Hydropneumatic suspension

Hydro Pneumatic offers potentially very high resiliency, variable ground clearance as well as automatic level control, adjusts itself to the vehicle load.

Construction

Instead of conventional mechanical springs and shock absorbers, each wheel is connected through the piston of a hydraulic cylinder with a spring loaded ball. It is filled with nitrogen under high pressure, which is separated by a membrane from the hydraulic fluid acting on it. This gas represents the actual spring element represents the connection to the wheel is made ​​of a variable amount of hydraulic fluid, which transfers the wheel movements via the hydraulic cylinder on the diaphragm spring of the associated ball. The amount of the hydraulic fluid in the wheel cylinders of the respective axis from an automatic control valve, the so-called height corrector influences. The hydraulic fluid is provided for this purpose by a high pressure pump permanently available. The vibration damping is realized by flexible valve plate and bypass holes, which slow down the flow of liquid to the spring ball.

The system replaces this way both conventional springs and shock absorbers and works by his constant pressure feed even with leakage backlash, ensuring at all times a fully effective damping is ensured.

Function

As long as the hydraulic system is pressurized, there is no suspension travel available. After starting the engine the built from the hydraulic pump pressure over the height correctors lifts the vehicle to: The body is on the - adjustable at Citroën's hydropneumatic - "ideal height" ( formerly known as " ground clearance ") is raised on the wheels of the full suspension travel. stand

A suspension goes through loading of the vehicle " to its knees ". This effect must be limited by a high enough spring rate for steel springs normally to maintain a certain residual spring travel. The suspension is then, but unloaded harder than necessary, and it is when the load increases less and less suspension travel available. As a result the road situation deteriorates progressively, the body has practically never their "ideal height".

This is different with the hydropneumatic: load in the car, the gas spring ball is compressed, its volume decreases, the body sinks. This is compensated by the said height corrector means of a regulation by Nachleiten of fluid under pressure into the suspension cylinder again. The reduced volume of gas is replaced by the same volume of liquid, the body reaches again as said ideal height.

Now since absorption of the spring travel is compensated under load, the suspension in its operating point, the ideal height, are very " soft " designed. An important property of the resilient nitrogen gas is also taken advantage of the law of Boyle -Mariotte, after which the product of pressure and volume of a given quantity of gas is always constant. The more the gas is compressed, its volume is so the lower, the higher the pressure force is required to cause a certain wheel motion. The natural frequency of the vehicle remains constant, a substantial advantage also with conventional air springs. This results in a load-dependent, proportional hardening of the suspension, which also has a loaded vehicle by a firmer suspension now the same driving characteristics as the empty vehicle (as opposed to conventional steel suspension, the loaded significantly deteriorated driving characteristics have ).

The ride height can also be also set manually, for example for driving over difficult unpaved paths or for loading.

As a by- product of this suspension system results in the so-called " central hydraulics ". This refers to the integration of the steering and braking support in the high-pressure hydraulic system of the suspension, so that additional components such as a power steering pump and brake booster are expendable.

Rolling stock

In license have Rolls-Royce ( Silver Shadow and Bentley identical T1/T2) and Daimler -Benz (standard in the Mercedes- Benz 450 SEL 6.9 Mercedes- Benz W 116 series - 1975 to 1980 - and optional at extra cost in the 8 SEL -cylinder models of the Mercedes- Benz W 126 - 1979 to 1991) used the hydro pneumatic technology, but only for the purpose of suspension; Power steering and braking systems were not integrated by Rolls- Royce and Mercedes -Benz.

Hydro-pneumatic suspension models by Citroën were or are:

Initially put Citroën synthetic hydraulic liquids - first " RF Rouge " plant-based, later called LHS (liquid hydraulique synthetique ), recognizable by the red color. This tended, among others to become resinous.

From 1964, LHS was replaced by the colorless LHS2, but was hygroscopic and so made ​​if maintenance is insufficient for corrosion damage in the hydraulic system. In addition, these liquid coatings was very aggressive towards.

The final for all central hydraulic systems green LHM (liquid hydraulique minerals ) was introduced in 1966. LHM was not hygroscopic, which the life and durability of the systems significantly improved and the maintenance of the hydro- pneumatic on rare oil change ( Citroën DS: every 40,000 miles) reduced, clean the filter and occasional checking of spring ball pressure. And the paint was now no longer endangered in case of spilled drops.

With cessation of production of XM and Xantia in 2000, the era of central hydro pneumatic systems ended, as is the use of LHM.

The hydropneumatic Citroën C5 and C6 is no longer working with a source coupled to the vehicle engine, but with an electrically driven pump. Steering and braking system are of conventional design and is no longer connected as in previous designs with hydropneumatic, there is here no longer a " central hydraulics " the speech. The oil used is now called LDS.

Such a system can also be found in the all-wheel version of the Peugeot 405 ( rear axle only ).

Disadvantages of the system, and fiction

The high driving comfort makes for the unwonted hydropneumatics driver partially for an uncertain feeling, since mentally a soft steel suspension is associated with their handling disadvantages. The fact that the road situation remains stable even for the typical body movements, must " take - it " some time. The reaction of Citroën it was an increasing tightening of the chassis, but this only meant sacrificing comfort without additional performance and handling.

Today, older models can hold inexpensive repair. The slightly higher cost of the hydro- pneumatic offset by a very high level of comfort and safety. Without system pressure, ie when the engine is not running and so the high pressure pump is not driven, the towing of DS, ID, SM, GS / A, CX, BX, XM and Xantia is only possible because the chassis is not active, the car fast " touches " and only the parking brake is available ( which acts on these models to the front drive wheels ).

The safety logic of the brakes makes for a felt unlimited oil supplies with defective (open) brakes, which does lead to a significant pollution, but the vehicle can stop safely. If the primary circuit fails, the brake supply is taken from the suspension system: the vehicle then lowers Although every time the brakes a few millimeters, but it can be casually parked on normally traffic safe place.

The brakes with the Citroën DS, SM, GS / A, CX and BX is used to: instead of using a long pedal travel build up the brake pressure had to be opened with a short way only one valve ( pressure point brake). DS and SM had instead of a pedal even just a rubber button, also called " brake fungus ". In the later Citroen Xantia, XM of the pedal travel was virtually simulated. This central high-pressure brake was very sensitive and aggressive; normal in conventional vehicles pressure on the brake led here to a screeching halt. The following Citroën C5 and C6 have a market-standard servo brake.

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