BMW 3: System overview

G20 PHEV, components of refrigerant circuit

1. Combined expansion and shutoff valve (interior)
2. Combined expansion and shutoff valve (high-voltage battery unit)
3. High-voltage battery unit
4. Refrigerant lines to high-voltage battery unit
5. Low-pressure connection (for drawing off, evacuating and filling)
6. Refrigerant pressure sensor
7. High-pressure connection (for drawing off, evacuating and filling)
8. Coolant-based air conditioning condenser (coolant-refrigerant-heat exchanger)
9. Electric A/C compressor (EKK)
10. Evaporator (vehicle interior)

System overview of climate control

1. Dryer flask
2. Coolant-based air conditioning condenser (coolant-refrigerant-heat exchanger)
3. Electric A/C compressor (EKK)
4. Blower for passenger compartment
5. Combined expansion and shutoff valve, passenger compartment
6. Evaporator, passenger compartment
7. High-voltage battery unit
8. Combined expansion and shutoff valve, high-voltage battery unit
9. Heat exchanger

The two graphics above show the refrigerant circuits in the G20 PHEV. The refrigerant circuit for cooling the high-voltage battery unit is switched parallel to the refrigerant circuit for cooling the vehicle interior.

Its temperature has a decisive influence on the service life of the high-voltage battery unit. The cells of the high-voltage battery unit should not deliver their power or absorb electrical power at too high or too low a temperature. The optimal cell temperature is approximately 77 ºF (25 ºC); the battery cells should not exceed a maximum temperature of 104 ºF (40 ºC).

R1234yf is used as the refrigerant depending on the market; it circulates in a circuit, absorbing heat at one point in the system and releasing it at another point. The heat from the vehicle interior and the high-voltage battery unit is transferred to the coolant in the coolant-based air conditioning condenser.

When the air-conditioning is activated for the vehicle interior, or when cooling power is requested for the high-voltage battery unit, the electrically driven air conditioning compressor is switched on, and the system supplies the corresponding point with cold. The vehicle interior cooling and the cooling of the high-voltage battery can be operated independently of each other. The electric A/C compressor draws the energy required from the high-voltage battery unit. Only refrigerant oils approved by BMW for electric A/C compressors may be used as lubricating materials. So that the battery cooling and the vehicle interior cooling can be operated independently of each other, special expansion and shutoff valves are integrated in the refrigerant circuit. The shutoff valves have only the "closed" and "open" positions. The amount of flowing refrigerant is adjusted thermally (depending on the refrigerant temperature) via the expansion valves. The two expansion and shutoff valves open only the portion of the circuit that is actually required. This ensures high efficiency and proper control characteristics of the system.

The combined expansion and shutoff valve upstream of the evaporator is activated by the Electrical Machine Electronics (EME); the combined expansion and shutoff valve for the high-voltage battery unit is activated by the battery management electronics (SME). When the shutoff valve in the corresponding refrigerant circuit is electrically actuated, and thus opened, liquid refrigerant flows into the cooling unit via the expansion valve and evaporates. As part of this process, it removes heat energy from its environment. The electric A/C compressor compresses the refrigerant again, and it returns to liquid physical state in the coolant-based air conditioning condenser. As a result, the refrigerant is once again able to absorb heat energy. The following table shows how the valves and the electric A/C compressor are controlled.

The request whether and how much cooling power is required is measured and determined by the IHKA control unit. On the one hand, the request can come directly from the customer to cool the passenger compartment. On the other hand, the battery management electronics control unit can send a request for the high-voltage battery unit to be cooled as a data bus message to the IHKA control unit. The IHKA control unit coordinates these cooling requirements and controls the electric A/C compressor via the LIN bus. The cooling requirements are prioritized depending on the temperature, for example at a high ambient temperature and very warm passenger compartment, a higher cooling power is demanded with higher priority. If the desired temperature is reached, the cooling power is reduced to maintain the temperature and set to a lower priority.

It is similar for the temperature of the battery cells. If the battery cells heat up to temperature of approximately 86 ºF (30 ºC), cooling of the high-voltage battery unit already starts. The cooling requested by the battery management electronics control unit has an even lower priority here. It can for instance be declined by the high-voltage power management. At a higher cell temperature, the cooling request for the high-voltage battery unit receives top priority and is always carried out.

Additionally, the speed of the electric A/C compressor is limited for acoustic reasons. This occurs as a function of the blower setting and the driving speed.

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