The device and principle of operation of the hydropneumatic suspension Hydractive
Suspension and steering,  Car device

The device and principle of operation of the hydropneumatic suspension Hydractive

Every year, car manufacturers improve their car models, making some changes in the design and layout of the latest generation vehicles. Some updates can be received by the following auto systems:

  • Cooling (the device of the classical cooling system, as well as some of its modifications, is described in a separate article);
  • Lubricants (its purpose and principle of operation are discussed in detail here);
  • Ignition (about her exists another review);
  • Fuel (it is considered in detail separately);
  • Various modifications of all-wheel drive, for example, xDrive, which read more about here.

Depending on the layout and the purpose of homologation, a car can receive updates to absolutely any system, even one that is not mandatory for modern vehicles. in a separate review).

One of the most important systems that ensure the safe and comfortable movement of a car is its suspension. The classic version is considered in detail here... Developing new suspension modifications, each manufacturer strives to bring their products as close as possible to the ideal, capable of adapting to different road conditions and meeting the needs of any, even the most sophisticated driver. For this, for example, active suspension systems have been developed (read about it separately).

The device and principle of operation of the hydropneumatic suspension Hydractive

In this review, we will focus on one of the successful suspension modifications used in many Citroen models, as well as some other automakers. This is the Hydractive hydropneumatic suspension. Let's discuss what its peculiarity is, how it works and how it works. We will also consider what its malfunctions are, and what are its advantages and disadvantages.

What is a hydropneumatic car suspension

Any modification of the suspension is primarily intended to improve the dynamic characteristics of the car (its stability when cornering and when performing sharp maneuvers), as well as to increase comfort for everyone who is in the cabin during the trip. The hydropneumatic suspension is no exception.

This is a type of suspension, the design of which implies the presence of additional elements that allow you to change the elasticity of the system. This, depending on the conditions on the road, allows the car to swing less (rigidity is necessary for high-speed sports driving) or to provide maximum softness to the transport.

Also, this system allows you to change the ground clearance (about what it is, how it is measured, and also what role it has for the car, read in another review) of the car, not only to stabilize it, but also to give the vehicle originality, as, for example, in lowriders (read about this style of auto-tuning here).

In short, this suspension differs from its usual counterpart in that it does not use any standard elastic element, for example, a spring, shock absorber or torsion bar. The scheme of such a suspension will necessarily include several spheres that are filled with a gas or a certain liquid.

Between these cavities is an elastic, strong membrane that prevents mixing of these different media. Each sphere is filled with liquid to a certain extent, which allows you to change the mode of operation of the suspension (it will react differently to unevenness of the road). The change in the stiffness of the suspension occurs due to the fact that the piston changes the pressure in the circuit, due to which compression or weakening of the effect of the gas filling the working circuit of the sphere occurs through the membrane.

The device and principle of operation of the hydropneumatic suspension Hydractive

The hydraulic circuit is automatically controlled. In a modern car equipped with this system, the position of the body is corrected electronically. The height of the car is determined by such parameters as the speed of the car, the condition of the road surface, etc. Depending on the car model, it can use its own sensor or sensor, which is designed for the operation of another car system.

The Hydractive system is considered one of the most effective and progressive, despite the fact that the technology is more than 70 years old. Before considering which cars the hydropneumatic type of suspension can be installed on, and what is the principle of its operation, we will consider how this development appeared.

The history of the appearance of the Citroen hydraulic suspension

The history of the development of a hydraulic version of this auto system began in 1954 with the release of the first car with such a suspension. It was the Citroen Traction Avante. This model received hydraulic shock-absorbing elements (they were installed on the rear section of the machine instead of springs). This modification was later used in DS models.

The device and principle of operation of the hydropneumatic suspension Hydractive

But at that time this system could not be called hydropneumatic. The hydropneumatically adaptive suspension, now called the Hydractive, first appeared on the Activa concept car. A working system was demonstrated in the 88th year of the last century. During the entire production period, Hydractive has changed two generations, and today the third generation of the device is used on some machine models.

The development was based on the principle of functioning of different types of suspensions used in heavy vehicles, including heavy military equipment. The novelty, adapted for the first time for passenger transport, caused great delight among auto correspondents and specialists in the world of the automotive industry. By the way, the adaptive suspension is not the only revolutionary development that Citroen has introduced into its models.

Adaptive light (headlights turn to the side where the steering gear or each steering wheel turns) was another advanced development that was introduced in the 1968 Citroen DS model. Details about this system are described in another review... In combination with this system, the body, capable of lifting, as well as the softest and smoothest operation of the dampers, brought the car an unprecedented glory. Even today, it is a coveted item that some car collectors would like to acquire.

The device and principle of operation of the hydropneumatic suspension Hydractive

Modern models now use the third generation of the system, regardless of whether the car is rear-wheel drive or front-wheel drive. We will talk about the differences between previous designs a little later. Now let's consider what principle the modern system has.

How the Hydractive suspension works

The hydropneumatic suspension is based on the principle of the action of hydraulics on the actuator, as, for example, in the brake system (it is described in detail in another review). As mentioned earlier, instead of springs and shock absorbers, such a system uses a sphere, which is filled with nitrogen under high pressure. This parameter depends on the weight of the car, and sometimes it can reach 100 atm.

Inside each sphere is an elastic yet highly durable membrane that separates the gas and hydraulic circuits. In earlier generations of hydraulic suspension, automobile oil with a mineral composition was used (for more information on the types of auto oils, read here). It was from the LHM category and was green. The latest generations of the system use a synthetic orange analogue (type LDS for hydraulic installations).

Two types of spheres are installed in the car: working and accumulating. One work area is dedicated to a separate wheel. The accumulation sphere is connected with the workers by a common highway. In the working containers in the lower part there is a hole for the hydraulic cylinder rod (it must lift the car body to the required height or lower it).

The suspension works by changing the pressure of the working fluid. Gas is used as an elastic element, filling the space in the upper part of the sphere above the membrane. To prevent hydraulic oil from flowing from one working sphere to another on its own (because of this, a strong body roll would be observed), the manufacturer uses holes with a certain section in the system, as well as petal-type valves.

The device and principle of operation of the hydropneumatic suspension Hydractive

The peculiarity of calibrated holes is that they create viscous friction (hydraulic oil has a density much higher than water, therefore it is not able to freely flow from cavity to cavity through narrow channels - this requires a lot of pressure). During operation, the oil heats up, which leads to its expansion and dampens the resulting vibrations.

Instead of a classic shock absorber (read about its structure and principle of operation separately) a hydraulic strut is used. The oil in it does not foam or boil. Gas-filled shock absorbers now have the same principle (read about which shock absorbers are better: gas or oil, read in another article). This design allows the device to work for a long time under heavy loads. Moreover, little in this design does not lose its properties, even if it gets very hot.

Different operating conditions of the system require their own oil pressure and the rate of creation of the desired pressure. This process is multistage in the system. The smoothness of the piston stroke depends on the opening of a particular valve. You can also change the stiffness of the suspension by installing an additional sphere.

In the latest modifications, this process is regulated by directional stability sensors, and in some cars the manufacturer even provided for manual adaptation (in this case, the cost of the system will not be so expensive).

The line works only when the engine is running. The control electronics of many cars allows you to change the position of the body in four modes. The first is the lowest ground clearance. This makes it easier to load the vehicle. The latter is the largest ground clearance. In this case, it is easier for the vehicle to overcome off-road conditions.

True, the quality of the passage of obstacles by the car directly depends on the type of rear part of the suspension - a transverse beam or a multi-link structure. The other two modes simply provide the comfort the driver wants, but usually there are no major differences between them.

If the hydropneumatics simply increases the distance between the body and the crossbeam, the vehicle's passability in most cases practically does not change - the car can hook on an obstacle with the beam. More efficient use of hydropneumatics is observed when using a multi-link design. In this case, the clearance really changes. An example of this is the adaptive suspension in the latest generation Land Rover Defender (a test drive of this model can be read here).

The increase in pressure in the line is provided by an oil pump. The height relief is provided by the corresponding valve. To increase ground clearance, the electronics activate the pump and it pumps additional oil into the central sphere. As soon as the pressure in the line reaches the required parameter, the valve is activated and the pump is turned off.

When the driver presses the gas pedal more sharply and the car picks up speed, the electronics registers the vehicle's acceleration. If you leave the ground clearance high, the aerodynamics will harm the vehicle (read more about aerodynamics in another article). For this reason, the electronics initiate the release of the oil pressure in the circuit via the return line. This brings the vehicle closer to the ground and the air flow pushes it closer to the road.

The device and principle of operation of the hydropneumatic suspension Hydractive

The system changes the ground clearance 15 millimeters lower when the car accelerates to speeds in excess of 110 kilometers per hour. An important prerequisite for this is the quality of the road surface (to determine this, there is, for example, a vehicle stability control system). In case of poor road surface and speed, below 60 km / h, the car rises by 20 millimeters. If the car is loaded, the electronics also pump oil in the highway so that the body maintains its position relative to the road.

Another option available to some types of models equipped with the Hydractive system is the ability to eliminate the roll of the car during high-speed cornering. In this case, the control unit determines to what extent a certain part of the suspension is loaded, and, using the relief valves, changes the pressure on each wheel. A similar process occurs to eliminate pecks when the machine stops abruptly.

Main suspension elements Hydractive

The hydropneumatic suspension scheme consists of:

  • Hydropneumatic wheel struts (working area of ​​a single wheel);
  • Accumulator (central sphere). It accumulates a reserve amount of oil for the operation of all areas;
  • Additional areas that regulate the stiffness of the suspension;
  • A pump that pumps the working fluid into separate circuits. The device was originally mechanical, but the latest generation uses an electric pump;
  • Valves and pressure regulators that are combined into separate modules or platforms. Each valve and regulator block is responsible for its own assembly. There is one such site for each axis;
  • Hydraulic line, which unites all regulating and executive elements;
  • Safety, regulating and bypass valves associated with the braking system and power steering (such an arrangement in certain varieties was used in the first and second generations, and in the third they are absent, since this system is now independent);
  • An electronic control unit, which, in accordance with the signals received from the sensors of this and other systems, activates the programmed algorithm and sends a signal to the pump or regulators;
  • Body position sensors installed in the front and rear of the vehicle.

Generations of Hydractive suspension

The modernization of each generation was carried out in order to increase the reliability and develop the functionality of the system. Initially, the hydraulic line was combined with the brake system and power steering. The last generation received contours independent of these nodes. Due to this, the failure of one of the listed systems does not affect the performance of the suspension.

Consider the distinctive features of each of the existing generations of hydropneumatic car suspension.

XNUMXst generation

Despite the fact that the development appeared in the 50s of the last century, the system got into mass production in 1990. This suspension modification was included with some Citroen models, such as the XM or Xantia.

The device and principle of operation of the hydropneumatic suspension Hydractive

As we discussed earlier, the first generations of systems were combined with brake and power steering hydraulics. In the first generation of the system, the suspension could be adjusted to two modes:

  • Automatic... Sensors record various parameters of the car, for example, the position of the accelerator pedal, the pressure in the brakes, the position of the steering wheel, and so on. As the name of the mode suggests, the electronics independently determined what the pressure in the highway should be in order to achieve the ideal balance between comfort and safety during the trip;
  • Sports... This is a mode adapted for dynamic driving. In addition to the vehicle height, the system also changed the hardness of the damper elements.

XNUMXnd generation

As a result of the modernization, the manufacturer changed some parameters of the automatic mode. In the second generation, it was called comfortable. It made it possible not only to change the ground clearance of the car, but also briefly the stiffness of the dampers when the car entered a turn at speed or accelerated.

The presence of such a function allowed the driver not to change the electronics settings if he drove the car more dynamically for a short time. An example of such situations is a sharp maneuver when avoiding an obstacle or overtaking another vehicle.

Another innovation that was made by the suspension developers is the additional area in which a check valve was installed. This additional component made it possible to maintain a high head in the line for a longer time.

The peculiarity of this arrangement was that the pressure in the system was maintained for more than one week, and for this the car owner did not need to start the engine so that the pump would pump oil into the reservoir.

The device and principle of operation of the hydropneumatic suspension Hydractive

The Hydractive-2 system was used on Xantia models produced since 1994. One year later, this suspension modification appeared in the Citroen XM.

III generation

In 2001, the Hydractive hydropneumatic suspension underwent a major upgrade. It began to be used in the French automaker's C5 models. Among the updates are the following features:

  1. Changed hydraulic circuit. Now the braking system is not part of the line (these circuits have individual reservoirs, as well as tubes). Thanks to this, the suspension scheme has become a little simpler - there is no need to control the pressure in two systems that are different from each other, using different pressure of the working fluid (for the brake system to work, there is no need for a large pressure brake fluid).
  2. In the settings of operating modes, the option to manually set the required parameter has been removed. Each individual mode is leveled exclusively by electronics.
  3. Automation independently lowers the ground clearance by 15mm relative to the standard position (set by the manufacturer - in each model it has its own), if the car accelerates faster than 110 kilometers / hour. When slowing down to a speed in the range of 60-70 km / h, the ground clearance increases by 13-20 millimeters (depending on the car model) relative to the standard value.
The device and principle of operation of the hydropneumatic suspension Hydractive

So that the electronics can correctly adjust the body height, the control unit collects signals from sensors that determine:

  • Vehicle speed;
  • The height of the front of the body;
  • Rear body height;
  • Additionally - signals from the exchange rate stability system sensors, if it is present in a specific car model.

In addition to the standard third generation in the expensive C5 configuration, as well as the basic C6 equipment, the automaker uses the Hydractive3 + version of the hydropneumatic suspension. The main differences between this option and the standard analogue are:

  1. The driver can choose between two suspension modes. The first one is comfortable. It is softer, but it can change its stiffness for a short time depending on the situation on the road and the driver's actions. The second is dynamic. These are sporty suspension settings that feature increased damping stiffness.
  2. Improved system response algorithms - electronics better determine the optimal clearance. To do this, the control unit receives signals about the current transport speed, the position of the body in front and behind, the position of the steering wheel, acceleration in the longitudinal and cross-section, loads on the damper suspension elements (this allows you to determine the quality of the road surface), as well as the position of the throttle (in detail about what is a throttle valve in a car is told separately).

Repair and parts price

Like any other system that provides automatic control of various parameters of the car, the Hydractive hydropneumatic suspension costs a lot of money. It synchronizes the operation of many electronic devices, as well as hydraulics and pneumatics. A large number of valves and other mechanisms, on the operation of which the stability of the vehicle depends, are all units that require some maintenance, and in case of their failure, also expensive repairs.

Here are just some prices for hydropneumatic repair:

  • Replacing the hydraulic prop will cost about $ 30;
  • The front stiffness regulator changes for about 65 cu;
  • To change the front sphere, the motorist will have to part with 10 dollars;
  • Refueling a serviceable but unpressurized unit costs about $ 20-30.

Moreover, these are only the prices of some service stations for the work itself. If we talk about the cost of parts, then this is also not a cheap pleasure. For example, the cheapest hydraulic oil can be purchased for about $ 10. for one liter, and when repairing the system, this substance requires a decent amount. The oil pump, depending on the type of construction and car model, will cost about $ 85.

Most often in the system, a malfunction appears in spheres, high pressure pipes, pumps, valves and regulators. The cost of the sphere starts at $ 135, and if you do not buy the original part, it is one and a half times more expensive.

Often most suspension elements suffer from the effects of corrosion, as they are not protected by anything from dirt and moisture. The parts themselves are dismantled without significant effort, but everything is complicated by corrosion and boiling of bolts and nuts. Due to poor access to some fasteners, the cost of dismantling the assembly is often equated to the cost of the element itself.

The device and principle of operation of the hydropneumatic suspension Hydractive

Replacing the pipeline is another problem that can fall on the head of the car owner. The line connected to the pump, damaged by corrosion, cannot be removed without dismantling the other elements of the car located under the bottom. This pipeline runs under almost the entire car, and so that it does not damage the ground, it is installed as close to the bottom as possible.

Since the fasteners of other devices and structures are also not protected by anything from moisture and dirt, their dismantling can also be difficult. For this reason, at some service stations, motorists have to shell out about $ 300 for replacing a simple tube.

It is generally inappropriate to replace some system components with new ones. An example of this are platforms, or modules, that adjust the stiffness of the damper struts. Usually, in this case, the elements are simply repaired.

Before purchasing a vehicle with such a suspension, it is also necessary to take into account that the breakdown of one element is often accompanied by the failure of several mechanisms at once, so the motorist will have to pay dearly for the repair and maintenance of such a system. This is especially true when buying a used car. In such transport, one part after another will surely fail. Also, in comparison with the classic suspension, due to the large number of parts operating under heavy load, this system will have to be subjected to routine maintenance more often.

Benefits of hydropneumatic suspension

In theory, using gas in the suspension as a stop is ideal. This arrangement is devoid of constant internal friction, the gas does not have "fatigue" like metal in springs or springs, and its inertia is minimal. However, this is all in theory. Often, a development that is at the drawing stage requires changes when translating it into reality.

The very first obstacle that engineers face is the loss of suspension efficiency while implementing all the groundwork displayed on paper. For these reasons, the hydropneumatic version of the suspension has both advantages and disadvantages.

The device and principle of operation of the hydropneumatic suspension Hydractive

First, consider the advantages of such a suspension. These include:

  1. Maximum smoothness of the dampers. In this regard, for a long time, models produced by the French company Citroen (read about the history of this auto brand here), were considered the standard.
  2. It is easier for the driver to control his vehicle around corners while driving at high speed.
  3. The electronics are able to adapt the suspension to the driving style.
  4. The manufacturer guarantees that the system is capable of running up to 250 thousand kilometers (provided that a new car is bought, not a used one).
  5. In some models, the automaker has provided for manual adjustment of the body position relative to the road. But even the automatic mode does an excellent job of its function.
  6. In both manual and automatic modes, the system does an excellent job of adapting the rigidity of the work depending on the road situation.
  7. Compatible with most types of multi-link rear axle, as well as MacPherson struts used on the front of the car.

Disadvantages of hydropneumatic suspension

Despite the fact that the hydropneumatic suspension is capable of qualitatively changing its properties, it has several significant disadvantages, which is why most motorists do not consider purchasing vehicles with such a suspension. These disadvantages include:

  1. To realize the maximum effect of the work painted on the drawings, the manufacturer has to use special materials, as well as introduce innovative technologies into the production of his car models.
  2. A large number of regulators, valves and other elements necessary for the high-quality operation of the system are at the same time potential areas of possible breakdown.
  3. In the event of a breakdown, the repair is associated with the dismantling of adjacent vehicle components, which in some cases is extremely difficult to perform. Because of this, you need to look for a real specialist who can do all the work with high quality and not damage the machine.
  4. The entire assembly is expensive and, due to the large number of components, often requires maintenance and repair. This is especially true for cars bought on the secondary market (for details on what you need to pay attention to when buying a used car, read in another review).
  5. Due to the breakdown of such a suspension, the car cannot be operated, since the loss of pressure automatically leads to the disappearance of the damper functions of the system, which cannot be said about classical springs and shock absorbers - they simultaneously never suddenly fail.
  6. The system is often not as reliable as the automaker is convinced.
The device and principle of operation of the hydropneumatic suspension Hydractive

After Citroen began to increasingly encounter malfunctions of its development, it was decided to change this suspension to a classic analogue for models of the budget segment. Although the brand has not completely abandoned the production of the system. Its different variants can be seen on premium cars of other auto brands.

This development is almost impossible to find in ordinary production cars. Most often, premium and luxury cars such as Mercedes-Benz, Bentley and Rolls-Royce are equipped with such a suspension. Over the years, hydropneumatic suspension has been fitted to the Lexus LX570 luxury SUV.

If we talk about Citroen C5, for which the latest generation of Hydractive was developed, now only a pneumatic analogue is used in these cars. Details about how such a suspension works, as well as how it works, is described in another article... The French automaker made this decision in order to reduce the cost of production and sale of the popular model.

So, the hydropneumatic suspension allows you to change the height of the car, as well as the stiffness of the damper units. As an alternative, some manufacturers use magnetic suspension modifications for these purposes. They are described in detail in another review.

In conclusion, we offer a short video comparison of some effective designs of suspensions, including the hydropneumatic version:

⚫CAPABLE OF WITHstanding EVERYTHING! UNUSUAL CAR SUSPENSION.

2 comment

  • Erling Bush.

    Is it true that the development of Citroën's unique suspension system started with a director demanding that a system be developed so that he could be transported / driven across a frozen plow field without losing his cigar box? V h Erling Busch.

  • Chuchin

    I had heard that it was said of the 2CV that it should be able to carry a basket of eggs across a plowed field without any breaking.

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