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.
Preheating:
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.
Fig. 99: Preheating time control
0 2 4 6 8 10 -20 -10 0 10 20 30 40 50 Kühlmitteltemperatur [˚C] Vorglühzeit [s] Coolant temperature (°C) T ime (s) KT-4112
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.
Note:
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 electron- ics - 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.
Post-heating:
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.
Fig. 100: Post-heating time control
If the coolant temperature sensor is defective the intake-air temperature is used as the substitute input variable.
0 50 100 150 200 250 -20 -10 0 10 20 30 40 50 Kühlmitteltemperatur [˚C] Nachglühzeit [s] Coolant temperature (°C) T ime (s) KT-4112
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:
Fig. 101: Sequence diagram/activation, damping-controlled hydraulic mount
The engine mounts are switched to "hard" in the event of a defective road speed sensor or water temperature sensor.
Switching value
Remarks
Engine speed 900 rpm Hysteresis (+ 50 rpm)
Vehicle speed 60 km/h Hysteresis (+ 5 km/h)
Power supply (DDE) Vehicle speed v Engine speed n
Engine mount soft (idle speed) Engine mount hard n > 950 v > 65 v < 60 n < 900 KT-210
Electric engine fan control
Electric engine fan control calculates the setpoint speed for the electric fan.
The engine-fan setpoint speed is derived from a characteristic curve as a function of:
• Water temperature • Current road speed
The fan output stage requested by the air-conditioning control unit via the CAN bus is also taken into account as a variable. Fan run-on after engine shutdown is limited by either the coolant temperature or a maximum run-on time.
A dummy value is used if the coolant temperature sensor is defective.
Electric fuel pump/pressure control valve (EKP/DRV) activation
The electric fuel pump (EKP) and the rail pressure control valve (DRV) are similar in terms of activation.
Both can be energised for only a certain period of time (depends on coolant temperature: characteristic curve) in order to
avoid thermal damage due to deficient cooling or useless pumping (EKP) against an inactive high pressure pump with the engine stopped.
Power is applied to the electric fuel pump for a period of 60 sec- onds when terminal 15 is switched on. The rail pressure control valve is activated at 1 kHz.
On exceeding the minimum speed of 50 rpm, both components are permanently activated until terminal 15 is switched off again. In the event of a crash, the control unit of the Multiple Restraint System (MRS) generates a signal which is carried by the
instrument cluster and CAN telegram to the DDE. This signal results in shutdown of the electric fuel pumps. The pumps cannot be reactivated until the terminal 15 changes state from OFF to ON.
A/C compressor activation
Activation and deactivation of the air-conditioning compressor depends on a number of vehicle and control-unit states. When high torque is needed for brief period (pulling away from rest, accelerating) the air-conditioning compressor control logic shuts down the air-conditioning compressor to ensure that sufficient engine torque is available.
Activation comprises two subfunctions: • Air-conditioning compressor logic • A/C compressor activation
Air-conditioning compressor logic
The logic checks all conditions which prevent activation of the air-conditioning compressor:
• Pulling away from rest (vehicle speed, accelerator pedal, engine speed)
• Acceleration (change in accelerator pedal position, vehicle speed, engine speed)
• Oil temperature • Coolant temperature • Engine starting procedure
• System fault (defect in road-speed sensor or accelerator pedal faulty)
• A dummy value is used if the oil temperature sensor or the coolant temperature sensor is faulty
A/C compressor activation
Activation drives the output as a function of: • Air conditioning request via CAN and • A/C compressor logic