Hydro pneumatic is the name given to a suspension system invented by Citroën and fitted to Citroën cars, as well as being adapted by other car manufacturers, notably Mercedes-Benz and Peugeot.
This system uses a belt driven pump from the engine to pressurise special LHM fluid around the hydraulic system to the brakes, suspension and power steering. It can also power any number of features such as the clutch, turning headlamps and even power windows.
LHM stands for 'Liquide Minerale Hydraulique' and is a mineral oil, quite close to automatic transmission oils. The suspension setup is referred to as 'oléopneumatique' in early literature, pointing to oil and air as its main components.
As well as powering the brakes, the suspension system features self-levelling and variable ride height. The height can be set in an extra high position to facilitate wheel changing, or just raised mode to give some off-road ability.
There have been many improvements to this system over the years, including variable ride firmness and active control of body roll. The latest incarnation features single pump-sphere combinations on each wheel.
Citroën first debuted this system in 1952 in the Traction Avant. The first full implementation was in the advanced DS in 1955.
Major milestones of the hydro pneumatics setup were:
1952 Traction Avant 15H: Rear suspension
1955 DS: Suspension, power steering, brakes and optionally the gearbox/clutch assembly powered by high pressure
1970 GS: Adaptation of the hydro pneumatic suspension to a small car
1970 SM: Variable speed auto returning power steering, dubbed Diravi, and hydraulically powered directional high beams
1974 The Mercedes-Benz 450SEL 6.9 becomes the first Mercedes-Benz automobile to use a variation that utilized a pump driven by the engine's timing chain instead of an external belt. This adaptation was used strictly by the suspension. Power steering and brakes were traditional hydraulic- and vacuum-powered respectively
1989 XM: electronic regulation of the hydro pneumatic system; sensors measure acceleration and other factors and dynamically alter suspension characteristics
1990 Peugeot 405 Mi16x4: first Peugeot equipped with rear hydro pneumatic suspension
1993 Xantia: Optional 'Activa' system, eliminating body roll by acting on torsion bars. An 'Activa' equipped Xantia was able to reach more than 1g lateral acceleration
2001 C5: No more central hydraulic pressure generation; combined pump/spheres at each wheel
At the heart of the system, acting as pressure sink as well as suspension elements, are the so called 'spheres'. They consist of a metal ball, open to the bottom, with a flexible membrane separating top and bottom. The top is filled with nitrogen at high pressure, the bottom connects to the car's LHM fluid circuit. (See hydraulic accumulator).
The high pressure pump powered by the engine pressurizes the circuit and an accumulator sphere. This part of the circuit is between 150 and 180 bars. It powers the front brakes via a security valve, and depending on type, can power steering, clutch, gear change etc.
Pressure goes from this circuit to the wheel spheres, pressurizing the bottom part of the spheres and rods connected to the wheel suspension. Suspension works by the rod pushing LHM into the sphere, compacting the nitrogen in the upper part of the sphere. Height levelling works by height correctors connected to the wheel suspensions. These height connectors allow for more fluid to travel to the rod/sphere system when detecting that the suspension is lower than its expected ride height (e.g. the car is loaded). Height correctors act with some delay in order not to correct regular suspension movements. Rear brakes are powered from the rear suspension sphere. Because the pressure there is proportional to the load, so is the braking power.
The whole high pressure part of the system is manufactured without any seals, it relies on metal tubing and extremely small tolerances for tightness under pressure. The only plastic/rubber parts are return tubes, catching seeping fluid around the suspension rods, from the brakes etc. Ironically these are the typical failure points for the system.
Spheres are subject to wear and pressure loss, mostly from nitrogen diffusing through the membrane. They typically keep between 60000 and 100000 km. Spheres originally used to have a valve on top and be rechargeable. Newer spheres do not have this valve anymore, but it can be retrofitted. Though a rechargeable sphere has a longer lifespan, the membranes will eventually wear out. A ruptured membrane means suspension loss at the attached wheel, or in the case of the accumulator sphere, reliance on the high pressure pump as the only source of braking pressure to the front wheels.
Compact suspension design
Excellent ride comfort (unless the suspensions spheres need re-gassing)
Maintains ride height independent of load
Large loads make no real changes to the handling
You can feel individual in owning a car so different to the herd
Poor road surfaces feel much like good roads and can be driven accordingly
Non-Citroën or Mercedes-Benz specialists are terrified of the system
Reliability has often been called into question - mostly in vain, except for very old and badly maintained vehicles
Body roll can be a severe problem on Citroëns (except Activa)
The road can feel quite distant
Road noise travels quite easily into the cabin on Citroëns
A few people complain of car sickness
Partial failure of the system is more disastrous than in normal cars and is more likely as time and corrosion take their toll. This is more likely with the Citroën belt-driven system as opposed to the chain-driven system found on Mercedes-Benz
Although physically no more complex than a traditional gas strut and coil spring set up, Hydro pneumatics are not as intuitive to understand. This leads to people feeling wary of the technology and in turn reluctant to purchase older cars (without warranties) that use it. Therefore it can be very cheap to pick up an old Citroën that features this magic carpet ride.
It is fair to say that as well as having a negative impact on Citroëns reputation for reliability, this system does at least re-enforce the legend of Citroën as an automobile innovator.
The accumulator sphere typically has enough pressure left for multiple hard braking, even if the engine (and attached high pressure pump) stops working.
Hydro pneumatic cars can drive on 3 wheels. One can remove one rear wheel, and the car will be able to compensate by pressurising the other 3 wheels higher.
Maseratis in the 1970s borrowed some parts of the system for braking and power steering, but the only Maserati which did use a hydro pneumatic suspension was the Quattroporte II. However, Rolls Royce cars used the system in combination with a standard suspension.
The first hydro pneumatic cars used a different liquid called LHS (liquide hydraulique synthétique). This was thought to have superior properties over LHM, but it turned out to eat the rubber of the return tubes when aging, thoroughly immobilising thousands of cars.
Hydro pneumatic parts being machined to the 1/1000th of a millimetre, were the only automotive parts being used in this way without parts matching (finding parts of corresponding tolerances by trial and error), up to the 1980s.
The "poor man's" alternative to hydro pneumatic is British Leyland's Hydragas system, which uses pre-pressurised spheres which are interconnected between axles.