Hydropneumatics: the combination of hydraulics and pneumatics

Hydraulics is the transmission of power through fluids, such as in a hydraulic cylinder, while pneumatics is the transmission of power through gases. Hydropneumatics refers to a combination of both types of power transmission.

Hydropneumatics: Properties of hydraulics and pneumatics

Hydraulics is characterized by two special properties:

• High and simple multiplication of forces
• Virtually lossless transmission of forces

Pneumatics also transmits forces through media, and its properties are:

• Particularly simple installation
• Fast response times
• Damped power transmission

Hydropneumatics combines these properties. This is advantageous in certain applications.

Possibilities of hydropneumatic systems

In hydropneumatic systems, also known as “hybrid systems,” the combinations of hydraulics and pneumatics are essentially used for two applications:

• Accumulators
• Shock absorption

Another property of hybrid systems is that only the hydraulic side is supplied by a pressure pump. The pneumatic components are usually pre-charged once and then only recharged during inspection intervals.

Hydropneumatic accumulators

Hydropneumatic accumulators are used for emergency operation features and to increase efficiency. Typical emergency systems include bladder or spherical accumulators. Piston accumulators are primarily used in energy-saving systems.

Hydropneumatics: Technology of emergency accumulators

A bladder accumulator consists of a rubber bladder that is pre-charged with gas. The rubber bladder is encased in a steel shell. When hydraulic oil is filled between the rubber membrane and the steel wall, the rubber bladder is compressed. The gas pressure inside the bladder continues to rise. Once the gas pressure has reached the desired level, the bladder accumulator is closed with a ball valve. The hydraulic system now has a small amount of residual pressure available for practically any length of time if the main pump fails. This allows important functions to be carried out even after a power failure and enables the hydraulic system to be shut down in a controlled manner.

Hydropneumatics: Technology of hydropneumatic shock absorption

Diaphragm accumulators are mostly used for hydropneumatic shock absorption. They also consist of a hollow steel sphere or a cylinder. A horizontally installed rubber diaphragm is located in the center. The upper side of the diaphragm accumulator is filled with gas and pre-charged. The lower side is filled with hydraulic oil. Unlike pressure accumulators, the diaphragm accumulator remains permanently connected to the hydraulic system. Pressure fluctuations in the hydraulic system are absorbed by the pre-charged gas and gently cushioned.

This technology is particularly well known from “Citroën” models with hydropneumatic suspension.