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Sunday, January 27, 2008

Tesla Roadster: In Detail

Simple Powertrain Improves Efficiency And Reliability For Tesla Roadster

Every moving part of a vehicle's propulsion system reduces its efficiency, so Tesla Motors's electric motor has only one moving part. Compare that to a typical American sports car's V-8 engine, with eight pistons, eight connecting rods, a crankshaft, 32 valves, oil pumps, valve springs and many more moving parts. Which one is likely to be more efficient? And which one is going to be more reliable?

The electric motor's efficiency rating is likely to be as high as 95 percent, while the internal combustion engine's efficiency is 20 percent or less. With this efficiency, the Tesla Roadster achieves the equivalent of 135 mpg.

That efficiency doesn't slow this sleek sports car down, either. Estimated 0-60 mph time is around four seconds, and with an electric motor, the torque is instantaneous, throw-you-back-in-the-seat responsive.

The 3-phase, 4-pole AC induction motor has a rotating magnetic field. Through electromagnetic induction, the rotating magnetic field induces a current in the conductors in the rotor, which in turn sets up a counterbalancing magnetic field that causes the rotor to turn in the direction the field is rotating. The rotor, which is made of copper and non-magnetized steel, provides power through the transmission to drive the rear wheels of the Tesla Roadster. The top speed for the motor is 13,500 rpm, putting it in Formula One territory, about twice the typical "red-line" of IC engines. The motor can deliver about 185 kW to the transmission.

The computer-controlled system for the motor, known as the Power Electronics Module (PEM), is an integrated power inverter and high-rate charging system. The motor can provide negative torque, otherwise known as regenerative braking, to enhance range. During charging the maximum input power is 240 volts at 70 amps AC 45-65 Hz or about 17 kilowatts. The PEM is air-cooled.

The unique two-speed electrically actuated manual transmission is also designed for optimum efficiency, with an integral free differential and final drive. There is no external oil cooling required. The transmission is cooled internally with a splash lubrication system. The gearshift is electro-mechanical with an electro-mechanical transmission lock (park position) on the final drive. There is no clutch. Torque change and speed matching for shifting is done using computer software. Maximum torque is 260Nm and maximum input speed is 13,500 rpm. Since electric motors can run in either direction, there's no need for the additional weight from a "reverse" gear; a simple switching arrangement reverses the motor. An on-board controller provides traction control.

High Performance Energy Storage System Is Key Driver For Tesla Roadster

What has 6,831 non-moving parts and powers a car? It's the unique Energy Storage System of the new Tesla Roadster.

The 6,831 non-moving parts are cells slightly larger than a typical AA battery. The large number of small cells allows Tesla Motors's engineers to create an energy storage system around fixed points on the chassis, ensuring optimum efficiency in packaging.

Tesla Motors CTO JB Straubel said, "Maximizing battery run-time is critical for an electric vehicle, but we've been able to do just that with the Tesla Roadster while minimizing weight and maximizing safety. The proprietary technology developed at Tesla Motors for our Energy Storage System is critical to the success of the Tesla Roadster."

The Energy Storage System (ESS) provides power to the entire vehicle, including the motor. It comprises a durable and tamper-resistant enclosure, the 6,831 cells, mechanical structure to mount the batteries, electrical interconnection between the cells, interconnection to the power electronics unit, a network of microprocessors for maintaining charge balance and temperature monitoring, a cooling system, and an independent safety system that is designed to isolate high voltage outside the enclosure under a variety of detectable safety situations.

A cooling system is controlled by the vehicle electronics. It uses a secondary loop in the cabin air conditioning system to provide chilled coolant which is circulated throughout the ESS. A resistive heater is used to heat the batteries in extreme cold conditions.

Batteries using the same lithium-ion technology used in Tesla Motors's vehicles have proven themselves over years of usage in modern electronic devices such as cell phones, camcorders, MP3 players and other digital devices. Unlike previous-generation nickel-cadmium and nickel-metal-hydride batteries, lithium-ion batteries have no "memory," and are good for 500 complete charge/discharge cycles before replacement may be required.

Lithium-ion cells are rechargeable and provide one of the best energy-to-weight ratios and a slow loss of charge when not in use. Pioneered by Gilbert N. Lewis in 1912, the first commercially available lithium-ion cell was created by Sony in 1991.

Lithium-ion technology is improving rapidly with the latest breakthroughs focusing on reducing charging time, and improving the capacity of cells. Tesla Motors continues to explore improvements in energy storage technologies and is actively working with several lithium-ion cell manufacturers.

Tesla Roadster Has Safety Built In

For automakers the world over, keeping a vehicle's occupants safe is a key factor when designing a car, particularly a performance electric car like the Tesla Roadster.

Tesla Motors's designers and engineers have gone to great lengths to ensure that not only is the Tesla Roadster safe to drive, but also when charging the performance electric car, at home or on the road. Their goal is to not only meet, but to surpass the rigorous standards of the Federal Motor Vehicle Safety Standards, or FMVSS, as implemented by the National Highway Traffic Safety Administration (NHTSA).

"The Tesla Roadster will meet or exceed the FMVSS criteria, period," said Tesla Motors CEO Martin Eberhard. "From traditional items like seat belts, air bags, and anti-lock brakes, to EV-specific standards including electrical disconnect systems and color-coding high voltage wires, I expect this car to be a model for vehicles anywhere, electric or not."

NHTSA has a legislative mandate to issue Federal Motor Vehicle Safety Standards and Regulations to which vehicle manufacturers, like Tesla Motors, must conform and certify compliance. These Federal safety standards are regulations written in terms of minimum safety performance requirements for motor vehicles or equipment.

The FMVSS includes crash testing in order to fulfill the "occupant crash protection" standard (part 571, standard no. 208), which specifies performance requirements for the protection of vehicle occupants in crashes. Its purpose is to specify vehicle crashworthiness requirements in test crashes and static airbag deployment tests. This standard also specifies equipment guidelines for active and passive restraint systems.

The FMVSS also has standards specific to electric vehicles, including redundant, independent electrical disconnect systems, activated when the car's electrical system has been compromised by a ground fault, too much current, over- or under-voltage, overheating, liquid in the energy storage system (ESS) or in the case of an accident.

The Tesla Roadster's high voltage wires are color-coded in accordance with the FMVSS and feature double insulation. All high-voltage enclosures, including the ESS, feature tamper-proof packaging and warning labels.

The ESS, which provides power to the entire vehicle, features an independent safety system designed to disconnect power outside the ESS enclosure under a variety of detectable safety-threatening situations.

The Tesla Roadster comes complete with its Electric Vehicle Service Equipment (EVSE), a home-based charging system. The EVSE is only to be installed by a qualified electrical technician, and also features an automatic system to switch off the charging current prior to the removal of the charge lead connector. The home system also includes a smoke detector that will discontinue charging and cut off current to the charging system if smoke is detected. Similarly the home system is equipped with a system that detects any unusual tension on the charging cable (such as another car running over the cable) and will cut off current passing through the cable. The Tesla Roadster is equipped with a safety interface that prevents the car from being driven or even moving if the charging system is connected to the cars charge port.

More traditional vehicle safety apparatus include self-tensioning driver and passenger seat belts and shoulder straps. An audible alert and instrument panel warning light will activate in the event the driver's seat belt is unfastened while driver key is inserted.

Passive safety is provided through energy-absorbing structure built into the front and rear of the vehicle, also known as "crumple zones," and driver and passenger air bags are also standard. The passenger compartment is encompassed by extruded aluminum door beams and high level side beams.

Enhanced safety features include the antilock braking system and traction control. An interlock system prevents the car from rolling or accidentally accelerating until the driver has turned the car on, fastened the seat belt, and shifted from park.

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