Hubbry Logo
Hydropneumatic suspensionHydropneumatic suspensionMain
Open search
Hydropneumatic suspension
Community hub
Hydropneumatic suspension
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Hydropneumatic suspension
Hydropneumatic suspension
Hydropneumatic suspension
View on Wikipedia
from Wikipedia
High position
Low position
Citroën suspension sphere
Challenger 2, main battle tank of the British army, uses hydropneumatic suspension for better crew comfort and increased firing accuracy.

Hydropneumatic suspension is a type of motor vehicle suspension system, invented by Paul Magès, produced by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers. Similar systems are also widely used on modern tanks and other large military vehicles. The suspension was referred to as Suspension oléopneumatique [fr] in early literature, pointing to oil and air as its main components.

The purpose of this system is to provide a sensitive, dynamic and high-capacity suspension that offers superior ride quality on a variety of surfaces. A hydropneumatic system combines the advantages of hydraulic systems and pneumatic systems so that gas absorbs excessive force and liquid in hydraulics directly transfers force. The suspension system usually features both self-leveling and driver-variable ride height, to provide extra clearance in rough terrain.

This type of suspension for automobiles was inspired by the pneumatic suspension used for aircraft landing gear, which was also partly filled with oil for lubrication and to prevent gas leakage, as patented in 1933 by the same company. The principles illustrated by the successful use of hydropneumatic suspension are now used in a broad range of applications, such as aircraft oleo struts and gas-filled automobile shock absorbers.

Description

[edit]

Hydropneumatic suspension is a type of motor vehicle suspension system, invented by Paul Magès, produced by Citroën, and fitted to Citroën cars. The suspension was referred to as Suspension oléopneumatique [fr] in early literature, pointing to oil and air as its main components.[1][2]

The system was also used under licence by other car manufacturers, notably Rolls-Royce (Silver Shadow), BMW 5 Series (E34) Touring, Maserati (Quattroporte II) and Peugeot.[citation needed] It was also used on Berliet trucks and has been used on Mercedes-Benz cars, where it is known as Active Body Control.[3] The Toyota Soarer UZZ32 "Limited" was fitted with a fully integrated four-wheel steering and a complex, computer-controlled hydraulic Toyota Active Control Suspension in 1991. Similar systems are also widely used on modern tanks and other large military vehicles.

Effects

[edit]

The purpose of this system is to provide a sensitive, dynamic and high-capacity suspension that offers superior ride quality on a variety of surfaces.[4] The suspension system usually features both self-leveling and driver-variable ride height, to provide extra clearance in rough terrain.[5] Hydropneumatic suspension has a number of natural advantages over steel springs, generally recognized in the auto industry.[6] In a hydropneumatic system, gas absorbs excessive force, whereas liquid in hydraulics directly transfers force, which combines the advantages of two technological principles:

  • Hydraulic systems use torque multiplication in an easy way, independent of the distance between the input and output, without the need for mechanical gears or levers.
  • Pneumatic systems are based on the fact that gas is compressible, so equipment is less subject to shock damage.

Suspension and springing technology is not generally well understood by consumers, leading to a public perception that hydropneumatics are merely "good for comfort".[citation needed] They also have advantages related to handling and control efficiency, solving a number of problems inherent in steel springs that suspension designers have previously struggled to eliminate.[7] Although auto manufacturers understood the inherent advantages over steel springs, there were two problems. First, it was patented by the inventor, and second, it had a perceived element of complexity, so automakers like Mercedes-Benz, British Leyland (Hydrolastic, Hydragas), and Lincoln sought to create simpler variants using a compressed air suspension.[8][9]

Citroën's application of the system had the disadvantage that only garages equipped with special tools and knowledge were qualified to work on the cars, making them radically different from ordinary cars with common mechanicals.[10] France was noted for the poor quality of its roads after World War II, but the hydropneumatic suspension as fitted to the Citroën ID/DS and later cars reportedly ensured a smooth and stable ride there.[4][11][12]

Hydropneumatic suspension offers no natural roll stiffness. There have been many improvements to the system over the years, including steel anti-roll bars, variable ride firmness (Hydractive), and active control of body roll (Citroën Activa).[13]

Basic mechanical layout

[edit]
Blue: nitrogen gas; gold: hydraulic fluid under pressure from the engine-driven pump

This system uses a belt- or camshaft-driven pump from the engine to pressurise a special hydraulic fluid, which then powers the brakes, suspension and power steering.[7][14] It can also power any number of features such as the clutch, turning headlamps and even power windows.[7]

Nitrogen is used as the trapped gas to be compressed, since it is unlikely to cause corrosion. The actuation of the nitrogen spring reservoir is performed through an incompressible hydraulic fluid inside a suspension cylinder.[4] By adjusting the filled fluid volume within the cylinder, a leveling functionality is implemented.[4] The nitrogen gas within the suspension sphere is separated from the hydraulic oil by a rubber membrane.[4]

History

[edit]
1954 Citroën Traction Avant 15CVH – high position

Citroën first introduced this system in 1954 on the rear suspension of the Traction Avant.[15] The first four-wheel implementation was in the advanced DS in 1955.[16] This type of suspension for automobiles was inspired by the pneumatic suspension used for aircraft landing gear, which was also partly filled with oil for lubrication and to prevent gas leakage, as patented in 1933 by the same company.[17] Other modifications followed, with design changes such as the 1960 "double stage oleo-pneumatic shock absorber" patented by Peter Fullam John and Stephan Gyurik.[18]

Major milestones of the hydropneumatics design were:

  • During World War II, Paul Magès, an employee of Citroën, with no formal training in engineering, secretly develops the concept of an oil and air suspension to combine a new level of softness with vehicle control and self-levelling.[19]
  • 1954 Traction Avant 15H: Rear suspension, using LHS hydraulic fluid.
  • 1955 Citroën DS: Suspension, power steering, brakes and gearbox/clutch assembly powered by high pressure hydraulic assistance. A belt-driven seven-piston pump, similar in size to a power steering pump, generates this pressure when the engine is running.[20]
  • 1960 The United States Patent and Trademark Office issues U.S. patent 2959410A for a double stage oleo-pneumatic shock absorber using concepts very similar to those developed earlier by Paul Magès – Patent forms the basis for aircraft oleo struts and gas-filled shock absorbers[18]
  • 1965 Rolls-Royce licenses Citroën technology for the suspension of the new Silver Shadow[21]
  • 1967 The superior non-hygroscopic LHM mineral fluid is introduced
  • 1969 Citroën M35: The Citroën M35 was a coupé derived from the Ami 8, and equipped with a Wankel engine and a hydropneumatic suspension. The bodies were produced by Heuliez from 1969 to 1971.
  • 1969 National Highway Traffic Safety Administration legalizes LHM mineral fluid in the United States
  • 1970 Citroën GS: Adaptation of the hydropneumatic suspension to a small car
  • 1970 Citroën SM: Variable speed auto-returning power steering, dubbed DIRAVI, and hydraulically actuated directional high beams. The beams of all six headlights are maintained parallel to the road surface by a hydraulic system separate from the directional long range high beams. The headlights' steering and leveling systems are totally separate from the central system that powers the suspension, steering and brakes and use a different fluid, a glycerine type.
  • 1972 BMW E12 5-series released with optional hydropneumatic rear suspension. Coil springs are retained, though softer than conventional coils for the same car. This system was offered in most BMW 5-, 6-, and 7-series models, as well as the E30 Touring (station wagon/estate), into the 1990s when it was replaced with an air suspension. Until late 1987, the hydraulic circuit was separate from the power steering, and the pump electrically powered.
  • 1974 National Highway Traffic Safety Administration bans vehicles with height adjustable suspension, impacting consumers in the United States. Ban repealed 1981.
  • 1974 Citroën CX: The car was one of the most modern of its time, combining Citroën's unique hydro-pneumatic integral self-leveling suspension and speed-adjustable DIRAVI power steering (first introduced on the Citroën SM). The suspension was attached to sub frames that were fitted to the body through flexible mountings, to improve even more the ride quality and to reduce road noise. The British magazine Car described the sensation of driving a CX as hovering over road irregularities, much like a ship traversing above the ocean floor.
  • 1974 Maserati Quattroporte II: was on an extended Citroën SM chassis, available since Citroën had purchased the Italian company and was the only Maserati Quattroporte to feature hydropneumatic suspension and front-wheel drive
  • 1975 The Mercedes-Benz 450SEL 6.9 W116 replaces the air suspension of the 6.3 with hydropneumatic suspension, with the pump driven by the engine's timing chain instead of an external belt. This adaptation was used only for the suspension. Power steering and brakes were conventional hydraulic- and vacuum-powered, respectively.
  • 1980 Mercedes-Benz W126 500SEL used hydropneumatic suspension as optional, later this system was available on 420SEL and 560SEL models.
  • 1983 Citroën BX, built as a 4WD in 1990[22]
  • 1984 Mercedes-Benz W124 selected models of E class had this technology (rear only hydraulic suspension) height adjustable suspension and self-levelling suspension mixed with coil springs.
  • 1987 BMW E30 3-series Touring (station wagon/estate) begins production in July, offering the same self-leveling hydropneumatic rear suspension as previous BMW, with the difference that the pump is a parallel circuit on the belt-driven steering assist pump, and shares its fluid. Starting in September, the E32 7-series (in production since June '86) switches to this pump from the previous electric pump. The BMW E34 5-series begins production in November, also with this new pump.
  • 1989 Citroën XM: Hydractive Suspension, electronic regulation of the hydropneumatic system; sensors measure acceleration and other factors [23]
  • 1990 Peugeot 405 Mi16x4: first Peugeot equipped with rear hydropneumatic suspension[citation needed]
  • 1990 JCB Fastrac high speed agricultural tractor uses this system for its rear suspension.[citation needed]
  • 1991 Toyota Soarer UZZ32 used hydraulic struts controlled by an array of sensors with yaw velocity sensors, vertical G sensors, height sensors, wheel speed sensors, longitudinal and lateral G sensors) that detected cornering, acceleration and braking force.
  • 1993 Citroën Xantia used hydropneumatic, on 1995 Optional Activa (active suspension) system, eliminating body roll by acting on anti-roll bars.[23] A Xantia Activa was able to reach more than 1g lateral acceleration, and still holds the record speed (85 km/h (53 mph)) through the moose test maneuver, due to its active anti-roll bars.[24] This test is conducted by the magazine Teknikens Värld's, as a test of avoiding a moose in the road. The second place car, Porsche 997 GT3 RS was able to manage 82 km/h (51 mph).[25][23]
  • 1995 Mercedes-Benz E-Class (W210) on estate (wagon) models on rear suspension used hydraulic suspension with spheres height adjustable suspension and self-levelling suspension mixed with coil springs.
  • 1999 Mercedes-Benz CL-Class (C215) and Mercedes-Benz S-Class (W220) introduce optional Active Body Control – an electronically controlled hydropneumatic system [26]
  • 2001 Citroën C5: Hydractive 3 removes the need for central hydraulic pressure generation; combined pump/sphere unit for the suspension only and with electric height adjustment sensors. Hydractive 3+ was available on some models[citation needed]
  • 2005 Citroën C6: An improved version of the C5 system known as Hydractive 3+ (also fitted to some C5 models), C6 with a V6 engine was fitted with AMVAR version of Hydractive 3+ (sometimes called Hydractive 4)[citation needed]
  • 2007 Citroën C5 II: Hydractive 3+ as optional on Exclusive models. other versions of the car have normal spring suspension.
  • 2008 JCB Fastrac high speed 7000 series agricultural tractors now use this system for front and rear suspension.[citation needed]
  • 2019 Mercedes-Benz 450 GLE introduces eActive Body Control on a Sport utility vehicle, discarding mechanical roll bars, notably enhancing performance.[27]
  • 2023 BYD Auto introduces advanced active hydropneumatic suspension systems on the Yangwang U8 SUV and U9 sportscar. The suspension features the ability to drive with only three wheels fitted, and jump in the air while parked remaining level.[28]

Functioning

[edit]
Diagram of the Hydractive system, showing centre spheres and stiffness valves

At the heart of the system, acting as pressure sink as well as suspension elements, are the so-called spheres, five or six in all; one per wheel and one main accumulator as well as a dedicated brake accumulator on some models. On later cars fitted with Hydractive or Activa suspension, there may be as many as ten spheres. Spheres consist of a hollow metal ball, open to the bottom, with a flexible Desmopan rubber membrane, fixed at the 'equator' inside, separating top and bottom. The top is filled with nitrogen at high pressure, up to 75 bar; the bottom connects to the car's hydraulic fluid circuit. The high pressure pump, powered by the engine, pressurizes the hydraulic fluid (LHM – liquide hydraulique minéral) and an accumulator sphere maintains a reserve of hydraulic power. This part of the circuit is at between 150 and 180 bars. It powers the front brakes first, prioritised via a security valve, and depending on type of vehicle, can power the steering, clutch, gear selector, etc.

Pressure flows from the hydraulic circuit to the suspension cylinders, pressurizing the bottom part of the spheres and suspension cylinders. Suspension works by means of a piston forcing LHM into the sphere, compressing the nitrogen in the upper part of the sphere; damping is provided by a two-way 'leaf valve' in the opening of the sphere. LHM has to squeeze back and forth through this valve which causes resistance and controls the suspension movements. It is the simplest damper and one of the most efficient. Ride height correction (self leveling) is achieved by height corrector valves connected to the anti-roll bar, front and rear. When the car is too low, the height corrector valve opens to allow more fluid into the suspension cylinder (e.g., the car is loaded). When the car is too high (e.g. after unloading) fluid is returned to the system reservoir via low-pressure return lines. Height correctors act with some delay in order not to correct regular suspension movements. The rear brakes are powered from the rear suspension circuit. Because the pressure there is proportional to the load, so is the braking power.

Working fluid

[edit]

Citroën quickly realized that standard brake fluid was not ideally suited to high-pressure hydraulics, and developed a special red-coloured hydraulic fluid named Liquide Hydraulique Synthétique (LHS), which they used from 1954 to 1967. The chief problem with LHS was that it absorbed moisture and dust from the air, which caused corrosion in the system. Most hydraulic brake systems are sealed from the outside air by a rubber diaphragm in the reservoir filler cap, but the Citroën system had to be vented to allow the fluid level in the reservoir to rise and fall, thus it was not hermetically sealed. Consequently, each time the suspension would rise, the fluid level in the reservoir dropped, drawing in fresh moisture-laden air. The large surface of the fluid in the reservoir readily absorbed moisture. Since the system recirculates fluid continually through the reservoir, all the fluid was repeatedly exposed to the air and its moisture content.

LHM reservoir and green suspension sphere in a Citroën Xantia

To overcome these shortcomings of LHS, Citroën developed a new green fluid, LHM (Liquide Hydraulique Minéral). LHM is a mineral oil, quite close to automatic transmission fluid. Mineral oil is hydrophobic, unlike standard brake fluid; therefore, water-vapour bubbles do not form in the system, as would be the case with standard brake fluid, creating a "spongy" brake feel. Use of mineral oil has thus spread beyond Citroën, Rolls-Royce, Peugeot, and Mercedes-Benz, to include Jaguar, Audi, and BMW.[29]

LHM, being a mineral oil, absorbs only an infinitesimal proportion of moisture, plus it contains corrosion inhibitors. The dust inhalation problem continued, so a filter assembly was fitted into the hydraulic reservoir. Cleaning the filters and changing the fluid at the recommended intervals removes most dust and wear particles from the system, ensuring the longevity of the system. Failure to keep the oil clean is the main cause of problems. It is also imperative to always use the correct fluid for the system; the two types of fluids and their associated system components are not interchangeable. If the wrong type of fluid is used, the system must be drained and rinsed with Hydraflush (Total's Hydraurincage), before draining again and filling with the correct fluid. These procedures are clearly described in DIY manuals obtainable from automotive retailers.

The latest Citroën cars with Hydractive 3 suspension have a new orange coloured LDS hydraulic fluid. This lasts longer and requires less frequent attention. It conforms to DIN 51524-3 for HVLP.[30]

Manufacturing

[edit]

The whole high-pressure part of the system is manufactured from steel tubing of small diameter, connected to valve control units by Lockheed-type pipe unions with special seals made from Desmopan, a type of polyurethane thermoplastic compatible with the LHM fluid. The moving parts of the system, e.g., suspension struts and steering ramm, are sealed by contact seals between the cylinder and piston for tightness under pressure. The other plastic and rubber parts are return tubes from valves, such as the brake control and height corrector valves, also catching seeping fluid around the suspension push-rods. Height corrector, brake master valve and steering valve spools, and hydraulic pump pistons have extremely small clearances (one to three micrometres) within their cylinders, permitting only a very low leakage rate. The metal and alloy parts of the system rarely fail, even after excessively high mileages, but the elastomer components (especially those exposed to the air) can harden and leak, typical failure points for the system.

Spheres are not subject to mechanical wear but suffer pressure loss due to the pressurised nitrogen diffusing through the membrane. They can, however, be recharged, which is cheaper than replacing them. When Citroën designed their Hydractive 3 suspension they redesigned the spheres with new nylon membranes, which greatly slow the rate of diffusion. These are recognisable by their grey colouring.

Classic (non-saucer) green- and grey-coloured suspension spheres typically last between 60,000 and 100,000 km. Spheres originally had a threaded plug on top for recharging. Newer ('saucer') spheres do not have this plug, but it can be retrofitted, enabling them to be recharged with gas. The sphere membrane has an indefinite life unless run at low pressure, which leads to rupture. Timely recharging, approximately every three years, is thus vital. A ruptured membrane means suspension loss at the attached wheel; however, ride height is unaffected. With no springing other than the (slight) flexibility of tyres, hitting a pothole with a flat sphere can bend the suspension parts or dent a wheel rim. In the case of main accumulator sphere failure, the high-pressure pump is the only source of braking pressure for the front wheels. Some older cars had a separate front-brake accumulator on power-steering models.

The old LHS and LHS2 (coloured red) cars used a different elastomer in the diaphragms and seals that is not compatible with green LHM. The orange LDS fluid in Hydractive cars is also incompatible with other fluids.

Legacy

[edit]

The principles illustrated by the successful use of hydropneumatic suspension are now used in a broad range of applications, such as aircraft oleo struts and gas filled automobile shock absorbers, first patented in the U.S. in 1934[31] by Cleveland Pneumatic Tool Co. Similar systems are also widely used on modern tanks and other large military vehicles.

Hydractive

[edit]

Hydractive Suspension is an automotive technology introduced by Citroën in 1990. The prototype debuted in 1988 on the Citroën Activa concept. It describes a development of the 1954 hydropneumatic suspension design using additional electronic sensors and driver control of suspension performance. The driver can make the suspension stiffen (sport mode) or ride in outstanding comfort (soft mode). Sensors in the steering, brakes, suspension, throttle pedal and gearbox feed information on the car's speed, acceleration, and road conditions to an on-board computer, which in turn activates or deactivates an extra pair of suspension spheres on the circuit, to enable either a smoother, more supple ride or tighter handling in corners. On the Activa and Activa 2, the car leaned inwards by one degree in turns – Citroën acknowledged that this was somewhat of a marketing gimmick, and that a lean of zero degrees was optimal.[32]

An additional, perhaps unexpected, benefit of active suspension is that fuel consumption and tire wear is reduced overall. The negative camber designed into most suspensions in order to maximize the size of the contact patch when turning causes tire scrub, which wears out tires and increases fuel consumption.[32]

Hydractive 1 and Hydractive 2

[edit]

Citroën Hydractive (and later Hydractive 2) suspension was available on several models, including the XM and Xantia, which had a more advanced sub-model known as the Activa. The first Hydractive suspension systems (now known as Hydractive 1) had two user presets, Sport and Auto. In the Sport setting the car's suspension was always kept in its firmest mode. In the Auto setting, the suspension was switched from soft to firm mode temporarily when a speed-dependent threshold in accelerator pedal movement, brake pressure, steering wheel angle, or body movement was detected by one of several sensors.[23]

In Hydractive 2, the preset names were changed to Sport and Normal. In this new version the Sport setting would no longer keep the suspension system in firm mode, but instead lowered the thresholds significantly for any of the sensor readings also used in Normal mode, allowing for a similar level of body firmness during cornering and acceleration, without the sacrifice in ride quality the Sport mode in Hydractive 1 systems had caused.

Whenever the Hydractive 1 or 2 computers received abnormal sensor information, often caused by malfunctioning electrical contacts, the car's suspension system would be forced into its firm setting for the remainder of the ride.

Starting with Xantia model year 1994 and XM model year 1995, all models featured an additional sphere and valve that together functioned as a pressure reservoir for rear brakes because of new hydraulic locks, letting the car retain normal ride height for several weeks without running the engine. Correctly called the SC/MAC sphere, it often became known as the 'anti-sink' sphere, because of its ability to better maintain rear suspension height.

Hydractive 3

[edit]

The 2001 Citroën C5 has continued development of Hydractive suspension with Hydractive 3. Compared to earlier cars, the C5 stays at normal ride height even when the engine is turned off for an extended period, through the use of electronics. The C5 also uses orange synthetic hydraulic fluid named LDS in place of the green LHM mineral oil used in millions of hydropneumatic vehicles.[30]

A further improved Hydractive 3+ variation was for cars with top engines on the Citroën C5 and in 2005 was standard on the Citroën C6. Hydractive 3+ systems contain additional spheres that can be engaged and disengaged via a Sport button, resulting in a firmer ride.

The Hydractive 3 hydraulic suspension has two automatic modes:

  • Motorway position (lowering by 15 mm of the vehicle height above 110 km/h)
  • Poor road surface position (raising by 13 mm of the vehicle height below 70 km/h)

The BHI of the Hydractive 3 suspension calculates the optimum vehicle height, using the following information:

  • Vehicle speed
  • Front and rear vehicle heights

The 3+ Hydractive hydraulic suspension has three automatic modes:

  • Motorway position (lowering by 15 mm of the vehicle height above 110 km/h)
  • Poor road surface position (raising by 13 mm of the vehicle height below 70 km/h)
  • Comfort or dynamic suspension (variation of suspension firmness)

The BHI of the 3+ Hydractive suspension calculates the optimum vehicle height, using the following information:

  • Vehicle speed
  • Front and rear vehicle heights
  • Rotation speed of steering wheel
  • Angle of rake of steering wheel
  • Vehicle's longitudinal acceleration
  • Vehicle's lateral acceleration
  • Speed of suspension travel
  • Movement of the accelerator throttle

C5 I (2001–2004)

C5 II (2004–2007)

  • Hydractive hydraulic suspension 3: EW7J4, EW10A, DV6TED4 and DW10BTED4 engines.
  • Hydractive hydraulic suspension 3+: ES9A and DW12TED4 engines (prior to RPO No 10645).

C6 (2005–2012)

  • Hydractive hydraulic suspension 3+: Standard on all models.

C5 III X7 (2007–2017)

  • Hydractive hydraulic suspension 3+: Depends on country and trim.

See also

[edit]
  • Hydrolastic – a type of automotive suspension system used in many cars produced by British Leyland and its successor companies.
  • Hydragas – is an improved form of Hydrolastic, using nitrogen-pressurised gas springs, rather than rubber.
  • Hydraulic recoil mechanism – uses the same principal for artillery.
  • Oleo strut – suspension for most large aircraft, using the same physical properties of air and hydraulic fluid.
  • Active Body Control – ABC, is the Mercedes-Benz brand name used to describe hydropneumatic fully active suspension, that allows control of the vehicle body motions and therefore virtually eliminates body roll in many driving situations including cornering, accelerating, and braking.
  • Air suspension – a type of vehicle suspension powered by an electric or engine-driven air pump or compressor. This compressor pumps the air into a flexible bellows, usually made from textile-reinforced rubber. The air pressure inflates the bellows, and raises the chassis from the axle.
  • Electronic Air Suspension (EAS) is the air suspension system installed on the second version of the Range Rover. Five suspension heights are offered by this system.

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Hydropneumatic suspension

View on Grokipedia
from Grokipedia
Hydropneumatic suspension is a type of vehicle suspension system that integrates and pressurized gas, typically , to serve as both the spring and damper mechanism, replacing conventional mechanical springs and shock absorbers. This design enables nonlinear stiffness that adapts to varying loads, providing self-leveling capabilities and adjustable for enhanced stability and comfort over uneven terrain. Invented by French engineer Paul Magès at during the 1940s amid secrecy, the system was first implemented in production vehicles on the in 1955, marking a pioneering advancement in . Key components include a high-pressure driven by the engine, accumulators or spheres containing the gas-fluid interface for spring action, height-correcting valves, and interconnected struts that allow fluid transfer for load distribution. The system's advantages encompass superior , reduced body roll and pitching, and integration with other hydraulic functions like and braking in models, contributing to exceptional handling and ride quality. Widely adopted by across models from the DS to the C5 and C6, with discontinuation in 2017 following the end of C5 production, hydropneumatic suspension has also found applications in military vehicles, , and construction equipment for its robustness in demanding conditions.

Overview

Description

Hydropneumatic suspension is a specialized suspension system that combines with pressurized gas to deliver automatic vehicle leveling, superior ride comfort, and adjustable height control. The design leverages the compressibility of the gas as a progressive spring medium while using the fluid for damping and load distribution, creating a responsive oleo-pneumatic mechanism. This technology was pioneered by engineer Paul Magès at during the 1940s, adapting oleo-pneumatic principles initially developed for aircraft to automotive applications. Magès' innovation focused on integrating high-pressure to replace conventional mechanical springs, enabling dynamic performance without rigid components. Hydropneumatic systems have found use in passenger cars and luxury vehicles for refined handling, as well as in tanks and heavy machinery like dump trucks and all-terrain cranes, where they enhance stability under extreme loads and terrains. In contrast to passive or air bag suspensions, hydropneumatic setups utilize interconnected hydraulic circuits for real-time adaptability, allowing the system to respond instantly to changes in vehicle attitude and road conditions. This includes a self-leveling feature that maintains consistent under varying payloads.

Benefits and Effects

Hydropneumatic suspension delivers exceptional ride quality, characterized by smoothness over bumps and reduced pitch and roll motions, owing to the compliant nature of gas-filled spheres combined with hydraulic . This results in a "flying " sensation for occupants, where road imperfections are isolated effectively from the vehicle's body, enhancing overall comfort during varied driving conditions. The system's self-leveling capability maintains a constant regardless of load variations, such as passenger or cargo changes, which optimizes , ensures proper headlight alignment, and preserves handling consistency. Height adjustment features further allow drivers to raise the vehicle for off-road clearance or lower it for highway efficiency, contributing to versatile performance across scenarios. Safety benefits include inherent anti-squat and anti-dive properties that minimize body pitch during and braking, alongside variable damping that enhances cornering stability by adjusting to road forces. These effects reduce the risk of loss of control and improve braking efficiency by linking suspension response to load distribution. Quantitative assessments highlight substantial improvements, with nitrogen spheres providing approximately six times the flexibility of steel springs, leading to significant reductions in vertical acceleration transmitted to the vehicle body. However, the system's higher complexity can result in elevated repair costs due to specialized components.

Design and Operation

Mechanical Components

The key mechanical components of a hydropneumatic suspension system include hydraulic spheres, struts, a high-pressure pump, distributor valves, and height corrector sensors, which together form the core physical structure for vehicle support and leveling. These elements replace traditional coil springs and separate shock absorbers with an integrated hydraulic setup, where fluid and gas interact within a closed circuit. Hydraulic spheres act as both accumulators for energy storage and suspension elements, typically constructed from steel housings that enclose a volume of nitrogen gas separated from the hydraulic fluid by a rubber or polyurethane diaphragm. The diaphragm, often made from materials like Urepan® or Desmopan® polyurethane for high tensile strength and low gas permeability, prevents mixing while allowing compression of the gas chamber to provide progressive spring rates. Suspension spheres are mounted directly at each wheel, while a central accumulator sphere maintains overall system pressure, with nominal volumes of about 385 cm³ and pressures around 5.7 MPa. Struts serve as the primary load-bearing units, integrating functions for both spring and , typically using for the cylinder body and piston assembly. In front suspensions, such as McPherson designs, the strut houses the , , and an attached , with internal elastic diaphragms and orifices to control fluid flow for . Rear struts or arms connect similarly, linking the vehicle's body to the wheels via these hydraulic elements. The high-pressure pump, engine-driven and usually vane-type, circulates the —such as the green-dyed LHM —throughout the system at up to 17.5 MPa to ensure consistent operation. Distributor valves, including anti-pitch and safety variants, are precision-machined blocks that regulate distribution to the struts and spheres, prioritizing functions like braking if pressure drops. Height corrector sensors, mechanical devices often linked to suspension arms or anti-roll bars, monitor via displacement and activate valves to add or remove for automatic leveling under varying loads. The layout features a centralized hydraulic circuit with high-pressure lines interconnecting the , valves, and all four wheels, where each strut's feeds into its dedicated suspension containing the gas-charged diaphragm. This interconnected design allows uniform pressure distribution, with return lines for low-pressure recirculation back to a . In the basic 1950s configuration, as first implemented in production vehicles, the system used purely mechanical linkages and valves without electronic aids. Later evolutions incorporated electronic sensors alongside these core mechanical parts for refined control, though the fundamental and rubber components remained central.

Functioning Principles

Hydropneumatic suspension operates by integrating dynamics with pneumatic elasticity to manage vehicle , load distribution, and . The core mechanism relies on a , driven by the , which pressurizes special fluid—typically to 150-180 bar—to transfer forces between the wheels and the vehicle body. This pressurized fluid fills hydraulic cylinders at each wheel, while nitrogen-charged spheres act as accumulators, providing a progressive spring rate through gas compression. The spheres separate the incompressible from the compressible gas via a flexible diaphragm, allowing the gas to absorb impacts by varying volume and pressure without direct contact. Self-leveling is achieved through height sensors, often mechanical correctors linked to the suspension linkages, that monitor the vehicle's relative to the axles. When a change in load or road conditions alters the height—such as by more than 20 mm over approximately 5 seconds—the sensors actuate control valves to either pump additional into the cylinders or release excess back to a , restoring equilibrium and maintaining a constant height independent of variations. This process ensures balanced weight distribution across all wheels without manual adjustment. Damping in the system arises from the controlled flow of hydraulic fluid through orifices and valves, where the fluid's viscosity dissipates oscillatory energy as heat via throttling losses. The damper valves, integrated into the spheres or struts, restrict fluid movement during compression and rebound, with the pressure drop Δp\Delta p proportional to the square of the volume flow rate, providing velocity-sensitive resistance that reduces vibrations. The nitrogen spheres contribute to the progressive nature of the spring rate, derived from the polytropic compression of the gas. The effective spring rate KK is given by: K=γPA2VK = \frac{\gamma P A^2}{V} where γ\gamma is the polytropic index (typically around 1.3 for nitrogen), PP is the gas pressure, AA is the piston area, and VV is the gas volume. This formulation yields a nonlinear stiffness that increases with deflection, enhancing stability under varying loads. In basic configurations, anti-roll is minimized by the progressive stiffness of the spheres and vehicle geometry, while load compensation is handled by the self-leveling mechanism's redistribution of fluid pressure, maintaining even axle loading without altering the overall system equilibrium. Advanced variants incorporate active anti-roll features, such as hydraulic control of anti-roll bars. In failure modes, a rupture—often due to diaphragm failure after extended use—allows to escape and to fill the gas chamber, resulting in loss of spring elasticity and progressive sagging of the affected corner to the bump stops. This creates a limp mode where the remains drivable on the hydraulic struts alone, though ride quality and handling are compromised until repair. or failures may similarly lead to loss, but reserve capacity in the main accumulator provides temporary operation.

Working Fluid

The working fluid in hydropneumatic suspension systems is a specialized hydraulic oil that serves as both a medium and a agent, requiring low for precise control, adequate for system components, and stability across a wide range. Early implementations, starting with the 1955 , utilized Liquide Hydraulique Végétal (LHV), a vegetable-based derived from , which was red in color and provided good but suffered from hygroscopicity leading to in humid environments. This was succeeded in late 1964 by Liquide Hydraulique Synthétique (LHS), a synthetic glycol-based also red in color, with a kinematic of 14.5–16.5 cSt at 40°C (approximately 40–50 cSt at 20°C), designed for better compatibility with EPDM seals but still requiring frequent changes every 18,000 miles or annually to mitigate absorption and risks. From 1967 onward, the system transitioned to Liquide Hydraulique Minéral (LHM), a green-dyed oil-based with additives for enhanced stability, marking a significant toward non-hygroscopic performance to reduce . LHM exhibits a kinematic of about 18 cSt at 40°C and 6.3 cSt at 100°C, a high of approximately 355 for consistent flow across temperatures from -40°C to 100°C, a exceeding 250°C, and compatibility with or Viton rubber seals, enabling longer service intervals of 24,000 miles between changes. Its near-zero ensures efficient force transmission without significant volume change under pressure, while its formulation provides inherent for the high-pressure and fire resistance due to the elevated boiling and flash points compared to glycol alternatives. Annual fluid inspections are recommended to verify condition and prevent degradation, particularly in vehicles with extended use. In 2001, with the introduction of the Hydractive 3 system in models like the and C6, the fluid evolved to Liquide Hydraulique de Synthèse (LDS), a fully synthetic orange-colored oil optimized for electronic integration and extreme conditions. LDS maintains a kinematic viscosity of 18 mm²/s at 40°C and 5.9 mm²/s at 100°C, with an exceptionally high of 320, a of -51°C, and operational stability up to 130°C system temperatures, offering superior anti-wear, anti-corrosion, and lubricating properties for pumps and valves. It is incompatible with prior mineral fluids like LHM, necessitating a complete system flush during conversion. Regarding environmental considerations, the original vegetable-based LHV offered relatively high biodegradability, breaking down more readily in and than later formulations, though its hygroscopic nature complicated safe disposal. In contrast, LHM and its variants, while formulated with inhibitors for durability, have moderate biodegradability—meeting some eco-label criteria but requiring proper or disposal at authorized facilities to minimize and contamination, as they are not fully rapid-degraders like vegetable oils. LDS, as a synthetic, prioritizes performance stability over inherent biodegradability, with disposal guided by local regulations to avoid environmental release; however, its longer lifespan reduces overall fluid consumption and waste.

Historical Development

Invention and Early Innovations

The hydropneumatic suspension system was invented by Paul Magès, a self-taught who joined the company in as a draftsman. Drawing inspiration from oleopneumatic systems developed by Georges Messier in the 1920s for aircraft and early automotive experiments, Magès sought to create a self-leveling suspension that combined for with compressed gas for springing, eliminating traditional metal springs. His work began in earnest in 1942 when director Pierre-Jules Boulanger tasked him with improving braking and suspension for post-war vehicles, leading to secretive development during due to material shortages and German occupation. Early prototypes emerged in the late , with a functional version installed on a by 1949 to test load-leveling and ride comfort. These initial setups demonstrated the system's ability to maintain constant ground clearance under varying loads but faced significant hurdles, including persistent fluid leaks from high-pressure hydraulic lines and the overall mechanical complexity, which deterred many engineers who viewed it as impractical for . Magès iterated on designs, incorporating accumulators—spherical reservoirs of gas separated from the by a diaphragm—to provide progressive and prevent oil-gas mixing. Key patents for the system were filed by in the early 1950s, protecting the integrated hydraulic circuit that powered suspension, , and braking functions from a single engine-driven . A major pre-production milestone came in 1954, when a simplified rear-only hydropneumatic setup was fitted to the 15/6H model as a , allowing real-world validation of correction and ride before broader . This partial adoption marked the transition from experimental prototypes to viable automotive technology, addressing earlier leak issues through refined seals and materials.

Automotive Implementations

The hydropneumatic suspension system achieved its first major commercial success with Citroën's adoption in passenger vehicles, beginning with the 1955 DS, which was the first production car to feature a full implementation of the technology across all four wheels, providing exceptional ride comfort and self-leveling capabilities. This innovation was carried forward to subsequent models, including the 1970 SM, where it was integrated with advanced features like variable-assist to enhance handling in a context. By 1974, the CX further refined the system for a mid-size sedan, maintaining the signature smooth ride while incorporating updated hydraulic components for improved durability. Throughout the and , hydropneumatic suspension became a hallmark of Citroën's lineup, appearing as standard equipment on upper-trim and flagship models such as the GS/GSA, BX, XM, and Xantia, underscoring the company's commitment to superior ride quality until the late . Citroën's technology gained broader industry recognition through licensing agreements with luxury automakers seeking to elevate ride refinement. In 1965, Rolls-Royce licensed the system for the Silver Shadow, applying it from 1965 to 1980 to deliver unparalleled isolation from road imperfections in a high-end sedan, marking a departure from the brand's traditional coil-spring setups. Mercedes-Benz followed suit in 1975 with the 450SEL 6.9, incorporating a Citroën-derived full four-wheel hydropneumatic suspension to complement its powerful and provide self-leveling under heavy loads, a feature that distinguished it among contemporaries. Similarly, the 1974 Maserati Quattroporte II adopted the system via Citroën's ownership of , utilizing an extended SM chassis with and hydropneumatic elements for a compliant ride in a sporting saloon produced until 1978. Within the PSA Group, Peugeot selectively integrated hydropneumatic components, starting with the 1990 405 Mi16x4, which featured rear-only hydropneumatic suspension for automatic load leveling in its all-wheel-drive variant, enhancing stability without the full-system complexity. This partial adoption continued in higher-end models, though the 1990 605 relied on conventional shared with the platform, forgoing full hydropneumatics to prioritize cost efficiency in the executive segment. The system's decline in passenger cars stemmed from escalating costs and complexity, which proved challenging in an era of platform-sharing and emissions regulations. PSA Peugeot Citroën phased out hydropneumatic suspension across its lineup by 2017, with the marking the final model to offer it, as the group shifted toward more standardized, electronically controlled alternatives to reduce production expenses and improve reliability.

Applications Beyond Cars

Hydropneumatic suspension systems have found significant application in military vehicles, particularly tanks, where they enhance mobility across rugged terrains while improving crew comfort and operational stability. The French light tank, originally produced from the 1950s, underwent upgrades in the late 1980s that incorporated hydropneumatic suspension units to replace earlier torsion bar systems, allowing for greater wheel travel and better absorption of shocks on uneven ground. This adaptation proved beneficial for reconnaissance and rapid deployment in rough environments, reducing fatigue during extended missions. Similarly, the Leclerc , introduced in the 1990s, features a hydropneumatic suspension developed by Société d'Applications des Machines Motrices (SAMM), which enables the vehicle to maintain a low profile for stealth while providing exceptional cross-country performance and precise gun stabilization during movement. These systems allow the Leclerc to achieve off-road speeds up to 60 km/h with minimal vibration transmission to the crew, thereby sustaining accuracy in fire control and overall mission endurance. In heavy machinery, hydropneumatic suspension has been employed to address demands for self-leveling and stability under variable loads, particularly in agricultural and equipment. The series of high-speed tractors, introduced in the 1990s, utilizes advanced hydropneumatic suspension on both axles to deliver superior ride comfort and handling at speeds exceeding 50 km/h on fields or roads, while automatically adjusting to payload changes for enhanced traction and reduced . This design supports the tractors' role in versatile farming operations, where it mitigates operator fatigue over long hours and maintains stability during towing heavy implements. In earthmoving machinery, such as certain excavators and loaders, hydropneumatic systems have been developed to handle high-speed traversal of uneven sites, with early implementations in the demonstrating their ability to withstand speeds up to 30 mph over rough terrain while protecting components from excessive wear. The technology's origins trace back to aircraft landing gear, where oleo-pneumatic shock absorbers—precursors to modern hydropneumatic designs—were pioneered in the early to cushion high-impact landings and absorb energy from compressed gas and . Adaptations beyond ground vehicles remain limited; in rail applications, multi-cylinder hydropneumatic suspensions have been explored for road-rail hybrid vehicles to improve ride quality and adaptability to mixed terrains, though widespread adoption is constrained by infrastructure demands. Marine uses are similarly niche, primarily in military amphibious assault vehicles like the U.S. Marine Corps' Assault Amphibian Vehicle, where in-arm hydropneumatic units provide adjustable for transitions between water and land, ensuring durability during high-load beach landings. Across these non-automotive fields, hydropneumatic suspension offers key advantages, including automatic height and stiffness adjustment to extreme loads—up to several tons in tanks or machinery—while providing progressive that isolates and enhances longevity of undercarriage components in harsh conditions. This self-leveling capability ensures consistent stability on slopes or during payload shifts, outperforming rigid or mechanical alternatives in and operator , as evidenced by reduced needs in prolonged field operations.

Advanced Variants

Hydractive Systems Overview

Hydractive systems represent an evolution of the hydropneumatic suspension, introducing electronic control to enhance adaptability and performance. Debuting in 1989 on the , these systems added computer-managed variable damping, allowing the suspension to switch between soft and firm ride modes for optimized comfort and handling. At the core of Hydractive upgrades are sensors monitoring key , including speed, , braking , and body movement, which feed data to an (ECU). This ECU activates valves—typically two, one for the front and one for the rear—to adjust flow in real-time, altering characteristics by connecting or isolating accumulator spheres. The Hydractive family progressed from the basic electronic implementation in Hydractive 1 to more sophisticated iterations, influencing Citroën's upper-lineup models such as the XM (1989–2000), Xantia (1993–2001), and C5 (2001–2017). These systems provided a balance of ride quality and dynamic response, becoming a signature feature in Citroën's executive vehicles during this period. While retaining the hydraulic core of traditional hydropneumatic suspension for inherent self-leveling and comfort, Hydractive introduces adaptive logic that enables sportier handling by firming the ride during cornering or high-speed maneuvers, reducing body roll without sacrificing everyday compliance.

Hydractive 1 and 2

Hydractive 1, introduced in 1989 on the , represented the first electronically controlled evolution of the hydropneumatic suspension, enabling automatic switching between comfort-oriented soft mode and sport-oriented firm mode to balance ride quality and handling. In soft mode, flows freely to all spheres on each for enhanced compliance and reduced resonance, while firm mode isolates the central sphere via an electro-, stiffening the setup and providing hydraulic anti-roll control through a center that limits cross- fluid transfer. The system relied on monitoring angle and rotational speed, accelerator pedal position, brake pressure, vehicle speed, and body movement to trigger mode changes, processed by an onboard computer for anticipatory adjustments. However, early implementations faced reliability challenges, particularly with the center and electro-, which could fail and lock the suspension in firm mode, leading to a harsh ride, often exacerbated by faulty multi-point connections or poor earthing in the computer. Hydractive 2, deployed from 1991 on updated Citroën XM models and the 1993 Xantia, built upon its predecessor with refined electro-hydraulic distributors and enhanced sensor integration for smoother transitions and more precise control. It introduced variable height adjustment modes, including a low-ride setting activated at highway speeds above approximately 110 km/h for improved aerodynamics and stability, managed through height correctors responsive to load and velocity. The system utilized up to seven sensors—including steering movement and speed, accelerator pedal dynamics, brake pressure (triggering at over 35 bar), vehicle speed, and body movement (detecting up to 180 mm displacement in 30 steps)—feeding data to a Texas Instruments-based ECU that processed inputs in under 25 ms to vary damping between modes via dual stiffness states and adjustable roll control. Like its forebear, it employed LHM mineral-based hydraulic fluid for compatibility with the high-pressure circuit operating at 150-180 bar, ensuring reliable fluid dynamics across components. Both variants were exclusive to PSA Group's Citroën lineup, with Hydractive 1 limited to the XM from 1989 to 1991 and Hydractive 2 extending to later XM iterations through 1998 alongside the Xantia until its phase-out. The ECU in Hydractive 2 incorporated diagnostic capabilities to flag inconsistencies, such as sensor faults or valve blockages, mitigating some of the reliability concerns from Hydractive 1 through updated parameters and faster response times under 0.05 seconds.

Hydractive 3

Hydractive 3, the third generation of Citroën's adaptive hydropneumatic suspension system, was introduced in 2001 on the first-generation and later featured on the top-range from 2005 to 2012. This iteration evolved from earlier Hydractive systems by incorporating advanced electronic controls and a specialized to enhance compatibility with vehicle electronics. The system utilized LDS (Low Dielectric Synthetic) , which features a low constant to prevent interference with onboard electrical components, alongside progressive that automatically adjusts between comfort and dynamic modes for optimized ride quality and handling. Key features of Hydractive 3 included automatic ride height reduction at higher speeds to improve aerodynamic stability and , seven suspension spheres (four main spheres, two anti-roll spheres, and one additional accumulator sphere per axle configuration), and electrovalves with response times under 100 milliseconds for rapid adjustments. The spheres, filled with gas and , provided variable spring rates, while the progressive allowed seamless transitions in stiffness based on driving dynamics, reducing body roll and pitch without compromising comfort. These enhancements contributed to a more refined suspension behavior compared to predecessors, with the system capable of maintaining consistent performance across varied loads. The employed an expanded array of , including height at each , a angle , longitudinal and transverse accelerometers, and a yaw rate , to monitor and road conditions in real time. Integrated with the vehicle's multiplexed network for data on brake pressure and engine speed, the (ECU) enabled adaptive responses to inputs like cornering forces or surface irregularities, switching modes instantaneously to prioritize either ride comfort or sporty handling. Reliability was improved over prior Hydractive versions through redesigned components and the durable LDS fluid, requiring no for up to 200,000 km or five years. Hydractive 3 marked the final major iteration of Citroën's hydropneumatic technology under PSA Peugeot Citroën, with its last implementation on the produced from 2006 to 2012. Following the C6's discontinuation, PSA phased out hydropneumatic systems in favor of conventional suspensions and newer hydraulic cushion technologies to reduce complexity and costs, ending a era of specialized engineering that began in the .

Modern Revivals

In recent years, hydropneumatic suspension principles have seen revival through integration into electric and hybrid vehicles, particularly emphasizing off-road capabilities and advanced control systems. The BYD Yangwang U8, an electric SUV launched in 2023, incorporates the DiSus-P intelligent hydraulic body control system, which utilizes hydropneumatic elements for dynamic height adjustment, vehicle leveling on uneven terrain, and specialized modes like "tank turn" that enable zero-radius pivoting by independently lifting wheels. This system seamlessly integrates with the vehicle's electric powertrain and battery, optimizing energy efficiency by reducing reliance on mechanical components during suspension adjustments. Similarly, the Yangwang U9 supercar, also introduced in 2023, employs the DiSus-X variant, combining hydropneumatic hydraulics with air suspension for extreme maneuvers such as vertical jumps and three-wheel driving, enhancing stability in high-performance electric applications. Beyond consumer vehicles, hydropneumatic systems have been updated in luxury and military contexts. The Mercedes-Maybach GLS, in its 2020s iterations, features E-ACTIVE BODY CONTROL, an electrohydraulically actuated hydropneumatic suspension that hybridizes with air springs for adaptive damping and adjustment up to 3.5 inches, providing superior comfort and handling in premium SUVs. In military applications, the French tank upgrade, unveiled in 2023, retains and refines its original hydropneumatic suspension for improved cross-country mobility, achieving a top speed of 72 km/h while maintaining stability over obstacles up to 1.25 meters high, with electronic enhancements for better terrain adaptation. These revivals incorporate innovations such as fully electronic controls without mechanical linkages, as seen in the DiSus system's sensor-driven actuators that respond in milliseconds to road inputs, improving precision and reducing weight compared to traditional setups. Post-2020 patents, including BYD's filings for hydropneumatic energy recovery in EV suspensions, focus on EV compatibility by minimizing fluid volume through compact accumulators and regenerative that recapture suspension motion energy to recharge batteries in off-road scenarios. Looking ahead, hydropneumatic suspensions hold potential for autonomous , where multi-mode electronic controls enable dynamic load balancing to compensate for shifting payloads from passengers or , as explored in adaptive strategies for uneven urban environments. Environmental considerations are also advancing, with biodegradable synthetic hydraulic fluids (e.g., polyalkylene glycol-based formulations meeting 301B standards) increasingly adopted to reduce ecological impact in case of leaks, as patented for shock and suspension applications (WO2012058737A2, 2012, with ongoing adaptations).

Production and Legacy

Manufacturing Processes

The manufacturing of hydropneumatic suspension components involves specialized processes to ensure the system's ability to handle high pressures and maintain separation between hydraulic fluid and gas. Steel spheres, central to the suspension, contain a flexible Desmopan rubber membrane, a polyurethane material compatible with the hydraulic fluid, to separate the nitrogen gas chamber from the fluid side; spheres are charged with dry nitrogen at approximately 75 bar, capable of withstanding system pressures up to 180 bar. Hydraulic tubing, which connects the spheres, cylinders, and valves throughout the , is produced from seamless low-carbon tubing designed for high-pressure applications. This tubing undergoes cold drawing to achieve precise dimensions and wall thickness, followed by to fit the vehicle's and flaring at the ends to form leak-proof connections compatible with the 's pipe unions. Valves and distributors incorporate Desmopan seals and O-rings for control and include two-way leaf valves that provide by regulating flow between the sphere and working cylinder. Assembly of the hydropneumatic system occurred on dedicated production lines at facilities, such as the PSA plant in , , from the through the , where components were integrated into the before final vehicle assembly. Key steps include mounting the spheres directly onto MacPherson struts at the front or trailing arms at the rear, connecting them via the hydraulic tubing, and installing height correctors linked to the anti-roll bars for load leveling. The system is filled with LHM (Liquide Hydraulique Minéral). Assembly took place at the PSA plant in , , until discontinuation in 2017. The specialized nature of these processes resulted in higher manufacturing costs than those for conventional coil-spring suspensions, though at PSA plants achieved that made the system viable for high-volume models like the DS and XM.

Maintenance and Longevity

Routine maintenance for hydropneumatic suspension systems primarily involves periodic checks and replacements of and suspension spheres to ensure optimal performance and prevent failures. The , typically LHM or LDS mineral oil, should be inspected annually for levels and contamination, with a full replacement recommended every five years or as indicated by discoloration from green to yellow or black. A complete fluid change requires approximately 3-5 liters, depending on the model, including draining the , cleaning filters, and the system to remove air. Sphere pressure tests, which assess charge integrity using a pressure gauge, are advised every 80,000-100,000 km or 5-6 years to detect gradual gas that could compromise ride quality. Common issues in hydropneumatic suspensions often stem from component wear under prolonged use, with sphere fatigue being prevalent due to nitrogen leakage through the rubber diaphragm, typically manifesting after 100,000-200,000 km and leading to a harsher ride or sagging. Pump wear from continuous operation can cause inconsistent delivery, while leaks from degraded seals in cylinders or hoses result in fluid loss and erratic height adjustment; these are diagnosed through of and height corrector function. The system's longevity is enhanced by its robust design, capable of exceeding 300,000 km with diligent care, as evidenced by high-mileage models like the BX maintaining functionality well beyond standard expectations. Post-2017, aftermarket parts such as spheres, pumps, and seals remain widely available as of 2025 from specialized suppliers, supporting continued repairs despite the system's discontinuation in production vehicles. Hydropneumatic suspension has left a lasting legacy by pioneering self-leveling and adaptive concepts that influenced modern technologies, such as electromagnetic and hydraulic systems in luxury vehicles for improved ride control. Repair expertise for these systems is largely concentrated among specialists familiar with hydraulic diagnostics, given the complexity beyond standard mechanical services.

References

  1. https://nimbus-suspensions.com/oleo-pneumatic-technology
  2. https://classicsworld.co.uk/guides/how-it-works-citroens-fluid-suspension/
  3. https://www.engineeringletters.com/issues_v31/issue_2/EL_31_2_23.pdf
  4. http://www.citroenet.org.uk/miscellaneous/hydraulics/hydraulics-18.html
  5. https://www.hydac.com/en-us/hydropneumatic-suspension-systems-increasing-your-machines-driving-performance/
  6. https://www.argo-hytos.com/news/archive/archive/hydro-pneumatic-suspension-systems-faster-and-more-cost-effective-development.html
  7. https://www.researchgate.net/publication/299188982_Analysis_of_Structural_and_Material_Aspects_of_Selected_Elements_of_a_Hydropneumatic_Suspension_System_in_a_Passenger_Car
  8. https://citroen.tramontana.co.hu/en/suspension/hydropneumatic-suspension
  9. https://beckassets.blob.core.windows.net/product/readingsample/841441/9783642151460_excerpt_001.pdf
  10. https://www.sciencedirect.com/topics/engineering/gas-spring
  11. https://www.machinerylubrication.com/Read/31775/fire-resistant-fluids
  12. http://www.citrogsa.com/tony.html
  13. https://www.oqvalue.nl/wp-content/uploads/Shell_LHM-S_TDS.pdf
  14. https://totaloilnz.co.nz/technical-documents/4203.pdf
  15. https://www.oilonline.store/totaltotal-lhm-plus
  16. https://ateupwithmotor.com/model-histories/citroen-ds/
  17. https://citroenvie.com/citroens-innovative-hydropneumatic-supension-a-result-of-adoptive-engineering/
  18. https://www.autoevolution.com/news/citroen-hydropneumatic-suspension-explained-49954.html
  19. http://www.citroenet.org.uk/passenger-cars/michelin/sm/sm-01.html
  20. https://www.hagerty.com/media/automotive-history/in-le-retroviseur-the-citroen-cx-at-50/
  21. https://en.escuderia.com/70-anos-de-citroen-hidroneumaticos-todos-los-modelos-con-esta-tecnologia/
  22. https://hypcars.eu/linguistics-ans-history/
  23. https://www.octane-magazine.com/articles/features/mercedes-benz-450sel-6-9-class-act/
  24. https://www.maserati.com/global/en/brand/maserati-classic-cars/quattroporte/quattroporte-ii
  25. https://www.gotothegrid.com/en/blog/the-peugeot-405-mi16-french-icon-of-the-90s
  26. https://driventowrite.com/2024/01/15/troubles-come-in-sixes-part-one-peugeot-605/
  27. https://www.autonews.com/article/20150703/ANE/150709991/citroen-will-drop-hydropneumatic-suspension/
  28. https://citroenvie.com/the-end-of-hydropneumatics-for-citroen/
  29. http://www.army-guide.com/eng/product1390.html
  30. https://www.forecastinternational.com/archive/disp_old_pdf.cfm?ARC_ID=1107
  31. https://www.army-technology.com/projects/leclerc/
  32. http://www.army-guide.com/eng/product144.html
  33. https://www.defenseadvancement.com/projects/leclerc-main-battle-tank/
  34. https://www.jcb.com/en-US/products/machines/agricultural-tractors/8000-series/
  35. https://www.sae.org/papers/development-a-hydropneumatic-suspension-system-earthmoving-machine-660613
  36. https://www.mdpi.com/2076-3417/11/5/2221
  37. https://apps.dtic.mil/sti/citations/ADA205094
  38. https://micro.hendrickson-intl.com/military/45745-295.pdf
  39. http://www.citroenet.org.uk/passenger-cars/psa/xm/xm-01.html
  40. https://en.passionnement-citroen.com/post/citroen-innovations-the-legendary-citroen-hydraulic-suspension
  41. https://www.autozine.org/technical_school/suspension/tech_suspension3.htm
  42. http://www.citroenet.org.uk/miscellaneous/hydraulics/hydraulics-1.html
  43. https://www.sphere-discount.com/gb/content/99-qu-est-ce-que-le-systeme-hydractive-citroen-
  44. http://www.mycitroen.dk/library/xm/the%20citroen%20xm%20internet%20reference.pdf
  45. http://www.ckc.dk/pubs/technical-training-citroenhydractive-ii-xm-xantia-document-ref-no-62304.pdf
  46. https://www.citroenkerho.fi/xantia/pdf/ohjeet/Hydractive_2.pdf
  47. https://autocatalogarchive.com/wp-content/uploads/2023/02/Citroen-C5-2001-AU.pdf
  48. https://www.eurol.com/en/shop/products/E113673-eurol-lds-fluid/1858
  49. http://www.citroenet.org.uk/miscellaneous/hydraulics/hydraulics-13.html
  50. http://www.citroenet.org.uk/passenger-cars/psa/c5/c5tech1.html
  51. https://www.sphere-discount.com/gb/hydractive-3/51076-7-spheres-for-citroen-c5-hydractive-3.html
  52. https://www.autoevolution.com/news/this-is-not-the-final-citroen-c6-ever-made-yet-it-costs-149900-108436.html
  53. https://www.byd.com/us/news-list/BYD-Reveals-DiSus-Intelligent-Body-Control-System-Exclusively-for-New-Energy-Vehicles
  54. https://carnewschina.com/2024/02/14/discussing-disus-byds-world-class-suspension-system/
  55. https://media.mbusa.com/releases/release-b91f8b669cfb266e3c9808008e05d2cb-the-new-mercedes-maybach-gls
  56. https://armyrecognition.com/news/defense-web-tv/exclusive-report-discover-new-french-leclerc-xlr-one-of-most-modern-battle-tanks-in-the-world
  57. https://www.mdpi.com/2076-3417/15/12/6871
  58. https://patents.google.com/patent/WO2012058737A2/en
  59. https://www.industrialtube.com/seamless/
  60. https://citroen-owners-club.co.uk/citroen/topic/2580-how-often-should-i-change-lhm-fluid/
  61. https://frenchcarforum.co.uk/forum/viewtopic.php?t=22491
  62. https://www.chandlermotorcompany.co.uk/hydropneumatic-suspension-care-expert-advice-from-your-citroen-dealer-in-bristol/
  63. https://frenchcarforum.co.uk/forum/viewtopic.php?t=81179
  64. https://www.extrica.com/article/15773
  65. https://aerosus.com/citroen.html
  66. https://www.carolenash.com/news/classic-car-news/detail/citroen-hydropneumatic-suspension-system-innovative-design
Add your contribution
Related Hubs
User Avatar
No comments yet.