A different riff on your basic two-seat electric car

  • 25-Sep-2012 05:03 EDT
Mute_Perspektive vorne oben.jpg

TUM's MUTE is designed to be a next-generation people's car for the next generation of Central Europeans.

The way Markus Lienkamp sees it, today’s electric car technology “is a complete disaster.” What he terms "electromobility" is impeded at the moment by range anxiety, prohibitive costs, and inconvenient charging. “And a lot of people are losing a lot of money on electric cars,” noted the Chair of Automotive Engineering at the Technical University of Munich (TUM).

That’s why Lienkamp is leading the Visio.M project, a German government-supported R&D effort to produce a low-cost, high-utility electric car that might attract the interest of the middle European mass market—a next-generation people’s car of sorts. “The idea was to take an integrated approach using appropriate high-tech solutions to produce an inexpensive product that still provides high levels of design and function,” he explained.

The TUM-based project, which began in 2010, has so far produced a sporty concept coupe that should not only be cheaper than existing EVs but should also be lighter as well, at only 400 kg (882 lb) with no battery.

The power for the MUTE prototype, which TUM displayed at 2011’s Frankfurt Motor Show, derives power from a small 15-kW electric motor. The modest powertrain would allow it to qualify in the European L7e regulation category as a "heavy quadricycle." Notably, the MUTE includes a one-time-use, zinc-air battery booster as a built-in emergency range-extender that adds 30 km (40% extra overall) if necessary.

No showstoppers

The German Federal Ministry of Education and Research recently extended the research project two and a half years with €10.8 million ($13.8 million) to help prove that electromobility is not only feasible from a technical point of view but also from an ecological and economic standpoint, he said. The academic/industrial collaboration will build another compact car prototype in which all the systems, especially the safety technologies, have been fully optimized and tested. “We have to prove out all the systems to make sure that there are no showstoppers,” he said.

The Visio.M project leverages the expertise of specialists and some 200 assistants and students across 20 departments at TUM. The cooperative program also receives donated components and parts from numerous suppliers such as AutoLiv, Siemens, TUV SUD, and Continental and expert services from two domestic automotive OEMs. For the MUTE, Daimler engineers consulted on safety issues, while BMW handling specialists assisted with driving dynamics. BMW also opened up its Aschheim test track near Munich for handling tests.

Cut car use in two

When his team first addressed the program target issues, Lienkamp said, “the main goal was minimize the total cost of ownership.” The researchers realized that the driving requirements of the average Central European owner could be split in two: “Long-distance trips on vacation to Alps or the coast, which would be undertaken in the big family car, and short-distance trips for around-town excursions and commuting, which accounts for 80% or 90% of trips.” The TUM planners focused on an EV with a 100 km (65 mi) minimum range and a maximum speed of 120 km/h (75 mph), deemed enough for short stints on the autobahn.

“These consumers also need lots of luggage and storage space,” he added, “enough for two suitcases and a couple of boxes.” As a result, the MUTE features about 500 L (17.7 ft³) of space.

Efficiency by any means

Weight management is key to the energy-efficient design, Lienkamp continued. More weight requires a larger capacity battery pack for the same degree of autonomy, which leads to significantly higher costs. More weight also entails a less dynamic driving experience and a rise in engine power.

“But we are aiming for an affordable car, one that’s fun to drive,” so limits must be imposed. Keeping the design energy-efficient, he said, meant making sure that the battery stayed small.

The MUTE’s central-mounted motor is a rear-drive 15-kW synchronous type powered by a 10 kW·h lithium-ion battery pack (containing 1232 Type 18650 cells in 11 modules). Charging is said to take 3 to 4 h at 230 V.

The designers crafted a clean and rather flat aerodynamic shape with a small frontal area. The streamlined fastback form with smooth underbody and integrated diffuser has a drag coefficient of only 0.27. Its elongated dimensions not only provide for large collapsible impact-crush zones front and rear but also for extra luggage space and a more comfortable ride, he said.

Other measures were taken to shave weight without adversely affecting vehicle safety. The structure is a lightweight aluminum spaceframe (with some prototype carbon-fiber composite parts) that resembles an Audi A8's with aluminum castings at the joints connecting formed aluminum tubes. The resulting rigid passenger cell provides for good passive safety performance.

Thin, low-rolling resistance tires at 115/70R16 help cut road friction losses.

The minimalistic design approach also posed some novel realities. For example, “we knew that in winter, heating the car would cut the driving range in half, so we installed an ethanol-burner to warm the passenger compartment using radiant heating.” The bioethanol unit, which has three times the energy density of the lithium-ion battery, is a good choice from a carbon dioxide footprint and weight viewpoint, he said.

Torque-vector power delivery

The TUM researchers also used a specially developed active torque-vectoring differential drive that helps the relatively small electric power system deliver unexpectedly good driving performance. The differential contains a small electric motor that runs as both a motor and generator to distribute optimal power to the two rear wheels. This technology improves the vehicle’s ability to maintain traction in poor weather conditions, for instance. A slipping wheel gets less torque, effectively braking it, while the opposite wheel gets greater torque, boosting stability and balancing the power output. The MUTE reportedly demonstrated outstanding dynamic but stable cornering performance on the BMW test track.

The torque-vectoring technology can recover as much as twice the energy during braking compared to vehicles operating without the system. It can recuperate more energy on curves.

The addition of electronic stability control to a low center of mass and a balanced 45/55 (front/rear) load distribution helps keep the car running reliably as directed.

The steering wheel, pedals, controls, and seat position can be adjusted individually so that the head of the driver remains at the same point within the cockpit. This leads to an optimal, unchanging operational viewpoint and better safety.

A central touchpad on the instrument panels serves as the user interface for radio and infotainment channels and to a navigation system that chooses the route based on energy-efficiency calculations. The driver can also check the battery charging status remotely via a smartphone.

Lienkamp acknowledged that one issue that requires further study is the behavior of light, stiffly constructed vehicles in collisions with more massive vehicles. In some impacts, the entire structure could bounce rather than deform locally to absorb energy, a tricky long-discussed safety concern with lightweight vehicles.

Although neither the MUTE nor the forthcoming Visio.M will be put into production, the attempt at a fresh EV design may constitute a first step toward a true electric people’s car that drivers find attractive, effective, affordable, and desirable.

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