The reverse side of the test rig with a control unit and a laptop equipped with LabView

Comfortable Temperatures for Electric Cars

Steinbeis experts participate in ZIM thermal management project

E-Car-Tech produces conversion kits that allow vehicles with combustion engines to be refitted with electric motors. One issue in this area comes with thermal management, which is not only different but also more complex. Working alongside experts at the Steinbeis Innovation Center for Development Technology, a so-called Thermobox is being developed as part of a project under the Central Innovation Program for SMEs (ZIM).

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Regardless of outside temperatures, the ideal temperature for batteries in electric cars is between 25°C and 30°C. If possible, the  power electronics for the motor as well as the charger system should be kept under 60°C and the electric motor should be kept below 110°C. One way to maintain the right temperatures is to use the oversized radiators already fitted in vehicles, but the car interior also needs to be heated or cooled.

To develop a system that addresses all of these challenges, a unit based on an inversion heat pump is being developed as part of an ongoing ZIM Thermobox project with the support of the Steinbeis Innovation Center for Development Technology. The new system is able to produce heat or cold by toggling an expansion valve, thereby alternating the function of the heat exchanger and turning the condenser into an evaporator – and vice versa. Thanks to a series of changeover valves, water pumps, and an intelligent control system, any heat source or heat sink can be connected to the Thermobox.

Operating conditions on a test rig

The first step was to develop a laboratory test rig to simulate a diverse array of operating conditions. Instead of using a vehicle interior, a battery, and a control unit with a motor, the test rig is fitted with water-to-water heat exchangers. These make it possible to use flow rates and temperature measurements to determine how much heat enters or exits individual heat-consuming components and gauge their heating and cooling requirements. Given that the motor, control unit, and charging electronics only require cooling – and are exposed to relatively high temperatures – they have been combined to form a single heat-consuming unit.

The original radiator always remains in vehicles after conversion from a combustion engine to an electric drive; just like the coolant reservoir, there is also one each included in the test rig. The heat pump has many more components than the system that will ultimately be installed in vehicles, but therefore presents a multitude of different scenarios which can be tested. For example, it allows the steam pressure in the system, and thus temperatures in the condenser, to be regulated. In addition, the rotational speed of the compressor and all pumps can be controlled using pulse width modulation (PWM). A lot of sensors on different positions of the system allow complex measurements.  Some of those components such as valves and sensors are going to be  included in the vehicle, although many more are undergoing testing on the test rig in order to simulate all kinds of operating conditions.

On the back side of the rig, sensors, pumps, and the heat pump have been incorporated into a block diagram, which uses LEDs to display the operating status. Similar to a vehicle, the test rig is also fitted with its own high-voltage power supply and 12-volt system. All sensors and actuators are linked via a controller area network (CAN) bus. This allows the testing facility to be controlled via LabView, a graphical programming environment. Work is currently underway to add some  other displays to the test rig. The testing results will also show whether it will be necessary to use an electric heat exchanger in the car later on.

In essence, the operating status of the thermal management system is a function of outside temperatures and the amount of load placed on the drive unit. Similar to the situation later in vehicles, the temperatures of all components consuming heat are considered control variables. The E-Car-Tech conversion kits ensure the maximum load placed on batteries is 1C , not only to ensure electrical load is kept to a minimum, but also to extend battery life. This means there is very little self-heating of the batteries during operation; they are almost exclusively influenced by external temperatures. This gave rise to the following:

  • Outdoor temperatures of -40°C to +18°C: interior heating and battery heating via the heat pump (HP), HP generates heat, remaining components are cool
  • Outdoor temperatures of +18°C to +22°C: cooling only via vehicle radiator, HP off
  • Outdoor temperatures of +22°C to +24°C: interior cooling via HP, remaining components are temperature-controlled via the vehicle radiator
  • Outdoor temperatures of +24°C to +50°C: interior and battery cooling via HP, , remaining components are temperature-controlled via the vehicle radiator
  • Outdoor temperatures of +50°C to +70°C: all components are cooled via HP

Thinking again and making improvements

 Steinbeis expert Professor Matthias Vogel is already in a position to draw initial conclusions: “Everything needs to be smaller! It needs a much more compact heat pump and the valves should be combined to form a valve terminal.” The  goal is to use  only one supply and return line  for the individual components . Inevitably, the heat pump and the valve terminal will have to be positioned in different places within the vehicle for space reasons. The original plan was to position everything in the Thermobox.


Prof. Matthias Vogel (author)
Steinbeis Innovation Center Development Technology (Oberndorf)