Compressed air is produced by a single-stage piston compressor with integrated air drier.
To prevent internal contamination of the air springs and the air drier cartridge, compressor design is of the dry-running type.
Lifetime-lubricated bearings and a piston ring made of polytetrafluorethylene ensure a long service life.
Design and Function
The Solenoid for Level Control System N111 (drain valve), a pneumatic drain valve with pressure limiting valve, and three non-return valves are integrated in the air drier housing.
To protect the compressor against overheating, it is turned off if excess temperature occurs.
SSP275/032
Pneumatic Drain Valve with Pressure Limiting Valve Solenoid for
Level Control System N111 (Drain Valve)
Pressure Connection
Drain Connection
Non-Return Valve 2
Electric Motor
Intake Fitting Lifting Piston Piston Ring Diaphragm Valve
“Closed”
Cylinder Non-Return Valve 1 Air Drier
Non-Return Valve 3
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Intake/Compression Cycles
During the upward movement of the piston, air is drawn into the compressor crankcase through the intake fitting via the silencer/filter. Air in the cylinder is
compressed above the piston and flows into the air drier via non-return valve 1.
Design and Function
The compressed and dried air flows via non-return valve 2 and the pressure connection to the valves and the
pressure accumulator.
SSP275/039
Air Drier
Pressure Connection
Non-Return Valve 2
Compressor Crankcase
Intake Fitting Piston Moves Upward Non-Return Valve 1
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Bypass Air Flow
During the downward movement of the piston, air drawn into the crankcase bypasses the diaphragm valve and flows into the cylinder.
Design and Function
SSP275/040
Fill/Lift Cycles
To fill the air springs and thus raise the vehicle, the Level Control System Control Module J197 activates the compressor relay (Relay for Compressor Level Control System J403) and the four air spring suspension strut valves at the same time.
Piston Moves Downward Bypass Air Flow Diaphragm Valve “Open”
Cylinder
45
Drain/Lowering Cycles
(Front Axle Components Shown) Left Front Suspension Strut Valve N148, Right Front Suspension Strut Valve N149, and Solenoid for Level Control System N111 (drain valve) are activated (open)
Design and Function
during the drain cycle. The air spring pressure flows toward the pneumatic drain valve and from there via the air drier, the pressure limiting valve, and the
silencer/filter into the spare-wheel well in the luggage compartment.
Drain Cycle Pneumatic Diagram (Front Axle Components Shown)
SSP275/042 SSP275/041
Pressure Limiting Valve and Solenoid for Level Control System N111 (Drain Valve) “Open”
To Silencer/Filter
Air Drier Pneumatic Drain Valve “Open”
M
Signals from Level Control System Control Module J197
Signal from Level Control System Control Module J197 Signal from Relay for Compressor
Level Control System J403
Right Front Suspension Strut Valve N149
Left Front Suspension Strut Valve N148
Non-Return Valve 2 Non-Return
Valve 3
Drain Restrictor Air Drier
Non-Return Valve 1 Silencer/Filter
Solenoid for Level Control System N111 (Drain Valve) Pneumatic
Drain Valve
Design and Function
Pneumatic Drain Valve
The pneumatic drain valve performs two functions:
• Residual pressure maintenance
• Pressure limitation
To prevent damage to the air springs (flexible air bladders), a specific minimum pressure of at least 51 psi (350 kPa) must be maintained. This is referred to as the air spring residual pressure.
The maintenance residual pressure ensures that pressure in the air spring system does not drop below 51 psi (350 kPa) during pressure relief during normal operation.
Residual pressure cannot be maintained if there is a leak in the system upstream of the pneumatic drain valve.
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When an air spring pressure of more than 51 psi (350 kPa) is applied, the valve body lifts against the spring force of the two valve springs and opens valve seats 1 and 2.
The air spring pressure is then admitted into the air drier via the flow restrictor and non-return valve 3.
After passing through the air drier, the air passes the open valve seat of the pressure limiting valve and through the silencer/filter in the spare-wheel well in the luggage compartment.
A sharp decrease in pressure downstream of the flow restrictor leads to a reduction in relative atmospheric humidity, thereby increasing the amount of moisture that will be absorbed by the air that is discharged into the luggage compartment.
SSP275/043
To Silencer/Filter
Valve Seat 2 Valve Seat 1 Air Drier Non-Return Valve 3 Flow Restrictor
Valve Body
Pressure Limiting Valve “Open”
Pneumatic Drain Valve
Solenoid for Level Control System N111 (Drain Valve) “Open”
Line to Solenoid Valve Block
M
Design and Function
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Pressure Limiting Valve
The pressure limiting valve protects the system against excessively high pressures, as for example if the compressor fails to cut out due to a defective contact in the Relay for Compressor Level Control System J403 or a defective Level Control System Control Module J197.
In such a case, the pressure limiting valve will open against the spring force when the pressure exceeds approximately 290 psi (2000 kPa), and air conveyed by the compressor will escape through the silencer/filter.
SSP275/044
Pressure Limiting Valve Actuation Pneumatic Diagram
Intake Fitting Lifting Piston
Pressure Limiting Valve “Open”
To Silencer/Filter
No “Drain Valve Open” Signal from
Level Control System Control Module J197 No “Cut Out” Signal from Relay for
Compressor Level Control System J403
Silencer/Filter
Pneumatic Drain Valve with Pressure Limiting Valve
Compressor
SSP275/044a
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Air Drier
The air in the self-leveling air suspension system must be dehumidified to avoid problems caused by the condensation of water in the system:
• Corrosion
• Freezing
An air drier is used to dehumidify the air.
The air drier uses a regenerative process to remove moisture from the air.
The air compressed in the self-leveling air suspension system is routed through a silicate granulate and dried in the process.
This granulate is able to absorb
atmospheric humidity amounting to over 20% of its natural weight, depending on temperature.
When the air in the system is released to lower the air springs, it flows back through the silicate granulate and is discharged to the atmosphere.
During this phase, the dry air extracts the moisture from the granulate that was absorbed during the intake cycle.
Design and Function
As a result of this regenerative process, the air drier requires no maintenance. It is not subject to a replacement interval.
Since the air drier is only regenerated by discharged air, the compressor must not be used to fill other vessels with compressed air.
Moisture in the system signifies that the air drier has failed or the system is
otherwise malfunctioning.
SSP275/045
Air Drier with Silicate Granulate Filling
SSP275/033