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The Citroën Technical Guide

by Zeljko Nastasic´ and Gábor Deák Jahn

There are many car manufacturers, makes, models and versions on the road today but— as we all know— none of them compares to Citroën in its engineering excellence, especially regarding suspension comfort, roadholding, and stability.

In this book we tried to describe how the various subsystems work. We never intended to replace service manuals or similar technical instructions. Illustrations are schematic, focusing on the principles of operation rather than on minute details of implementation.

This guide is not linked to any specific Citroën model but describes all systems and solutions used on a large number of cars from the glorious line of DS, ID, CX, GS, GSA, BX, XM, Xantia, Xsara and the C5.

Table of Contents

Fuel Injection

Electronic Fuel Injection

The Otto engine needs a mixture of fuel and air for its operation. It would be the task of the fuel supply— carburetor or injection— to provide the engine with the ideal mixture. Unfortunately, there is no such thing as an ideal mixture.

Perfect combustion, as chemistry calls it, would require air and fuel in proportion of 14.7 parts to 1 (this is the stochiometric ratio). While this might be satisfactory for the scientists, the real-life conditions of a vehicle call for slightly different characteristics.

We use the ratio of actual mixture to the stochiometric mixture, called lambda (l), to describe the composition of the mixture entering the engine: l= 1 denotes the chemically ideal mixture, l< 1 means rich, l> 1 is lean.

The best performance would require a slightly rich mix-ture, with the lambda around 0.9, while fuel economy would need a slightly lean one, between 1.1 and 1.3. Some harmful components in exhaust gas would reduce in quantity between lambda values of 1 to 1.2, others below 0.8 or above 1.4. And if this is not yet enough, a cold engine re-quires a very rich mixture to keep running. After warming up, the mixture can return to normal, but the temperature of the incoming air still plays a significant role: the cooler the air, the denser it becomes, and this influences the lambda ratio as well.

All these requirements are impossible to satisfy with sim-pler mechanical devices like carburetors. Electronic fuel in-jection provides a system that can measure the many cir-cumstances the engine is operating in and decide on the amount of fuel (in other words, the lambda ratio) entering the engine. By carefully adjusting the internal rules of this device, manufacturers can adapt the characteristic of the fuel injection to the actual requirements: a sporty GTi would demand rather different settings than a city car; be-sides, catalytic converters have their own demands that, as we will later see, upset the applecart quite vehemently.

Earlier, fuel injection systems only knew about fuel, the ignition was supplied by traditional methods. Later on, these systems (now called engine management systems) took on the duty of generating the sparks as well. But even with this second incarnation, the fuel injection part re-mained practically the same, thus the following section ap-plies to both kind of systems.

Fuel injection

The two most important inputs describing the actual oper-ating condition of the engine, thus determining the fuel de-mand are the engine speed (revolution) and engine load. The engine speed can be measured easily on systems using traditional ignition: the ignition primary circuit gener-ates pulses with their frequency proportional to engine speed (the tachometer uses this same signal to show the rpm to the driver). When the injection system provides the ignition as well, it cannot at the same time rely on it, so an additional sensor is used instead.

The engine load is usually determined by measuring the quantity of air the engine tries to suck in. There are various methods of attaining this: earlier systems used a flap which is deflected by the air flowing through the sensor— the angle of deflection is proportional to the amount of air passing through (air flow sensor, AFS). Later systems used a pressure sensor measuring the pressure inside the inlet manifold (manifold absolute pressure, MAP sensor). Yet another 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 temperature at a constant value above the temperature of the incoming air, the mass of air can be determined. Some sim-pler systems do not even measure the amount of air but use a pre-stored table in their computer to approximate it based upon the engine speed and the position of the throttle pedal— not that accurate but certainly much cheaper.

Under ideal conditions, these two inputs would already be enough to control the engine. A large table can be set up, like the one illustrated here (of course, this is an illustration only, the actual values mean nothing here), and for any pair of incoming engine speed and load values the necessary fuel amount can be determined. By keeping the pressure of fuel constant behind the injector valves, the amount of fuel injected depends solely on the time period the injectors are opened for, hence, the table can contain injector opening times.

Amount of fuel injected Engine load
0%5%…100%
Engine speed idle 3 3 … 3
850 rpm 4 5 … 5
900 rpm 5 6 … 7
… … … … …
6,000 rpm 9 8 … 10

And this is exactly how it is done in modern injection systems: the controlling microcomputer keeps a lookup table like this to determine the base pulse width. Earlier systems were constructed from discrete, analog elements, not like a small computer; a more or less equivalent circuit made of various hybrid resistance arrays and semiconductors were used for the same purpose.

Chip tuning, by the way, is the simple operation of replacing the said table with another one, yielding different characteristics (usually to gain power, allowing for worse fuel economy). As the computer stores this table in a programmable memory— similar in function to the BIOS in desktop computers—, replacing it is possible. The earlier systems with analog circuits cannot be modified that easily.

So, we obtained the base pulse width from the table but as the operating conditions of automotive engines are hardly ideal for any reasonable amount of time, several corrections have to be applied. Our air flow meter measures the volume of the air but we would need to know the mass of the air to calculate the required lambda ratio— remember, colder air is denser, thus the same volume contains more gas, requiring more fuel to provide the same mixture. To accomplish this, the injection system uses an air temperature sensor (ATS)— although on some systems it measures not the air but the fuel-air mixture— and lengthens the injector pulse width according to this input (except for the case of the airflow meter using a heated wire, this one takes the air temperature into account automatically, consequently, there is no need for correction).

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

The position of the throttle pedal is communicated to the computer by a throttle position switch (TS) or throttle potentiometer (TP). These devices signal both fully open and fully closed (idling) throttle positions. When the pedal is fully depressed, the computer makes the mixture richer to provide good acceleration performance.

Idle speed is more complicated: the throttle is closed, so there has to be a bypass to let the engine receive fuel to run. In simpler systems this bypass is constant (but manu-ally adjustable to set the correct idle speed) in a warm engine, providing a fixed amount of air, although the computer can decide on a varying amount of fuel to be injected. Later systems generally use a controlling device changing the cross section of the bypass, regulating the amount of a ir coming through (these systems often have no facility to adjust the idle speed, the computer knows the correct revolution and maintains it without any help from mechanical de-vices). The controlling device can either be an idle speed control valve (ISCV) or an idle control stepper motor (ICSM). The first one can only open or close the idle bypass, so any regulation must be done by rapidly opening and closing it by the computer, the second one can gradually change the bypass, hence fine tuning is easier and smoother.

Just like the choke on carburetors, there is a complete subsystem dealing with cold start and warm up, as the requirements under such circumstances are so different from the normal operation that they cannot be fulfilled by the regular control. The ECU monitors the ignition key switch to learn when the engine is started, then looks for the input from the coolant temperature sensor (CTS) to see whether this is a cold start or a warm one. If the coolant fluid is measured cold, a special warm-up sequence will be started.

The engine needs significantly more fuel, a richer mixture during this period. This extra fuel is used for two purposes: first, part of the fuel injected is condensed on the cold walls of the engine, second, to ensure better lubrication, the engine should run at an elevated revolution during this period.

There are two ways to provide more fuel: through the usual injectors, making the computer inject more gas than normal, or by using an additional cold start injector (CSV)— there is only one such injector even in multipoint systems. This injector is fed through a temperature-timer switch, protruding into the coolant just like the CTS, plus it is heated by its own electric heater. The injector operates as long as the ignition key is in the starting position but its behavior later on is governed by the timer switch. The colder the engine initially is, the longer it stays closed to let the cold start injector do its job. In a warm engine (above 40 °C) it does not close at all.

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

From that point, the surplus is dictated by the warming of the engine, communicated by the CTS to the computer. EFI systems without an idle speed control device often use an electromechanical auxiliary air valve (AAV). This valve, which is fully open when the engine is still cold but will close gradually as it warms up, lets an additional amount of air measured by the AFS pass through the system. Because it is measured, it tricks the computer into providing more fuel. The valve is heated by its own heating element as well as the engine, thus it closes shortly.

The injectors are electrovalves. As with any electromagnet, there is a small time delay between the arrival of the control signal and the actual opening of the valve due to the build-up of electromagnetic fields. The length of this delay depends heavily on the voltage the injectors are fed with. The same pulse width would result in shorter opening time, hence less fuel injected if the battery voltage drops below nominal (which is often the case on cold mornings). The injection computer therefore has to sense the battery voltage and to lengthen the injector pulse width if necessary.

The final, total pulse width (also called injector duty cycle) is calculated by summing up all these values received: the base pulse width from the RPM/ AFS table lookup, the various correction factors based on the temperature sen-sors, throttle position and the like, plus finally, the voltage correction.

As the computer has already calculated the exact amount of fuel to be injected, there is only one task left: actually injecting it. There are two possible ways: to inject the fuel into the common part of the inlet, still before the throttle butterfly, or to inject them close to the inlet valves, individually to each cylinder. Depending on the solution cho-sen, the system will be called monopoint or multipoint. Monopoint fuel injection requires a single common injector; the smaller cost and simpler setup makes it more common on smaller engines (in the case of Citroëns, the 1380 ccm ones). In all cases, the computer actually calculates the half of the fuel amount required as it will be injected in two installments, once for each revolution of the engine.

The injectors of the multipoint system can be operated simultaneously or individually. Previous Citroëns on the road today still use simultaneous operation. Individual cylinder injection, however, holds great potential— just to name one, some of the cylinders of a larger engine can be temporarily shut off by cutting off their fuel supply if the car is operating at partial load, saving a considerable amount of fuel—, so we are sure to meet this sort of fuel injection systems in the future.

All systems— regardless of the number of injectors— use a similar fuel supply layout. The fuel is drawn from the tank by a continuously operating fuel pump, transported via a filter to the injectors, then back to the tank. There is a pressure regulator in the circuit as well to keep the pressure of the fuel at a constant pressure above that in the inlet manifold (this regulator is a separate unit on multipoint systems while integrated into the injector on monopoint ones). As the pressure difference between the two sides of the injectors are constant, the amount of fuel injected depends solely on the opening time of the injectors. The pressure used in contemporary EFI systems is 3 to 5 bars.

This is practically all there is to it, there are only a couple of safety and economy features in addition. If the engine revolution exceeds a certain limit (between 1,200 and 1,500 usually) and the throttle is closed— this is called deceleration—, the momentum of the car is sufficient to rotate the engine through the wheels. To save fuel, the injection is cut off. As soon as the engine speed drops below the limit or the throttle is opened, the injection is reintroduced— supposedly smoothly and gradually, however, many drivers complain about some jerkiness.

To avoid prolonged operation at revolutions exceeding the specification of the engine, the injection is cut off above a maximum engine speed (6,000-7,000 rpm, depending on the engine). And finally, to avoid the hazard of fire in a crash and the fuel squirting from the injection system with the engine stopped or possibly destroyed, the relay of the injectors is controlled by the ECU, allowing fuel injection only when the ignition (or the signal of the corresponding sensor) is present.

Who will light our fire?

Models with simpler fuel injection have traditional (electronic) ignition systems which are practically equivalent to the solution used on cars with carburetors.

The distributor has two purposes: generating the driving signal for the ignition system and to distribute the high voltage to the four cylinders in turn. This two parts inside the distributor are electrically separate but mechanically coupled— both are driven by the camshaft to keep them in sync with the strokes of the engine.

The ignition signal thus starts from the distributor. A magnetic induction sensor (consisting of a rotating four-sided magnet and a pick-up coil) sends a pulse to the ignition module at each firing point. This pulse will be switched to the ignition coil (an autotransformer; auto here does not mean that it is manufactured for automotive use, auto-transformers have their primary and secondary coils connected) by a power transistor inside the module. The current change in the primary coil induces very high voltage spikes in the secondary circuit. These spikes then go back to the HT part of the distributor which in turn sends them to the spark plug of the actual cylinder requiring the spark.

It takes some time for the spark to ignite the fuel-air mixture inside the combustion chamber: this means that the spark has to arrive slightly before the piston reaches its top position (top dead center, TDC), so that it will receive the downward force of the detonation in the right moment. However, as the engine speed increases, so does the speed of the piston or the distance it travels during a given period of time. Therefore, the exact time of the spark has to be advanced as the revolution increases. Traditional systems do this by adding a vacuum line connecting the inlet manifold to the distributor. As the vacuum increases with the engine revolution, its sucking force rotates the inner part of the distributor slightly away from its original position, causing all its timing devices switch earlier, as required by the value of the timing advance.

Clever systems can get away without a distributor: some CXs have such an ignition setup. This systems has two ignition coils, both serving two spark plugs at the same time. These two spark plugs belong to cylinders whose pistons move in unison: one is compressing, the other exhausting. Although both plugs generate sparks at the same time, the one in the exhausting cylinder will be wasted.

Two birds with one stone

We made the ignition seem too simple in the previous section. While it works as described, there are many factors to be considered if we want to build a modern ignition system. For instance, the timing advance depends not only on engine speed but on many other factors as well: engine load, engine temperature and to some extent, the air temperature.

Just like the carburetor was not really good at deciding the amount of fuel required by the engine, the traditional ignition is similarly not perfect in estimating the timing advance and other characteristics of the sparks needed. An electronic system similar to the one used for fuel injection shows clear advantages over any earlier system.

And as they use about the same sensors and rely on each other, what could be more logical than to integrate them into a common system, elegantly called an engine management system?

If we compare the schematics of the corresponding EFI and EMS systems, they look almost the same. There are two notable differences: the small arrow on the line connecting the ECU to the distributor has changed its direction and a new sensor, a crank angle sensor (CAS) has appeared. Both changes have to do with the fact that the enhanced system, whose new task is to generate the ignition signals as well, cannot at the same time build on them as inputs. This new sensor— practically a replacement for the induction magnet in the distributor of earlier systems— informs the computer of both engine speed and camshaft position.

The flywheel has steel pins set into its periphery. As it rotates, the inductive magnet of the CAS sends pulses to the computer. Two of the pins are missing and this hole passes before the sensor just as the first piston reaches its TDC position. The missing pins cause a variance in the sensor output that can be read by the ECU easily.

The rest is the same: the base pulse width is calculated based on the CAS and AFS/ MAP sensors. The correction factors— air temperature, idle or full load, starting, warming up, battery voltage— sum up into an additional pulse width. Besides, the same input signals (AFS, CAS, CTS and TS/ TP) are used for another lookup in a table, yielding the correct dwell time and timing advance for the ignition. The dwell period remains practically constant but the duty cycle varies with the chaging engine speed. The ignition signal is amplified and sent to a distributor containing only secondary HT components: it does not create the ignition signal only routes the HT current to each spark plug in firing order.

Some systems also have a knock sensor (KS), sensing the engine vibration associated with pre-ignition (so-called pinking). If this occurs, the ignition timing is retarded to avoid engine damage.

Think green

As we saw, fuel injection and engine management systems are capable of determining the ideal amount of fuel to be injected, depending on the conditions of operation and several other factors in the engine. It is capable of deciding on lean mixture for general, partial load to save fuel, or on rich mixture when performance considerations call for this.

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

Ideal combustion would not generate polluting materials in the exhaust gas. Fuel is a mixture of various hydrocarbons (C n H m ), which when burned together with the oxygen (O 2 ) of the air, should transform to carbon-dioxide (CO 2 ) and water vapor (H 2 O). However, combustion is never ideal, besides, fuel contains many additives: the exhaust gas, in addition to the products mentioned, has various byproducts as well, some of them toxic: carbon-monoxide (CO), various unburned hydrocarbons (C n H m ), nitrogen-oxides (NO x ) and lead (Pb) in various substances coming from the anti-knock additives found in the fuel.

The relative amount of these byproducts depend on the lambda ratio of the air-fuel mixture burned. As shown on the diagram, a value between 1.2 and 1.3 would give a relatively low percentage of toxic byproducts while, as we can recall, being a lean mixture would be in the right direction towards fuel economy.

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

      2 CO + O 2 W 2 CO 2 (oxidation)
      2 C 2 H 6 + 7 O 2 W 4 CO 2 + 6 H 2 O (oxidation)
      2 NO + 2 CO W N 2 + 2 CO 2 (reduction)

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

If you compare this diagram with the previous one, you will see that the real gain is the supression of nitrogen-oxides. CO and C m H n will be reduced as well, although to a much lesser extent. Nevertheless, the overall reduction in polluting byproducts is quite high, amounting up to 90 percent. Lead substances are not considered as lead must not reach the converter anyway, it would clog the fine pores of the converter in no time. The fuel used in cars equipped with a catalytic converter has to be completely free of lead.

But there is something of even greater consequence depicted on the diagram: to keep the amount of pollutants down, the lambda has to be kept inside a very small value range, practically at l= 1 all the time. If the lambda drops just a fraction below 1, the CO emission rises sharply, while a small step above 1 skyrockets the NO x emission. The main task of the fuel injection is therefore to ensure that the air-fuel mixture sticks to the stochiometric ratio all the time. This means higher consumption than the one of a car with fuel injection without a converter to start with.

There are situations where this lambda cannot be observed. A cold engine will simply stall without a much richer mixture, thus the cold start mechanism does not obey the lambda control. The catalytic converter does not work at all below 250 °C, so this is not a significant compromise (its normal operating temperature is 400 to 800 °C, above 800 °C is already harmful; unburned fuel getting into the exhaust and detonating inside the converter could cause overheating, thus ignition and similar problems has to be rectified as soon as possible in catalytic cars).

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

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

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

The HPi engine

Earlier engines used sophisticated electronic circuitry to maintain the stoechiometric ratio of air and fuel (14.7 parts to 1, or as usually expressed, a lambda value of one). This chemically ideal mixture is not actually that ideal for real-life engine applications — the major reason for sticking to it was the proliferation of catalytic converters which can only operate on such mixture.

The new engine technology introduces the concept of stratified load and leaner mixture. Below a specific limit — 3,500 rpm — the engine leaves the usual stoechiometric mode and burns lean.

The upper part of the basic EW 2 liter engine was redesigned to incorporate a new, deviated jet combustion chamber. The injectors are placed to inject the fuel slantwise, directly into the combustion area. An off-center bowl in the piston positioned opposite the injector directs the fuel stream backwards, towards the sparking plug.

The specific shape of the admission ducts create a rotational movement of the mixture (called reverse tumble). After the compression phase, the fuel-air mixture is injected into this pre-established zone. The internal aerodynamics of the combustion chamber direct this deviated jet of fuel to the vicinity of the spark plugs.

For the mixture of air and fuel to be physically inflammable, there has to be a certain amount of fuel present. By concentrating the air-fuel mixture around the spark plug while filling the rest of the combustion chamber with air — providing a stratified load — , less fuelcan stull be sufficient to ensure the ignition and the combustion.

Other improvements like keeping the throttle butterfly wide open reduces the so-called pumping losses, the effort required for the admission of the air into the combustion chamber.

In addition to this, nearly a third of the exhaust gases are recirculated.

The driving force

The completely new control system — developed jointly with Siemens — controls all aspects of the engine operation: stratified mode, pollution control, European-standard diagnostic functions and the details of changeover between the operating modes.

The injectors inject a fine spray of mixture in a jet with 70 degree cone angle, swirling around the axis. The pressure of the fuel varies from 30 to 100 bar (in contrast to the traditional 3.5 bar of earlier systems), adjusted all the time to the prevalent needs of the engine.

The high pressure injection pump delivering the fuel is innovative in engine technology but it builds on the wellknown and proven concepts of Citroën's high pressure suspension pumps: three axial pistons driven by an inclined rotating plate, driven directly by the camshaft. As a result, this high pressure injection pump is lighter and more compact than any of it's competitors, by a factor of two.

Drive by wire

The butterfly is electrically controlled by the injection computer, without any direct link to the throttle pedal. In stratified mode, the butterfly is kept open to provide an excess of air. At idle speed, the butterfly is already open at 20°. During the transition from stratified to stoechiometric (homogeneous) mode or during acceleration, the system controls the butterfly to ensure a smooth transition, completely unnoticable for the driver.

Ignition

The spark plugs have three different ignition levels. When operating in the stratified load mode, the plugs receive twice the energy than at full load.

An engine operating on lean mixture also generates an excess of oxygen. A new, specific after-treatment systems had to be developed in lieu of the traditional three-way catalytic converters to cope with the specific conditions of this operating mode, to reduce the amount of carbon monoxide (CO), unburned hydrocarbons (HC) and nitrogen oxides (NOx) generated. Although the nitrogen oxides themselves are reduced thanks to the recirculation of the exhaust gas, the remaining pollutants cannot be filtered using the traditional technology, due to the mixture being very far from lambda 1.

The new after-treatment device works sequentially, in two stages. The pre-catalyst treats the CO and HC, the main catalyst stores and releases the NOx.

The platinum active coating of the main catalyst fully oxides the nitrogen oxides into nitrogen dioxide (NO2) and then stored in the form of nitrates, fixed to a barium alkaline-earthy metal salt. Periodically — on average for three seconds every minute — the nitrates collected are released to briefly increase the richness of the combustion mixture. During this operation, the concentration of CO and HC increases. Acting as reducers, they chemically reduce the nitrogen oxides into nitrogen (N2) on the rhodium coating in the pre-catalyst.

Diesel engines

Diesel oil has been a contender to gasoline for many decades. Earlier diesel engines were not re-fined enough to win the hearts of many drivers but recent advances in technology made these en-gines not only a worthy competitor in all areas but in some features — fuel economy or low end torque, to name just two — even exceeding the characteristics of their gasoline counterparts. And in addition to the general technological advantages, Citroën's diesel engines have a widely ac-cepted reputation — even among people blaming the quirkiness of its suspension or other fea-tures — of being excellent and robust.

As it is widely known, diesel engines have no ignition to initiate their internal combustion, they rely on the self-combustion of the diesel oil entering into a cylinder filled with hot air. Due to this principle of operation, the supply of the fuel has to comply with much more demanding requirements than it is necessary in the case of gasoline engines.

Unlike in the gasoline engine, not a mixture but air enters into the cylinders via the inlet valves. During the adiabatic compression all the energy absorbed is used to increase the temperature of the gas. The small droplets of fuel will be injected at high velocity near the end of the compression stroke into this heated gas still in motion. As they start to evaporate, they form a combustible mixture with the air present which self-ignites at around 800 °C.

This self-ignition, however, is not instantaneous. The longer the delay between the start of the injection and the actual ignition (which depends on the chemical quality of the diesel oil, indicated by the cetane number), the more fuel will enter the cylinder, leading to harsher combustion, with the characteristic knocking sound. Only with the careful harmonization of all aspects— beginning of injection, the distribution of the amount injected in time, the mixing of the fuel and air— can the combustion be kept at optimal level.

Small diesel engines suitable for cars were made possible by a modification to the basic principle, that allowed these stringent parameters to be considerably relaxed. It includes a separate swirl chamber connected to the cylinder via a restrictor orifice. The air compressed by the piston in the cylinder enters this chamber through the orifice, starting to swirl intensively. The fuel will then be injected into this swirl, and the starting ignition propels the fuel-air mixture still incompletely burned into the cylinder where it will mix with the air, continue and finish the combustion process. Using a prechamber results in smaller ignition delay, softer combustion, with less noise and physical strain on the engine parts, but introduces some loss of energy because of the current of air having to pass between the chambers. Citroën engines of this type use a tangentially connected spherical prechamber.

As diesel engine evolution continued, better simulation and modeling techniques became available, which, together with the improvements in fuel injection technology, lessened or removed the problems initially solved by the introduction of the prechamber. The direct injection engines of today have no prechamber, instead, the piston has a specially formed swirl area embedded in its face.

Mechanical injection

Although the basic principles of fuel injection are similar to what we have already discussed for gasoline engines, there are some notable differences. First of all, diesel engines op-erate without restricting the amount of air entering the en-gine: there is no throttle, the only means of regulating the engine is to vary the amount of fuel injected.

The fuel is injected into the engine, creating a combustible mixture in the same place it is going to be burned. Because the forming of this mixture results in its self-combustion, the diesel injection system is, in essence, an ignition control system. Unlike on the gasoline engine, fuel injection and ignition cannot be separated in a diesel engine. The complete mechanical injection system is built into a single unit which can be divided into five individual— although interconnected— subsystems:

The diesel fuel is drawn — through a filter — from the tank by the low pressure pump [1] operated by the engine. A pressure regulating valve [2] ensures that the fuel pressure will not exceed a preset limit; when the pressure reaches this value, the valve opens and lets the fuel flow back to the primary side of the pump.

The piston [6] of the high pressure part is driven through a coupling [4] consisting of a cam disc and four cam rollers. The piston rotates together with the shaft coming from the engine but the coupling adds a horizontal, alternating movement as well: for each turn, the shaft and the piston [6] performs four push-pull cycles.

It is the pushing movement of this piston [6] that creates the high pressure and sends the fuel to the injectors. The fuel, provided by the pump [1] arrives through the fuel stop electro-valve [17], which is constantly open while the ignition switch is on but cuts the fuel path when it is turned off.

First, the piston [6] is pulled back by the coupling [4], letting the fuel enter the chamber and the longitudinal bore inside the piston. As the side outlets are blocked by the regulator collar [5], the fuel stays inside the chamber (phase 1).

In the next phase, the piston rotates and closes the in-gress of fuel from the stop valve [17]. On the other side of the piston, the high pressure outlet opens but as the fuel is not yet under pressure, it will stay in the chamber.

In phase 3 the piston is energetically pushed by the cam disc and rollers of the coupling [4], injecting the fuel stored in the chamber into the output line with a significant force.

As the piston [6] moves to the right, at some point the side outlets will emerge from under the regulator collar [5] — the fuel injection into the real output will stop immediately, and the rest of the fuel stored in the chamber will leave through this path of lesser resistance. This is phase 4, the end of the injection cycle.

Actually, this operation is repeated four times for each revolution of the incoming shaft. There are four high pressure outlets radially around the piston, each serving a given cylinder. As the outlet slot [19] of the piston turns around, it allows only one of the outlets to receive the fuel.

The pressure valves [7] serve to drop the pressure in the injector lines once the injection cycle is over. To reduce the cavitation caused by the pressure waves generated by the rapid closing of the injector valves, a ball valve minimizing the back flow is also used.

The length of phase 3, thus the amount of fuel injected depends on the position of the collar [5]. If it is pushed to the right, it will cover the side outlets for a longer time, resulting in a longer injection phase, and vice versa. If it stays in the leftmost position, no fuel will be injected at all.

And this is exactly what the regulator part does: it moves this collar [5] to the left and to the right, as the actual requirements dictate. The lever [9] attached to the collar is rotated around its pivot by several contributing forces. The two main inputs are the position of the accelerator pedal as communicated through a regulator spring [12] and the actual engine speed, driving a centrifugal device [8] via a pair of gears [3]. The higher the engine speed, the more the shaft [20] protrudes to the right, pushing on the lever [10].

When the engine is being started, the centrifugal device [8] and the shaft [20] are in their neutral position. The starting lever [10] — pushed into its starting position by a spring [11] — sets the position of the collar [5] to supply the amount of fuel needed for the starting.

As the engine starts to rotate, a relatively low speed will already generate a large enough force in the centrifugal device [8] to push the shaft [20] and overcome the force of the rather weak spring [11]. This will rotate the lever [10], moving the collar [5] to the left, setting the amount of fuel required for idling. The accelerator pedal is in the idle position as well, dictated by the adjustment screw [14]. The idle spring [13] keeps the regulator in equilibrium.

Normally, the amount of fuel will be regulated by the position of the pedal as both springs [11] and [13] are fully compressed and do not take an active part in the process. When the driver pushes on the pedal, the regulating spring [12] stretches, both levers [9] and [10] rotate and move the collar [5] to the right, to allow the maximum amount of fuel to be in-jected. As the actual engine speed catches up, the centrifugal device [8] opens up, pushing the shaft [20] to the right, countering the previous force, gradually returning the collar [5] towards the no fuel position, until the point is reached where the amount of fuel injected maintains the equilibrium. When the driver releases the pedal, the inverse of this process takes place. During deceleration — pedal at idle, engine rotated by the momentum of the car — the fuel is cut off completely.

Without such regulation, if enough fuel is provided to overcome the engine load, it would continue accelerating until self-destruction (this is called engine runaway). Speed regulation is a feedback mechanism comparing the actual speed of the engine to the one dictated by the gas pedal and modifies the amount of fuel as necessary. If either the engine speed changes (because of varying load, going over a hill, for instance) or the driver modifies the position of the accelerator pedal, the regulation kicks in, adding more or less fuel, until a new equilibrium is reached. If the engine is powerful enough to cope with the load, keeping the pedal in a constant position means constant cruising speed in a diesel car; gasoline vehicles need speed regulated fly-by-wire systems or cruise controls to achieve the same.

The excess fuel will finally leave the pump unit through an overflow valve, flowing back to the fuel tank.

Something needs to be corrected...

The chemistry involved in the combustion dictates some parameters of fuel injection, the most important being the smoke limit, the maximum amount of fuel injected into a given amount of air, that results in combustion without resulting in soot particles. Although gasoline engines also have this limit, they normally operate with a constant fuel to air mixture that automatically places the amount of fuel below this critical limit. Diesel engines, in contrast, operate with a variable fuel to air mixture, using this very variation for power regulation. With diesel fuel observing the smoke limit is a much stricter task because once soot starts to develop, this changes the character of the combustion itself, resulting in a sudden and huge increase in the amount of particulates — a bit like a chain reaction.

Because the maximum amount of fuel injected depends on how far the lever [10] is allowed to rotate counter-clockwise, the inability of the pump to inject too much fuel, thereby crossing the smoke limit, is insured by an end stop [21] for this lever. This very basic means of smoke limit correction, adjusted for worst case conditions, was developed further on turbocharged engines, and still further on electronically controlled injection systems.

Timing is of enormous importance in a diesel engine. During the stroke of combustion, several events take place in close succession: the fuel injection system starts its delivery, then the fuel is actually injected (the time elapsed between these two is the injection delay), slightly later the fuel will self-ignite (this delay is the ignition delay), then the injection will stop but the combustion is still raging, first reaching its maximum, then dying away slowly (on the scale of milliseconds, that is).

Just like in a gasoline engine, the ignition delay remains constant while the engine speed changes. The fuel has to ignite before the piston passes its TDC position, but with the increasing engine speed, the distance the piston travels during a given period of time becomes longer. Therefore, the injection has to be advanced in time to catch the piston still in time. The injection adjuster [15] feeds on the fuel pressure provided by the pump [1], proportional to the engine speed.

This will move the piston, which in turn, through the levers, modifies the relative position of the cam rollers to the cam disc inside the coupling [4], increasing or decreasing the phase difference between the revolutions of the engine and the rotating-alternating movement of the distributor piston [6].

Some engines also have additional minor correction mechanisms [16] that modify the idle speed and timing depending on engine temperature, to provide better cold start performance. The engine temperature is measured indirectly, through the coolant acting on cylinder and piston-like elements filled with paraffin. As the paraffin expands or contracts as the coolant temperature dictates, the transformed mechanical movement, coupled through cables to two movable end stops for both the lever [9] and the injection adjuster [15], modifies the idle speed and the injection timing of the engine. Because correct timing depends on temperature, the corrections, although relatively slight, insure that the amount of fuel injected as well as the timing provide better combustion and lower pollution when the engine is started and operated at low temperatures. They do not have any effect once the engine reaches the normal operating temperature.

Now that the correct amount of fuel is carefully determined and the necessary high pressure generated by the pump, it has to be injected into the swirl chamber. The pressurized fuel entering the injector through a filter [1] tries to press the piston [2] upwards but a spring [3] counters this force. As soon as the pressure exceeds the force of the spring (which can be adjusted by placing appropriately sized shims behind it), the piston jumps up and the fuel rushes into the swirl chamber through the small orifice now opened. After the injection pump closes its pressure valve at the end of the injection period, the spring [3] pushes the piston [2] back, closing the orifice until the next injection cycle.

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

Turbo

More power requires more fuel. An efficient way to boost the performance is to provide both more air and fuel to the engine. The exhaust gases rushing out from the engine waste a great deal of energy; a turbocharger [4] spun by the exhaust flow taps into this source of energy to provide added pressure in the air inlet. Diesel engines are particularly well suited for turbocharging. Gasoline engines may not have the inlet pressure raised too much because the air and fuel mixture may subsequently self-ignite when it is not supposed to, and instead of burning controllably, detonate. In a diesel such a situation is not possible because the fuel is injected only when combustion should actually happen in the first place. As a result, relatively high inlet pressures can be used, considerably improving the power output of a diesel engine, and with proper attention to the subtleties of the design, engine efficiency and fuel consumption.

On its own, once the amount and pressure in the exhaust manifold reaches a level high enough to power it, with the engine fully loaded, the turbine would spin proportionally to engine speed squared, because both the pressure and the volume of the air pumped into the engine are increasing.

Because the engine is required to deliver as much torque as possible at the widest possible range of engine revolution, the requirements on the turbine are somewhat contradictory. If the turbo is made very small and light, it will spin up very quickly due to its low mass and inertia, ensuring its full benefit already at low rpms. However, with a moderate increase in engine speed, the rotational speed of the turbine (note the quadratic relationship) would become excessively high. When the turbine blade speed approaches the speed of sound, a supersonic wave effect occurs that can abruptly leave it without any load, at which point runaway would occur, resulting in severe damage to the turbine.

On the other hand, if the turbine was dimensioned so that even at the highest engine speed it is still operating within safe limits, it would not be useful at all in the middle range where the engine is most often used. A compromise can be achieved using an overpressure valve, the wastegate valve [5]. The turbo pressure is constantly monitored by this valve opening above a set pressure limit, letting the exhaust escape through a bypass. This avoids turbo runaway by making the turbo rotational speed proportional to that of the engine, once the limit pressure is reached. This way the quick spin-up resulting from the quadratic relationship can be preserved while the turbocharging effect is extended over a significant percentage of the usable engine speed range— typically the higher 70-80%. But it comes at a price: because of the simplicity of such a regulation, the limit pressure is dictated by the maximum turbine speed, which is usually calculated for maximum engine speed plus a safety margin. The maximum pressure is already reached at lower engine and turbine speeds, where the turbine could conceivably still provide more pressure because of a lesser demand for air volume. Although with a simple wastegate a certain amount of the turbocharging potential is lost, the increase in power output is still substantial.

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

Essentially, a turbocharged diesel engine runs in two different modes: atmospheric pressure or turbo-charged. The atmospheric pressure mode prevails while the exhaust gas produced is not yet sufficient to power the turbine (below a given engine speed and load). Once this limit is crossed and the turbine starts generating higher than atmospheric pressure, the engine is running in turbocharged mode.

The injection pump regulator needs to know about the changes in the inlet pressure, because those changes mean differences in the amount of air entering the engine. And this also means that the upper limit of fuel injected needs to be changed correspondingly. These injection systems are tuned for the turbo producing the rated waste pressure (also known as full boost). However, the amount of fuel injected during the atmospheric mode of the engine — before the turbo kicks in — has to be reduced in order to avoid crossing the smoke limit. The turbo pressure drives a limiter in the injection pump: with the increasing pressure the piston [21] moves down. Its varying diameter forces the lever [22] rotate around its pivot, which then acts as a stop to limit the allowed range of operation of the regulator lever [9], limiting the amount of fuel to be injected.

Towards a cleaner world

Exhaust Gas Recycling (EGR) systems were used — depending on the market — as add-on units. An electronic unit measuring the coolant temperature and the position of the gas pedal control on the pump (with a potentiometer fitted to the top of the control lever) controls a valve which lets part of the exhaust gas get back into the inlet.

Post-glowing is also used as a pollution reducing mechanism. A definite post-glow phase, lasting for up to minutes is usually controlled by a combination of a timer and the engine coolant temperature: either the timeout of 4 minutes runs out or the engine reaches 50 °C. An additional mechanism prevents post-glowing if the engine was not actually started.

Electronic Diesel Control

Just like it is the case with gasoline engines and carburetors, a mechanical device — even one as complicated as a diesel injection pump — cannot match the versatility and sensibility of a microcomputer coupled with various sensors, applying sophisticated rules to regulate the whole process of fuel injection.

The only input a mechanical pump can measure is the engine speed. The amount of air entering into the engine, unfortunately, is far from being proportional to engine speed, and the turbo or the intercooler disturbs this relationship even further. As the injection always has to inject less fuel than the amount which would already generate smoke, the mechanical pump — capable only of a crude approximation of what is actually going on in the engine— wastes a significant amount of air, just to be of the safe side.

The satisfactory combustion in diesel engines relies on the exhaust as well — if this is plugged up, more of the exhaust gases stay in the cylinder, allowing less fresh air to enter. A mechanically controlled injection pump has no feedback from the engine (except for the engine speed) — it will simply pump too much fuel into the engine, resulting in black smoke. An electronically controlled injection pump, on the other hand, can tell how much air has actually entered by using a sensor (although only the latest systems use such a sensor).

There are also other factors never considered by a mechanical system. The details of the combustion process depend heavily on the chemical characteristics of the fuel. The ignition delay, as we have already seen, depends on the cetane number of the diesel oil. In spite of the fact that correct timing has a paramount influence on the performance and the low pollutant level of a diesel engine, the mechanical system can have no information about this very important input factor. Less essential but still important is the temperature of the incoming air. With measuring all the circumstances and conditions in and around the engine (air, engine and fuel temperatures), the injection system can achieve better characteristics, lower fuel consumption and less pollution.

All in all, the electronically controlled injection pump not only adds precision to the injection process as its gasoline counterpart does but introduces completely new methods of regulation; therefore it represents a much larger leap forwards than fuel injection in gasoline engines. In spite of this, it is quite similar to its mechanical predecessor. From the five subparts, four remain practically the same, only the regulator is replaced with a simple electromagnetic actuator that changes the position of the same regulator collar [5] as in the mechanical pump, in order to regulate the amount of fuel to be injected.

The real advantage over the former, mechanical pumps is that an electronic device, a small microcomputer can handle any complex relationship between the input values and the required output. With mechanical systems, only simple correction rules are possible, and as the rules get more complicated, the mechanics quickly becomes unfeasible. In con-trast to this, the ECU just have to store a set of characteristic curves digitized into lookup tables, describing the amount of fuel to be injected using three parameters: engine speed (measured by a flywheel inductive magnet), coolant temperature (measured by a sensor protruding into the coolant liquid), air temperature (measured by a sensor in the air inlet).

The newer HDi engines use an air mass sensor using a heated platinum wire (as that mentioned on page 5). Having the exact amount of air to enter the engine, these latest EDC systems can deliver true closed loop regulation.

A potentiometer attached to the accelerator pedal sends information about the pedal position to the computer. This signal is used as the main input, conveying the intentions of the driver. The ECU uses this sensor to learn about special conditions like idle speed or full load as well. Air temperature is measured by a sensor in the inlet manifold (but if the air mass is measured by a heated platinum wire sensor, this already provides the necessary air temperature correction, thus there is no need for an additional sensor).

The ECU stores the basic engine characteristics, the intrinsic relationship between the air intake and the engine speed (plus the manifold pressure if a turbo is fitted). The values obtained from this table are corrected according to the inputs of the various sensors, in order to arrive at a basic timing and smoke limit value. The actual amount of fuel injected and the accurate timing are a function of these results and the position of the accelerator pedal.

The final amount of fuel calculated will be used to control the electric actuator [8] which — by moving a lever [10] — changes the position of the regulating collar [5]. To ensure the necessary precision, the factual position is reported back to the computer using a potentiometer.

As we have already mentioned, the exact timing of the injection is of utmost importance in a diesel engine. The electronic system uses a needle movement sensor built into one of the injectors (the other are assumed to work completely simultaneously) notifying the computer about the precise time of the beginning of the injection. Should there be any time difference between the factual and designated opening times, the electro-valve ß of the injection ad-juster à will receive a correction signal until the difference disappears. If the electro-valve is completely open, the injection start will be delayed, if it is closed, the start time will be advanced. To achieve the timing required, the valve is driven with a modulated pulse signal, with the duty cycle (on-off ratio) determined by the ECU.

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

The computer has extensive self-diagnostic functionality. Many sensors can be substituted with standard input values in case of a failure (serious errors will light up the diagnostic warning light on the dashboard). Some sensors can even be simulated using other sensors— for instance, the role of a failing engine speed sensor might be filled in by the signal generated from the needle movement sensor.

As there is no standalone ignition in a diesel engine, the only way to stop it is to cut off the fuel supply. The mechanical default position of the actuator 8 is the position where no fuel enters the injectors at all; this is where it returns when the computer receives no more voltage from the battery, the ignition switch having turned off.

As it has already been mentioned, the inlet pressure is one of the principal EDC parameters for a turbocharged engine. Later Citroën turbocharged diesels— starting with the 2.5 TD engine of the XM— pioneered variable turbo pressure technology. The wastegate on these turbines has several actuators, fed with the turbo pressure through electric valves. The ECU, based on the relevant engine operation parameters obtained from the sensors, controls these actua-tors in various combinations, providing a selection of two or three different wastegate limit pressures. This lets the system ease the compromise between the turbo pressure and turbine speed: the pressure is kept at the usual value for higher engine speeds (limited by the maximum turbine speed) but is allowed to go higher than that in the middle rpm ranges, adding a significant amount of torque in the range where it is most needed.

Green versus Black

Diesel oil, just like gasoline, is a mixture of various hydrocarbons (C n H m ), and burned together with the oxygen (O 2 )of the air, transforms to carbon-dioxide (CO 2 ) and water vapor (H 2 O). However, as the combustion is never ideal, the exhaust gas also contains various byproduct gases: carbon-monoxide (CO), various unburned hydrocarbons (C n H m ), nitrogen-oxides (NO x ). The relatively high lambda value a die-sel engine is operating with reduces the hydrocarbon and carbon-monoxide content to 10– 15%, and the amount of nitrogen-oxides to 30– 35% of the corresponding figures measured in gasoline engines without a catalytic converter. The sulphur content of the fuel— drastically reduced during the recent decades— is responsible for the emission of sulphur-dioxide (SO 2 ) and sulphuric acid (H 2 SO 4 ).

Conversely, these engines emit 10– 20 times more particulates— or black soot— than gasoline engines. These are unburned or incompletely burned hydrocarbons attached to large particles of carbon. These substances are mainly aldehydes and aromatic hydrocarbons; while the first only smells bad, the second is highly carcinogenic.

The much higher amount of particulates is due to the different combustion process. The various aspects of mixture formation, ignition and burning occur simultaneously, they are not independent but influence each other. The distribution of fuel is not homogenous inside the cylinder, in zones where the fuel is richer the combustion only takes place near the outer perimeter of the tiny fuel droplets, producing elemental carbon. If this carbon will not be burned later because of insufficient mixing, local oxygen shortage (large fuel droplets due to insufficient fuel atomization, caused by worn injectors) or the combustion stopping in cooler zones inside the cylinder, it will appear as soot in the exhaust. The diameter of these small particles is between 0.01 and 10 mm, the majority being under 1 mm. Keeping the amount of fuel injected below the smoke limit— the lambda value where the particulate generation starts to rise extremely— is essential.

Similarly to gasoline engines, the exhaust gas can be post-processed to reduce the amount of pollutants even further. There are two different devices that can be used:

Diesel Direct Injection

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

Soot burning was experimented with a lot but was never made practical before HDI due to a too low exhaust temper-ature. The particle filter would need heating to a very high temperature and that was deemed to be too dangerous. Even with cerine additives, essentially, there would have to be a separate small burner to heat up the filter, which is again another system that can go wrong. HDI essentially in-tegrates a burner by alowing post-injection, something that is simply impossible for injection systems derived from a classical pump due to teh timing required. I think that for soot management it is enough to write that the smoke limit control is vastly improved by the better regulation of the EDC.

Other things like controlled swirl and multi-valve technol-ogy, also pioneered by Citroën (XM 2.1 TD!) should be men-tioned. The catalytic converter section remains unchanged.

And, of course, there should be an "In addition to the pol-lution management implemented on mechanical injection systems" sentence somewhere in there, since proper cold start corrections and EGR are implemented in EDC units by default.

Suspension

A Suspension Primer

From the early days of the automobile — and even before, in the time of horse-drawn carts — it was already well known that the body of the car, hous-ing both the passengers and the load, has to be decoupled from the unevenness of the road sur-face.

This isolation is much more than a question of comfort. The vertical force of the jolts caused by the repeating bumps and holes of the road surface are proportional to the square of the vehicle speed. With the high speeds we drive at to-day, this would result in unbearable shock to both people and the mechanical parts of the car. Jolts in the body also make it more difficult to control the vehicle.

Consequently, there has to be an elastic medium be-tween the body and the wheels, however, the elasticity and other features of this suspension medium are governed by many, mostly contradicting factors.

The softer, more elastic the spring, the less the sus-pended body will be shaken by various jolts. For the sake of comfort, we would thus need the softest spring possible. Unfortunately, too soft a spring will collapse under a given weight, losing all its elasticity. The elasticity of the spring would need to be determined as a function of the weight carried but the weight is never constant: there is a wide range of possible load requirements for any car. On one hand, a hard suspension will not be sensitive to load varia-tions but being hard, will not fulfill its designated purpose, either. A soft suspension, on the other hand, is comfortable but its behavior will change significantly on any load varia-tion. To cope with this contradicting requirements, an elas-tic medium of decreasing flexibility would be required: such a spring will become harder as the weight to be carried in-creases.

When the spring is compressed under the weight of the load, it's not only its flexibility that changes. The spring de-flects, causing the clearance between the car and the road surface decrease, although a constant clearance would be a prerequisite of stable handling and roadholding. At first sight, this pushes us towards harder springs: soft springs would result in excessive variations of vertical position —un-less, of course, we can use some other mechanism to en-sure a constant ground clearance.

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

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

Any vertical jolt would thus cause such oscillations: the upward ones are transmitted to the car body while the downward ones make the wheels bounce, losing contact with and adhesion to the road surface. The first is only dis-comforting, but the second is plainly dangerous. In addi-tion, it's not only the spring that oscillates; the tires contain air which is a highly elastic spring medium. Oscillation in it-self causes unwanted motion but when the corrugation of the road surface happens to coincide with the period of the suspension oscillations, it might lead to synchronous reso-nance, a detrimental situation leading to serious damages in the suspension elements.

Mass in motion can also be viewed as a source for kinetic energy; because of this, moving parts of the suspension are often reduced in weight to decrease this portion of the stored energy, and this in turn eases the requirements on the dampers as they have to dissipate less unwanted energy as heat. This solution, however, often shifts the frequency of the self-oscillation of the suspension upwards. Unfortu-nately, occupants are more sensitive to higher frequencies reducing comfort (mostly adding noise), so this is an area where compromise is needed.

Conventional suspension systems use a second element, a shock absorber to dampen these oscillations. The ab-sorber uses friction to drain some of the energy stored in the spring in order to decrease the oscillations. Being an ad-ditional element presents new challenges: the characteris-tics of both the spring and the absorber have to be matched carefully to obtain any acceptable results. The ab-sorber ought to be both soft and hard at the same time: a soft absorber suppresses the bumps of the road but does not decrease the oscillations satisfactorily while a hard ab-sorber reduces the oscillations but lets the passengers feel the unevenness of the road too much. Due to this contradic-tion, conventionally suspended cars have no alternative but to find a compromise between the two, according to the in-tended purpose of the car: sport versions are harder but of-fer better roadholding, luxurious models sacrifice roadhold-ing for increased comfort. This contradiction clearly calls for a unified component serving both as a spring and an absorber, harmonizing the requirements.

Hydropneumatic Suspension

As we saw, the ideal suspension would require elasticity decreasing with the load, constant ground clearance, shock absorbers integrated into the suspension— all these beyond the obvi-ous independent suspension for all wheels. And this is exactly what Citroën's unique hydropneu-matic suspension offers.

According to the Boyle– Mariotte formula defined in the 17th century, the pressure and the volume of a mass of gas are inversely proportional at a constant temperature. There-fore, by keeping the mass of the gas constant and changing the volume of its container, its pressure can be controlled (the usual pneumatic suspensions operate on the opposite principle: air is admitted or withdrawn from the system by compressors and exhaust valves, modifying its mass while keeping the volume constant).

The volume changes are controlled by hydraulics, a tech-nology in widespread use in every branch of the industry. As liquids are non-compressible, any amount of liquid intro-duced at one end of a hydraulic line will appear immedi-ately at the other end (this phenomenon was first formu-lated by Blaise Pascal). Using this principle, motion can be transmitted, multiplied or divided (according to the relative sizes of the operation cylinders), with velocity increased or decreased (using varying cross sections in the tubing), to any distance desired, over lines routed freely.

Hydraulics are immensely useful, very efficient, reliable, simple to use, and— due to their widespread deployment— relatively cheap. It is no wonder that it is used for many pur-poses even in the most conventional vehicles: shock absorb-ers, brake circuit and power assisted steering being the most trivial examples; however, Citroën is the only one to use it for the suspension.

The First Embodiment

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

The hydropneumatic spring-absorber unit uses an inert gas, nitrogen (colored blue on the illustrations) as its spring medium, resulting in very soft springing. The flexibility of the gas decreases as the increasing load compresses the suspension pistons, reducing the vol-ume of the gas and adding to its pressure. The damping effect is obtained by forcing the fluid (colored in green) pass through a two-way restrictor unit between the cylinder and the sphere. This effect provides a very sensitive, fast and progressive damping to reduce any unwanted oscillations.

There are many great advantages to this hydropneumatic suspension. First, by adding or removing fluid from the suspension units (practically, by adjusting the length of the hydraulic strut), ground clearance can be kept constant under any load variations. Although this might not seem very important at first sight, it means that the suspension geometry is also constant— in other words, the handling of the car does not depend on the load.

The compressed gas has a variable spring effect, becoming harder as the load increases. This compensation for the increasing load keeps the resonance frequency of the suspension nearly constant. As a consequence, the same excitation in the suspension moves the same amount of fluid through the dampers regardless of load (which is not the case with conventional springs). The working range of the dampers becomes much smaller and this fact makes the use of a simple damper element very effective.

This basically constant suspension resonance frequency also contributes to the consistent behaviour independent of the load. In essence, it ensures that both the road con-tact and the feeling transmitted to the driver remains al-ways the same. This is something absolutely unique: all con-ventional suspensions have an optimum point around aver-age load; when carrying more or fewer passengers or load than this average value, the han-dling characteristics change, not sel-dom so radically that the car be-comes utterly dangerous to drive.

Another advantage is the limited but very useful anti-dive behav-ior: this is essential for efficient braking with a basically very soft suspension. The center of mass of the car moves much less than usual, hence the braking force is distributed more evenly. Manufacturers of cars with conventional suspension and braking only start to add brake force distributors to their ve-hicles these days. The first DS did have a force distributor but Citroën later realized that the suspension, with the addi-tion of a single pipe, can fulfill its role entirely.

The height correction and the constant connection be-tween the left and right side of the suspension has another important implication: lower difference in forces on the wheels. Coupled with variable damping this keeps the wheels in contact with the road at all times, which in turn maximizes the tractive forces on the tires— braking while turning still leaves the vehicle with the grip of all four wheels: this is essential for security in low adherence condi-tions, such as ice, snow, rain, mud.

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

For instance, if the front wheels run into a pothole and the rear wheels go over a bump, the car will rotate around its transversal axis. The angle of rotation remains relatively small as the length of the car is its largest dimension; the higher weights like the engine bay are far from the centre of mass, resulting in a large inertial torque to counter outside forces. If all suspension elements of the wheels were con-nected hydraulically, the vehicle would absorb the bumps very efficiently (the rear struts compressed by the bump would deliver fluid into the front struts, resulting in immedi-ate compensation: the rear would sink, the front would rise, restoring the horizontal position of the car). Unfortu-nately, this would also lead to slow transversal (dive and squat) oscillations, made even worse by acceleration, decel-eration and varying distribution of weight inside the cabin.

As the inertia of the car body around its transversal axis is basically sufficient to counter the effect of longitudinal bumps, the front and rear suspension circuits are sepa-rated. The active height correction of the system acts as a further a non-linear stabilizer both countering dive and squat, and solving weight distribution problems.

On the other hand, if the bumps are transversal— for in-stance, a pothole under the right wheel and a bump under the left one—, the car will rotate around its longitudinal axis. Being much less wide than long, the angle of rotation will be higher and the inertial torque is considerably lower to counter this kind of rotation. Completely independent sides would result in very little damping of roll movements: the low inertia provided by the body would find the reac-tion of the suspension too stiff. Hence, the two sides in the hydropneumatic suspension are interconnected, providing a push-pull operation of the two sides. The interconnection has special damping elements which react differently to dif-ferent fluid movements between the sides: to quick suspen-sion movements caused by potholes and bumps, or to slower changes occuring when driving in a curve.

To counter body roll resulting from the second, an addi-tional element, an anti-roll bar is also needed. The effects of roll could be eliminated if the center of the roll could be identical to the center of the mass. As this is not possible, the opposite approach of moving the center of roll away from the center of mass could also help overcome body roll by increasing the opposing torque. This is the role of the anti-roll bar: similarly to a bike leaning into a curve, it lifts the inner side of the wheel, using the force on the outer edge, and this moves the center of roll outwards. In other words, the wheels and suspension elements do have roll, the role of the anti-roll bar is to isolate this roll from the body which should remain, ideally, horizontal. To accom-plish this, the bar cannot be completely rigid (it has to ab-sorb the road undulations without transfering them to the body), a torsion spring is the usual solution. Such anti-roll bars are used on conventional spring sus-pension

systems as well, however, there are substantial dif-ferences in the way the bar interacts with the rest of the suspension on Citroëns. In a spring system, there is a considerable amount of interaction, a significant flow of energy in both directions between the suspension and the bar. The shock absorbers have to provide the damping for the anti-roll bar, introducing yet another interaction (in the hydraulic setup this is catered for by the damping inside the connection line between the sides).

Consequently, the hydropneumatic suspension has much less interdependence and compromise between damping, countering roll, squat and dive. In addition, it can provide solutions which are simply unfeasible mechanically in a conventional suspension. Cars with steel springs always have roll, including diagonal one, induced by undulations of the road— their anti-roll bar represent a constant mechanical connection between the sides, unable to differentiate between bumps and curves. Citroëns, on the other hand, have a varying interconnection depending on fluid movement— this is very easy to accomplish with hydraulics but extremely complicated with springs.

The only disavantage is that damping occurs further from the source of the disturbance, and due to the good conductivity of sound via the hydraulic lines, this results in slightly more noise. The same effect makes the hydropneumatic suspension somewhat noisier than a conventional one. However, good sound insulation inside the cabin can help overcome this small annoyance.

This suspension layout reduces the sensitivity to underinflated or blown tires and cross-wind. Even with largely uneven braking forces on the two sides the car will not pull to either side.

Although the hydropneumatic spring-absorber unit is an integrated unit from a technical point of view, hydraulics make it possible to place some hydraulic parts (for instance, the center spheres on Hydractive systems) in different locations, reducing the amount of sprung mass. Conven-tional springs have a considerable mass of their own while the mass of the nitrogen in the spheres is practically negligible. Even adding the mass of the fluid moving around in the system, the sum remains much below that of a steel spring. Hydropneumatic struts can be kept relatively small by increasing the operating pressure, which decreases the diameter of the struts. The automatic height correction reduces the mass further because the basic suspension mechanics can be simpler, without requiring multilinks and similar components.

The brakes share the mineral fluid with the suspension. This fluid boils at a very high temperature, therefore it provides great resistance to vapor lock. Due to the proportional regulation a hydropneumatical Citroën can keep braking as long as there is anything left of the brake pad. Even if the liquid starts to boil, there will be no vapor lock as the pressure is automatically released and remains proportional to the braking effort applied by the driver.

This system is often criticized for being overly complicated and prone to error, none of which accusations is true. The suspension is actually quite simple when considering its extra services in comparison to a conventional system and experience shows that the whole system is very reliable. The perfect functioning of the system relies mainly on the prescribed cleaning of the system and the change of the hydraulic fluid— adhering to these simple prescriptions can make the system very reliable.

A typical example: the BX

Finally, there are no forces in the suspension when the circuit is depressurised, allowing very easy and safe servicing of the relevant suspension and transmission parts.

Modern spring suspension systems are in fact capable of achieving some of these results. For instance, variable diameter or pitch springs coupled with hydraulic shock absorbers (incidentally, with a similar internal geometry as the damper elements used in Citroën spheres) behave similarly to these hydropneumatic units. The main difference is that even if these elements would be practically identical, all other functionality that comes either for free or at a small additional cost in Citroën systems— constant height, anti-dive, brake force regulation and so on—, require complex and expensive additional systems.

The illustration shows the basic layout of the suspension (differences on models fitted with power steering or ABS will be described in the corresponding chapters). Most components have an output line to collect leakage (which is intentional to keep the elements lubricated) and return it to the reservoir— although the outputs are indicated, the lines themselves are omitted for the sake of clarity. In reality, they are grouped together and go back to the reservoir.

The high pressure supply subsystem consists of a five-piston volumetric high pressure pump drawing the mineral suspension liquid called LHM from the reservoir. The fluid under pressure is stored in the main accumulator. It is the task of a pressure regulator— built into the same unit with the accumulator— to admit fluid into the accumulator as soon as the pressure drops below the minimum value of 145 bar; as soon as the pressure reaches 170 bar, the regulator closes and the fluid continues its idle circulation from the pump, immediately back to the reservoir. On simpler models the output marked with an asterisk is omitted and it goes to the return ouput inside the regulator unit instead, as shown by the dashed line. On models fitted with power assisted steering (DIRASS) this interconnecting line is missing and both outputs are used independently.

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

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

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

The characteristic ticking which can be heard in Citroëns is the sound of the regulator pistons quickly moving one af-ter the other, in quick succession: 2 down, 1 down, 2 up. The opposite tick— 1 up, when the regulator is switched on to replenish the accumulator— is much softer.

The interconnection 6 is normally closed. Opening it lets all the fluid stored under pressure return back to the LHM reservoir— this is the way the system is depressurized when any of the suspension elements need servicing.

The liquid— supplied to the rest of the system from the main accumula-tor— passes through a security valve whose task is to ensure safety by feed-ing the brake circuits first. The front brake circuit is always open but the other two outputs are blocked by a pis-ton. If the pressure in the main circuit exceeds 100 bar, the fluid pushes the piston back against the force of the spring, opening up the suspension outputs as well. The electrical switch for the low hydraulic pressure warning lamp on the dashboard is built into this valve as well. This way, a sudden failure of the pump or the belt driving it will not leave the car without sufficient braking power.

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

The mechanical connection between the anti-roll bar and the height corrector is not a rigid linkage but has some free play. Just before the height corrector, the connecting rod coming from the anti-roll bar hooks into a small window on the corrector side. Small movements of the control rod do not change the position of the height corrector, only those are large enough to exceed this free play. In addition, the corrector has its internal (albeit low) resistance, besides, all rods are somewhat elastic, so in the end, all these factors make the height correction system filter out the higher frequency components of the suspension movement.

Observing an initial threshold which has to be crossed before any correction occurs not only reduces the strain and wear on the correctors but also prevents the system from developing self-oscillation. A powered system provides amplification and any feedback mechanism with a delay— such as the height correction— could potentially result in oscillations. The initial threshold ensures that there is no feedback, and consequently, no oscillation when the required correction is too small.

The next circuit is the rear suspension. Its layout and operation is identical to the front one, having its own height corrector.

The first circuit, as already mentioned, feeds the front brakes. The liquid under pressure flows into the brake compensator valve, operated by the brake pedal. In its neutral position, the brake circuits are connected to the re-turn lines to ensure that the brakes are not under pressure. When the driver pushes on the pedal, this moves the first piston, closing the return output and opening up the outlet going to the front brake cylinders.

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

The damping elements in the sphere supports consist of a central hole which is always open and addi-tional small holes closed and opened by a spring as the flow of the hydraulic liquid dictates. Slower suspension movements like body roll, squat or dive result in a slower flow of the liquid and the smaller dy-namic pressure differences are not sufficient to bend the spring cover open over the additional holes. The damping effect is therefore only determined by the diameter of the center hole.

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

The additional holes are located in a circle around the center hole. There are two spring covers, one on each side, but they do not cover all the holes equally. Half of the holes (actually, every second one) are slightly enlarged on one side, the remaining half on the other side. By carefully ad-justing the size of the holes, the designers could fine tune the damping factors independently for both directions of strut travel.

Hydractive I

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

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

During normal driving, the computer keeps the suspen-sion in soft mode most of the time but— based on the input provided by many sensors (steering wheel, accelerator pedal, body movement, road speed and brake), including the Sport/ Comfort switch on the dashboard— the suspen-sion ECU decides when to switch to hard mode; in other words, when to deactivate the additional spheres for extra roadholding and safety.

When the driver selects the Sport setting, the suspension is switched to hard mode constantly. This setting is not what any Citroën driver would call comfortable… The suc-cessor system, Hydractive II overcomes this limitation.

The layout of the system (front suspension)

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

  1. A standard Citroën sphere base which fits a sphere without a damper block. The sphere volume and pressure differ for the front and rear, as well as according to the model of the car;
  2. A hydraulically controlled isolation valve that con-nects or isolates the sphere from the rest of the suspension, modifying the string constant of the suspension;
  3. A ball and piston valve arrangement that limits fluid cross-flow between the left and right suspension struts in case of body roll. This valve is disabled for suspension height corrections, in order to guarantee that the fluid pressure in the corner struts remains equalized;
  4. Two damping elements similar to those used on the corner spheres, acting as dampers for the center one;
  5. An electrically controlled valve driven by the suspen-hydropneumatic sion ECU. In order to reduce heat build-up, the computer uses pulse width modulation to achieve a constant current through the coil. The initial voltage is higher to make the valve react quicker but it is reduced to a smaller value once the inductive effects have been overcome, should the valve stay on for a long enough time. The valve is capable of being on indefinitely when driven with this sustained current.

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

Soft, hard, soft, hard…

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

While the computer keeps the suspension in soft mode, the electro-valve 5 is energized by the ECU, opening the feed pressure onto the isolation valve piston 2 and by moving it, connecting the center sphere 1 to the rest of the sus-pension. The fluid in the suspension has to pass through two damping elements 4 (one for each strut connection). When both struts move in unison, the center sphere behaves as a standard sphere with a damper hole twice as large as a single damper element, but when the car starts to roll, the fluid has to move from one strut to the other, passing through both damper elements consecutively. In addition to this double damping, the sphere 1 itself acts as a damping string, absorbing quick changes in pressure between the two dampers. This dampens the body roll to some extent even in soft mode.

Whenever the computer feels it necessary to switch to hard mode, it closes the electro-valve 5, not allowing the main feed pressure to move the isolation piston 2. The pres-sure inside the center sphere 1, always higher than that of the return path under normal operating conditions, will move the control piston into a position which closes off the center sphere completely. The remaining pressure in this sphere remains unknown but as the main circuit pressure might change while the suspension is in hard mode (due to either the dynamics of the suspension— acceleration, braking, movement due to uneven surface— or the vehicle height altered by the driver), the computer equalizes the pressure periodically by enabling the control block to assume the soft position for a short period of time.

Hard mode serves three reasons. First, it provides higher resistance to body roll. The cross-flow of LHM from one strut to the other has to pass through both damper blocks as in soft mode, but it is additionally limited using the piston and ball valve 3, now switched into the hydraulic circuit between the damper elements instead of the center sphere. The ball is positioned in the fluid so that any crossflow moves the ball and thus limits the flow, dampening the body roll as well.

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

Third, hard mode not only limits the suspension travel between the body to the road but between the suspension elements and the body. Its aim is to reduce suspension movement at the cost of comfort but to gain safety, limiting the influence of the body movement to steering, very important in extreme situations like a flat tire.

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

At the same time when the body roll is present, the car might need to change the ground clearance as well: for instance, when braking in a curve. The valve 3 therefore has an additional pis-ton which lets the LHM flow between the circuits of the struts and of the height corrector. If the body has to be raised, the pressure in the height correctors will be higher than that in the suspension. This higher pressure pushes the piston, which in turn dislodges the ball and the pressure will raise equally in both struts (without dislodging the ball, only one of the struts would receive the fluid, resulting in incorrect operation).

If the body has to be lowered, the higher pressure in the struts will dislodge the ball again, opening the piston towards the return line ad the fluid will escape from both struts, lowering the vehicle.

Sensory perceptions

The computer of the suspension system takes its input signals from the various sensors and based on a set of rules, dynamically activates the electric valve.

There are eleven inputs to the ECU. First, the Comfort/ Sport switch on the dashboard, enabling the driver to choose between the two settings. The status light on the instrument panel informs about the setting selected (it does not indicate the mode the suspension is currently in).

The second input comes from a vehicle speed sensor. This inductive magnet tachogenerator generates 4 pulses per rotation, that is approximately 5 pulses per meter traveled (although this depends somewhat on tire size). It is located on the gearbox where the speedometer cable attaches, or in some versions, on the cable itself. The ECU determines the acceleration of the car by evaluating changes in vehicle speed for the duration of one second.

Another input arrives from the steering wheel angle and speed sensor, an optoelectronic device consisting of two infrared light beams, interrupted by a rotating disc with 28 holes. The ECU senses the quadrature signal changes of both sensors to effectively increase the resolution of the sensor (28 pulses per steering wheel revolution) by a factor of four. This produces one edge change every 3.214 degrees of steering wheel rotation. The direction of turning can be determined by the sequence of the edge changes.

To make decisions, the computer needs to know the straight ahead position of the steering wheel. The sensor does not have a built-in zero position (as it would not always work, due to misalignment and wear in the mechanical components). The computer uses heuristics instead:

First, the straight line position is assumed if the vehicle speed is above 30 km/ h and the steering wheel position was not changed (an error margin of up to 4 pulses is allowed) for the last 90 seconds. Second, we know the maximum number of pulses in both directions from the center (lock to lock angle divided by two). If the steering wheel is found to turn more than this value (an error of up to 4 pulses is accepted here, too), this is a clear indication of an incorrect center reference: in this case the center position will be adjusted by the surplus.

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

A similar sensor informs the computer about the movement of the car body. Two infrared beams, the disc having 45 notches, similarly quadrupled by the ECU. Excessively long intervals are considered coming from slow height changes resulting from the driver selecting a different height setting, and are consequently discarded.

The sensor is connected to the front anti-roll bar, to the right of the height corrector linkage. Due to its location, it is capable of detecting both squat and dive, and to some extent, body roll. But as the sensor is mounted off-center, its sensitivity to roll is about three times less than the sensitivity to squat and dive. In all directions, it can measure both movement amplitude and speed of movement, using the same process as the steering wheel sensor does.

The throttle pedal position sensor is located below the dashboard, right next to the pedal mechanism, where the pedal can operate its sprung lever as it moves. The sensor is a potentiometer with an integrated serial resistor in the wiper's circuit.

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

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

The door/ tailgate open switches are located on the door frame and in the boot latch. The door switches are all wired together in parallel and connected to one input line (and routed to the interior light dimmer and timer as well). The tailgate switch is connected to the other input line (and routed to the boot light and the tailgate opened detection input for the status display on the dashboard, too; the door open and bonnet open signals for the status display are generated by a separate set of switches, independent of the ones used for the suspension).

The usual ignition switch provides a power-on signal, triggering and internal reset and self diagnostic run in the ECU. Turning the ignition on and off also triggers internal events that guarantee proper pressure equalization between the center and corner spheres.

The brain behind the suspension

The ECU is a small microcomputer sensing the input signals coming from the various sensors. A very interesting and important aspect of the system is that it uses the driver of the car as a major part of its intelligence, making the operation very simple but effective. To achieve this, most of the sensors read the controls the driver operates.

The software contains the description of various conditions (status of the input lines and internal timers) governing when to activate-deactivate the electrovalve switching the suspension to either hard or soft mode. These conditions can be formulated as rules.

Every main input sensor has an associated rule: when the value collected from the sensor exceeds a specific threshold, the suspension is put into hard mode and the com-puter starts a timeout counter. For the suspension to return to soft mode at the end of the timeout period, the threshold must not be exceeded again during this time. If it was exceeded, the suspension stays in hard mode and the timeout starts all over again.

There are four additional rules overriding the normal op-eration— even if the sensor inputs call for a generic rule to be applied, these four conditions are checked first:

As already mentioned, the steering wheel sensor is used to derive two inputs values: steering wheel speed and an-gle. These values are treated separately with the purpose of calculating the lateral acceleration of the vehicle (vehicle speed, steering angle) and the potential change in this ac-celeration (vehicle speed, steering wheel speed). It is seem-ingly done this way to save memory which would otherwise be required for a full three-parameter lookup (based on ve-hicle speed, steering wheel angle, steering wheel speed). The steering wheel sensor rules actually give a measure of potential body roll. Body roll is significantly reduced in hard mode, consequently, the rules were set up to ensure that the body roll is minimized when there is potential for it, still the suspension stays soft to absorb bumps when there is no body roll caused by the vehicle changing direction.

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

The table below shows the thresholds of steering wheel angle and rotating speed. If any of these values exceed the threshold for the actual vehicle speed, the sus-pension will switch to hard mode; it will revert to soft when the corresponding value drops below the threshold for at least 1 second if the switching was triggered by the steering wheel angle and 2 seconds if triggered by the rotational speed:

Vehicle speed (km/ h)Steering wheel angle (deg)
< 30 always soft
31– 40130
41– 60100
61– 8052
81– 10040
101– 12018
121– 14015
141 >8
Vehicle speed (km/ h)Steering wheel speed (deg/ s)
< 30 always soft
31– 60196
61– 100167
101– 120139
121 >128

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

The body movement speed is used as the parameter for the activation of two types of corrections:

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

Once any of these conditions are met, the suspension will revert to normal operation, with thresholds restored according to the table. Exceeding any of these thresholds will force the suspension into hard more. The computer checks every 0.8 seconds whether the conditions forcing the suspension into hard mode are still present, and if so, the system stays in hard mode.

Suspension down > 13 pulses, timeout 1 sec
Suspension up > 9 pulses, timeout 1 sec
Suspension change speed between 30 and 50 ms AND
Durchfederung > 3 pulses, timeout 1 sec

Vehicle speed (km/ h)Dive (mm)Squat (mm)Steering wh pos (deg)
< 30———
Vehicle speed (km/ h)Dive (mm)Squat (mm)Steering wh pos (deg)
< 30———

The values delivered by the throttle pedal sensor are used with reference to the vehicle speed in order to anticipate the vehicle dynamics as a result of acceleration or deceleration. The rules for this sensor represent a reaction to probable vehicle squat (on acceleration) or dive (on deceleration). Both are significantly reduced when the suspension is in hard mode.

The suspension ECU quantizes the pedal position into five discrete steps: 0, 30, 40, 50 and 60 percent of the complete pedal travel. The computer measures the time elapsed as the pedal travels from one step to the next in either direction. If this time is inside the intervals shown in the table, the suspension will switch to hard mode. It will revert to soft if the pedal movement becomes slower for at least the duration of the timeout specified:

Pedal press speed (ms)Timeout (s)
< 1001
101– 1502
Pedal release speed (ms)Timeout (s)
< 1001
101– 2002

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

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

It is important to note that the suspension will switch to soft mode even with the ignition switch tur