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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 Brake s, 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 ( 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. HISTORY 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:
FUNCTIONING 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 LHM fluid circuit. (See '' Hydraulic Accumulator ''). The high pressure pump powered by the engine pressurizes the circuit and an Accumulator Sphere . This part of the circuit is between 150 and 180 bars. It powers the front brakes first, prioritised via a security valve, and depending on type, can power 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 the rod pushing LHM into the sphere, compacting the nitrogen in the upper part of the sphere, the 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. 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 (ie 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 , Aud i, and BMW . 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 extremely small tolerances 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. 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 subject to no mechanical wear but suffer pressure loss, mostly from nitrogen naturally diffusing through the membrane. They typically last between 60,000 and 100,000 km. Spheres originally used to have a valve on top and be rechargeable. Newer spheres do not have this valve anymore, but it can be retrofitted. Though a rechargeable sphere has a longer lifespan, the membranes will eventually wear out, though this can take over 20 years. A ruptured membrane means suspension loss at the attached wheel, however ride height is unaffected. Or in the case of the accumulator sphere, reliance on the high pressure pump as the only source of braking pressure to the front wheels. ADVANTAGES
DISADVANTAGES
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. 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 3 The 2003 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 . TRIVIA
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