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 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 in early literature, pointing to oil and air as its main components.
The system was also used under licence by other car manufacturers, notably Rolls-Royce, BMW 5 Series Touring, Maserati 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, 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, and active control of body roll.

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:

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 self-levelling.
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 seven-piston pump, similar in size to a power steering pump, generates this pressure when the engine is running.
1960 The United States Patent and Trademark Office issues for a double stage oleo-pneumatic shock absorber using concepts very similar to those developed earlier by Paul MagèsPatent forms the basis for aircraft oleo struts and gas-filled shock absorbers
1965 Rolls-Royce licenses Citroën technology for the suspension of the new Silver Shadow
1967 The superior non-hygroscopic [|LHM] mineral fluid is introduced
1969 Citroën M35: The Citroën M35 was a coupé derived from the Ami 8, and equipped with a Wankel engine and a hydropneumatic suspension. The bodies were produced by Heuliez from 1969 to 1971.
1969 National Highway Traffic Safety Administration legalizes LHM mineral fluid in the United States
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. The beams of all six headlights are maintained parallel to the road surface by a hydraulic system separate from the directional long range high beams. The headlights' steering and leveling systems are totally separate from the central system that powers the suspension, steering and brakes and use a different fluid, a glycerine type.
1972 BMW E12 5-series released with optional hydropneumatic rear suspension. Coil springs are retained, though softer than conventional coils for the same car. This system was offered in most BMW 5-, 6-, and 7-series models, as well as the E30 Touring, into the 1990s when it was replaced with an air suspension. Until late 1987, the hydraulic circuit was separate from the power steering, and the pump electrically powered.
1974 National Highway Traffic Safety Administration bans vehicles with height adjustable suspension, impacting consumers in the United States. Ban repealed 1981.
1974 Citroën CX: The car was one of the most modern of its time, combining Citroën's unique hydro-pneumatic integral self-leveling suspension and speed-adjustable DIRAVI power steering. The suspension was attached to sub frames that were fitted to the body through flexible mountings, to improve even more the ride quality and to reduce road noise. The British magazine Car described the sensation of driving a CX as hovering over road irregularities, much like a ship traversing above the ocean floor.
1974 Maserati Quattroporte II: was on an extended Citroën SM chassis, available since Citroën had purchased the Italian company and was the only Maserati Quattroporte to feature hydropneumatic suspension and front-wheel drive
1975 The Mercedes-Benz 450SEL 6.9 W116 replaces the air suspension of the 6.3 with hydropneumatic suspension, 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.
1980 Mercedes-Benz W126 500SEL used hydropneumatic suspension as optional, later this system was available on 420SEL and 560SEL models.
1983 Citroën BX, built as a 4WD in 1990
1984 Mercedes-Benz W124 selected models of E class had this technology height adjustable suspension and self-levelling suspension mixed with coil springs.
1987 BMW E30 3-series Touring begins production in July, offering the same self-leveling hydropneumatic rear suspension as previous BMW, with the difference that the pump is a parallel circuit on the belt-driven steering assist pump, and shares its fluid. Starting in September, the E32 7-series switches to this pump from the previous electric pump. The BMW E34 5-series begins production in November, also with this new pump.
1989 Citroën XM: Hydractive Suspension, 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.
1991 Toyota Soarer UZZ32 used hydraulic struts controlled by an array of sensors with yaw velocity sensors, vertical G sensors, height sensors, wheel speed sensors, longitudinal and lateral G sensors) that detected cornering, acceleration and braking force.
1993 Citroën Xantia used hydropneumatic, on 1995 Optional Activa system, eliminating body roll by acting on anti-roll bars. A Xantia Activa was able to reach more than 1g lateral acceleration, and still holds the record speed through the moose test maneuver, due to its active anti-roll bars. This test is conducted by the magazine Teknikens Värld's, as a test of avoiding a moose in the road. The second place car, Porsche 997 GT3 RS was able to manage.
1995 Mercedes-Benz E-Class on estate models on rear suspension used hydraulic suspension with spheres height adjustable suspension and self-levelling suspension mixed with coil springs.
1999 Mercedes-Benz CL-Class and Mercedes-Benz S-Class introduce optional Active Body Controlan electronically controlled hydropneumatic system
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. Hydractive 3+ was available on some models
2005 Citroën C6: An improved version of the C5 system known as Hydractive 3+, C6 with a V6 engine was fitted with AMVAR version of Hydractive 3+
2007 Citroën C5 II: Hydractive 3+ as optional on Exclusive models. other versions of the car have normal spring suspension.
2008 JCB Fastrac high speed 7000 series agricultural tractors now use this system for front and rear suspension.
2019 Mercedes-Benz 450 GLE introduces eActive Body Control on a Sport utility vehicle, discarding mechanical roll bars, notably enhancing performance.
2023 BYD Auto introduces advanced active hydropneumatic suspension systems on the Yangwang U8 SUV and U9 sportscar. The suspension features the ability to drive with only three wheels fitted, and jump in the air while parked remaining level.