Solar EV thunder Down Under

  • 31-Jul-2013 11:13 EDT
LEAD IMAGE - UofM solar Generation racecar.jpg

Generation, the University of Michigan's 2013 World Solar Challenge racecar, at its unveiling with the engineering team (seen in background) in Detroit in the late spring. Note offset cockpit and single pylon for the car's left-side road wheels.

Finishing in third place over the course of five World Solar Challenge races has the University of Michigan College of Engineering's team stoked that 2013 is finally going to be its year atop the podium at the premier event for sunlight-powered EVs.

“We feel confident about Generation, our new car, which is in many ways a fairly radical departure from Quantum, its predecessor,” said Eric Hausman, the senior in Industrial and Operations Engineering who is the team’s Project Manager. “We believe our chances in the race are excellent—and we’ll be on four wheels this time.”

Hausman was referring to the major rules change for the week-long, 1860-mi (2990-km) endurance race across the Australian outback that is run every other fall. (It kicks off Oct. 6 in Darwin.) The new-for-2013 rules mandate that the solar-powered cars have four wheels rather than the three permitted previously. It’s the most significant World Solar rules change since 2007, when the driver moved from lying down to seated.

“The rules were released just over a year ago, and they really surprised us,” said Hausman, who spoke with SAE Momentum by phone in late June from the team’s development shop in Ann Arbor. “We speculated there would be some change after the last race, perhaps intended to level the playing field.”

But Michigan’s 100-strong team, which won its seventh American Solar Challenge championship in 2012, was already working through design iterations for its 2013 car when the four-wheel rules were announced. “We believe the asymmetrical shape and new geometry we came up with will be our competitive edge—the most efficient car possible with four wheels,” Hausman said.

New cockpit configuration

Because solar irradiation is the only fuel permitted—and the vehicles must be as aerodynamic and light as possible for the daily 9 h of racing—World Solar cars typically resemble a rolling airfoil, with their horizontal upper-body surface containing the solar panel array. The design question then becomes where to put the driver’s tiny cockpit.

On the two-wheels-forward Quantum that Michigan used in the previous (2011) Australia race, the cockpit was located near the rear of the airfoil, on the car’s centerline. It gave a distinctly “cab rearward” look. By comparison, Generation’s cockpit is close to the middle of the vehicle and offset to the left. It is part of a large fairing that also incorporates the two left-side wheels. On the right side of the car, each wheel has its own separate, smaller fairing.

The asymmetrical design allows the driver’s seat (dubbed the “butt bucket”) to be tucked up completely into the carbon-fiber monocoque tub. This helped create an aerodynamically cleaner underbody compared with Quantum. Hausman explained that before the 2013 rules were released, his team had decided not to use again the “window” fairings that enabled Quantum’s wheel pylons to open up, allowing the wheels to turn. That design had its share of reliability issues—which the new offset configuration is expected to rectify.

And by placing the canopy to one side, the new layout also reduces shading of the solar cells—important for optimizing Generation’s driving-range capability in the grueling race. (The upper body assembly also tilts up to a high angle for service access and for charging the battery when the car is parked.)

The canopy has a longer sweep than on the old car due to new forward-vision regulations. The driver must be able to see ground level 11 m (36 ft) in front of the car, as well as every point between 0.70 m (2.3 ft) below eye level and 0.70 m above eye level at a distance of 4.0 m (13 ft) from the driver's eyes, at every forward angle.

With Generation’s driver seated on the left, most of the car’s heaviest equipment is fitted on the right side to maintain stability. The car is the same length and height as Quantum but is approximately 12 in (305 mm) narrower.

Rolling resistance was inevitably increased by the additional wheel.

“There isn’t really a good way to compensate for that,” Hausman noted. “We thought of putting two tires right next to each other in the rear, but the rules don't allow it,” he said. The team is using custom designed, low-rolling-resistance Michelin tires, but most of the other top teams will probably have them as well. The completed car spent time in Ford’s wind tunnel last summer to fine-tune aerodynamics.

Consolidated CAD

Michigan’s solar car team, founded 23 years ago, is structured in four divisions: engineering, business, operations, and strategy. Within engineering, there are several subdivisions including mechanical, aerodynamics, micro-electrical, and power-electrical. Typically students decide to join either the Formula SAE car or Baja SAE teams or the Solar team; all three occupy the same large vehicle-development building on campus.

“This year we built really strong ties with the Formula SAE and Baja teams,” Hausman said. “We bounce ideas off each other; they know a lot about making parts that are easily manufactured, while we have carbon-fiber knowledge. We’re proud of our inter-team relationships and sharing, which make us stronger overall. The Baja team doesn’t have a national championship, but they won the Ironman. It’d be awesome to have at least three world championship racing teams from Michigan!”

Sharing computer tools may be the next step for the three vehicle teams. For the Generation car’s development, the Solar team switched to Siemens NX for its CAD/CAE platform.

“Previously we used [Dassault Systèmes’] SolidWorks and some NX for the aero body design, and a host of other suites for some of the smaller electrical systems,” Hausman explained. “This year we consolidated on NX, and Siemens came onboard as one of our largest Platinum supporters. We’re excited to have them because NX really helped us create a better model of the car.

“Instead of going from one software package to another and trying to convert things, we now can put everything in one file and make sure it all will fit and work right before manufacturing anything,” he said.

According to Hausman, the Generation powertrain is little changed from that used in Quantum. Its basic architecture includes silicon-type photovoltaic solar cells, a Li-ion battery, and a brushless dc traction motor located in a wheel hub.

“We’ve done a bit of work on the solar panel array to increase efficiency,” he said. Like any good competitor, he provided few pre-race details about the powertrain hardware and performance. Hausman believes Generation is capable of beating Quantum’s on-the-road efficiency, which averaged 60 mph (97 km/h) on 1.6 kW—power equivalent to a hair dryer—over the total distance. Quantum’s solar array provided 80-90% of the car’s motive energy during the race, with the remainder coming from the onboard battery. Its sun-to-wheels efficiency was better than 93%.

Hausman admitted that adding a fourth wheel and its attendant structure made maintaining mass parity with the Quantum challenging. As of mid-summer, the team was still optimizing a few components and so didn’t have a final curb weight. He estimated the total weight of machine and driver will be the same as Quantum (about 550 lb/249 kg). Of that, the Li-ion battery is allowed to weigh up to 21 kg (46 lb).

1100-mi shakedown test

During the summer, the Michigan team put the new car through a series of tests at the Chrysler Proving Grounds in Chelsea, MI, and its annual “mock race,” an 1100-mi (1770-km) road test that spans four days and takes the car through rural Ohio, Indiana, and western Michigan. The route is designed to help the team prepare itself, and the car, for the Australian course in October. This year a bug in the car’s electronics vexed engineers through an entire night until a fix was found the next morning.

Hausman expects Michigan’s toughest competitors to be teams from Tokai University in Japan (the 2011 World Solar champions) and Nuon, the Dutch team from the Delft University of Technology (TU Delft) that finished second in 2011.

To follow the University of Michigan team during the Oct. 6-13 race, go to http://solarcar.engin.umich.edu/. Also see http://www.worldsolarchallenge.org/.

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