Given that it is likely that PSA are going to drop hydropneumatic
suspension in the not too distant future, I thought it might be
worthwhile to devote an article to the subject of hydropneumatics.
Etymologically, since the system employs a liquid (oil) and gas
(nitrogen), it should be called ‘oleopneumatic’ (in English) and
indeed, in the early days, CitroŽn used both olťopneumatique and
Since the early fifties, CitroŽn have been using hydropneumatic
suspension, and, unlike other systems such as BMC/BL’s
Hydrolastic/Hydragas set up (which, unlike the CitroŽn system was
unpressurised and interconnected front to rear and is therefore in
concept closer to the A Series’ set up) it was a whole-car solution
which can include the brakes, steering, clutch and gearchange as well
as the suspension itself. Research was also undertaken into using
the system to operate the wipers, window lifters, anti-roll, a rear
spoiler that was activated during braking to increase rear wheel
downforce and even a gearboxless hydrostatic transmission system.
The suspension system provides a soft, comfortable, yet well-controlled
ride. The nitrogen springing medium is approximately six times more
flexible than a conventional steel system, so self-leveling is
incorporated to allow the vehicle to cope with the extraordinary
suppleness provided. In the early fifties, France was noted for
particularly poor road quality and therefore the only way to maintain a
relatively high speed in a vehicle was if it could easily absorb road
irregularities. It was this need that also drove the development of the
2CV's interconnected suspension system. To a large extent, the
hydropneumatic system resolves the dichotomy between firm suspension
for taut, responsive handling and soft suspension for comfort.
core technology of hydropneumatic suspension is as you might guess from
the name, the use of a hydraulic fluid and gas. The system has
often been misdecribed as ‘air suspension’.
the system is quite complex, the underlying principles are actually
rather simple. Gases are compressible whereas fluids are
not. The gas acts as the spring and is housed in metal spheres
which contain a flexible membrane or diaphragm that allows the fluid
that occupies the rest of the sphere to compress the gas – rather like
squeezing an inflated balloon. Pressurised hydraulic fluid
compresses the gas, and then as hydraulic pressure drops, the gas
pushes the fluid back. This is the hydropneumatic equivalent to a
conventional spring being compressed and then rebounding. The
harder the gas is compressed, the stiffer the ‘spring’ becomes – this
is called ‘infinite rising rate suspension’. Conventional springs
maintain pretty much the same ‘stiffness’ whether fully compressed or
The fluid acts as a lever or strut to transmit the vertical wheel
movement brought about by irregularities in the road surface to the
pneumatic spring. Contained within the sphere is a piston and
cylinder and inevitably, there is some fluid leakage. It is for
this reason that the hydraulic system is pressurised.
system is powered by an engine driven hydraulic pump which provides
fluid under pressure to an accumulator (another hydropneumatic sphere)
where it is stored ready to be delivered to the system that requires
it, be it suspension, brakes, steering or gearchange. The
accumulator serves two main purposes – it evens out the inevitable
variations in pressure that are inherent in the design of the pump and
evens out the variations in pressure caused by varying demands on the
system – in other words it acts rather like a battery. It also
provides a reserve of pressure in the event of the pump failing or the
a wheel hits a bump it rises and via a mechanical link it pushes the
suspension piston back and this squeezes fluid through a tiny hole
(effectively a damper valve) in the sphere to let the gas spring absorb
the energy of the bump. Once the car is over the bump, the gas pushes
the diaphragm back out, pushing the fluid down and thereby pushing the
wheel down to the ground.
such a soft suspension, it was necessary to fit height correctors at
both ends of the car to correct for long-term/static errors in height
brought about by varying loads and fluid seepage. The height correctors
are linked to the suspension and work by measuring the height at the
middle of the anti-rollbar, by automatically taking the average of the
left and right wheel height on the axle and adjusting the height
accordingly. By taking the average, this prevents spurious
reactions to body roll - it can only make both sides on each axle go up
or down together.
Additionally the height correctors are fitted with a hydraulic damping
chamber which restricts and delays their movement – it typically takes
a suspension movement of at least 20mm in one direction for at least 5
seconds before the height corrector responds. Even fully bottoming the
suspension still takes at least 5 seconds for a response. This prevents
the height correctors from responding to bumps or road undulations,
(both of which would be undesirable). However, prolonged heavy
acceleration of more than 5 seconds (particularly noticeable on an
automatic) causes a height correction response - an undesirable side
further advantage of hydraulic suspension is that the car is able to
link its braking effort to the weight on the wheels. This was achieved
by operating the rear brakes from the rear suspension hydraulics and
varying the pressure according to the load on the rear wheels.
Under heavy braking when weight transfer towards the front of the car
occurs, there is less pressure on the rear suspension. The suspension
then exerts less pressure on its fluid, and as weight and grip diminish
on the wheels, so does the braking effort, thus the hydropneumatic
system prevents rear wheel lock ups and since the rear brakes use the
rear suspension fluid, the tail is pulled down allowing for level
In 1988, CitroŽn showed the Activa concept car which was fitted with
the first major development to the hydropneumatic suspension
system. It employed electronics to control the suspension. Each
axle had several suspension spheres that are controlled by data
(vertical bodywork movement, angle of steering wheel, rate of steering
wheel movement, vehicle speed, vehicle acceleration/deceleration,
braking) picked by sensors and sent to a computer. As the vehicle speed
increases, the suspension stiffens. The suspension works on the basis
of flexible response and an instantly variable damping system.
Electronic control allowed the implementation of other features such as
automatically varying the ride height and angle of attack as a factor
of speed to improve the car’s Cd. And raising the vehicle when at a
standstill to make it easier for passengers to get in and out.
The suspension also corrects for dynamic shifts in lateral loads with
an active anti-roll system made up of hydraulic actuators on the front
and rear anti-roll bars which lets it corner on the level, irrespective
of vehicle speed and the sharpness of the bend.
The following year, the XM was launched, fitted with much of the
technology showcased in Activa. The system was called Hydractive
and used a computer to take readings from the chassis and control
systems. An additional sphere was fitted to each axle and this
was switched in and out of circuit using servo valves controlled by the
computer. When the additional sphere is in circuit, the
suspension ‘sees’ a larger volume of gas than it does when it is out of
circuit. This provides a soft ride for cruising. When the
sphere is switched out of circuit, this provides a stiffer, sportier
suspension for faster harder driving. A switch was fitted to
allow the driver to choose between ‘normal’ and ‘sport’ modes – in the
‘sport’ position, the additional spheres were permanently switched out
Unfortunately, the system was plagued with electrical problems due,
mainly, to poor earthing.
Hydractive 2 was a refinement of Hydractve and was fitted to post 1993
XMs and high specification Xantias. The biggest changes were to
the manual mode switching which, in ‘sport’ mode, instead of switching
the additional sphere out of circuit, altered the parameters for doing
so and also changed these parameters in ‘normal’ mode. The
electrical problems were also tackled. Constant state changes
occurred within 0.05 seconds.
Hydropneumatic cars feature cross piping between the two wheels on
each axle. This piping was of very narrow diameter and had
a theoretical downside insamuch as this could create a lot of very slow
body roll since there is no damping control of the flow of oil from one
side to the other, other than some restriction caused by the small pipe
diameter – ‘theoretical’ since in practice, the cross-flow is so
minimal that the effect can be discounted in normal use. A side
effect of this cross piping is that it gives the suspension very soft
compliance for slow roll movements. Conventional springs resist
such movements but in an hydropneumatic car it is necessary to fit a
rollbar - without rollbars the suspension would be completely unstable
in the roll axis - you could sit on the left and it would go right down
and the other side would go right up. On later cars, the pipe diameter
was increased and the roll bar became even more essential in providing
long term roll stiffness.
benefit of the cross-piping between left and right suspension units on
the same axle is that the system "pre-sets" the car for bumps in the
road, keeping the car on an even keel. The height corrector
connects to a T-junction of this cross piping, but when the height
corrector is "closed" (which is nearly all the time while driving) it
represents a dead end, so only the piping from left to right comes into
play. When the wheel on one side hits a bump some oil will flow into
the sphere on that side via the damping valve, and some will flow
across to the other side and extend the wheel on that side, which gives
a degree of roll stabilising response. This tends to make the car more
steady in the roll axis, and reduces side to side rocking motions on
Hydractive 2 overcomes these shortcomings by modifying the side to side
connection – the pipe diameter is increased from 3.5mm to 10mm, and
once again there is an additional sphere, an on/off valve, and two
damper valves. In the "soft mode" (selected dynamically by the
suspension computer) this additional middle sphere is connected in
circuit and provides additional springing, via the two damping valves
in the unit. It effectively increases the “volume” of the
spheres. The system has two parallel paths for the oil to flow
for each bump, with different damping rates. The damper valves in the
spheres on Hydractive 2 are very stiff, while the ones in the middle
unit are softer, giving a net result of three stage damping in the soft
mode, and two stage damping in the firm mode. Any body roll requires
oil to either flow into and out of the very stiff damping valves in the
strut spheres - where the opening thresholds are above that produced by
roll movement - or to flow from side to side - where it must pass
through two damping valves in series in the centre unit. This means
roll movements are hydraulically damped in Hydractive systems, unlike
Hydropneumatic. This contributes towards the reduced roll on later
models like XM and Xantia.
of the large gauge of pipe there is greater instantaneous fluid flow
when hitting large bumps and the roll axis stability of the car is
improved over older models. In the "firm mode", again selected
dynamically by the computer based on inputs such as steering wheel
angle, speed of movement of the steering wheel, brake pedal movement
and rate of application, body roll and road speed, the central sphere
is isolated, completely blocking the transverse flow of oil which gives
stiffer springing, much stiffer damping, and much reduced body roll.
In 1990, CitroŽn showed the Activa 2 concept car which was equipped
with anti-roll suspension (SC. CAR) which enabled the car to corner on
the level, thus improving safety and increasing driving pleasure. When
the system detected a curve, the on-board computer immediately
increased roll stiffness. If the curve continued and the passenger
cabin tilted to an angle of 0.3 deg, two hydraulic jacks kicked in to
help restore the vehicle’s balance. Roll was eliminated. This
system was subsequently adopted for the Xantia Activa.
Fitted to the C5, the Hydractive 3 system adapts automatically,
instantly and continuously to both the style of driving and to the
state of the road. The system adapts damping force and flexibility
through its two settings: comfort suspension and dynamic suspension.
Each axle is equipped with a third sphere and a stiffness regulator.
The operating principle of the hydractive system is to isolate the
third sphere in dynamic mode or to bring it into service in comfort
mode. To do this, the system receives data from the height sensor,
steering wheel sensor and information on brake pressure and engine
the real-time management of these two settings, the system is able to
control roll, pitch, heave and yaw.
This sytem is the same as Hydractive 3 with the addition of an ‘auto
adaptive’ feature which works by measuring the the longitudinal and
transversal acceleration values calculated by the built-in
hydroelectronic unit and is fitted to the C6 and some C5s. These are collected and filtered over a
period of around one minute. The self-adapting suspension is thereby
able to define the criterion of "driving style". For a dynamic style of
driving, the suspension will adopt the dynamic setting more frequently,
thus providing a customised response.
driver can manually choose to give priority to the dynamic setting
Hydractive automatically adapts the height of the vehicle according to
its speed and the state of the road.
built-in hydroelectronic unit comprises three parts:
An electronic unit with a host of control laws stored in its memory,
controlling the electric motor and the stand-alone pressure generator
in accordance with the information delivered by:
• two new electric height sensors positioned on the
front and rear anti-roll bars, which precisely measure variations in
the body's height and speed of movement,
• a sensor measuring the steering wheel angle and
velocity of angular displacement,
• the multiplexed network and built-in systems
interface (BSI) concerning the rate at which the accelerator pedal is
pressed down or released, vehicle speed, pressure on the brake pedal
and engine speed.
electric motor drives the pump and comes into use as required. There is
a stand-alone pressure generator that groups flow, safety and
anti-dive/anti-squat functions, and is equipped with a pump and four
A hydropneumatic accumulator attached to the pump
regulates pressure variations and reduces operating noise.
and rear suspension circuits each have two electrovalves, one dedicated
to intake and the other to return of hydraulic fluid. The return
electrovalves are equipped with a non-return valve for the
anti-dive/anti-squat function with switching of this feature occurring
virtually instantaneously (15 ms).
suspension was fitted to the 15CV H Traction (at the rear), the D
Series, the SM, the GS, Birotor and GSA, the CX, the BX and most Xantias. The Hydractive system was fitted to series 1 XMs.
Hydractive 2 was fitted to top of the range Xantias and series 2 XMs.
SC. CAR was fitted to the Xantia Activa. Hydractive 3 was fitted
to most series 1 C5s and Hydractive 3+ was fitted to some series 1 and
2 C5s and to the C6 .
Whole car solutions
As mentioned at the beginning of this article, hydropneumatics can
provide a ‘whole car’ solution and this was indeed the case where the
DS was concerned. Hydraulic power was used to operate the
steering, brakes, clutch and gearchange. Early IDs featured
unassisted steering and a conventional brake operating system.
The braking system fitted to the DS and later IDs was fully-powered as
opposed to servo-assisted – the brake button operated a sleeve valve
that allowed fluid under pressure to operate the brakes. The SM,
CX and some LHD V6 XMs were fitted with DIRAVI (Varipower) steering
which used the hydraulic system to provide variable powered return of
the wheels to the straight ahead position which, coupled with very
high-geared powered steering led to very quick steering with consistent
feel at all speeds.
C5 and C6 retained hydropneumatics for the suspension only.
Braking and steering are conventional (in CitroŽn marketing speak, this
is called ‘decentralisation’).
H Van ambulances were also fitted with hydropneumatic suspension at the
rear and the Belphťgor trucks used high pressure hydraulics to operate
the brakes. Rolls Royce also used the system and during CitroŽn’s
ownership of Maserati, high pressure hydraulics were used on the
Bora, Merak and Khamsin for brakes, pedal box and seat adjustment,
headlight raising and clutch while the Khamsin also made use of the
SM's DIRAVI steering.
to Nigel Wild for his technical input.