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