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According to wikipedia,

A hydropneumatic system combines the advantages of two technological principles:

  • Hydraulic systems use torque multiplication in an easy way, independent of the distance between the input and output, without the need for mechanical gears or levers
  • Pneumatic systems are based on the fact that gas is compressible, so equipment is less subject to shock damage.
  • Gas absorbs excessive force, whereas fluid in hydraulics directly transfers force

What purpose do the hydraulics serve? I understand that the air would serve as a cushion, but why the hydraulics?

  • And I also understand that hydraulics would transmit the energy... – rotaredom Nov 21 '16 at 13:50
  • I would bet it has to do with the hydraulic portion going through valves to create the main dampening effect. Gas valving would not be nearly as easy to make it work well. – Pᴀᴜʟsᴛᴇʀ2 Nov 21 '16 at 13:51
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As your quote says:

Hydraulic systems use torque multiplication in an easy way, independent of the distance between the input and output, without the need for mechanical gears or levers

This allows you to install the gas reservoir independently of the shock absorbers. E.g. Citroën installed their gas reservoirs inside the engine bay and trunk, near the suspension turret.

It also allows you to e.g. vary the ride height without changing the suspension characteristics: you can inject more hydraulic fluid, but leave the gas system at the same pressure.

And the force multiplication is used to lower the pressure in the system: the membrane in the gas sphere is about 10 times larger than the diameter of the shock absorber, so pressure can be 10 times lower for the same carrying capacity.

It may be easier to run a hydraulic high-pressure pump in a car than it'd be to run an air compressor rated for the same kind of pressure (~100 bar for the Citroen system), but that's just speculation on my part.

  • So it uses more or less oil compared to air to produce more or less suspension? – rotaredom Nov 21 '16 at 13:54
  • it varies the amount of oil to vary the ride height. The suspension stiffness is regulated by valves that limit the oil flow. – Hobbes Nov 21 '16 at 14:00

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