Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, 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, where it is known as Active Body Control. 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. The suspension was referred to as oléopneumatique in early literature, pointing to oil and air as its main components.
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 by Cleveland Pneumatic Tool Co. 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. Other modifications followed, with design changes such as the 1960 "Double stage oleo-pneumatic shock absorber" patented by Peter Fullam John and Stephan Gyurik.
Hydropneumatic 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 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.
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.
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.
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 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.
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).
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:
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 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.
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 (Liquide Hydraulique Synthétique), 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 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.
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.
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 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) 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 deflation. 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 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 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.
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.
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.
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.
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:
The BHI of the Hydractive 3 suspension calculates the optimum vehicle height, using the following information:
The 3+ Hydractive hydraulic suspension has 3 automatic modes:
The BHI of the 3+ Hydractive suspension calculates the optimum vehicle height, using the following information:
C5 I (2001-2004)
C5 I facelift (2004-2007)
C5 II (2007-2017)
Magès was the man behind the hydropneumatic 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.