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 obvious independent suspension for all wheels. And this is exactly what Citroën’s unique hydropneumatic 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. Therefore, 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its pressure can be controlled (the usual pneumatic suspensions operate on the opposite principle: air is admitted or withdrawn from the system by compressors and exhaust valves, modifying its mass while keeping the volume constant).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 technology in widespread use in every branch of the industry. As liquids are non-compressible, any amount of liquid introduced at one end of a hydraulic line will appear immediately at the other end (this phenomenon was first formulated 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 purposes even in the most conventional vehicles: shock absorbers, 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 volume 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 contact and the feeling transmitted to the driver remains always the same. This is something absolutely unique: all conventional suspensions have an optimum point around average load; when carrying more or fewer passengers or load than this average value, the handling characteristics change, not seldom so radically that the car becomes utterly dangerous to drive.

Another advantage is the limited but very useful anti-dive behavior: 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 vehicles these days. The first DS did have a force distributor but Citroën later realized that the suspension, with the addition of a single pipe, can fulfill its role entirely.

The height correction and the constant connection between 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 conditions, such as ice, snow, rain, mud.

The steady connection between the sides requires an external 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 ideally 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 connected 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 immediate compensation: the rear would sink, the front would rise, restoring the horizontal position of the car). Unfortunately, this would also lead to slow transversal (dive and squat) oscillations, made even worse by acceleration, deceleration 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 separated. 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 instance, 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 reaction 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 different fluid movements between the sides: to quick suspension 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 additional 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 accomplish this, the bar cannot be completely rigid (it has to absorb 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 suspension systems as well, however, there are substantial differences 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. Conventional 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.

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.

A typical example: the BX

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 connection (dashed line).

Shortly, as the suspension and braking circuits start to deplete 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 after 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 accumulator—passes through a security valve whose task is to ensure safety by feeding the brake circuits first. The front brake circuit is always open but the other two outputs are blocked by a piston. 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 return 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 piston which works similarly for the rear brakes, although those are not fed directly from the security valve but receive their supply from the rear suspension (later brake valves have three pistons but their method of operation is practically the same). In consequence, the braking force at the rear depends on the load: the more the back of the car is loaded, the stronger the rear brakes work. Actually, on a Citroën mostly used to carry only its driver, without much load in the trunk, the rear brake pads and disks wear much slower than those in the front.

The damping elements in the sphere supports consist of a central hole which is always open and additional 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 dynamic 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 adjusting the size of the holes, the designers could fine tune the damping factors independently for both directions of strut travel.