Hydropneumatic suspension : automobile type of suspension, derived from the oil and air Suspension, developped at the point by Citroen and equipping the vehicles with the high and average-high ranges of the manufacturer. It currently equips the Citroen C5 and C6.

More than 9 million vehicles employing it were produced to date. One finds it in a marginal way on the back Essieu of some Rolls-Royce like on that of the Peugeot 405 MI16. Similar systems are also employed on some military vehicles like the Char Leclerc.

Like any other suspension, its role is to filter the irregularities of the road not to transmit them to the case of the vehicle and to deaden the associated movements. Moreover, the use of the hydraulic high pressure makes it possible to ensure a constant height the top of the ground and this, in all circumstances.

This system uses a hydraulic Pompe, driven by the engine, coupled to an accumulator which ensures the distribution of the hydraulic fluid (different during time: LHS, LHS2, LHM, and LDS today on the C5 and C6) under high pressure in all the hydraulic system. This pressure allows, according to the vehicles and the times, the assistance of the direction, the old Frein, the ordering of the clutch and the order of Gear box. Several improvements with this system were made during time: like Hydractive or SC-CAR.

History

This device was invented by Paul Magès Research department S Citroen the shortly after the Second world war. ; 1954 Traction 6-H

One will retain that the first vehicle equipped with this hydropneumatic suspension was the 6-H in 1954 (Traction 15-Six-H). Only the axle postpones is equipped with it, the hydraulic fluid is of LHS.
Mais the first vehicle of great series equipped with a hydropneumatic suspension will be the Citroen DS 19 presented to the living room of Paris 1955. The 6-H being intended only for the test of the production of such a system into large série.
It should be noted that at the origin, Paul Magès envisaged his use on the 2CV.

; 1955 DS 19

It is thus with the Motor Show of October 1955 with Paris that the " was revealed; bombe". The use of hydraulics was pushed to the extreme. Nonglad to ensure a Handling fantastic and unknown and a comfort until now, the whole with a constant height above the ground, the use of the high pressure made it possible to the engineers to develop:
- power-assisted steering (big first),
- braking assisted hydraulically qualified surpuissant (to be noted that the DS 19 is the first car with being equipped with an independent brake system for the nose gear wheels and the aft wheels), and a regulation of the pressure of braking on the rear wheel-axle unit according to the load of the back part of the vehicle preventing with the aft wheels from blocking itself before the nose gear wheels: guarantee safety,
- a ordering of the clutch and a semi-automatic box hydraulically-operated.

; 1957 ID 19

ID 19 is intended to the owners of Traction 11 not having the means of treating to the luxurious DS 19. It has a stripped hydraulic system: only the suspension remains hydraulic, braking (ndm: until 1961 dates to which it becomes with hydraulic assistance), the direction as well as the ordering of gear box are traditional (thus a third pedal for the ordering of the clutch appears). The liquid employed is always red LHS of first generation.

; 1965 LHS2

Change of fluid on all vehicles DS and ID: passage to the colorless LHS2 less corrosive than the first version.

; 1966 LHM

New change of fluid: adoption of the LHM (Hydraulic fluid Mineral) of green color. It will equip all the vehicles until the appearance of the C5 in 2000.
Mercedes-Benz presents the 300SEL 6.3 with a suspension activates pneumatic conceived to escape the patents from Citroen.

; 1970 Citroen GS

Birth of the first vehicle of average range to being equipped with the suspension and hydraulic braking resulting from ID.

; 1970 Citroen SM

At the exit of the SM , the latter is equipped with the suspension and braking resulting from his/her cousin DS as well as few innovations: hydraulic direction with variable assistance according to speed and recall controlled in straight line named DIRAVI which will equip some CX and some rare XM, hydraulically-operated directional headlights and not mechanics as on DS according to 1967 and with corrector of plate to prevent the oscillations of the beam of light.

; 1975 Citroen CX

Naissance of the CX which succeeds DS but loses the semi-automatic hydraulic gear box with the profit of a conventional 5-speed gearbox with clutch with hand drive operation.

; 1982 BX

Replacement of GSA by BX: it takes again the hydraulic equipment of GS and lays out for certain models of a direction with hydraulic assistance. It adopts finally a forage ladder, which was so much lacking with the CX.

; 1989 XM Birth of the XM downward of the CX. It presents a new version of the named hydropneumatic suspension Hydractive . The use of a third sphere per axle coupled with electronics allows him various operating processes: normal or sport. The automatic passage in firm mode is modified by the pressure of the “button sport” by the driver, according to the speed of entry in turn in particular, for a better control of the catch of rolling.

; 1990 Peugeot 405 Mi16x4

First and alone Peugeot equipped with the hydropneumatic suspension on the rear axle.

; 1993 Xantia

It replaces growing old BX and inaugurates for the top-of-the-range models a new version of Hydractive: Hydractive II. This evolution will also equip in same time all the range XM. Other Xantia being equipped with a hydropneumatic suspension similar to BX. The nose gear is of type Pseudo Mc Pherson as on the BX, the rear wheel-axle unit is equipped with a deformable cross-piece (or train autodirectionnel developped at the point on the ZX).

; 1994 Xantia

Xantia inaugurates the system SC-MAC (System Citroen de Maintien of Constant Plate). It removes the phenomenon of depression of the hydropneumatic suspension cars at the time of a prolonged stop.

; 1995 Xantia Activa

It is the first vehicle in the world to be transfered completely flat thanks to the adoption of the suspension Hydractive II coupled with the system SC-CAR (Citroen System of the Active Control of Rolling), 40 years result of research launched by Paul Magès on the anti-roll one. System considered diverting for the average driver which will not be taken again on other models of the mark.

; 2000 Citroen C5

It replaces Xantia while waiting for the advent of very awaited the Citroen C6 (replacing of the XM). It inaugurates a new version of Hydractive. All is new: abandonment of liquid green LHM with profit of liquid of synthesis LDS (Liquid Of Synthesis), abandonment of hydraulic pump involved by engine with profit of unit compact pump-accumulator electric, abandonment of braking assisted by hydraulic fluid of suspension (good-bye the limitation of braking ordered by the suspension) due to EU law since the latter obliges the brake system to be completely uncoupled from all the remainder (to use a pump and an additional accumulator not to order that the brakes would be too expensive), the assistance of the direction is traditional but hydraulic (more DIRAVI), the spheres are completely modified and become oval all in using a new membrane in Nylon (Citroen source), abandonment of the mechanical correctors height, this last function becoming electronic and allowing the vehicle to lower themselves by 10 mm to the top of 110 km/h and to take again its normal position in lower part of 90 km/h. To note that on very battered ground and at low speed the height of the vehicle is increased by 20 Misters C5 is not equipped with Activa. It on the other hand is equipped with Hydractive III which gives him, according to Citroen, 80% of the performances of Activa in term of limitation of rolling.

; 2006 Citroen C6

the vehicles V6 are equipped with a Hydractive suspension directly resulting from the Citroen C5 but presenting some changes, exit them of Activa technology: controlled damping in 16 states allowing up to 400 changes of states a second from which calculation is different for each wheel and holding account from the general evolution of the car on the road; a fourth sphere equips the axles before and back allowing a greater flexibility in flexible mode.

Operation

The principle of the system is based on the use of spheres filled with separate nitrogen of the liquid Hydraulique by a membrane which was at the beginning out of nylon rubber then.

In the middle of the system, is the pump high pressure delivering more than 170 bars of pressure. The pressure is generated by the accumulator (sphere) coupled with the line breaker whose role is to distribute to all the bodies of the hydraulic system a pressure varying between 130 and 170 bars in all circumstances.

A traditional vehicle (GS, BX) has a sphere by wheel equipped with a shock absorber slowing down the passages with the fluid instead of a spring and with a traditional shock absorber. A corrector height directly actuated by the anti-roll bar allows the vehicle to keep a constant height in all circumstances (control height of case). The use of this device allowed to the engineers the use of a suspension with very great flexibility, guarantees comfort. Priority is given to braking so that in the event of loss of pressure, the driver can keep the control the speed of the vehicle and to stop. The use of anti-roll bars was constrained but contrary with the will of Paul Magès to put on DS a system the anti-roll one.

Maintenance

The spheres flee naturally in time because them Azote passes through metal and the membrane. Their lifespan is of 50.000 km for the vehicles before 2000; on the C5 and the C6, their lifespan passed to 5 years or 200.000 km.

An increase in the stiffness of the suspension announces l'" usure" spheres and requires their refill or their change. Hydraulic fluid LHM is to be changed every 80.000 km, its purity being guarantor of the longevity of the whole of the hydraulic system.

Disadvantages

• maintenance requires a qualified mechanic or good knowledge.
• forgotten maintenance is very expensive.
• the spheres lose their nitrogen.
• Feeling of slow Pitching on the DS which can give the sea sickness.
• the loss (very rare) of pressure does not make it possible any more the car to roll.

Impact on Citroen

Thanks to the hydraulic suspension, Citroen was forged at the time an brand image based on the innovation.

This system returned very expensive to Citroen: indeed, although having exceptional characteristics, this type of hydraulic technique was not spread in the car as much as one could have awaited it: in fact, the constraints of manufacturing in series are very heavy (machining of the pumps to the fraction of micron!) and expensive for mean advantages. Hydraulics tends to being replaced by the electric one of allied power to electronics.

Impact on the world

The twenty years in advance of the DS are not any more and the other manufacturers made part of their delay concerning the comfort and the handling of the vehicles.

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