Hydractive I - CITROEN DS series Technical Manual

U
The Citroën Guide

Hydractive I

The Hydractive I suspension system appeared
with the XM. Unlike the simpler hydropneumatic
suspension used on the DS, GS/GSA, CX, BX and
some XMs, this one has two modes of operation,
soft and hard. The suspension functions in soft
mode but it will be switched to the hard mode
when the computer deems this necessary for the
sake of roadholding and safety.
To achieve this, the first hydractive system adds two spheres
(one for each axle) and an electric valve to the struts and
spheres of the standard hydropneumatic setup.
The layout of the system (front suspension)
return
strut & sphere
rear
suspension
5
feed from
security valve
electro-valve
The illustration only depicts the differences to the standard
hydropneumatic layout already presented in the previous
section:
1 A standard Citroën sphere base which fits a sphere
without a damper block. The sphere volume and pres-
sure differ for the front and rear, as well as according to
the model of the car;
2 A hydraulically controlled isolation valve that con-
nects or isolates the sphere from the rest of the suspen-
sion, modifying the string constant of the suspension;
3 A ball and piston valve arrangement that limits fluid
cross-flow between the left and right suspension struts
in case of body roll. This valve is disabled for suspension
height corrections, in order to guarantee that the fluid
pressure in the corner struts remains equalized;
4 Two damping elements similar to those used on the
corner spheres, acting as dampers for the center one;
Suspension: Hydractive I
return
control from
computer
During normal driving, the computer keeps the suspen-
sion in soft mode most of the time but—based on the input
provided by many sensors (steering wheel, accelerator
pedal, body movement, road speed and brake), including
the Sport/Comfort switch on the dashboard—the suspen-
sion ECU decides when to switch to hard mode; in other
words, when to deactivate the additional spheres for extra
roadholding and safety.
When the driver selects the Sport setting, the suspension
is switched to hard mode constantly. This setting is not
what any Citroën driver would call comfortable... The suc-
cessor system, Hydractive II overcomes this limitation.
1
suspension
control block
2
4 4
3
5 An electrically controlled valve driven by the suspen-
sion ECU. In order to reduce heat build-up, the computer
uses pulse width modulation to achieve a constant cur-
rent through the coil. The initial voltage is higher to
make the valve react quicker but it is reduced to a smaller
value once the inductive effects have been overcome,
should the valve stay on for a long enough time. The
valve is capable of being on indefinitely when driven with
this sustained current.
The front and rear suspension circuits are identical and the
same electrovalve serves both subsystems.
Soft, hard, soft, hard...
The default electrical mode of the suspension, when the
electro-valve 5 is not energized, is hard.
???
return
strut & sphere
height
corrector
return
24
nitrogen
LHM
moving parts