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This article originally appeared in the CitroŽnian, the magazine of the CitroŽn Car Club

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 hydropneumatique.

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.

The 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’.

Although 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 not. 
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.

The 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 engine stalling.

When 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.

With 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 effect.

A 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 braking.

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 of circuit.
Unfortunately, the system was plagued with electrical problems due, mainly, to poor earthing.

Hydractive 2
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.

The 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 transverse undulations.

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.

Because 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.

Activa 2
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.

Hydractive 3
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 speed.

Through the real-time management of these two settings, the system is able to control roll, pitch, heave and yaw.

Hydractive 3+
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.

The driver can manually choose to give priority to the dynamic setting (sports position).

Third-Generation Hydractive automatically adapts the height of the vehicle according to its speed and the state of the road.

The 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.

An 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 electrovalves.
 A hydropneumatic accumulator attached to the pump regulates pressure variations and reduces operating noise.
The front 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).

Hydropneumatic 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.

The C5 and C6 retained hydropneumatics for the suspension only.  Braking and steering are conventional (in CitroŽn marketing speak, this is called ‘decentralisation’).

Some 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.

Thanks to Nigel Wild for his technical input.

© 2014 CitroŽnŽt/© CitroŽn Car Club