Hydropneumatics

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The hydropneumatics is a suspension system that performs the functions of damping and suspension by means of hydraulics and pneumatics . It was developed in the 1950s by Paul Magès for the passenger cars of the French car manufacturer Citroën . In 1989 the system was supplemented by an electronic control system, see Hydractive .

The hydropneumatics potentially offer comfortable, soft suspension with a progressive characteristic curve, variable ground clearance and automatic level control , so it adapts to the vehicle load.

construction

Parts of the hydropneumatic system with storage tank (left) and spring ball (green)

Instead of classic mechanical springs and shock absorbers , each wheel is connected to a spring ball by the piston of a hydraulic cylinder . This is filled with nitrogen under high pressure, which is separated from the hydraulic fluid acting on it by a membrane . This gas represents the actual suspension element. The connection to the wheel is established via a variable amount of hydraulic fluid, which transfers the wheel movements via the hydraulic cylinder to the membrane of the associated spring ball. The amount of hydraulic fluid in the wheel cylinders of the respective axle is influenced by an automatic regulating valve, the so-called height corrector . The hydraulic fluid is permanently made available for this purpose by a high-pressure pump. The vibration damping is effected by flexible valve plates and bypass holes, which brake the flow of the liquid to the spring ball.

In this way, the system replaces both conventional springs and shock absorbers and, thanks to its constant pressure supply, works without play even in the event of leakage, which ensures fully effective damping at all times.

function

As long as the hydraulic system is depressurized, there is no spring travel available. After the engine has started, the pressure built up by the hydraulic pump raises the vehicle via the height correctors: the body is raised to the ideal height (previously known as ground clearance), which was adjustable with Citroën's hydropneumatic system, at which the full suspension travel is available to the wheels.

A suspension goes "on its knees" when the vehicle is loaded. With steel springs, this effect must normally be limited by a sufficiently high spring stiffness in order to maintain a certain residual spring travel. The suspension is then unloaded harder than necessary, and there is less and less spring travel available with increasing load. As a result, the road holding deteriorates increasingly, the body practically never has its ideal height.

The situation is different with hydropneumatics: When the vehicle is loaded, the gas in the spring ball is compressed, its volume decreases and the body sinks. As a result, the height corrector feeds hydraulic fluid under pressure into the wheel suspension cylinder via a valve. The reduced volume of the gas is replaced by the same volume of liquid, so the body reaches the ideal height again. The suspension is regulated to maintain constant ground clearance .

Compression of the gas in the spring ball during compression

Since the consumption of the spring travel is compensated for under load, the suspension can be designed to be very soft at its working point, the ideal height. An important property of the resilient nitrogen gas is also used, Boyle-Mariotte's law , according to which the product of pressure and volume of a given amount of gas is always constant. The more the gas is compressed, i.e. the smaller its volume, the more pressure is required to bring about a certain wheel movement. The natural frequency of the vehicle increases with the load in hydropneumatics = air spring with constant gas weight, a major disadvantage compared to conventional air suspension = air spring with constant gas volume. The result is a load-dependent quadratic increase in the spring stiffness, which means that a loaded vehicle has stiffer suspension than the empty vehicle. This is the reason why vehicles with hydropneumatics are designed to be relatively soft when empty so that they do not appear too hard when loaded. In contrast, vehicles with conventional linear steel suspension are designed to be significantly harder so that the suspension does not become too soft when loaded. The vehicle level can also be adjusted manually, for example to drive over difficult unpaved roads or for loading.

The so-called central hydraulics is a by-product of this suspension system. This refers to the integration of the steering and braking assistance into the high-pressure hydraulic system of the suspension, which means that additional components such as a steering servo pump and a brake booster are unnecessary.

vehicles

The hydropneumatics are typical for the larger Citroën cars . The system was first used experimentally on prototypes of the 2CV, then in series on the rear axle of the Citroën 15CV of the last year of construction. It was officially introduced and known with the Citroën DS (1955 to 1975).

Under license have Rolls-Royce (Rolls-Royce Silver Spirit and in the Corniche and the Camargue (from model year 1981)) and Daimler-Benz (standard in the Mercedes-Benz 450 SEL 6.9 of the Mercedes-Benz 116 series  - 1975-1980 - , optionally for a surcharge in the 8-cylinder SEL models of the Mercedes-Benz 126 series  - 1979 to 1991 - as well as standard on the rear axle of the 124 series T-model  - 1985 to 1996) the hydropneumatic technology is used, but only for Suspension; Power steering and braking systems were not integrated by Rolls-Royce and Mercedes-Benz.

Citroën models with hydropneumatic suspension are:

Initially, Citroën used synthetic hydraulic fluids - initially plant-based HF Rouge, later the so-called LHS (liquide hydraulique synthetique), recognizable by its red color. Among other things, this tended to gum up.

From 1964 LHS was replaced by the colorless LHS2, which, however, was hygroscopic and thus caused corrosion damage in the hydraulic system if insufficiently maintained. In addition, this liquid was very aggressive towards paints.

The final green LHM (liquide hydraulique minerale) for all central hydraulic systems was introduced in 1966. LHM was no longer hygroscopic, which significantly improved the service life and operational stability of the systems and reduced maintenance work on the hydropneumatics to infrequent oil changes (Citroën DS: every 40,000 km), cleaning of the filter and occasional checking of the spring ball pressure, the paint was now with spilled drops also no longer endangered.

In the Citroën XM (starting from 1989) the hydropneumatics were equipped for the first time with the electronic control Hydractive .

With the cessation of production of XM and Xantia in 2000, the era of central hydropneumatic systems ended, as did the use of LHM.

The hydropneumatics in the Citroën C5 and C6 no longer work with a pump driven by the vehicle engine, but with an electrically driven pump. To control the oil pressure for setting the normal body height, mechanical height correction valves through which hydraulic oil flows are no longer used, but electronic height sensors that transmit their data to a suspension control unit and thus activate electromagnetic valves. The steering and braking system are conventional and no longer connected to the hydropneumatics as in earlier designs, it is no longer a central hydraulic system. The oil used is now called LDS.

Such a system can also be found on the rear axle of the all-wheel drive version of the Peugeot 405 .

The last Citroën with hydropneumatics was produced in August 2017.

Disadvantages of the system

The high level of driving comfort sometimes creates an unsafe feeling for drivers who are not used to hydropneumatics, as a soft steel suspension is mentally associated with its handling disadvantages. It takes some time to see that the road holding remains stable even with typical body movements. Citroën's reaction to this was an increasing tightening of the chassis, which, however, only meant a loss of comfort without further improving handling.

Today older models can be maintained inexpensively. The somewhat higher costs of hydropneumatics are offset by a very high gain in comfort and safety. Without system pressure, i.e. if the engine is not running and the high-pressure pump is not being driven, the towing of DS, ID, SM, GS / A, CX, BX, XM and Xantia is only possible to a limited extent, since the chassis is not active , the car touches down quickly and only the parking brake is available (which acts on the front drive wheels on these models).

The safety logic of the brakes ensures an unlimited supply of oil in the event of defective (open) brakes, which, although it leads to considerable environmental pollution, allows the vehicle to stop safely. If the primary circuit fails, the brake supply is taken over by the suspension system: the vehicle then lowers a few millimeters with every braking operation, but it can be parked in a traffic-safe place as normal.

Braking with the Citroën DS , SM, GS / A, CX and BX takes getting used to: Instead of building up brake pressure with a long pedal travel, only one valve had to be opened with a short travel (pressure point brake). DS (Pallas equipment) and SM only had a rubber button instead of a pedal, also known as a brake button. The pedal travel was simulated on the later Citroën Xantia and XM. This central high pressure brake was very sensitive and sensitive; Normal pressure on the brakes in conventional vehicles led to an emergency stop. The following Citroën C5 and C6 have a standard servo brake.

Web links

Commons : hydropneumatics  - collection of images, videos, and audio files

Individual evidence

  1. ↑ Daniel Puiboube: The big Citroën DS book. Edition Heel, Königswinter 1999, ISBN 3-89365-742-8 , p. 15.
  2. ↑ Revue de la presse, No 8 | 64 ° année, Août 2017, p. 4, "Citroën arrête la suspension hydropneumatique"