Hydropneumatic Suspension

Hydropneumatic suspension is a type of automotive suspension system, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers, notably Rolls-Royce, and Peugeot. It was also used on Berliet trucks and is since recently used on Mercedes-Benz cars. Similar systems are also used on some 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. 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.

The nitrogen gas 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 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 a perceived element of complexity, so automakers like Mercedes-Benz, British Leyland (Hydrolastic, Hydragas), and Lincoln have sought to create simpler variants using a compressed air suspension. The system of the inventor Citroën had a disadvantage that only garages equipped with special tools and knowhow were qualified to work on the cars, making them seem radically different from ordinary cars with common mechanicals.

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. The suspension system usually features driver-variable ride height, to provide extra clearance in rough terrain.

There have been many improvements to this system over the years, including variable ride firmness (Hydractive) and active control of body roll (Citroën Activa). The latest incarnation features a simplified single pump-accumulator sphere combination. Auto manufacturers are still trying to catch up with the combination of features offered by this 1954 suspension system, typically by adding layers of complexity to an ordinary steel spring mechanical system.