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Hydropneumatic is a type of automotive suspension system, invented by Citroën, and fitted to Citroën cars, as well as being adapted by other car manufacturers, notably Rolls-Royce, Mercedes-Benz and Peugeot. It was also used on Berliet trucks. Similar systems are also used on some military vehicles.

The purpose of this system is to provide a soft, comfortable, yet well-controlled ride quality. Its nitrogen springing 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, so the only way to maintain relatively high speed in a vehicle was if it could easily absorb road irregularities.

While the system has inherent advantages over steel springs, generally recognized in the auto industry, it also has an element of complexity, so automakers like Mercedes-Benz, British Leyland (Hydrolastic, Hydragas), and Lincoln have sought to create simpler variants.

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. The suspension system usually features driver-variable ride height, to provide extra clearance in rough terrain.

The suspension setup is referred to as 'oléopneumatique' in early literature, pointing to oil and air as its main components.

There have been many improvements to this system over the years, including variable ride firmness (Hydractive) and active control of body roll (Citroën Activa). The latest incarnation features a simplified single pump-accumulator sphere combination.

The system had one key negative impact on the inventor, Citroën - only specialist garages were qualified to work on the cars - making them seem radically different from ordinary cars with common mechanicals.

Auto manufacturers are still trying to catch up with the combination of features offered by this 1955 suspension system, typically by adding layers of complexity to an ordinary steel spring mechanical system.

[edit] History

1954 Citroën Traction Avant 15CVH - high position

Citroën first introduced this system in 1954 on the rear suspension of the Traction Avant. The first full 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 air/oil suspension to combine a new level of softness with vehicle control and self-leveling
• 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.
• 1962 Morris Motor Company introduces the BMC ADO16 ('1100') with hydrolastic suspension
• 1964 Mercedes-Benz introduces the 600 with air suspension designed to avoid Citroën patents
• 1965 Rolls-Royce licenses Citroën technology for the suspension of the new Silver Shadow
• 1966 Mercedes-Benz introduces the 6.3 also with air suspension
• 1967 The superior LHM mineral fluid is introduced (LHM = the French for mineral hydraulic fluid)
• 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 The Mercedes-Benz 450SEL 6.9 becomes the first hydropneumatic Mercedes-Benz automobile, 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: 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.
• 1993 Citroën Xantia: Optional 'Activa' (active suspension) system, eliminating body roll by acting on torsion bars. An 'Activa' equipped Xantia 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+
• 2008 JCB Fastrac high speed 7000 series agricultural tractors now use this system for front and rear suspension.

[edit] Functioning

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 antisink or Activa suspension, there may be as many as nine spheres. They 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 circuit and an accumulator sphere. 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, gearchange, etc.

Pressure goes from this circuit to the wheel spheres, pressurizing the bottom part of the spheres and rods connected to the wheel suspension. Suspension works by means of the rod pushing 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. Car height correcting works by height correctors connected to the anti-roll bar, front and rear. These height correctors allow for more fluid to travel under pressure to the rod/sphere system when detecting that the suspension is lower than its expected ride height (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. Rear brakes are powered from the rear suspension spheres. Because the pressure there is proportional to the load, so is the braking power.

[edit] LHM

Citroën quickly realized that standard brake fluid was not ideally suited to high pressure hydraulics. They invented a new, green fluid, LHM. LHM stands for Liquide Hydraulique Minéral and 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 used to 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.

The chief problem with LHS (the fluid previously used, similar to conventional DOT3 brake fluid), is that the water it absorbs produces 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 was never sealed. Each time the car "got up" the fluid level in the reservoir dropped, drawing in fresh moisture-laden air and dust. The large surface of the fluid in the reservoir readily absorbed moisture. And, as the pump recirculates fluid continually through the reservoir, all the fluid was repeatedly exposed to the air and its moisture content. LHM, being a mineral oil, absorbs only an infinitesimal proportion of moisture, plus it contains corrosion inhibitors. The dust inhalation problem continues. Changing the fluid at the recommended intervals removes most dust and wear particles from the system, helping preserve the parts.

[edit] Manufacturing

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, mostly from nitrogen diffusing through the membrane. They 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 can be retrofitted. The membrane has an indefinite life unless run at low pressure, which leads to rupture. Timely recharging is thus vital. A ruptured membrane means suspension loss at the attached wheel; however, ride height is unaffected. With no springing but the tires and parts' (slight) flexibility, 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. The older cars had a separate front brake accumulator on power steering models. NOTE: the old LHS and LHS2 ("brake fluid") cars used a different rubber in the diaphragms and seals that is NOT compatible with LHM.

[edit] Advantages

Hydropneumatics have a number of natural advantages over steel springs that are poorly understood, leading to general public perception that hydropenumatics are basically "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.

• Hydropneumatic is naturally a progressive spring-rate suspension, i.e. the more it is compressed, the harder it becomes. This results in the suspension being extremely soft around its initial course (softer than a steel spring) but getting harder and harder as compressed (more than a steel spring). This is because of the properties of gas: halve its volume, and its pressure doubles. When the suspension operates, the ram is pushing oil into the sphere altering its gas volume (and therefore the pressure). This natural principle of hydropneumatics has not been met so far by any other type of suspension. The nearest is steel springs with a softer course and a harder course (two different spring rates, while hydropneumatics offer an infinite number of rates). Usually steel-sprung cars are either too soft ("comfortable"), or too stiff ("sporty"), or some intermediate compromise, while hydropneumatics offer "two cars in one".
• This advantage pays off in a spectacular way when slaloming: the swinging speeds and acceleration patterns of the body of a hydropneumatic car offer ideal body control, and "load" the tyres in an ideal linear-like manner, helping to get the most out of them. A steel-sprung car acts more like a violently-swinging pendulum, "crashing" on its tyres (and abusing them) when leaning from side to side.
• The same natural law governing gasses also ensures that the suspension's spring-rate (hardness) is continuously adapted to the weight it has to carry, and to infinite positions. For example, when the car is standing empty, the pressure within its spheres is in balance. If one passenger enters the car, this pressure becomes higher by the value of his weight (the gas in the spheres compressed to an equal degree, i.e. has now become "harder"). The car will have lost some height, so the self-leveling system immediately reacts and brings the car up to the predetermined ride height. The result is that the spring rate is kept constant, regardless of the load of the car. I.e., a car with 4 passengers and full payload will be equally well controlled as a car with just one passenger (bar the tyres, which of course remain the same dimension!). With a steel-spring car, either the car would be set up to be comfortable with 1-2 passengers but getting too soft as more weight is added (becoming uncontrollable under full payload), or it would be too stiff with 1-2 passengers and okay on full payload.
• This effect is especially pronounced at the rear axle, where the designer of a steel-sprung car has to make the greatest compromise: the rear suspension has to be able to deal satisfactorily with a large range of load. Because of the above propriety of hydropneumatics, Citroens can have a rear that is set very soft; one can easily push the empty car down with his hand. When load is added, it stiffens as much as necessary. Steel-sprung cars need to have rear springs much stiffer than necessary for average daily driving.
• The self-leveling system makes it such that there's always and at any time an equal travel available for suspension compression and extension, no matter the car's load. Citroen have calculated that the ideal suspension should have at least about 18 cm of motion range, i.e. 9 cm each way, for achieving effective continuous contact between pavement and tyres (by absorbing any road unevenness). With a "height corrector" for each axle, the car suspension always remains at its ideal middle position, providing a steady compression and extension course, no matter the car's load. As you load a steel-sprung car, its bump absorption capability becomes totally asymmetrical (too small a compression margin and too much of an extension course available, and the suspension moves far from its ideal operating angles, reducing lateral/longitudinal grip, etc).
• Very importantly, the continuous self-levelling function also rids suspension design of a number of unwanted compromises that commonly designers of steel-sprung cars have to incorporate: as the suspension is always functioning around one predetermined position, no matter the car's load, the various suspension-geometry issues become a much more simple equation to solve. A hydropneumatic suspension operates from its ideal angles at all times and conditions.
• The suspension being self-leveling, the possibility opens for dynamic height control. This has actually been implememented in Citroens from the C5 I and onwards: the cars are programmed to lower by about 1 cm above some specified speed, thus reducing aerodynamic resistance, improving fuel economy and increasing high-speed stability.
• Ride height is manually adjustable (in all Citroens) in 4 positions: "low", "drive", "mid-high", "highest". Select "Low" for loading heavy items. Select "Highest" to unload them. Select "Mid-high" or "Highest" to tackle some road obstruction (flooding, pavements, off-road, etc).
• Because the suspension doesn't need to be set stiff to overcome all sorts of restrictions imposed by the steel spring, the ride comfort is excellent (the ride is described as floating above the road surface), with the difference that the suspension never 'wallows' uncontrollably like an equally soft car on springs would do. This preserves precise handling and road-holding (like a sports car). Orthopedic doctors advice that patients with spinal injury or disk problems can only drive Citroens with hydropneumatic suspension. The legendary Rolls-Royce comfort is only due to this system equally fitted to millions of Citroens sold to this day since 1955 with the DS. The possibility of having such a soft suspension but NOT the uncontrolled wallowing (e.g., like many older American cars) is due to another natural property of nitrogen gas: it inherently has much less endogenous friction than steel. In other words, it is relatively much more neutral, more inactive. Think of a leaf-spring being compressed: leave it free and it will pop and spend some time vibrating around its centre position till it comes to rest. This is due to its internal mechanical friction (due to the material), and this is why cars need shock dampers. A steel spring performs much better (which is why it has long replaced leaf-springs in cars). Gas however behaves even better than the spring, in fact it represents a leap in effectiveness: compress a gas and release it: the gas will want to only return to its initial volume, not much more. Thus damping can be also reduced, resulting in an unearthy softness.
• This inertia of the gas is also the reason why on a hydropneumatic Citroen the driver will many times not even realise the event of a blown tyre—if not for the added noise—which can cause dynamic unrest and prove fatal on a steel-sprung car.
• The low-frequency wallowing characteristic of older Citroens (down to the BX) is due to the gas inertia as described above, and a weak damping. Commonly manufacturers believe that the optimal undulating frequency for a car's suspension is the one of human walking, i.e. about 0.2 cycles per second (0.2 Hz), thus many cars have this "rubbery", abrupt suspension feeling, which tends to increase as speed increases. Citroen tuned their cars to undulate at 1.6 Hz early on (DS, CX). Gradually they have brought this frequency down, with the Hydractive XM being set at 0.6 Hz (in the "soft" mode; in "hard", the car is sprung and damped like a sports car). The gas gives infinite possibilities: the operational frequency is just adjusted with the dampers (which look like disks the size of a large coin).
• The legendary comfort of Citroens is also due to another factor, which is a specific choice in the set up of the system: Citroen chose to hydraulically interconnect each wheel at the same axis. Thus the two front wheels are connected with each other. And so are the rear wheels. This has a very specific result for body control: when one of the two wheels meets, say, a bump, that wheel will tend to compress and absorb the bump. In the degree not the entire height of the bump can be absorbed, the car's body will lift at that side. In the same time, through the antiroll bar, this wheel will exert force at the other side wheel and tend to also lift it in the same direction (compress the suspension). This is where the hydraulic interconnection of these two comes to play. In an ordinary steel-sprung car, this antiroll bar effect would mean that the car's body at the bump-absorbing side would lift, while it would tend to drop at the other side. This amplifies the bump's effect of destabilisng the car and creating discomfort. In the Citroen system, the antiroll bar will also tend to do the same, but it is stopped by the hydraulic oil that is sent from the bump-side wheel at the moment it is pushed upwards to absorb the bump. Thus the suspension at the other side will receive a force opposing that of the antiroll bar, and of a degree of definition equal to the size of the bump. This in effect translates into a much reduced overall feeling of impact from any bump, as the shock is effectively automatically shared with the wheel at the other side of each axis. In practice this feels as if the whole front or the whole rear is slightly lifting, rather than the car receiving an unpleasant shock from one side. Another solution, as e.g. implemented in the Austin Princess, is to interconnect the front and the rear wheel of each side. This has also beneficial results comfort-wise (the front and the rear share the impact forces) but also unwanted counter-effects on braking and accelerating: in braking the car tends to nose-dive (all hydraulic oil is pushed to the rear). In the Citroen setup the adverse effect is that the car tends to roll a lot (the oil is pushed from one side to the other) however this is filtered by the antiroll bar (there's no equivalent item on the Princess to filter front-rear balance). This effect in Citroens, perceived by many as the only potential disadvantage of the hydropneumatic suspension (although it has not real effects on the lateral accelerations the car can achieve), has been totally put under control with the advent of the Hydractive systems, from the XM onwards.
• The master cylinder ("brake doseur valve") on Citroens includes specific solutions that take advantage of the hydropneumatic suspension, e.g. the pressure for rear braking is taken directly from the rear suspension (they are hydraulically connected). This makes that the braking force of the rear axle is continuously adjusted to the weight it carries (as pressure within rear suspension is equal to the weight it carries). So the rear brakes will come in significantly harder the more the rear is loaded.
• It is a matter of minutes for the home mechanic to replace an old sphere—which includes the "spring" and "damper" in one—using a simple tool that can be homemade. Also, one is free to try various sphere combinations on his car, selecting from the several varieties available for the various Citroen models, thus going for a more "comfortable" or more "sporty" set up.
• Compact suspension design.
• Maintenance, for a do-it-yourselfer, is relatively easy, once you know what you are doing. It doesn't require any specific tools.
• Inexpensive in mass production; for vehicles that would otherwise have a conventional power steering pump, hydropneumatic suspension adds no new equipment and in many cases results in a lower unsprung mass. Equally, the same system pressure produced from one central pump is used for braking (up to and including the Xantia).
• Upon body roll, the pressure exerted between the tires of the same axle is not subject to the same differential as on some other cars. The pressure in one suspension strut equals the pressure in the other through Pascal's law, potentially giving the 'light' tire more footprint pressure.
• Can be conveniently interconnected in the roll plane to improve roll stiffness and thus roll stability limit, especially for heavy vehicles.
• Can be connected in the pitch plane to improve braking dive and traction squat.
• If they are interconnected in the three-dimensional full car model, the interconnected hydro-pneumatic suspension could realize enhanced roll and pitch control during excitations arising from steering, braking/traction, road input and crosswind, as with the Hydractive arrangement
• Flexibility in the suspension strut design in the interconnected suspension system to realize desirable vertical, roll and pitch properties for different types of vehicles.
• Horizontal orientation of the rear suspension cylinders below the boot floor makes the full width of the boot is available for cargo.
• Mechanical steel spring suspension systems that try to replicate only some of the inherent advantages of hydropneumatic suspension (electronically adjustable shock absorbers) end up being lesser solutions and more complex to build and maintain than the straightforward hydropneumatic layout.
• People who are prepared to carry out simple maintenance can acquire a used luxury car for a fraction of the cost, as hydropneumatic suspension scares potential buyers and dealers despite more complex and maintenance-intensive systems on other cars. Very few of the units are repairable in the field: units are usually exchanged for new or remanufactured units. Pumps, height correctors, accumulators (including suspension "spheres"), steering units, etc. are not usually repaired by owner-mechanics, just replaced with the use of ordinary automotive mechanics' tools. Hydraulic fluid is drained and refilled with fresh, much like changing engine oil. Later Citroën automatic transmissions are conventional modern units similar to those of other makes.

[edit] Disadvantages

• Service sometimes requires a specifically trained mechanic.
• Hydropneumatic suspension systems are expensive to repair or replace, if poorly maintained or contaminated with incompatible fluids.
• Failure of the hydraulic system will cause a drop in ride height and, possibly, the failure of suspension completely, and the brakes will not work. However, an acute failure will not lead to acute brake failure as the accumulator sphere holds enough reserve pressure to ensure safe braking far beyond that needed to bring a vehicle with a failed system to a standstill.

[edit] Hydractive

Hydractive Suspension is a new automotive technology introduced by the French manufacturer Citroën in 1990. It describes a development of the 1955 Hydropneumatic suspension design using additional electronic sensors and driver control of suspension performance. The driver can make the vehicle 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 switched an extra pair of suspension spheres in or out of 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.

[edit] 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.[1]

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 model year 1995, specific Citroën XM models featured an additional sphere that functioned as a pressure reservoir, letting the car retain normal ride height for several weeks without running the engine.[2]

[edit] Hydractive 3

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 a new, incompatible orange fluid, rather than the familiar green LHM mineral oil used in millions of hydropneumatic vehicles.

An further improved Hydractive 3+ first appeared in 2005 on the Citroën C6.

[edit] See also

See Hydragas for a type of automotive suspension system used in many cars produced by British Leyland and its successor companies.

[edit] External links

• Detailed Explanation