Air suspension

Posted On December 9th, 2019

Air suspension (the common name is air suspension ) is a type of suspension that provides control of the level of the body relative to the road through the use of pneumatic elastic elements. Currently, air suspension is installed as an option on some models of business class cars and large SUVs (for example, Volkswagen Touareg, Audi Q7).

At its core, air suspension is not a separate type of vehicle suspension, because implemented with many suspension designs ( MacPherson , multi-link suspension , etc.). Currently, many car manufacturers use air suspension on their cars: Audi, Bentley, BMW, Lexus, GM, Ford, Land Rover, Mercedes-Benz, SsangYong, Subaru, Volkswagen. Some suspension designs have their own names, for example, Airmatic Dual Control from Mercedes-Benz.

The main advantages of the air suspension are comfort, geometric cross-country ability and car safety. Air suspension is usually used in combination with automatically adjustable shock absorbers. This design is called adaptive air suspension .

The air suspension includes pneumatic elastic elements on each wheel, an air supply module, a receiver and a control system.

Pneumatic elastic element performs the main function of the suspension – maintaining a certain level of the car body. This is achieved by changing the pressure and the corresponding air volume in the elastic elements.

Pneumatic elastic element diagram

The pneumatic resilient element consists of a housing with a guide, a cuff and a piston. Structurally, the pneumatic elastic element can be manufactured with a built-in shock absorber or installed separately. The elastic element, combined with a shock absorber, is called a pneumatic strut (similar to the MacPherson strut suspension strut).

The cuff of the pneumatic elastic element is made of a durable multilayer elastomer. In some designs of elastic elements, additional pneumatic accumulators are used. To maintain pressure during air leakage, a residual pressure valve can be installed in the elastic element.

The air supply module serves to supply elastic elements with air. It includes an electric motor, compressor and air dryer. Structurally, the module includes a block of electromagnetic valves of the suspension control system.

The receiver is a reservoir for air and provides clearance control when driving at low speed without turning on the compressor, as well as adjusting the position of the body in the parking lot.

The design and operation of adaptive suspension elements is discussed in a separate article.

The air supply module and air struts form a pneumatic suspension system. The system can be open or closed (closed). Preferred is a closed pneumatic system that provides minimal air loss, which means saving energy on its creation.

The creation and regulation of pressure in the pneumatic suspension system is carried out using an electronic control system, which includes input sensors, a control unit and actuators.

Input devices include sensors for body level, body acceleration, compressor temperature, system pressure, and a mode switch.

Using the switch on the dashboard, manual control of the body level is carried out. Sensors monitor system operation parameters and convert them into electrical signals.

The control unit converts the electrical signals of the input sensors into control actions on the actuators. In its work, the control unit interacts with the blocks of the engine control system , the system of directional stability.

The following actuators are used in the air suspension control system: valves of pneumatic elastic elements ( to create pressure ), exhaust valve ( to relieve pressure ), switching valve ( to maintain pressure in the receiver ), compressor enable relay. Structurally, all valves are concentrated in the solenoid valve unit located in the air supply module.

The principle of operation of the air suspension

In air suspension, as a rule, three control algorithms are implemented:

  • automatic maintenance of the level of the body;
  • forced change in body level;
  • automatic change in body level depending on speed.

Automatic maintenance of a certain level of the body in the air suspension is carried out regardless of the degree of congestion of the vehicle. Body level sensors constantly measure the distance from the wheels to the body. The measurement results are compared with a predetermined value. If the readings diverge, the electronic control unit activates the necessary actuators: elastic element valves for lifting, an exhaust valve for lowering the suspension.

Forcing a change in body height usually involves three levels: nominal, high and low. The nominal level is used to travel on ordinary roads at speeds up to 100 km / h. Low level is used for high-speed driving. An increased level is needed for travelling off-road and is sold at speeds up to 40 km / h. Body levels are set by the driver using the switch. The design of the air suspension of large SUVs provides an additional level for boarding passengers and loading luggage, which is implemented on a stationary car.

An automatic change in the level of the body depending on speed ensures the stability of the vehicle in motion. With increasing speed, the suspension control program translates the level of the body sequentially from higher to nominal and further, with increasing speed, to lower. With a decrease in speed, the system translates the body position from lowered to nominal.

The use of shock absorbers with an adjustable degree of damping significantly expands the characteristics of the air suspension, allowing in addition to the height of the body to change the stiffness of the suspension depending on driving conditions.

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