compression ratio BMW 745LI 2008 E66 Level Control System Manual

49
Level Control Systems
Post Mode
The Post-mode is adopted in order to compensate any inclination or to adjust the ride
height after driving and between the Pre-mode and Sleep mode.
The Post-mode is limited in time to 1 minute. The Post-mode is only executed if the engine
has been running before the system switches into this mode. If the engine has not been
previously running, the system switches directly from Pre-mode into Sleep mode.
The control operation is performed in a narrow tolerance band of +/- 6mm and is terminat-
ed at +/- 4mm. The fast signal filter is used.
In the event of an inclination (Kerb Mode), the control operation takes place for the nominal
heights applicable in this situation.
Pre-Mode
The Pre-mode is activated by the “Load Cutoff” signal (e.g. by opening the door or unlock-
ing with the remote control). The Pre-mode then stays set for 16 minutes and is restarted
with a change in status.
The ride height of the vehicle is monitored and evaluated with a wide tolerance band.
In Pre-mode, the vehicle is only controlled un to the nominal height if the level is significantly
below the nominal height. The control tolerance band is -40mm from the mean value for
the single axle air suspension and -20mm for the dual axle system. This control tolerance
ensures that the vehicle is only controlled up in the case of large loads in order to increase
the ground clearance prior to departure. Small loads give rise to small compression travel
and this is compensated only when the engine is started. This control setting helps reduce
the battery load.
With the single axle air suspension, the vehicle is controlled down when the mean value
derived from both ride height signals is > 0mm and one side is in excess of +10mm. With
twin axle air suspension, the vehicle is controlled down when one side is >15mm.
In this mode, only the mean value of the two height signals is considered when deciding
whether there is an need for control operation.
The control operation is executed as long as pressure is available in the accumulator. When
the accumulator is empty and the engine is turned off, the control operation is driven direct-
ly by the compressor. User-activated changes of ride level and filling of the accumulator are
not possible.
Control operations which were started in other modes are continued with the inner toler-
ance bands applicable to these modes.
There is no inclination identification in Pre-mode.

50
Level Control Systems
Normal
The normal mode is the starting point for the vehicle’s normal operating state. It is obtained
by way of the engine running signal.
Ride level compensation, changing the vehicle’s ride height and filling the accumulator are
possible. The compressor starts up as required.
A narrower tolerance band than that in Pre-mode cab be used because the battery capac-
ity does not have to be protected. The fast filter is used with a narrow tolerance band of
+/- 10mm. In this way, ride level compensation takes place outside a narrow band of 10+/-
10mm. The faster filter allows the system to respond immediately to changes in ride level.
Evaluation and control are performed separately for each wheel.
When a speed signal is recognized, the control unit switches into Drive mode. When the
vehicle is stopped, the control unit remains in Drive mode. The system switches back into
Normal mode when a door or the boot (trunk) lid is also opened. If none of the doors or
the boot lid is opened, the vehicle cannot be loaded or unloaded.
This prevents a control operation happening when the vehicle (for example) is stopped at
traffic lights and the ride height is above the mean axle due to the pitching motion on the
rear axle.
Drive
The Drive mode is activated for E39/E53 single axle air suspension when a speed signal of
>4km/h is recognized. The Drive mode is recognized from >1km/h for the E65/E66 single
axle air suspension and for the E53 twin-axle air suspension system.
Low pass filters are used. In this way, only changes in ride height over a prolonged period
of time (1000 seconds) are corrected. These are merely the changes in ride height, caused
by vehicle compression and a reduction in vehicle mass due to fuel consumption. The high
pass (fast) filter is used during the control operation. The slow filters are recognized at the
end of the control operation. The slow filters are re-initialized at the end of the control oper-
ation. The markedly dynamic height signals caused by uneven road surfaces are filtered
out.

51
Level Control Systems
Kerb (Curb) Mode
The Kerb mode prevents the inclination caused by the vehicle mounting an obstacle with
ine wheel from being compensated. Compensation would cause a renewed inclination of
the vehicle and result in a renewed control operation after the vehicle comes off the obsta-
cle.
The Kerb Mode is activated when the height difference between the left and right sides of
the vehicle is > 32mm for the E65/E66 with single axle air suspension and >24mm for the
E39/E53 with single axle air suspension and lasts longer than 0.9s. Twisting (also over both
axles) > 45mm must occur for the E53 with twin axle air suspension.
There must be no speed signal present. The system switches from single wheel control to
axle control.
The Kerb mode is quit when the difference between the left and right sides of the vehicle is
< 28mm for the single axle air suspension and lasts longer than 0.9s when the speed is
greater than > 1km/h.
If the system switches from Kerb mode to Sleep mode, this status is stored in the
EEPROM.
If the vehicle is loaded or unloaded in Kerb mode, the mean value of the axle is calculated
by the control unit. The value is calculated in the control unit from the changes in ride level
of the spring travel on the left and right sides.
A change in ride level is initiated if the mean value of compression or rebound at the axle is
outside the tolerance band of +/- 10mm. The left and right sides of the vehicle are raised
or lowered in parallel. The height difference between the two sides is maintained.
Curve
Since rolling motions have a direct impact on the measured ride level, an unwanted control
operation would be initiated during longer instances of cornering with an appropriate roll
angle in spite of the slow filtering of the Drive mode. The control operations during corner-
ing would cause displacement of the air volume from the outer side to the inner side of the
curve. Once the curve is completed, this would produce an inclination which would result
in a further control operation. The Curve mode prevents this control operation whereby
when cornering is recognized slow filtering is stopped and a potential control operation that
has started is terminated.
The Curve mode is activated for the E65/E66 single axle EHC and for the E53 twin axle
suspension for a lateral acceleration of > 2m/s
2and deactivated at < 1.5 m/s2.
The lateral acceleration is recorded by the rotation rate sensor.