Hydropneumatic suspension is a type of motor vehicle suspension system, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers, notably Rolls-Royce (Silver Shadow), Maserati (Quattroporte II) and Peugeot. It was also used on Berliet trucks and has more recently been used on Mercedes-Benz cars. Similar systems are also used on some military vehicles. The suspension was referred to as oléopneumatique in early literature, pointing to oil and air as its main components.
- 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.
- Gas absorbs excessive force, whereas fluid in hydraulics directly transfers force
The principles illustrated by the successful use of Hydro-pneumatic suspension are now used broad range of applications, like aircraft oleo struts and gas filled automobile shock absorbers, first patented in the U.S. in 1960.
Hydro-pneumatic suspension has a number of natural advantages over steel springs, generally recognized in the auto industry.
Suspension and springing technology is not generally well understood by consumers, leading to a public perception that hydropneumatics are merely "good for comfort". They actually also have great advantages related to car handling and control efficiency, solving a number of problems inherent with using steel springs that suspension designers have always dreamt they could eliminate.
Among auto manufacturers that understood the inherent advantages over steel springs, it faced two additional issues. 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 have sought to create simpler variants using a compressed air suspension.
Citroën's invention of the system had a disadvantage that only garages equipped with special tools and know-how were qualified to work on the cars, making them seem radically different from ordinary cars with common mechanicals.
The nitrogen gas (air) as spring medium is approximately six times more flexible than conventional steel, so self-leveling is incorporated to allow the vehicle to cope with the extraordinary suppleness provided. France was noted for poor road quality in the post-war years, but the hydropneumatic suspension as fitted to the Citroën ID/DS and later cars ensured a smooth and stable ride even when quickly traversing poorly surfaced roads.
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).
Basic mechanical layout
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. It can also power any number of features such as the clutch, turning headlamps and even power windows.
Nitrogen is used as the trapped gas to be compressed, since it is unlikely to cause corrosion. A nitrogen reservoir with variable volume yields a spring with non-linear force-deflection characteristics. In this way the resulting system does not possess any eigenfrequencies and associated dynamic instabilities, which need to be suppressed through extensive damping in conventional suspension systems. The actuation of the nitrogen spring reservoir is performed through an incompressible hydraulic fluid inside a suspension cylinder. By adjusting the filled fluid volume within the cylinder, a leveling functionality is implemented. The nitrogen gas within the suspension sphere is separated from the hydraulic oil through a rubber membrane.
Citroën first introduced this system in 1954 on the rear suspension of the Traction Avant. The first four wheel implementation was in the advanced DS in 1955. 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.
- 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 7-piston pump, similar in size to a power steering pump, generates this pressure when the engine is running.
- 1959 Borgward P100 introduced its Airswing hydro-pneumatic suspension. The first German-market passenger car with Air suspension.
- 1960 The United States Patent and Trademark Office office issues US 2959410 A 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
- 1962 Morris Motors Limited introduces the BMC ADO16 ('1100') with hydrolastic suspension
- 1962 Mercedes-Benz W112 300SE introduced an Air suspension designed by Mercedes and Bosch based on the Borgward P100 system.
- 1964 Mercedes-Benz introduces the 600 with a complex air suspension designed around Citroën patents
- 1965 Rolls-Royce licenses Citroën technology for the suspension of the new Silver Shadow
- 1967 The superior non-hygroscopic LHM mineral fluid is introduced
- 1968 Mercedes-Benz introduces the 6.3 also with air suspension
- 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
- 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 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.
- 1983 Citroën BX, built as a 4WD in 1990
- 1989 Citroën XM: Hydractive Suspension, electronic regulation of the hydropneumatic system; sensors measure acceleration and other factors
- 1990 Peugeot 405 Mi16x4: first Peugeot equipped with rear hydropneumatic suspension
- 1990 JCB Fastrac high speed agricultural tractor uses this system for its rear suspension.
- 1995 Citroën Xantia: Optional Activa (active suspension) system, eliminating body roll by acting on torsion bars. A Xantia Activa was able to reach more than 1g lateral acceleration
- 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
- 2005 Citroën C6: An improved version of the C5 system known as Hydractive 3+ (also fitted to some C5 models)
- 2008 JCB Fastrac high speed 7000 series agricultural tractors now use this system for front and rear suspension.
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) 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, compacting 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 levelling) 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.
LHS versus LHM
Citroën quickly realized that standard brake fluid was not ideally suited to high pressure hydraulics, and developed a special red coloured hydraulic fluid named 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.
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 not hygroscopic (i.e., it will not absorb water from the air), unlike standard brake fluid, so therefore gas 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.
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, 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.
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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 rubber, a type of rubber compatible with the LHM fluid. The moving parts of the system (e.g., suspension strut or steering ram) are sealed by contact seals between the cylinder and piston for tightness under pressure. The other plastic/rubber parts are return tubes from valves such as the brake control or 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 (1–3 micrometres) with 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 rubber 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 re-designed the spheres with new nylon membranes, which greatly slow the rate of deflation. These are recognisable by their grey colouring.
Older, green coloured, suspension spheres typically last between 60,000 and 100,000 km. Spheres once had a threaded plug on top for recharging. Newer spheres do not have this plug, but it can be retrofitted enabling them to be re-gassed. The sphere membrane has an indefinite life unless run at low pressure, which leads to rupture. Timely recharging, approximately every 3 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 rubber 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.
Hydractive Suspension is a new automotive technology introduced by the French manufacturer 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 on-board computers. Where appropriate, and within milliseconds, these computers switch an extra pair of suspension spheres in or out of the circuit, to allow the car a smooth supple ride in normal circumstances, or greater roll resistance for better handling in corners. This development keeps Citroën in the forefront of suspension design, given the widespread goal in the auto industry of an active suspension system. All auto suspension is a compromise between comfort and handling. Auto manufacturers try to balance these aims and locate new technologies that offer more of both.
Hydractive 1 and Hydractive 2
Citroën hydractive (Hydractive 1 and Hydractive 2) suspension was available on several models, including the XM and Xantia, which had a more advanced sub-model known as the Activa. Hydractive 1 suspension systems 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.
In Hydractive 2, the preset names were changed to Sport and Comfort. 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 Comfort 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 that 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.
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 fluid in place of the green LHM mineral oil used in millions of hydropneumatic vehicles.
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 2 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 3 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)
- Hydractive hydraulic suspension 3: EW7J4 and DW10TD engines.
- Hydractive hydraulic suspension 3+: EW10J4, EW10D, ES9J4S and DW12TED4 engines.
C5 II (2004-2008)
- Hydractive hydraulic suspension 3: EW7J4, EW10A, DV6TED4 and DW10BTED4 engines.
- Hydractive hydraulic suspension 3+: ES9A and DW12TED4 engines (prior to RPO No 10645).
- Hydragas - a type of automotive suspension system used in many cars produced by British Leyland and its successor companies
- Oleo strut - suspension for most large aircraft, using the same physical properties of air and hydraulic fluid
- Heißing, Bernd; Ersoy, Metin: Fahrwerkhandbuch - Grundlagen, Fahrdynamik, Komponenten, Systeme, Mechatronik, Perspektiven. Wiesbaden: Vieweg/Teubner, 2008
- Reynolds, John: Daring to be different p.75. Haynes, 2004
- "Citroën Faces – The men behind Citroën". The Citroën Source. Retrieved 2014-04-01.
Magès was the man behind the hydro-pneumatic suspension of the DS. His ideas were discovered by Pierre Boulanger by accident, and Boulanger was fascinated by them, even though the Citroën technicians considered them hopeless. Boulanger employed Magès in the development department. A decision, he never would regret. Paul Magès was a curious man, and he consumed all literature concerning wheel suspension, suspension in general and braking systems.
- Marius Venz; Copyright © 2010 Borgward Car Club of Australia Inc.
- Mercedes Air Suspension, By Martin WerminghausenWerminghausen, Martin. "Mercedes Air Suspension" (PDF). Mercedes Air Suspension Solutions. http://600airsuspension.com/resources/article-on-mercedes-air-suspension-by-martin-werminghausen.pdf. Retrieved 30 August 2014.
- LDS Fluid data sheet