BMW 3: Low coolant temperature

When the coolant temperature is low, for example after driving off, or in all-electric driving, the electric changeover valve is activated by the Body Domain Controller (BDC). The electrical changeover valve blocks the supply from the coolant circuit of the combustion engine. The coolant is now pumped to the electrical heating by the electric coolant pump, heated and conveyed to the heat exchanger.

Coolant temperature, high

The coolant heated by the combustion engine flows through the de-energized, open changeover valve and the electrical heating to the heat exchanger. There some of the heat is released into the air flowing through the heat exchanger and ultimately reaches the coolant circuit of the combustion engine again.

The electrical heating is switched off, but the electric coolant pump is active.

Heating control

The electric coolant pump and the electrical changeover valve are 12 V components which are activated by the Body Domain Controller (BDC).

The maximum electrical power of the electrical heating is 5.5 kW (280 V x 20 A). The electrical heating is implemented by three heater coils, which are each cycled with the same power, but phase offset, via pulse width modulation (PWM). The output from the heater coils is controlled by the switched-on period within the PWM signal. The heater coils are cycles within the electrical heating by electronic switches (power MOSFETs).

Heater coils in the electrical heating

1. Coolant feed line connection (from the electric coolant pump)
2. Coolant feed line connection (to the heat exchanger for heating system)
3. Coolant temperature sensor
4. High-voltage connector connection
5. Heater coil (three)

Block diagram for electrical heating

1. Low-voltage connector
2. Temperature sensor (control unit printed circuit board)
3. Coolant temperature sensor
4. Electric heating (control unit)
5. Convenience charging electronics (KLE)
6. Electric Motor Electronics (EME)
7. Battery management electronics (SME)
8. High-voltage battery unit
9. High-voltage connector at electrical heating
10. Hardware shutdown in the event of excessive current in heater coil 3
11. Hardware shutdown in the event of excessive current in heater coil 2
12. Hardware shutdown in the event of excessive current in heater coil 1
13. Electronic switch (Power MOSFET) for heater coil 1
14. Electronic switch (Power MOSFET) for heater coil 2
15. Electronic switch (Power MOSFET) for heater coil 3
16. Heater coil 1
17. Heater coil 2
18. Heater coil 3

The current through the individual strands is measured and controlled by the electrical heating control unit. A current of maximum 20 A flows in a voltage range from 250 V to 400 V. Above and below this voltage range the power is reduced. At increased power consumption the energy supply by the hardware switching is interrupted. This switching is designed so that even in the event of a fault in the control unit a power cut is achieved safely.

Inside the electrical heating a galvanic separation was realized between the high-voltage circuit and the low-voltage circuit.

The connections for local interconnect network bus and voltage supply (terminal 30B) are located at the low-voltage connector.

The high-voltage contacts of the round connector for the electrical heating are protected against contact. The high-voltage connector of the electrical heating is not part of the high-voltage interlock loop. A contact bridge is integrated in the high-voltage connector of the G20 PHEV, which interrupts the voltage supply to the control unit of the EH if the high-voltage connector is pulled off. This prevents power consumption of this high-voltage component.

Phase-offset pulse width modulation (PWM) of the three heater coils enables continuous heater output between approximately 550 W (equivalent to 10%)  and a maximum of 5.5 kW (equivalent to 100%). The request for switching on the heating comes from the IHKA control unit via LIN bus.

On reaching the maximum temperature, or exceeding the maximum permissible current level, the heater output is automatically restricted by the electrical heating.

The power of the electrical heating is also reduced in ECO PRO mode and from a certain state of charge of the high-voltage battery unit.

The electrical heating is switched off in the event of system faults.

The electrical heating is maintenance-free.

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