . . . . . . . . . . . . . . . . . . . 56heated wire AFS . . . . . . . . . . . . . . . . . . 6, 17height corrector . . . . . . . . . . . . . . . . 26, 29, 32

front . . . . . . . . . . . . . . . . . . . . . . . . 26high pressure pump

diesel . . . . . . . . . . . . . . . . . . . . . . . . 12hydraulics . . . . . . . . . . . . . . . . . . 25, 38, 43

HP brakes . . . . . . . . . . . . . . . . . . . . . 48-49Hydractive 3. . . . . . . . . . . . . . . . . . . . 38-39Hydractive I . . . . . . . . . . . . . . . . . . 27, 29, 31Hydractive II. . . . . . . . . . . . . . . . . . . . 32-33Hydractive sphere . . . . . . . . . . . . . . . . . 27, 32Hydractive valve

electric . . . . . . . . . . . . . . . . . . . . . 27, 32hydraulic . . . . . . . . . . . . . . . . . . . . 27, 32

ICSM. . . . . . . . . . . . . . . . . . . . . . . . . . 7idle control stepper motor . . . . . . . . . . . . . . . 7

Index63

Idle speed . . . . . . . . . . . . . . . . . . . . . . . 7idle speed control valve . . . . . . . . . . . . . . . . . 7ignition coil . . . . . . . . . . . . . . . . . . . . . . 9ignition delay

diesel . . . . . . . . . . . . . . . . . . . . . . . . 14gasoline . . . . . . . . . . . . . . . . . . . . . . . 9

ignition key switch . . . . . . . . . . . . . . . . . . . 7ignition switch . . . . . . . . . . . . . . . . 29, 31, 34injection adjuster . . . . . . . . . . . . . . . 12, 14, 18injection delay

diesel . . . . . . . . . . . . . . . . . . . . . . . . 14gasoline . . . . . . . . . . . . . . . . . . . . . . . 8

injectordiesel . . . . . . . . . . . . . . . . . . . . . . . . 15gasoline . . . . . . . . . . . . . . . . . . . . . . . 8

injector needle movement sensor . . . . . . . . . . . 18inlet manifold . . . . . . . . . . . . . . . . . . . . 8-9intercooler . . . . . . . . . . . . . . . . . . . . . . 15ISCV . . . . . . . . . . . . . . . . . . . . . . . . . . 7knock sensor . . . . . . . . . . . . . . . . . . . . . 10KS . . . . . . . . . . . . . . . . . . . . . . . . . . 10lambda

ratio . . . . . . . . . . . . . . . . . . . . . 6, 10, 18sensor . . . . . . . . . . . . . . . . . . . . . . . 10

LDS. . . . . . . . . . . . . . . . . . . . . . . . . . 38lead in gasoline . . . . . . . . . . . . . . . . . . . . 10leak returns . . . . . . . . . . . . . . . . . . . . . . 25main accumulator. . . . . . . . . . . . . . . . . 25, 42manifold absolute pressure . . . . . . . . . . . . . . . 6MAP sensor . . . . . . . . . . . . . . . . . . . . . . 6monopoint EFI/EMS . . . . . . . . . . . . . . . . . . 8multiplex message . . . . . . . . . . . . . . . . . . 53multiplex wiring . . . . . . . . . . . . . . . . . . . 52multipoint EFI/EMS. . . . . . . . . . . . . . . . . . . 8ORGA number . . . . . . . . . . . . . . . . . . . . 60OS . . . . . . . . . . . . . . . . . . . . . . . . . . 10oxygen sensor . . . . . . . . . . . . . . . . . . . . 10particulates . . . . . . . . . . . . . . . . . . . . 14, 18PAS . . . . . . . . . . . . . . . . . . . . . . . . 25, 42piston and ball valve . . . . . . . . . . . . . . . . . 28post-glowing . . . . . . . . . . . . . . . . . . . . . 16power assisted steering . . . . . . . . . . . . 25, 42-43power steering . . . . . . . . . . . . . . . . . . 25, 44prechamber . . . . . . . . . . . . . . . . . . . . . 12pressure regulator

EFI . . . . . . . . . . . . . . . . . . . . . . . . . . 8hydraulics. . . . . . . . . . . . . . . . . . . . 25, 42

regulator . . . . . . . . . . . . . . . . . . . 12-13, 17reservoir

hydraulics . . . . . . . . . . . . . . . . . . . . . 25RP number . . . . . . . . . . . . . . . . . . . . . . 60security valve . . . . . . . . . . . . . . . . . . . . . 26self-diagnostics . . . . . . . . . . . . . . . . . . . . 18smoke limit . . . . . . . . . . . . . . . . . . . . 14, 18spark plug . . . . . . . . . . . . . . . . . . . . . . . 9steering

centering pressure regulator . . . . . . . . . . . . 45control unit. . . . . . . . . . . . . . . . . . . . . 45control valve . . . . . . . . . . . . . . . . . . . . 42rack . . . . . . . . . . . . . . . . . . . . . . . . 45ram cylinder . . . . . . . . . . . . . . . . . . 42, 45wheel angle sensor . . . . . . . . . . . . . 29-30, 33

wheel centering device . . . . . . . . . . . . . . . 45wheel speed sensor . . . . . . . . . . . . . 29-30, 33

stiffness regulator . . . . . . . . . . . . . . . . . . 39suspension ECU . . . . . . . . . . . . . . 27-29, 32, 38suspension mode

hard . . . . . . . . . . . . . . . . . . . 28-29, 32, 39soft . . . . . . . . . . . . . . . 28-29, 31-32, 34, 39

suspension resonance frequency . . . . . . . . . . . 23suspension selector switch . . . . . . . . . . . 28, 31-32suspension status light . . . . . . . . . . . . . . . . 31swirl chamber . . . . . . . . . . . . . . . . . . . . 12TDC . . . . . . . . . . . . . . . . . . . . . . . . . . 9temperature-timer switch . . . . . . . . . . . . . . . 8throttle pedal position sensor . . . . . . . . . 29-30, 33throttle position switch . . . . . . . . . . . . . . . . . 7throttle potentiometer . . . . . . . . . . . . . . . . . 7timing advance . . . . . . . . . . . . . . . . . . . . 9top dead center . . . . . . . . . . . . . . . . . . . . 9TP . . . . . . . . . . . . . . . . . . . . . . . . . . . 7TS . . . . . . . . . . . . . . . . . . . . . . . . . . . 7turbocharger

diesel . . . . . . . . . . . . . . . . . . . . . . . . 15VAN . . . . . . . . . . . . . . . . . . . . . . . . . 52variable turbo pressure . . . . . . . . . . . . . . . . 18Vehicle Area Network . . . . . . . . . . . . . . . . . 52vehicle speed sensor. . . . . . . . . . . . . . . . 29, 33wastegate

diesel . . . . . . . . . . . . . . . . . . . . . . . . 15

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