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 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 two technological principles:
The suspension system usually features both self-leveling and driver-variable ride height, to provide extra clearance in rough terrain.
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.
Citroëns 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 cars 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, 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, 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, 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.
Complete article available at this page.
This post have 0 komentar
EmoticonEmoticon