1018 Electronic controlled pneumatic air suspension for on and off road use

A pneumatic (air) controlled suspension system provides a variable spring rate so that a constant suspension frequency is obtained between light and heavy load conditions. Additional telescopic dampers are also installed to improve the ride quality and comfort. A driver's height control is also provided to enable the body height to be adjusted for specific purposes such as loading the boot or cargo space, towing a trailer, driving over rough terrain, or muddy ground, or travelling though flooded areas.

10.18.1 Description of system (Fig. 10.127) The basic air suspension system consists of four rolling lobe air springs mounted between the chassis and each of the suspension lower swing arms. These springs work within an air pressure ranging between 6 and 10 bar, thus for low height settings and light loads the pressure needed may be down to 6 bar but for higher height settings and heavy loads the pressure could rise to just under 10 bar. Compressed air is supplied via a single cylinder compressor driven by an electric motor and a reservoir tank is provided to store compressed air for instant use. A unloader valve is provided to safe guard the compressor and system from overload. There are four ride height solenoid valves, one for each air spring unit and an inlet and outlet solenoid valve which controls the air supply to and from the system. In addition there are inlet and outlet exhaust solenoid valves which control the release of excess air to the atmosphere. An air silica gel drier dries the wet freshly compressed air before it passes though the various valves and enters the reservoir tank and air spring units.

There are various sensors and switches which provide essential information for the ECU to process and to make corrective decisions to the levelling of the vehicle's body, these are as follows:

Compressor pressure switch A compressor pressure switch monitors the reservoir tank air pressure and provides a signal to the electronic-control unit (ECU) to switch the compressor's electric motor on when the pressure drops below 7.5 bar and to switch it off as the pressure reaches 10 bar.

Height sensors The air spring unit height is monitored by individual height sensors which provide voltage signals to the ECU, the ECU then computes this informs to determine when the pre-deter-mined spring unit height has been reached, and when to switch 'on' or 'off the ride height solenoid valves. Each wheel suspension is constantly moving up and down so that the different heights measured over a period of 12 sec are recorded to enable the ECU to calculate if more or less air is needed to fill any of the rolling lobe springs.

Engine speed sensor The engine speed sensor takes its readings from the alternator; it is used to indicate if the engine is running as the ECU will switch 'off the compressor's electric motor if the engine is not running or if the engine speed drops below 500 rev/min to prevent the electric motor draining the battery.

Vehicle speed sensor The vehicle speed sensor takes its readings from the speedometer, to monitor road speed. When the vehicle's speed exceeds 80 km/h the ECU lowers the ride height by about 20 mm to reduce aerodynamic drag and to improve the road holding stability.

Hand brake switch The hand brake switch is used to inform the ECU that the vehicle has stopped and is stationary and only then can the lower access height setting mode be actuated for safety reasons if required.

Exhaust silencer

Electric motor

Engine Road switches speed speed sensor

Hand Foot brake brake switch switch

Fig. 10.127 Electronic controlled pneumatic (air) suspension

Exhaust silencer

Electric motor

Engine Road switches speed speed sensor

Hand Foot brake brake switch switch

Fig. 10.127 Electronic controlled pneumatic (air) suspension

Driver's function switches The driver's function switches are dash board mounted and are used to select the different height programmes in the ECU when in manual mode. These are standard ride height used for trailer towing, high profile mode for off-road driving and a low profile mode for load accessing.

10.18.2 Operating conditions (Fig. 10.127)

Compressor charging When the ignition switch is on and the engine is running the ECU signals the electric-motor relay switch to close and to energize the electric motor; the compressor will then rotate and begin to charge the reservoir tank via the reservoir non-return valve (NRV). Under these conditions both the inlet and the outlet solenoid valves are de-energized, thereby closing off the ride height solenoid valves and air spring units from the rest of the system. The exhaust outlet solenoid valve is de-energized and is closed, and the inlet solenoid valve is energized and therefore opens, thus permitting air pressure to force down the unloa-der valve diaphragm against the spring tension, hence closing the unloader valve and thus preventing air discharging into the atmosphere (opposite to that shown in Fig. 10.127). As the air pressure in the reservoir approaches 10 bar the compressor pressure switch signals the ECU to switch 'off' the compressor's electric motor.

Raising and lowering height levels If the rolling lobe air springs are in the low profile access mode or if there has been a loss or expulsion of air previously from the springs, then each height sensor will signal to the ECU to energize and open the inlet solenoid valve and each of the ride height solenoid valves, hence permitting the reservoir tank to discharge additional air into the four air spring bags. Once the air spring units have risen to their setting mode the ECU de-energizes and closes the inlet and ride height solenoid valves thereby retaining the air mass within the spring units. Should the air spring height be too high for a particular ride height setting due to a reduction in load or that a different height profile mode has been selected, then the ECU will energize and open the four ride height valves, the outlet solenoid valve, the inlet and outlet exhaust solenoid valves. Air will now be released from the air spring bags, where it then escapes to the compressors intake and intake filters and via the open unloader valve into the atmosphere; this will continue until the air springs are restored to their pre-programmed height at which point the ECU will cut 'off' the current supply to close all of these valves.

Individual spring levelling If one corner of the vehicle dips more than the others (see left hand top corner Fig. 10.127), possibly due to uneven passenger or load distribution, the relevant height sensors detect this in the from of a voltage change. This is therefore passed on to the ECU and accordingly energizes and opens the inlet solenoid valve and the appropriate ride height inlet solenoid valve; once again pressurized air will be delivered from the reservoir tank via the inlet NRV to the particular spring to compensate for the body sag until the body is level again.

Unloader valve action When the compressor is charging the system the exhaust inlet and outlet valves are open and closed respectively, if the discharge pressure from the compressor should exceed its maximum safe limit of around 10 bar, then air pressure acting with the spring underneath the diaphragm lifts it up against the resisting downward acting air pressure thus enabling the valve to open and to unload the system.

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