Cooling system, function

The cooling system in general

The vehicle's thermal management comprises the cooling of the fuel cell, climate control of the vehicle interior and the cooling of the component parts of the high-voltage on-board electrical system. The fuel cell system and the component parts of the high-voltage on-board electrical system are cooled by three closed, separate coolant circuits:

 High-temperature circuit

 Low-temperature circuit 1

 Low-temperature circuit 2

The high-temperature circuit takes over the cooling of all component parts of the fuel cell system that are involved in cold combustion as well as the climate control of the vehicle interior. The flow temperature of the fuel cell should be 63 °C, or 70 °C in winter mode. The flow temperature is in a range from 50 °C to 80 °C.

The low-temperature circuit 1 cools the electric machine (A79/1) and the fuel cell DC/DC converter control unit (N129/8), the energy distributor unit with integrated fuel cell management system control unit (N116/5) and the exhaust air turbocharger (A77) of the fuel cell system.

The low-temperature circuit 2 cools the high-voltage battery (A100g1), the DC/DC converter control unit (N83/1) and the alternating current charger for high-voltage batteries (N83/11).

Each of the low-temperature circuits has one and the high-temperature circuit two variable speed coolant pumps as well as various regulation valves.

The cooler for the high-temperature circuit, the two coolers of the low-temperature circuits and the condenser are integrated in one cooling module with side coolers. A fan motor located centrally behind it (M4/7) ventilates the cooling module. The fan motor and all coolant pumps are actuated by the powertrain control unit (N127) via the powertrain LIN (LIN C3). In the high-temperature circuit, a particulate filter protects the fuel cell from pollutants (> 100 µm). An ion exchanger ensures that the coolant maintains a low degree of conductivity (< 5 µS/cm).

 Only coolant that is specially suitable for fuel cell vehicles may be used. Refilling with commercially available coolants or water causes irreparable damage to the fuel cell.
Please observe the notes on fuel cell coolant: AH06.50­P­0001­01RFW.

 Extensive information on the fuel cell system and warnings when the fill level falls below that required are shown on the instrument cluster (A1).

Function of the high-temperature circuit

The fuel cell operates in a temperature range of approx. 80 °C and is therefore cooled by a separate high-temperature circuit. The PreCon function (pre-entry climate control) enables preheating of the fuel cell. A cold start is possible down to -25 °C.

The high-temperature circuit is regulated by the powertrain control unit. The coolant pressure in the cooling circuit is temperature-dependent and also depends on the speed of the fuel cell coolant pumps. The sensors integrated in the coolant pumps transmit the value of the coolant pressure to the powertrain control unit. If the value of the coolant pressure differential approaches 1.5 bar, the speed of the coolant pumps of the fuel cell is reduced, which may lead to a limitation in the power output of the fuel cell.

 The coolant pressure in the fuel cell may be a maximum of 1.5 bar above the gas pressure at that moment. If it exceeds it, this may lead to irreparable damage in the fuel cell.

The temperature of the high-temperature circuit is recorded via the following temperature sensors:

 High temperature coolant circuit temperature sensor 1 (B94/28)

 Cathode outlet temperature sensor (B94/26)

 Cathode inlet temperature sensor (B94/27)

The coolant level in the expansion reservoir of the high-temperature circuit is recorded by the high-temperature circuit coolant fill level sensor (S41/1).

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Low-temperature circuit 1

Low-temperature circuit 1 cools the electric machine and the fuel cell DC/DC converter control unit, the energy distributor unit with integrated fuel cell management system control unit and the exhaust air turbocharger of the fuel cell system.

Low-temperature circuit 1 is regulated by the powertrain control unit. This evaluates the data from the low-temperature coolant circuit 1 temperature sensor (B11/6) and actuates the rear electric machine coolant pump (M75/9) where necessary.

 The power electronics are also flowed through for low outside temperature, always with a minimum coolant volume flow (dependent on the coolant temperature).

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Low-temperature circuit 2

Low-temperature circuit 2 cools the high-voltage battery, the DC/DC converter and the alternating current charger for high-voltage batteries.

Low-temperature circuit 2 is regulated by the powertrain control unit. It evaluates the data from the low-temperature coolant circuit 2 temperature sensor (B11/7) and actuates the high-voltage battery coolant pump (M75/10) where necessary. According to the ambient temperature waste heat from the high-voltage battery is led away over the low-temperature circuit 2 cooler or over the heat exchanger attached to the refrigerant circuit. Low-temperature circuit 2 is regulated via the actuation of the high-voltage battery cooling switchover valve (Y140).

The heat exchanger cools the coolant by means of the refrigerant injected into the heat exchanger and evaporated. The cooled down coolant is subsequently available to low-temperature circuit 2. When the high-voltage battery is being charged, low-temperature circuit switchover valve 2 (Y73/2) is switched in the direction of the alternating current charger for the high-voltage battery and DC/DC converter control unit in moderate temperatures. The waste heat from the electronics is dissipated via the low-temperature circuit 2 cooler (1).

When the high-voltage battery is at low temperatures, the coolant is led via the heat exchanger blocked off from the high-voltage battery cooling expansion valve (Y110). In this case, the heat capacity of the high-voltage battery is used to cool the electronics of the alternating current charger for high-voltage batteries and of the DC/DC converter control unit.

 The energy management module in the powertrain control unit coordinates the energy flows of the fuel cell system. It is related to the electric variables, the interface to the battery management system control units (N82/2), power electronics and to the electric refrigerant compressor.

If the enable is awarded by the energy management the powertrain control unit makes the request to the climate control control unit (N22/1) via the CAN network for actuation of the electric refrigerant compressor. The climate control control unit then actuates the electric refrigerant compressor (A9/5) via the climate control LIN 2 (LIN B8­2). The high-voltage battery cooling expansion valve is opened by the battery management system control unit and the refrigerant flows through the heat exchanger. In doing so, thermal energy is removed from low-temperature circuit 2, the high-voltage battery and the battery management system control unit.

The cooling output is primarily dependent on the level of actuation to the electric refrigerant compressor. In idle and during an automatic engine stop, the output of the electric refrigerant compressor is limited to a maximum of approx. 2 kW. If the high-voltage battery's state of charge is very low, the output of the electric refrigerant compressor is regulated down to 0 kW.

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Overpressure protection function

Air and hydrogen diffuse into the coolant of the high-temperature circuit through the fuel cell’s fluid distributor plates. The gas input is dissipated to the high-temperature circuit expansion reservoir via a bleed line and led into the atmosphere in a defined way. An overpressure valve in the cover of the high-temperature circuit expansion reservoir is used for this. The overpressure valve opens at an overpressure > 0.8 bar. The escaping gas/air mix is diverted to the front-end assembly.