Hydropneumatic suspensions use pneumatic springs
The advantage of this is that the pressure in the fluid of the springs can be varied while the car runs.
The circuit comprises a reservoir
for the fluid (black box with "Achtung" written on it), a main accumulator
(pink sphere), four pneumatic springs (four spheres), a pump and two
height control valves :
schematic view of the hydropneumatic circuit
The suspension system is fed directly from the main accumulator. The feed immediately splits front and rear, each passing through a Height Control Valve. When each valve is activated, high-pressure fluid inflates the pair of load-bearing shocks. When the valve is in the neutral position, the pressure level remains constant between the pair. When the valve is deactivated, the fluid in the shock pair drains directly back to the reservoir.
Sharing pressure between left and right shocks provided many benefits. The tendency to equalize pressure between the two accomplishes a horizontal self-leveling, even at high speeds. This achieves a natural anti-roll effect and gave the relatively large and heavy saloons remarkably good cornering capabilities.
While sharing pressure left to right provided many benefits, it proved more advantageous to have pressure separated fore and aft. This was accomplished through the independent height control valves.
If the load on the rear of the car increased, the rear valve is activated and a greater volume of high-pressure fluid allowed into that pair.
The pressure inside the circuit is provided by a pump, driven by the engine's speed and is temporarily held in the main accumulator :
main accumulator working
The pump keeps rotating to provide a continuous flow of a high-pressure fluid and so maintains a constantly high pressure in the circuit :