Student Racing Series Impresses with Electric Motor Innovation

Enzo Ferrari, Ferry Porsche, Ferruccio Lamborghini – all of them built sports cars themselves because they could not find one that fulfilled their desires. Every year, the dream of seeing your own race car at the start-line becomes reality for the participants in the Formula Student race series. In line with contemporary concerns, the focus is on the electric engine. The eSleek14, belonging to the DHBW Engineering Stuttgart team, has two electric motors, each with 60 horsepower. The electricity that feeds this speed machine is derived from batteries fixed laterally inside the vehicle. The construction of the air cooling system was supported by EOS in its role as Additive Manufacturing expert.

Challenge
The drive unit of a vehicle with an electric motor is a complex construction. The actual drivetrain is simpler than with conventional combustion engines. This must, however, be set against the complications of integrating the energy-storage units. In the eSleek14 these are comprised of lithium-polymer cells. The 24 modules have a total capacity of 6.7 kWh. This powerhouse is contained in a battery-housing made from fibre glass reinforced plastic in a sandwich-type construction. An integrated battery management system (BMS) controls the charge and discharge of the individual cells.

Because of the cell chemistry, the lithium-ion batteries used are flammable. The physical protection of the packs of cells is just as essential as the reliable ventilation of the system as a whole. This is because, alongside damage, overheating can also lead to a fire. In connection with the BMS, it is therefore vital to keep the build-up of heat under control. At the same time, optimized heat dissipation guarantees the best possible performance of the energy supply, and ensures the provision of power to the electric motor.

Another ever-present factor in the minds of the developers is the weight of each component. The battery unit must be as small and densely packed as possible in order to provide maximum power while taking up the least amount of space. Simultaneously, a defined range must be ensured - the proverbial squaring of the circle. This was the challenge with which the constructors of the eSleek14 were faced. Their response also needed to meet the crash requirements of the formula series.

Solution
The construction of the entire battery system was carried out in a way that ensured both mechanical and electrical protection, as well as providing optimized cooling. Three continuous channels guarantee the supply and venting of air to and from the electricity storage unit by way of the in-flowing and out-flowing airstream. The cooling air enters from the front, is distributed along the cells via the three channels, is then reunited by a manifold, and finally is drawn back and up to be expelled by way of a radial fan – a technical masterpiece on the part of the constructors.

Such components are, however, not available off the shelf. Additive Manufacturing processes offer a particularly valuable option in this area, as confirms David Köhler, the man responsible for both the construction of the batteries for the 2013/2014 car and the cooling concept for the high-voltage device: "We decided to have the cooling duct additively manufactured and, thanks to EOS' technology, we had complete freedom of design. With such small quantities, injection-moulding didn't make sense, and we would anyway, have had to make changes to the construction design." For the 2013/2014 season, as in previous years, EOS trained the team in how best to exploit the advantages of Additive Manufacturing.

In the development of the battery container it was possible to realize not only the cooling channels between the individual modules, but also a cooling duct that directed the air at the end of the container, back outside in the most efficient way. In order to satisfy the strict internally set limits for the component weight, the team chose the lightweight fine polyamide PA 2200 for the construction. This material is characterized by its high rigidity and good thermo conductivity – perfect characteristics for application in motor racing.

Results
The air duct was produced using the EOS P 396, the laser beam melting the powdered material, layer by layer, to form the final component. Intensive component testing showed that the construction complied with all of the safety standards. The components successful application within the vehicle followed. The weight also met the desired criteria: the cooling duct component weighing just 77g. The importance of each gram is shown by the fact that the total weight of the eSleek14, at just 180kg, converts to a power/weight ratio of just 1.5 kg/hp. In comparison, each unit of horsepower in the Porsche 911 GT2 road version converts to 3kg in weight.

The team was able to increase the cooling performance by well over 100 %. In total, the temperature within the battery container fell from a high of 80 °C to just 50 °C. The distribution, in addition, is considerably more uniform. The results of the team at the Hockenheimring make it clear that nothing burns during the process of heat dissipation. Following a shock disqualification for excessive performance (!) in the acceleration test, the team showed its strengths in the endurance and efficiency competitions, with 4th place finishes in both categories. Despite the disqualification in the acceleration tests, the team was still able to achieve 7th place overall. Proof that the car is not only fast but also both reliable and efficient was provided by a race in Barcelona. "We kept pace with the best teams in the world: after Barcelona and Hockenheim we were in the top ten of the global rankings," summarizes Köhler with pride.

It is worth keeping an eye on the long-term results which will become clear in the coming years. In its role as a training race series for the next generation of engineers the Formula Student has a value that is difficult to quantify. "Today we are laying the foundations for the innovations of tomorrow." This is something of which Nikolai Zaepernick, head of Strategy and Business Development at EOS is certain. "For us this was a conscious choice: this type of application has great potential for use in electric road cars over the coming years."
The complex and lightweight component for the dissipation of heat reduced the top temperature within the container from 80 °C to just 50 °C. This translated to an increase in cooling performance of over 100% (courtesy of DHBW Engineering Stuttgart).
The complex and lightweight component for the dissipation of heat reduced the top temperature within the container from 80 °C to just 50 °C. This translated to an increase in cooling performance of over 100% (courtesy of DHBW Engineering Stuttgart).

"EOS' expert knowledge and technology have really helped us to get our electric powered race car eSleek14 on the track and recording great times. The advantages of Additive Manufacturing are very convincing: lightweight components, the fast realization of ideas, and all this possible for extremely small part quantities. The available materials mean that safety is never compromised. Thanks to the instruction received from EOS we were able to get the maximum out of the process – and prove ourselves out on the track."

David Köhler, Assistant Head of Battery Development for the race team 2013/2014

Short Profile
The Baden-Württemberg Cooperative State University sponsors the DHBW Engineering Stuttgart e. V. club, which every year, enters a new racing team for participation in the Formula Student Electric. Each season, different project teams construct a race car within the specifications set by the series.

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