Hydro-pneumatic is a type of automotive suspension system, invented by Citroën, and fitted to Citroën cars, as well as being adapted by other car manufacturers, notably Rolls-Royce, Mercedes-Benz and Peugeot. It was also used on Berliet trucks. Similar systems are also used on some military vehicles.
The purpose of this system is to provide a soft, comfortable, yet well-controlled ride quality. Its nitrogen springing 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 an element of complexity, so automakers like Mercedes-Benz, British Leyland (Hydrolastic, Hydragas), and Lincoln have sought to create simpler variants.
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.
The suspension setup is referred to as 'oléopneumatique' in early literature, pointing to oil and air as its main components.
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.
The system had one key negative impact on the inventor, Citroën - only specialist garages were qualified to work on the cars - making them seem radically different from ordinary cars with common mechanicals.
Auto manufacturers are still trying to catch up with the combination of features offered by this 1955 suspension system, typically by adding layers of complexity to an ordinary steel spring mechanical system.