M57/M67 – Boost-pressure control

Boost pressure control has the task of controlling the turbocharger with variable turbine geometry (VNT).
Boost pressure control comprises:
• Driving-style classification
• Setpoint calculation
• Boost-pressure control

Driving-style classification

This is the function which allows the control characteristic map and the boost-pressure setpoint characteristic map to be selected on the basis of the style of driving.
A distinction is drawn between two styles of driving:
• Sporty driving style
• Economical and ecological driving style

The style of driving is graded on the basis of:
• Accelerator pedal position
• Changes in accelerator pedal position
• Engine speed

The higher the result obtained from these three factors, the more sporty is the style of driving.

Constant switchover between the 2 characteristic maps is avoided by a dependent time constant and a hysteresis.

Coasting at high engine speed is not wrongly classified as sporty driving style because in this mode the driving style is frozen in the style most recently learned. The same applies to cruise control mode, idling and clutch disengaged.

An economically and ecologically acceptable driving style is set in cruise control mode.

Setpoint calculation

The absolute and relative setpoint boost pressures are derived from a setpoint characteristic map as a function of:
• Engine speed
• Injection quantity

The setpoint characteristic map is selected on the basis of driving style:
• Sporty driving style
• Economical and ecological driving style

The value, moreover, is corrected to allow for atmospheric pressure (sensor inside the electronic control unit), injection quantity and engine speed.

Boost-pressure control

Boost pressure is controlled by activation of the variable-geometry turbine (VNT) by means of an electropneumatic pressure transducer (M57) or the actuation unit (M67) in accordance with the setpoint. The boost-pressure sensor provides the feedback signal which notifies the DDE control unit of the actual boost pressure.

Control is a function of:
• Control deviation
• Engine speed
• Gear selected by driver

Boost pressure control is not always active. It is not activated before the quantity exceeds a speed-dependent threshold. This function, too, is regulated with the aid of on/off hysteresis.

Additional functions

The following additional functions are integrated into the
DDE control unit:
• Engine preheat period control
• Engine mount control
• Electric engine fan control
• Electric fuel pump/pressure control valve (EKP/DRV) activation
• A/C compressor activation

Engine preheat period control

The DDE control unit has integrated engine preheat period control which takes battery voltage into account. The engine heater plugs are energised via the heater plug relay.

Preheating and pre-start heating can be initiated when the engine is stopped and the coolant temperature renders these measures practical.

The preheating duration depends on the engine coolant temperature. Preheating enhances the engine’s starting
characteristics. The preheating duration is extended if temperature is low and the battery is not fully charged.

The driver is informed of the preheating procedure by the message “preheating” and should not start the engine before the visual indicator (basic instrument cluster – indicator lamp, IKE – test message) goes out.

The indicator lamp for the preheat time has a second function. If the indicator lights up while the engine is in operation it does not mean that the preheating relay has pulled up. Instead, it acts as a warning lamp in the DDE system (instrument cluster electronics – text message). In this case it provides information relating to a fault in the “electronic fuel injection system (DDE)”. The driver must proceed to the nearest authorised workshop.

The post-heating phase commences when the engine starts.
The purpose of this phase is to reduce engine noise, improve idling quality and reduce hydrocarbon emissions as soon after the start as possible by promoting efficient combustion. The post-heating duration depends on the coolant temperature.

If the coolant temperature sensor is defective the intake-air temperature is used as the substitute input variable.

Engine mount control

Engine mount control actuates the electric changeover valve (EUV) for the variable-damping engine mounts.
The electric changeover valve is switched to “soft” during engine start. After completion of the start phase, the engine
mount changeover takes place as a function of operating point and with an engine-speed-related hysteresis and a road speed-related hysteresis.
Activation of the damping-controlled hydraulic mounts by the DDE is based on the following parameters:

The engine mounts are switched to “hard” in the event of a defective road speed sensor or water temperature sensor.