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. 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 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.
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. The suspension system usually features both self-leveling and driver-variable ride height, to provide extra clearance in rough terrain. Hydropneumatic suspension has a number of natural advantages over steel springs, generally recognized in the auto industry. In a hydropneumatic system, gas absorbs excessive force, whereas liquid in hydraulics directly transfers force, which combines the advantages of two technological principles:
Hydraulic systems use torque multiplication in an easy way, independent of the distance between the input and output, without the need for mechanical gears or levers. Pneumatic systems are based on the fact that gas is compressible, so equipment is less subject to shock damage. 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. 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).
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. 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. 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 by a rubber membrane.
History
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. 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. Major milestones of the hydropneumatics design were:
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