Hydropneumatic Suspension Explained

In this brief article, we are going to discuss Hydropneumatic Suspension, its evolution, and common problems with hydropneumatic suspension.

What is a Hydropneumatic suspension?

The Hydorpneumatic Suspension system is based on the basic principles that:

  • Gas is compressible so that Gas absorbs excessive force and act like a Spring
  • Fluid in hydraulics is incompressible and directly transfers force

At the crux of the Hydropneumatic system is a “Sphere”, also called ‘Accumulator’, which is a metal container spherical in shape that functions as a Pressure Sink as well as a Suspension Stiffness Element. The Hydro-Pneumatic Suspension system uses trapped Nitrogen in a sphere along with hydraulic pressurized fluid, both separated by a flexible rubber membrane. 

The Accumulator is oriented such that the Nitrogen portion is on top, whereas the Hydraulic portion sits below. The bottom portion is connected to the Hydraulic circuit of the vehicle. In many cases, the same hydraulic circuit is connected to the Brakes, Clutch, and steering.

The compressible gas provides suspension stiffness acting against the hydraulic fluid pressure. The adjustment of hydraulic fluid volume within the sphere gives the suspension the ability to vary ride height in the Cylinder ram.

The suspension works by means of a piston-cylinder arrangement, called “Ram” that forces hydraulic fluid into the sphere as the wheel is in bump motion. The Nitrogen in the sphere gets compressed at this time and provides the “Spring” reaction to the wheel. As the wheel moves into the rebound condition, the hydraulic fluid is drawn back into the Ram. As the hydraulic fluid flows in and out of the Accumulator Sphere, damping action is achieved by a two-way ‘leaf valve’ located at the opening of the sphere. The leaf valve resists the flow of hydraulic fluid in and out of the sphere, this generating damping force.

Who invented the HydroPneumatic Suspension?

The hydropneumatic suspension was invented by Citroen back in  1954. Citroen is now regarded as the pioneer in bringing Hydropneumatic suspension systems into mass-production.  

The first Citroen car Model to implement hydropneumatic suspension was the Traction Avant in 1954. Initially, it was implemented on the rear suspension alone. Later on, the 4-wheeled version of the hydropneumatic suspension was launched in the Citroen DS in the 1955 model year.

Which vehicles have a Hydropneumatic Suspension?

The Hydropneumatic suspension technology has been licensed by Citroen to several car manufacturers. Some of the notable examples are the Roll Royce Silver Shadow, the Maseratti QuatroPorte II and some Mercedes Benz models. 

The Toyota Active Control Suspension in the Toyota Soarer was a very similar system which was hydropneumatic and had very advanced electronic control systems at the time of its launch in 1991.

Apart from Passenger vehicle applications, the hydropneumatic suspension approach has been extensively used in many modern military tanks.

What suspension system did the Citroen C5 have?

The first generation of the Citroen C5, called DC/DE, was launched in 2001. Along with the C5 launch was Citroen’s next iteration of its hydropneumatic suspension called ‘Hydractive 3’.

‘Hydractive’ was the name given to Citroen’s modern Active version of the basic Hydropneumatic suspension system. The Hydractive system’s first version was launched in 1990.  On-board sensors would transmit signals like the car’s speed, acceleration, and road conditions to the Electronic Control Unit (ECU). The ECU would then decide to either activate or deactivate an extra pair of suspension spheres within the hydraulic circuit to either provide a smoother ride or better handling according to what is best suited to the given driving condition.

What are the most common suspension issues seen in the Citroen C5?

The most common Hydraulic suspension issues seen in the Citroen C5 are:

  • Suspension Faut Message
  • Ram Cylinder/Strut Leak
  • Rear Suspension sitting low
  • Suspension ‘Stop’ message

Citroen C5 hydraulic Suspension

Suspension Fault Message

Symptoms:

  • Error message appears
  • Suspension does not change height
  • Hydraulic motor does not run; no sound

Troubleshooting Suspension Fault Message

  • The suspension is stuck at the lowest height setting and is unable to raise due to the hydraulic pump not working
  • The first most obvious to check would be the fuse for the hydraulic pump.
  • On the left hand side in the engine bay, find the hydraulic pump fuse and inspect whether it was blown.
  • In case it is blown, before replacing the fuse, it would be best to check for damage or residue left behind by the burnt fuse and clean it up.

Front Ram Cylinder Strut/Actuator Leak

Symptoms:

  • Poor Ride quality
  • Clunking noises 
  • Bump stop hitting often

Troubleshooting Front Cylinder ram Leak

  • There are a number of reasons why you find oil on the cylinder ram. 
    • if there is a block in the hydraulic inlet valve this could lead to oil droplets on the Cylinder
    • The is an actual deterioration of the Cylinder Ram in that the top seal would have failed
  • To check the hydraulic connector, just remove it and cut out the tip portion of the hosepipe that was inserted into the valve. Now insert the new tip of the hosepipe back into the valve
  • If it is concluded that the Cylinder ram was indeed the culprit, this part can be changed easily.
  • It is possible to assess the condition of the Cylinder ram externally. First unscrew the threaded bottom eye mount and inspect the condition of the rod bottom. If it had rusted, then it is a sign of deterioration in performance.
  • Remove the plastic cover and boot around the ram cylinder and check for leakages of hydraulic fluid.

Pro-Tip:

  • Before jacking the car, make sure the suspension is at its lowest ride-height setting. This is so that you can jack the wheels off of the ground faster and easier with lesser jack height.

Rear Suspension Sits lower than the front

Symptoms:

  • When trying to raise the suspension height, front alone rises
  • Rear sits at a lower height than the front
  • Usually seen in C5 second generation RT/RD (2007 – 2020)

Troubleshooting Rear Suspension Sits lower than the front

  • The vehicle display shows that the car is at normal ride height, but externally obvious that the rear suspension has not raised itself such a suspension fault could be occurring due to 
    • A lack of hydraulic pressure at the rear suspension due to some reason, or, 
    • The height sensors at the rear might be malfunctioning or out of calibration
    • A faulty Ram Cylinder
  • Out of the above 3 reasons, the first and most easy cause to eliminate would be the out-of-calibration height sensors
  • To confirm that it is indeed a sensor problem, you will need to plug-in your car to a diagnostic tool and access the Electronic suspension control. Within the module you can read all the height sensor values. If the rear ride height values reads a very illogical number, then it is definitely an issue with the ride height sensor of that particular corner suspension
  • At the rear underbody, the height sensor and hydraulic lines are fairly easy to locate and access the suspension control module. Here you will be able to access the ride height readings from the sensors for all the corners. 
  • The height sensor has an adjustable clamp fitted to the Anti-roll bar which can be loosened and rotated to the desirable position and re-tightened.
  • In order to adjust the car must be on a ramp at the rear end and suspension set at normal height
  • After loosening the clamp, rotate it slightly in small 1 degree angle increments and wait for 20 secs to see if the rear suspension raises. Fix the rotational position and re-tighten the clamp at this new position.
  • One more way to do this is to use the manufacturer recommended reference height sensor angle position against suspension height measured from a chassis reference point to a suspension reference point.

Suspension ‘Stop’ message

  • When trying to either raise or lower the suspension you see the suspension is unable to move
  • There is no sound of the hydraulic pump and 
  • The height control display reads a ‘Stop’ sign

Troubleshooting Suspension ‘Stop’ message

  • Suspension ‘Stop’ message often points to the hydraulic pump malfunctioning
  • If you hear no sound, you can first check whether the hydraulic fuse is blown or not
  • If the pump still doesn’t start after replacing the fuse, then the issue most likely will be a faulty pump
  • Be sure to drain the hydraulic fluid before starting to remove the hydraulic pump.

Conclusion

In this brief article, we have discussed Hydropneumatic Suspension, its evolution, and common problems with hydropneumatic suspension.

For any questions or comments, please feel free to get in touch with us.

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