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This article has been lifted, with permission, from Toaph's site. Although it relates to the D Series, the general principles apply to all hydropneumatic cars.

About the CitroŽn Hydraulic System

The principles of hydraulics have been present in automotive design since the first brake system distributed pressure to each wheel by means of compressing fluid rather than pulling cables or mechanical linkages. Hydraulic systems were further used in similar ways such as operating clutch mechanisms, and in new ways such as hydraulically dampened shock absorbers, power assisted steering, and automatic transmissions.The designers of the CitroŽn DS19 set out to use hydraulics in an all new way. Rather than have a number of independent hydraulic systems, each with its own fluid type, reservoir and pumping mechanism, the DS19 would have one master hydraulic system that would feed a universal fluid to specialized subsystems. This would simplify the design and create a more unified automobile.The master hydraulic system was kept at a constant pressure, fed by a pump that was powered by the engine itself by means of a belt (the way a conventional power steering unit is powered). Hydraulic pressure was distributed to the various subsystems as needed, and would always return to a common reservoir.The considerable amount of pressure that the engine was capable of generating gave the DS19 designers considerable creative latitude. They decided that not only would the hydraulic system dampen the suspension, but it would actually suspend the vehicle as well. Rather than employing springs or torsion bars, the designers of the DS19 made the hydraulic shocks "load-bearing." This would provide an incredibly smooth ride, and would allow certain functionality that would be impossible with conventional suspension methods. By regulating the volume of fluid distributed to the load-bearing shocks, it was possible to adjust the height at which the vehicle was suspended. By using the position of the suspension arms relative to the body as the regulating devise, it was possible for the vehicle to automatically level itself when exposed to an uneven load. By simply providing a control mechanism that effected an adjustment of the height regulation device, the overall riding height of the vehicle could be set to various levels.The designers of the DS19 redefined the automotive hydraulic paradigm. Beyond changing the way that the hydraulic systems were implemented, they came up with entirely new ways of using hydraulics, and were able to do things that had never been done before.

The High-Pressure Pump

The genesis of the hydraulic system is the reservoir. It must be large enough not only to contain the combined reservoirs of a conventional vehicle, but also all the fluid required to bring the suspension up to full height. It is located at front of the vehicle to the right and behind the spare tire.

At the heart of the pump is a simple piston. When the piston ascends, a valve opens to allow fluid to be drawn from the reservoir. When the piston descends, that valve closes and another opens allowing fluid to be forced out under pressure. Through the magic of hydraulic physics, this simple mechanism, smaller than a roll of dimes, is able to generate enough hydraulic pressure to hold the body aloft even under stress and heavy loads.

In order to generate an uninterrupted flow of pressurized fluid, the pump unit is actually comprised of seven pistons arranged in a circle. By rotating an armature, each piston is depressed in succession and a constant supply of fluid is produced. The armature is turned by a pulley that is run by a belt, and can be engaged or disengaged by means of an electro-magnet clutch.

Pressurized Spheres

A fundamental component of the CitroŽn hydraulic system is the pressurized sphere. There are six such units in the average DS19. They are used for a variety of purposes, but each one functions in exactly the same way.

Each unit separates into halves. They screw together to form a solid unit that has a perfectly spherical cavity inside. A hemispherical rubber bladder conforms to one half of the cavity, secured in the seam between the two halves. The cavity is then completely charged with compressed nitrogen creating a very stable pressurized environment. (Note: more modern sphere units to not unscrew into halves, but were maufactured as integrated units).

Hydraulic fluid can enter the unit through an orifice below the bladder. As a quantity of fluid is pressed into the cavity, the rubber bladder is forced upward further compressing the nitrogen gas. When pressure outside the unit decreases, the quantity of fluid will be forced back out of the cavity as the compressed nitrogen again attains equilibrium.

The Main Accumulator

The first use of a pressurized sphere is as the main accumulator. It collects high-pressure fluid from the pump and distributes it to the subsystems. It is connected to an electronic pressure regulator switch. When pressure is low, e.g. at start-up time, the switch is tripped, engaging the pump, filling the pressurized sphere with fluid. When the pressure inside the sphere reaches a certain point the switch is cut and the supply of high-pressure fluid stops.

Each of the subsystems draws more or less directly from this sphere as pressure is needed to perform its specific function. Even if there is a constant draw on the accumulated high-pressure fluid, the pump will only be engaged intermittently to ensure that the pressure in the accumulator never gets too low.

Subsystem Components

The three major subsystem components are the steering, the transmission, and the suspension & brakes. For the purposes of this discussion, each can be thought of simply as a "black box" that accepts high-pressure fluid from the main accumulator as needed, and returns it to the reservoir when finished.

The "Load-bearing" Shocks

Each load-bearing shock is a simple piston with a pressurized sphere on top of it. Hydraulic fluid can pass back and forth between the piston and the sphere. The pressure of the compressed nitrogen in the sphere counteracts the force of the weight of the body. In this way the spheres function as springs or torsion bars would in conventional cars. An iris in the orifice between the piston and the sphere produces a dampening effect.

The Suspension Subsystem

The suspension subsystem is fed directly from the main accumulator the way the other subsystems are. The feed immediately splits front and rear, each passing through a Height Control Valve. When each valve is activated, high-pressure fluid inflates the pair of load-bearing shocks. When the valve is in the neutral position, the pressure level remains constant between the pair. When the valve is deactivated, the fluid in the shock pair drains directly back to the reservoir.

Sharing pressure between left and right shocks provided many benefits. The tendency to equalize pressure between the two accomplished a horizontal self-leveling, even at high speeds. This achieved a natural anti-roll effect and gave the relatively large and heavy sedan remarkably good cornering capabilities.

While sharing pressure left to right provided many benefits, it proved more advantageous to have pressure separated fore and aft. This was accomplished through the independent height control valves. If the load on the rear of the car increased, the rear valve would be activated and a greater volume of high-pressure fluid would be allowed into that pair.

The Height Control Valves

The height control valves are actually quite simple devices. Each is operated by a mechanical arm that extends from the device. When the arm is pushed into the valve, a path is created between the high-pressure feed and the suspension elements. When the arm is pulled out from the valve, a path is created between the suspension elements and the reservoir. When the arm is in the neutral position, no fluid can flow in either direction. By mounting the valve on the frame and connecting the arm to the suspension, the self-leveling effect is achieved. If a heavy load is placed in the rear, for example, the suspension will force the valve arm inwards, and more high-pressure fluid will enter the rear suspension system until the valve arm returns to the proper position. The height adjustment lever under the dash adjusts the relationship between the valve arm and the suspension, thus effecting the height adjustment.

The Brake Subsystem

The brakes get their hydraulic pressure directly from the suspension. While the suspension subsystems are themselves isolated front and aft, so are the brake subsystems. Each pressure line comes into the brake "button" independently, and is sent independently to the front calipers and rear drums respectively.

The primary reason for taking the pressure from the suspension is safety. If there was ever a catastrophic loss of pressure in the vehicle, for example a failure of the pump, there would be adequate pressure in the suspension to power the brakes enough to bring the car to a stop. The side effect of this, however, is that the loss of pressure in the load-bearing shocks causes a slight but sudden loss of height. This effect is counter-acted in the front by introducing another pressurized sphere into the system. It is called the "brake accumulator," and it stores enough pressure that the front suspension height is not affected when the brakes are applied. There is no such accumulator in the rear, because the loss of suspension height is not a bad thing in this case. When brakes are applied at high speed, the vehicle naturally wants to "nose dive." By reducing the height of the rear suspension when the brakes are applied, the rear sinks slightly and counter-acts the dive effect.

The Big Picture

The end result of this unique and ingenious design is a flawlessly functioning network of inter-operating parts. The substystems interact with each other cooperatively to form a master system that accomplishes every function needed in a modern, luxury automobile. This holistic approach has created a vehicle that is almost more organism than machine.


© 1996 Toaph and 2010 Julian Marsh