Citroën hydraulics - cars with green blood.

One of the hallmarks of Citroën is the more or less advanced hydraulic system. The smaller Citroëns (2CV, AMI, ZX, AX ...) completely lack hydraulics except for a conventional braking system, but GSA has, like the GS, been fitted with a high-pressure system for both the suspension and the brakes. The system is used in all the larger Citroëns (DS/ID, CX, BX, XM ...); in many cases also supplying the servo-assisted steering and so-called active anti-roll systems. GS/GSA are, however, the smallest Citroëns that have been equipped with hydraulics.

The Citrën hydraulic system is often perceived as difficult and complex, but the principles are simple to understand, and the system has many advantages.

Hydropneumatic suspension

Hydropneumatic, i.e. liquid-gas, suspension is based on simple principles. First, create a soft suspension action by using pressurized gas contained in a "balloon" - the actual spring. However, the soft action causes the ride height to vary depending on car load, so, to compensate this, use a liquid-filled column to connect the "balloon" to the wheel. The amount of hydraulic liquid is regulated using a height corrector valve, which admits fluid into or releases fluid from the cylinder depending on whether the height is to be corrected upwards or downwards, respectively.

In practice the "balloon" is a sphere made of thick steel, in which there are two separate chambers, separated by a strong flexible diaphragm. The gas (nitrogen is used) is contained in the top chamber. The sphere is attached to a hydraulic cylinder so that the lower chamber can be filled with fluid which provides the physical connection to the wheel arm. In the bottom of the sphere there is a little spring-loaded valve which limits the flow of liquid, causing a damping effect just as conventional shock absorbers on a conventionally sprung car. In contrast to conventional shock absorbers, the spheres with their integral damper valves can be mounted in any position, for instance, completely horizontal, which means that the whole rear suspension fits under the floor, leaving a large bagage compartment.

The advantage of the hydropneumatic suspension system is the smooth ride and the fact that the ground clearance is not dependent on the load. There is also a control by the driver's seat where the ground clearance can be varied - from normal to a highest position, useful for negotiating bad roads, or to ease tyre and oil changes. The whole system is virtually maintenance-free, and the suspension does not have to be dismantleded periodically for shock absorber replacement.

One disadvantage is however that the gas tends to diffuse out of the upper chamber through the diaphragm, causing a lack of pressure, and when too much gas has escaped, the suspension tends to go hard. This usually doesn't occur until after about 10 years or 100 000 km. The spheres can be replaced or recharged, which however requires them to be removed from the car. On the GSA the front spheres are child's play to replace, but it's usually much worse at the rear. To alleviate this one can mount recharging valves on the spheres which makes it possible to recharge them in place. There are different opinions on how good such valves are (there is a risk of leakage through the valve) - on my car I have conventional spheres at the front because it's simple to replace them, whereas I have valve-equipped spheres at the rear to avoid the necessity of removal for recharging.

High-pressure braking system

Instead of a conventional brake main cylinder the brake pedals maneuvers a brake valve which simply regulates the pressure in the brake lines. The advantages of this system include the short travel of the brake pedal, resulting in accurate brake control and short reaction time, and the fact that any air present in the system is compressed by the high pressure, resulting in a delayed but not total loss of braking power like on conventional (low pressure) brakes.

The GSA is equipped with disc brakes all around, which are of a conventional type. The front brake discs aren't mounted by the wheels however, but rather inside, near the gearbox.

High-pressure feed

In order for the system to work, a high pressure must be generated somewhere. In the GSA this is done using a single-piston pump, which doesn't feed the system directly however. Instead there is a pressure accumlator, utilizing the same type of sphere as in the suspension (but without damper valve), which is connected to a pressure regulator. The regulator allows the pump to charge the accumulator, and cuts out the pump when a specified pressure has been attained. When hydraulic fluid is consumed (or rather, recirculated via) the suspension or braking system, the pressure gradually drops until the regulator cuts in the pump again for recharging.

The use of a regulator eases the mean load on the pump and provides a constant high pressure source.

Hydraulic fluid

The hydraulic fluid used in Citroën cars since the middle of the 1960's is a light green mineral oil called LHM - Liquide Huile Minerale. One of its special characterstics is that it doesn't go thick at low temperatures, and in contrast to the vegetable based oil used in conventional braking systems it is not water-absorbing. Nevertheless, the hydraulic fluid must be replaced from time to time because apart from its purely mechanical function it also continuously cleans the system.

A 3-liter tank provides a resovoir of hydraulic fluid, and also contains filters for removing impurities from the fluid.

Hydraulic lines

For transporting hydraulic fluid with a pressure of a few hundred bars conventional copper tubing is out, instead, plastic-clad (for protection) steel piping is used. To handle the recirculation of hydraulic fluid (for instance when the height corrector lowers the ride height) a system of return hoses is utilized. For various reasons most components in the system have return hoses (leakage lines from hydraulic cylinders, and the brake valve).

Two-circuit system

In order not to be stranded should the system start to leak somewhere, which is more likely in the rear sections since the hydraulic pipes are rather exposed on their way to the rear of the car, there is a priority valve which disconnects the rear of the system if the pressure should suddenly drop. The front brakes are mounted inside the car, by the gearbox and not by the wheels which among other things minimizes the risk of damage to these components.
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