Hydropneumatic

Hydropneumatic 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.
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(a.)
Pertaining to, or depending upon, both liquid and gaseous substances; as, hydropneumatic apparatus for collecting gases over water or other liquids.
  

A water system, usually small, in which a water pump is automatically controlled by the pressure in a compressed air tank.

IDROPNEUMATICO