Preheat, function

Function requirements for preheating, general points

 Circuit 15

 Preglow system functional

Preheating, general points

The preglow system warms up the engine combustion chambers to the ignition temperature required for ignition of the fuel/air mixture.

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At cold temperatures the vent line heater element (R39/1) is actuated by the CDI control unit (N3/9). This prevents freezing of the crankcase ventilation system and any associated engine damage.

Function sequence for preheating

The function sequence is subdivided into the following steps:

 Function sequence for preheating

 Function sequence for start-ready glowing

 Function sequence for start-up glowing

 Function sequence for afterglowing

 Function sequence for diagnostic glowing

 Function sequence for diesel particulate filter (DPF) preheating

 Function sequence for emergency glowing

Function sequence for preheating

The CDI control unit receives the "Circuit 15 ON" message over the drive train CAN (CAN C1) from the powertrain control unit (N127), suspension FlexRay (Flex E) and from the electronic ignition lock control unit (N73). The CDI control unit reads in the engine oil temperature over the engine oil temperature sensor (B1) and then actuates over the drive-LIN (LIN C1) the glow output stage (N14/3) to match the coolant temperature, and it actuates the preglow indicator lamp symbol over the drive train CAN, powertrain control unit, suspension FlexRay, electronic ignition lock control unit and the user interface CAN (CAN HMI). The glow output stage then actuates the glow plugs (R9) on the positive side.

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If an error occurs in the preglow system, at the glow plugs or in the lines, this is indicated by the preglow symbol, and it is stored in the CDI control unit.

Function sequence for start-ready glowing

Start-ready glowing ensures an adequately high temperature until the engine finally begins to run. If the CDI control unit receives the "Circuit 50 ON" information via the drive train CAN, the powertrain control unit, the suspension FlexRay from the electronic ignition lock control unit, it then continues to actuate the glow output stage for a specific period, irrespective of the engine oil temperature, and this in turn actuates the glow plugs.

After reaching the operational readiness temperature of 1000 °C, the preglow indicator lamp symbol then goes out.

Function sequence for start-up glowing

Start-up glowing is used to stabilize the cold start speed of the engine. If the CDI control unit receives the "circuit 61 ON" message from the alternator (G2) via the drive LIN, the glow output stage is actuated over the drive LIN. The glow output stage then actuates the glow plugs. In this way the first working cycles and the eventual running of the engine are supported.

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For alternators with alternator interface is the line

"circuit 61" is not needed. Using the information received by the CDI control unit through the alternator interface, the "circuit 61 signal" is simulated in the CDI control unit and sent through an output stage connected to ground (e.g. for the instrument cluster (A1).

Function sequence for afterglowing

Afterglowing improves engine running after a cold start and the engine warm-up characteristics.

After a successful engine start, the CDI control unit reads in the engine oil temperature via the engine oil temperature sensor and actuates the glow plugs via the glow output stage dependent on the engine oil temperature.

Function sequence for diagnostic glowing

This function is used to energize the glow plugs to a low temperature level and a system diagnosis is performed. This function is activated for troubleshooting purposes and when the preglow system has not been activated for a long period (no preheating required owing to high ambient temperature).

Function sequence for diesel particulate filter (DPF) preheating

The DPF glow function is used to raise the load and to stabilize combustion and thus to assist regeneration of the DPF. To do this the CDI control unit receives the message "circuit 50 ON" via the drive train CAN vom drivetrain control unit and the suspension FlexRay from the electronic ignition lock control unit. The CDI control unit then actuates the glow output stage through the drive-LIN.

Function sequence for emergency glowing

If a problem in communication occurs between the CDI control unit and the glow output stage so that the glow output stage does not recognize any activity for 2 s after activation via circuit 87, the emergency glowing function is actuated for 180 s. If communication is missing during a glowing process for more than 250 ms then the glowing process will be continued for 180 s.

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If no new glow request is received from the CDI control unit then emergency glowing stops after 180 s.

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The fuel filter condensation sensor with heating element (B76/1) is switched on and off over the glow output stage. The bi-metal switch switches the PTC heating element on at a temperature  –12 °C and off at a temperature  –6 °C. The PTC characteristics ensures that the surface temperature of the heating element is limited to 163°C.

 PTC

PTC means "Positive Temperature Coefficient", i.e.: The electrical resistance increases as the temperature increases (PTC thermistor).