How Does It Work?
by Don Sherman
Nikola Tesla, electricity's mad scientist, would be proud of what a clever gang of computer nerds and car geeks has wrought in his name. The Tesla Roadster is not only the first production car powered solely by lithium-ion batteries, it's also the first electric machine worthy of the passing lane.
BODY AND CHASSIS
While the Roadster's extruded aluminum spaceframe is similar in design and construction to the Lotus Elise, most structural components are specific to this car. The wheelbase is 2.0 inches longer to provide space for the Energy Storage System and additional legroom. Sill height is lower by 1.6 inches for easier entry, and bucket seats are an inch wider for improved comfort. The Roadster's overall length is greater by 6.4 inches, and its width is increased by 0.9 inch. Body panels are carbon-fiber moldings versus the Elise's fiberglass skin. While both Lotus-built cars share the same basic steering and suspension systems, the vehicles' spring, damper, and antiroll bar calibrations are tuned to suit each application.
1. MOTOR
The Tesla Roadster's 115-pound AC induction motor spins to 13,000 rpm, delivering a peak 248 hp (185 kW) at 8500 rpm. Its laminated-steel and copper-wedge rotor is supported by two sealed and permanently lubricated ceramic ball bearings. At least 205 lb-ft of torque is available from 0 to 6000 rpm. The exterior surfaces of the motor are air-cooled.
2. TRANSAXLE
The Roadster's transaxle, developed by a Tesla/Ricardo collaboration, is a dual-clutch, three-shaft, two-speed automatic. First gear is 4.20:1, second is 2.17:1, and the final-drive ratio is 3.41:1. The wet clutches are servo-operated and computer-controlled. The engineering goal of changing gears in 0.3 second had not been realized at the time of our test drive.
3. POWER ELECTRONICS MODULE
The inversion of the battery pack's DC voltage to the three-phase AC required by the motor is handled by power-switching semiconductors packed inside a "black" box that measures four feet by one foot by eight inches. The semiconductors - called Insulated Gate Bipolar Transistors (IGBTs) - were invented in 1982. Third-generation IGBTs now in use for hybrids and electric cars are fast-acting, rugged, and affordable. Recharging equipment is also contained within the black box.
4. BATTERY PACK
The Tesla Roadster's Energy Storage System mainly consists of a battery pack, a cooling system, and seventeen circuit boards. The battery array contains 6831 3.7-volt lithium-ion cells, each of which measures 2.56 inches in length by 0.71 inch in diameter (about 30 percent larger than a AA battery). A total of 69 cells connected in parallel make one brick. Nine bricks wired in series produce one sheet. Eleven sheets wired in series yield a battery pack providing a nominal 366 volts and 56 kilowatt-hours of electrical power. Propylene glycol circulated through aluminum tubing in contact with every cell keeps the battery pack's temperature within its set limits. The same AC compressor that maintains the Tesla's cockpit comfort removes heat from the battery-coolant circuit. A network of voltage, temperature, smoke, moisture, shock, and rollover sensors guard the pack so that power delivery can be interrupted in the event of a problem. Various fail-safe and limp-home modes exist so that some mobility is maintained even when recharging is temporarily prohibited. A normal recharge cycle ranges from forty hours using the 15-amp, 120-volt mobile (in-car) charger to three-and-a-half hours using a 70-amp, 240-volt home unit.