Thermal management – the unsung star of electric motors
Cold feet, a hot electric motor, with the battery somewhere in between: temperature control for electric cars is a complex and sensitive subject. This is where efficient multitasking of optimal thermal management works.
Somehow, we’ve gotten used to it: setting the temperature control of our car’s air conditioning to 20 degrees centigrade and letting the system take care of the rest. Whether it’s freezing cold outside or sweltering, the cabin remains comfortable. In many cars, the driver and passengers can even select different temperature levels – to suit their personal preferences.
Technology operates most efficiently in a special temperature window as well. Excessive variations may even cause damage. Since optimal temperature windows may vary, depending on the component assembly, thermal energy must be skilfully distributed or stored. This is a job for thermal management.
Thermal management modules for powertrains using gasoline engines have been in use since 2011: in Audi’s 1.8-l TFSI engine. Just by means of this technology the CO2 emissions of the vehicles using it were reduced by up to four per cent. Since then, the system has been subjected to regular development, especially for meeting the more complex requirements of electrified powertrains.
Current hybrid electric vehicles have up to three cooling circuits operating at different temperature levels: for the IC engine, the electric motor including power electronics, as well as the battery. Even so, these circuits are interlinked via heat exchangers. The individual components are no longer bound to performing fixed roles as heat sinks and heat sources, though can be swapped. In addition, their higher complexity required switching from central to decentralized thermal management for electrified powertrains.
Schaeffler’s thermal management system
Efficient heating, especially in combination with a heat pump, is decisive for the cabin and the battery of an electric car when starting the vehicle from cold. High cooling power is crucial at hot outdoor temperatures and during fast charging. The new integrated thermal management system from Schaeffler as the central control unit interconnects the coolant and refrigerant circuits. In addition, it uses the waste heat from the electric drive efficiently for optimized energy balance.
The system integrates two electric water pumps. The central electronics unit for the pumps and valves as well as the sensors and chiller as the refrigerant circuit interface. The compact design reduces the space requirement by up to 60 percent compared to conventional, non-integrated systems. At the same time, our new thermal management system with its hydraulically optimized design provides the basis for high overall efficiency.
Thermal management is even more important for fully electric vehicles
Because the ample supply of the IC engine’s waste heat as a source of heat does not exist in all-electric vehicles, optimized energy management is even more crucial. The objective of the development engineers is to achieve a maximum intersection of high powertrain efficiency as one set and ensured comfort functions and optimal component protection as the other – by means of sophisticated thermal management.
The interaction between the cooling and the refrigerant circuit in fully electric vehicles is particularly important due to its significantly higher impact on overall vehicle efficiency than in an ICE or hybrid electric powertrain. On cold winter days and nights, the heater in an electric car consumes a similar amount of energy to produce comfortable temperatures in the cabin as the car uses for propulsion. The more efficiently the heater can use the heat dissipated by other units, the higher the car’s range. A reversible heat pump in the refrigerant circuit delivers any additionally needed degrees centigrade or Fahrenheit. It can increase the electrical energy diverted from the traction battery by as much as a factor of five and use it not only for heating but also for cooling, as needed.
Ailing batteries due to fever or hypothermia
In contrast to a conventionally powered automobile, in which the IC engine is the central element in the powertrain, the battery – including thermal management – is the focus of attention in an electric vehicle. The energy storage device with its liquid electrolytes is highly temperature-sensitive and therefore typically has a dedicated cooling circuit. All of us are familiar with this phenomenon from our smartphones: battery power decreases in cold temperatures due to an increase of internal resistance. The same happens in an electric vehicle, whose energy storage device is nothing but an XXL-sized smartphone battery.
However, the decrease in usable capacity is just one of several issues. High charging currents at low, sub-zero temperatures can damage the battery due to the risk of the lithium ions forming metallic lithium, instead of depositing in the anode as desired, while the battery is being charged. This dreaded process is called “plating” and can lead to short circuits or even fires. Therefore, predictive thermal management moves the energy storage device into a “healthy” temperature window prior to the charging process.
The ideal temperature window of a lithium-ion battery ranges between 20 and 40 degrees centigrade. If the battery gets hotter it will not just lose energy but age as well. Due to the fact a normal cooling circuit will soon reach its limits at high ambient temperatures, a refrigerant circuit with a heat pump operating using the same principle as the air conditioning system for the cabin can be activated. The weight of a traction battery, amounting to several hundred kilograms, makes it more difficult to achieve an optimum thermal window. Here, extensive interlinking of the thermal management system and the vehicle control unit helps detect potential stress early and respond accordingly. For instance, if the driver uses GPS navigation to head for a fast-charging station the system can put the battery into the ideal temperature range for charging at maximum power input. The more data that is made available to the predictive management module the more effectively it can respond. Therefore, adequate computing power is becoming an increasingly important pillar of efficient thermal management.
Electric motors need cooling too
Even though the power dissipation of an electric motor is clearly less than that of an internal combustion engine, it does get hot as well, due to friction and current flow. Overheating of the motor may result in damage, for instance to the winding. Therefore, nearly all electric traction motors are now liquid-cooled. The same applies to the power electronics controlling the complex current flow of an electric vehicle. The cooling circuit controlled by the thermal management system dissipates this thermal energy and conducts it toward a place in the vehicle where it’s needed.
Schaeffler’s thermal management system controls the complex interaction between the cooling and the refrigerant circuit via a highly integrated cooling module consisting of pumps, valves and the coolant reservoir with a dedicated control unit, complemented by decentralized, smart valves that are coordinated centrally. Based on the prevailing temperatures and operating condition of the vehicle, the central electronics unit controls the actuators and pumps –also, continuously, the flow of thermal energy. This explains why Jochen Schröder, President of E-Mobility at Schaeffler, says: “Thermal management is the unsung star of the show when it comes to making vehicles even more efficient and user-friendly.”
The R&D Department Schaeffler