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EETE JULAUG 2014

E-car research project sets efficiency standards By Christoph Hammerschmidt The prototype of an electric car, developed by the Technical University of Munich along with a consortium of carmakers and technology companies, outlines some general design trends for electrical vehicles. The design Goal of Visio.M, as the project has been baptised, is two-fold: The research consortium wants to devise an electric vehicle that offers a maximum of energy efficiency and at the same time features a sporty driving behaviour, or, as the researchers put it, an “efficient car with the genes of a sportsman”, the latter one obviously an inheritance from BMW as one of the two carmakers who joined the project consortium as research partners. The other one, by the way, is Daimler. The Visio.M, though still in the prototype phase, is not just a research vehicle. Instead, it also has been designed with industrial production and low total cost of ownership (TCO) in mind. The vehicle will be as affordable Fig. 1: The Visio.M electric car prototype. as a subcompact car - with a purchase price of somewhat above 15.000 euros but extremely low energy and maintenance costs. These figures anticipate a serial production of at least 50.000 units per year - an ambitious but not unrealistic goal. With regard to the battery concept, the Munich research team followed the standard set by Tesla Motors: They use a battery block composed of small lithium-ion cells like they are used in laptop computers. According to Markus Lienkamp, professor at the Munich Technical University and head of the research team, this design offers the highest energy density currently available. “Tesla has the best batteries of all electric vehicles”, Lienkamp said. Though recently a number of alternative materials and battery concepts such as lithium-air Fig. 2: Power it like Tesla: the battery pack of the Visio.M is based on the same principles as Tesla Motors. or zinc-air made a splash in the media, these alternatives are years, if not decades away from industrialisation, Lienkamp said. “Over the past years, lithium-ion batteries achieved an improvement of 7 percent per year” Lienkamp said, adding that he believes doubling of capacity within ten years can be expected. The low total weight (including batteries) of 548 kg is owed to the aluminium body of the two-seater; in the final version due later this year, some body components will be made of carbon fibre. With such a low mass, the designers found a battery of just 13.5 kWh strong enough to bring the car with its relatively small engine of 15kW to a top speed of 120 kmph. Though these figures do not really make the race driver’s heart beat any faster, they have to be set in context with the efficiency goals, said research team member Patrick Stenner. Besides the low weight, another key feature to maximise the efficiency was the minimisation of drag and friction; the vehicle offers a drag coefficient of 0.24 - lower than any series vehicle today, as the team claims. The sportsman’s genes find their expression in the torque vectoring differential gear on the rear axle: Here, an additional microprocessor-controlled small motor of 1 kW maximum power allows active torque distribution to the traction wheels, giving the vehicle a rather sporty cornering behaviour. In addition, this concept is said to allow significantly better energy recuperation. As to the main motor, the researchers chose an induction motor, because this type offers better energy efficiency than a permanently excited synchronous motor, Stenner said. Another measure to reduce the energy consumption is the heater: instead of a heat pump which uses electric energy or a range extender (bad efficiency), the engineers put a small heater into the vehicle that burns bio ethanol. Besides BMW and Daimler, other consortium members are automotive suppliers Autoliv and Continental, chipmaker Texas Instruments, technology company Siemens and certification services provider TÜV Süd. The results of the Visio.M project and the final version of the vehicle will be introduced on October 22. 14 Electronic Engineering Times Europe July/August 2014 www.electronics-eetimes.com


EETE JULAUG 2014
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