Vehicles used for transport of loads have their efforts on the axles very close to the allowed or critical limits mainly during its travel on a bumpy surface or during cornering. In such cases the use of conventional suspension systems can increase the axle's overload phenomena. Hydropneumatic suspension leads to an even distribution of load per axle, thereby decreasing the overload problem and simultaneously increasing the efficiency and comfort levels.
About the Suspension System:
This suspension system was invented in the late 50's by Citroen® and has been fitted to many of their cars since. As its name suggests, its core technology and mainstay of its functionality is hydraulics. Superbly smooth suspension is provided by the fluid's interaction with a pressurized gas. This system is powered by a large hydraulic pump operated directly by the engine in much the same way as an alternator or an air- conditioner is, and provides fluid toan accumulator at a pressure, where it is stored ready to be delivered to servo a system.
The spheres are like the springs on your cars, and the struts and the hydraulic components that make the fluid act like a spring. There is a hydraulic component called an accumulator, which is gas under pressure in a bottle contained within a diaphragm, effectively a balloon which allows pressurized fluid to compress the gas, and then as pressure drops the gas pushes the fluid back to keep the system's pressure up. As you can see in the drawing, the pink gas(Nitrogen) is compressed when the pressure in the green fluid overcomes the gas pressure and pushes back the diaphragm, which compresses the gas. Then as the pressure of the fluid decreases, the gas pushes back the diaphragm and as the gas overcomes the fluid, it expels the fluid from the sphere, returning gas and fluid to equilibrium.
The difference, comparing with conventional suspension system is the gas spring instead of a mechanical spring, and the hydraulic fluid passing through the valve, where the energy is dissipated without using additional dampers, achieves the damping.
The most important item of this system is the gas chamber, therefore the stiffness will be defined basically, by the pressure and volume contained within the chamber. In some vehicle (like the Citroen® CX series), the chamber is built from two different parts that then join to take the shape of a sphere, and the gas is separated hydraulic fluid by a flexible diaphragm.
The hydraulic fluid that drains out of the system is stored in a reservoir and thereby returned to the system using a hydraulic pump, keeping the vehicle height constant throughout. A semi active control can be done through an adjustable valve that increases or decreases the damping. Controlling the level of the hydraulic fluid either manually or automatically may do an active suspension.
One or two of the more obvious ones are that since the system is hydraulic, the ride height can easily be altered, a trend low riders are now following on with in California, nearly fifty years later. Also, they could link the four corners together to make a system that prepared the car for the bump to keep it even and offer the passengers a smoother ride. Basically they put fancy valves called height correctors on the anti-roll bar. These were mounted in such a way that as the suspension twisted, this operated the valves that controlled the transfer of fluid to the struts
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