Hydragas is a type of automotive suspension system used in many cars produced by British Leyland and its successor companies.
Invented by famous British automotive engineer Alex Moulton, Hydragas is an evolution of the previous Hydrolastic system and was first introduced in 1973 in the Austin Allegro. Both systems attempt to address the ride-handling compromise of car suspension by interconnecting the suspension of the front and rear of the car in some way. Hydragas attempted to perform the same function and advantages as the famous hydro pneumatic system developed by Citroen, but without its attendant complexity.
The heart of the system are the displacer units, which are pressurised spheres containing nitrogen gas. These replace the conventional steel springs of a regular suspension design. The means for pressurising the gas in the displacers is done by pre-pressurising a hydraulic oil, and then connecting the displacer to its neighbour on the other axle. This is unlike the Citroen system, which uses hydraulic fluid continuously pressurised by an engine-driven pump and regulated by a central pressure vessel.
Despite early teething problems (the Allegro version of Hydragas was found seriously wanting), it was gradually developed into a very effective and efficient alternative to steel springs on later BL/Rover Group models such as the Austin Metro and MGF.