High pressure control
There are three subfunctions which make up high pressure control:
• Setpoint generation
• Rail-pressure control
• Rail-pressure monitoring
Setpoint generation
The setpoint is generated as a function of the engine’s operating point. On the basis of the operating point, the setpoint is corrected by a value which depends on the following parameters:
• Air pressure
• Water temperature
• Air temperature
The setpoint is then pegged in order to prevent spurious response on the part of the pressure monitoring sensor.
If the rail pressure sensor is defective a narrower range is imposed for limitation of the possible setpoints.
Rail pressure control
The sampling ratio for the pressure control valve (DRV) is subject to open-loop or closed-loop control, depending on the operating conditions. The rail pressure set in this way is in the range 200 – 1350 bar.
If for any reason (e.g. system fault, overspeed) the pressure control valve is held to minimum pressure, it is immediately controlled with the min. pulse duty factor.
Pressure control includes provision for testing electric shutoff (EAB) and the electric fuel pumps (EKP).
Rail-pressure monitoring
Rail-pressure monitoring is for detecting leaks and other faults in the high pressure system.
Two conditions must be satisfied for rail-pressure monitoring:
• Engine speed above a minimum value
• Rail pressure monitoring in normal control mode
The following are monitored:
• Rail pressure sensor
• System pressure
The system registers an impermissibly high rail pressure if rail pressure overshoots the maximum value.
• Rail minimum pressure
A fault signal is transmitted if the rail pressure is too low compared to an engine speed-dependent value from a comparison characteristic curve.
• Control deviations in the rail-pressure control loop
Elevated system pressure can be due to a sticky or stiff pressure control valve.
Reduced pressure or a volume imbalance in the rail can occur in certain instances such as those listed below:
• “Internal” and “external” leakage
• “Empty fuel tank”
• Fault in the low pressure system
• Injector leak-off excessive
• Injector control quantity too high
The following are reactions to a fault in the high pressure control system:
• Pressure control valve is de-energized
• Electric fuel shut-off (EAB) is activated (closes)
• Activation duration (pressure control valve) = 0 ms
• Bank 1/2 shut down
These measures shut down the engine.