Hydropneumatic suspension

Hydropneumatic suspension is a type of motor vehicle suspension system, invented by Paul Magès, produced by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers. Similar systems are also widely used on modern tanks and other large military vehicles. The suspension was referred to as Suspension 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 hydraulic systems and pneumatic systems so that gas absorbs excessive force and liquid in hydraulics directly transfers force. The suspension system usually features both self-leveling and driver-variable ride height, to provide extra clearance in rough terrain.

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. 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.

Description

Hydropneumatic suspension is a type of motor vehicle suspension system, invented by Paul Magès, produced by Citroën, and fitted to Citroën cars. The suspension was referred to as Suspension oléopneumatique in early literature, pointing to oil and air as its main components.[1][2]

Elements of the system were also used under licence by other car manufacturers, notably Rolls-Royce (Silver Shadow), BMW 5 Series (E34) Touring, Maserati (Quattroporte II) and Peugeot. It was also used on Berliet trucks and has been used on Mercedes-Benz

Effects

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.[4] The suspension system usually features both self-leveling and driver-variable ride height, to provide extra clearance in rough terrain.[5] Hydropneumatic suspension has a number of natural advantages over steel springs, generally recognized in the auto industry.[6] In a hydropneumatic system, gas absorbs excessive force, whereas liquid in hydraulics directly transfers force, which combines the advantages of two technological principles: 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.[7] 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

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.[7][14] It can also power any number of features such as the clutch, turning headlamps and even power windows.[7]

Nitrogen is used as the trapped gas to be compressed, since it is unlikely to cause corrosion. The actuation of the nitrogen spring reservoir is performed through an incompressible hydraulic fluid inside a suspension cylinder.[4] By adjusting the filled fluid volume within the cylinder, a leveling functionality is implemented.[4] The nitrogen gas within the suspension sphere is separated from the hydraulic oil by a rubber membrane.

History

Citroën first introduced this system in 1954 on the rear suspension of the Traction Avant.[15] The first four-wheel implementation was in the advanced DS in 1955.[16] 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.[17] Other modifications followed, with design changes such as the 1960 "double stage oleo-pneumatic shock absorber" patented by Peter Fullam John and Stephan Gyurik.[18]

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

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 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 leveling) 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.

Working fluid

Citroën quickly realized that standard brake fluid was not ideally suited to high-pressure hydraulics, and developed a special red-coloured hydraulic fluid named Liquide Hydraulique Synthétique (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, 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

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, a type of polyurethane thermoplastic compatible with the LHM fluid. The moving parts of the system, e.g., suspension struts and steering ramm, are sealed by contact seals between the cylinder and piston for tightness under pressure. The other plastic and rubber parts are return tubes from valves, such as the brake control and 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 (one to three 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 diffusion. 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 three 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.

Legacy

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[30] by Cleveland Pneumatic Tool Co. Similar systems are also widely used on modern tanks and other large military vehicles.

See also

• Hydrolastica type of automotive suspension system used in many cars produced by British Leyland and its successor companies.
• Hydragasis an improved form of Hydrolastic, using nitrogen-pressurised gas springs, rather than rubber.
• Hydraulic recoil mechanismuses the same principal for artillery.
• Oleo strutsuspension for most large aircraft, using the same physical properties of air and hydraulic fluid.
• Active Body ControlABC, is the Mercedes-Benz brand name used to describe hydropneumatic fully active suspension, that allows control of the vehicle body motions and therefore virtually eliminates body roll in many driving situations including cornering, accelerating, and braking.
• Air suspensiona type of vehicle suspension powered by an electric or engine-driven air pump or compressor. This compressor pumps the air into a flexible bellows, usually made from textile-reinforced rubber. The air pressure inflates the bellows, and raises the chassis from the axle.
• Electronic Air Suspension

External links

• The Citroën Technical Guide