Overview
This document contains information on:
General
Function requirements
Function
General
The air suspension is a single-chamber or multiple-chamber air suspension with ADS. The air suspension adapts both the damping and the suspension at each individual wheel to suit the ambient conditions. The level control system ensures that the ground clearance remains constant even when the vehicle is loaded.
The air suspension system basically consists of the following components:
On the front axle, suspension struts with integrated ADS
On the rear axle, air spring with ADS
Compressor unit with pressure sensor, valve unit and air suspension control unit
Level sensors for regulation of vehicle level and damping
Central reservoir for pressure storage
Function requirements
Ignition ON (circuit 15)
Function
Function of air spring
The compressed air trapped in the air springs of the suspension struts bears the weight of the vehicle. Depending on the vehicle load, the air is under a pressure of 7 bar to 11 bar in the normal driving state. Rolling motions are reduced by means of early spring progression in the tensile and compression ranges. For this, the suspension struts at the front and the shock absorbers at the rear each contain an integrated rebound buffer spring and a stop buffer. The multiple-chamber air suspension is used to realize various spring characteristics. For this purpose, the air suspension bellows consist of multiple air chambers, which are connected or disconnected to each other by switching the air suspension valves.
The full load-bearing air suspension system provides a soft base suspension with a low natural frequency, meaning that the full spring travel is always available, even when the vehicle is loaded.
Function of compressor unit with pressure sensor and valve unit
An electrically driven compressor unit supplies the air suspension with pressure. An air drier dehumidifies the air compressed by the compressor, and it is then regenerated by the system's dry air, after venting. Integrated into the compressor is a pressure sensor and a valve unit with electronically actuated valves and a power-output limitation valve. The pressure sensor monitors the suspension strut pressures when the valves are switched accordingly.
Function of air suspension control unit
The air suspension control unit regulates the air suspension. This essentially involves the processing of the following control variables:
Vehicle level
Vehicle speed
Ambient air temperature
The vehicle level is determined by four level sensors, two at the front axle and two at the rear axle. An automatic locking function is stored in the air suspension control unit. For example, in certain situations such as "wheel change with jack" or "vehicle on lift", all regulation of the vehicle level is suppressed. These situations are detected by the software.
Function of central reservoir
To ensure a reasonably short level control response time (e.g. after loading), the system is equipped with a pressure reservoir. The pressure sensor integrated into the compressor unit also monitors the reservoir pressure of the central reservoir when the valves are switched accordingly. The vehicle attitude and control processes are achieved by pumping air back and forth from the central reservoir to the air springs. The vehicle attitude is guaranteed by the valves assigned to each wheel. Actuation is pulsed at specific wheels to allow the vehicle to be aligned horizontally or by axle in order to minimize the stresses in the vehicle. The air suspension control unit selects the control strategy (by axle or by wheel) according to the driving situation.
Lowering or raising vehicle level
The air suspension lowers or raises the vehicle level according to requirements. For example, it lowers the vehicle level at higher speeds in order to reduce aerodynamic drag and improve fuel consumption. The raised levels and the "Highway" level are lowered automatically when defined speed thresholds are exceeded.
Function schematics | |||
Function schematic of air suspension | Model all (CAR) with code 489 (AIR BODY CONTROL) | ||
Additional basic functions | |||
Wake-up mode, basic function | Model all (CAR) | ||
Level control system, basic function | Model all (CAR) | ||
Raise/lower rear end, basic function | Model all (CAR) | ||
Control units | |||
Air suspension control unit, basic function | Model all (CAR) | ||
Components | |||
Air suspension system valve unit, basic function | Model all (CAR) |