The Formula Student team from Stuttgart solved the thermal stress issues in electric racers by creating an oil cooling system though additive manufacturing (AM). Article by EOS.
Racers must keep a cool head—and their cars should not overheat either. This applies equally to racing cars with combustion engines and electric motors. The difference: in fuel-fired racers the engine has to be tempered, in electric vehicles this must be considered in particular for the accumulator. The Formula Student team from Stuttgart has solved this task in the truest sense of the word with an additively manufactured oil cooling system and support from EOS.
A battery—as accumulators are called today—for an electric car has diva-like characteristics. It needs to be treated with caution. This applies not only to mechanical stress, but also to thermal stress: It doesn’t like temperatures that are too high or too low. The reason for this is the behaviour of the electron flow: If it is too cold, the electrons do not migrate fast enough for the maximum power output due to the higher internal resistance. If the temperature is too high, for example if the maximum power output is maintained for a longer period or if the climate is simply hot, there is a risk that membranes will be destroyed or that they will age more rapidly, even to the extent of the so-called thermal runaway.
In order to guarantee an optimum working range, appropriate systems are necessary; liquid-based solutions have the advantage that they can also heat the cells and thus maintain high performance – which is of course of central importance in racing. Oil cooling systems offer very good properties for the battery, but can only be realized with great effort using traditional construction methods: The filled quantity should be kept as low as possible in order to save weight. This also reduces space requirements, which plays a major role not only in tightly cut racing cars.
“In addition, the flow characteristics in the system are important for achieving a high volumetric flow rate,” says Florian Fröhlich from the Stuttgart Formula Student GreenTeam. “Several aspects have to be considered in order to secure an optimum flow velocity, including the expedient design and the lowest possible surface resistance.”
The aim of the racing team was to ensure that a major part of the fluid constantly circulates in the area of the cell flags. Additionally, as oil is quite aggressive, the chosen material must feature a certain level of chemical resistance, while at the same time it must follow the lightweight character of the entire project. High fire resistance is obligatory in racing anyway.
The young racing team set to work with this sporty technical wish list. Simulations on Computational Fluid Dynamics (CFD) resulted in the expedient design of the cooling system, which is made up of flux direction parts and inlet devices. The geometry was optimized in such a way, that a consistent flow is created through the outlets with their compact design and high surface quality. Due to the planned construction geometry and the incorporated hollow structures as well as, of course, the very small number of units, additive manufacturing was the best choice for the production process: The required flow properties would not have been reproducible with traditional methods.
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