MIT Solar Electric Vehicle Team (SEVT)

The Solar Electric Vehicle Team (SEVT) is a recognized student organization at the Massachusetts Institute of Technology, working under the auspices of the Edgerton Center. The team draws on a broad range of technical knowledge encompassing all fields of engineering and science. Team membership provides an intense educational experience which teaches practical skills impossible to communicate in the classroom environment, turning students into engineers. In addition to providing real-world design and manufacturing experience, involvement in the team develops project management and business skills. The team also gives students the opportunity to work closely with professors and members of the business community, including those on the board of consultants, when developing the vehicles.

The MIT Solar Electric Vehicle Team sets its goals beyond just winning races. The team is dedicated to promoting alternatively powered vehicles. Members participate in seminars, lectures, museum displays, conferences dedicated to alternative energy, and numerous Earth Day and ecological fairs. Team members answer questions about electric transportation at these events as well as during race-sponsored demonstrations.

Tesseract is a single-seat high performance solar race car. Because we’re trying to attain highway speeds on the same power as a hairdryer, everything on the car has a necessary and specific purpose. If it isn’t essential, we leave it out. All together, the car weighs about 375 lbs without driver.

How it works: Solar Array
The solar array consists of 2732 space grade triple-junction solar cells. These are the same cells that NASA puts on its satellites. We use them for the same reason they do, weight is at a premium, so we want the most power per area that we can get. The cells are connected into 40 different series strings of varying length on the car to optimize collection over the curved surface of Tesseract. Immediately after the solar array, there are 12 maximum-power-point trackers, which ensure we are drawing the max power possible from our array at any given time. These also act like a power conditioner to deal with the fluctuating battery voltage.

Motor -- Tesseract is powered by a 6 hp axial flux brushless DC motor. By using and axial flux configuration, the motor is thin and flat like a pancake, and can produce large amounts of torque. Because this motor was specially designed for solar cars, it can be run in a direct-drive configuration. There is no transmission, the motor is attached directly to the hub of the rear wheel. This motor is coupled with a special controller that monitors the rotor position and fires 3 phase bursts of DC energy in sequence. This eliminates the need for brushes, a large source of energy loss. As a system, the motor and controller peak at 94% efficiency from stored electrical energy to mechanical energy at the wheels. When compared with passenger cars, which have an efficiency of about 15% from stored energy in gasoline to mechanical energy at the wheels.

Batteries -- Tesseract uses 512 li-ion batteries. These are the same type of batteries found in most laptops, only laptops use 6 or 8 cells in a battery pack instead of 512. Our pack is broken down into twelve modules, which are each equivalent to a car battery, but only weigh 5 lbs each. Through an innovative pack design, we ventilate the batteries with even airflow to minimize temperature differences between the modules. Because li-ion batteries are so energy-dense, we build our own battery protection circuitry to monitor each module voltage and temperature. This data is fed into a power controller that will disconnect the batteries in case they near a dangerous operating condition.

Chassis and Suspension -- The car is held together by a chromoly steel space frame. This is employed to provide the driver with a safe roll cage while minimizing the weight of the structure. The frame weighs about 35 lbs, and is fabricated by the team. the suspension is a familiar design, double a-arms with coil-over shocks in the front, and a trailing arm with coil-over shock in the rear. It’s a car/mountain bike hybrid suspension, using lightweight bike shocks, while using steel control arms and aluminum uprights.

Steering and Brakes -- The driver controls the car with center mounted handlebars, much like that on a bicycle, that connect to a rack-and-pinion steering system. The car stops (only when necessary) with 4 mountain bike brakes on the front wheels connected to go-kart master cylinders and pedal. There are two redundant hydraulic systems so the car can still be stopped in case one system fails.

Body -- The body of Tesseract is made of Kevlar and carbon fiber fabric, Nomex honeycomb, and epoxy resin. By laminating these together in a special process and reinforcing them properly, the shell of Tesseract weighs only 60 lbs. The body was designed for super-streamlined aerodynamics, and has been thoroughly wind tunnel tested.

Control Electronics and Telemetry -- With all the subsystems of the car working, the driver has his work cut out for him — keeping the car on the road. This is because of a specially developed cruise control system developed by the team. The team’s tactical strategy is programmed into the cruise control, so the car adjusts speed according to race conditions. All the data from the car is collected over an industry standard CAN bus, and is relayed to the support crew, where the strategy team analyzes the data and picks the car’s target speed.