Electronic Design

Leading The Charge

The popularity of electric vehicles rises as battery and fuel-cell technologies improve.

Forget about those old gas guzzlers. Electric and hybrid vehicles are hitting the streets. A driving factor is consumer environmental concern, but improved technology has really made the difference—especially in motors, motor control, batteries, and power management. Now, the biggest challenge lies in matching consumer expectations and use to new products that don't operate like fossil-fuel-powered vehicles (see table).

Motor companies are meeting that challenge. Tesla Motors' all-electric Roadster rivals high-performance two-seaters from Porsche, Ferrari, and other sports car companies (Fig. 1). It does 0 to 60 mph in four seconds and has a range of 250 miles/charge, getting the equivalent of 135 mpg.

Tesla needs to check its rear-view mirror, though, for concept cars looking to steal its thunder—such as the Chevy Volt, which can run on electricity or biodiesel (see "Volt Charges Up The Crowd In Detroit") and the hydrogen-powered Honda FCX. And when it comes to hybrids, Toyota's popular Prius embodies the state of the art (see "Setting The Standard For Hybrid Cars").

It's Electric
Tesla's 2600-lb Roadster uses a custom 77-lb motor (Fig. 2). The three-phase, four-pole ac induction motor redlines at 13,500 rpm. It has a peak power of 248 hp (185 kW). Also, it's tied to a clutchless, manual two-speed gearbox that gives it that four-second 0- to 60-mph takeoff.

The Roadster doesn't have a reverse gear. Instead, it runs the motor in the opposite direction. It's probably the only high-performance vehicle that runs as fast in reverse as it does in drive. The motor also is used for regenerative braking.

Key to Tesla's system is the ESS (energy storage system). The 950-lb ESS packs an array of lithium-ion (Li-ion) batteries and a host of electronics, including 13 microprocessors that provide redundant support. Each 831 Li-ion cell is monitored and can be bypassed. The system is liquid-cooled so the electronics can optimize the thermal characteristics. It also checks for smoke, excessive heat, and other failure modes.

Drivers can charge the ESS with 220 V and Tesla's fast, self adjusting, onboard charger (Fig. 3). A full recharge takes 3.5 hours. Drivers also can use a standard 110-V circuit, but that may take as long as 12 hours, depending upon the amount of current that can be drawn. The breakaway charging cable has a number of safety interlock features. If you're looking to shut power companies completely out of your driving, check out solar cells for charging, if they can get about 56 kW.

David Vesrini of Tesla Motors estimates that the ESS locked up about 40% of the research dollars for the Roadster. He also notes that Li-ion batteries are improving at a rate of about 8% per year. This means that when the ESS and motor bearings need to be replaced at 100,000 miles, Roadster owners will essentially upgrade to a less expensive vehicle with even longer range.

But when it comes to R&D for electric vehicles, the overall economy and efficiency from source to use must be considered (Fig. 4). Even accounting for the power it draws from the grid, the Roadster tops its hybrid competitors with a 1.14km/MJ well-to-wheel efficiency. In addition to recharge time and travel distance, the Roadster leads the way in performance (Fig. 5), which will be a major selling point.

Starting at $92,000, the Roadster is expensive compared to most automobiles on the road. But it's a bargain compared to its competition. Code named DarkStar, the Roadster is just becoming available in limited quantities. And it's just the beginning for the company, whose WhiteStar project will be a four-door, five-passenger vehicle with a starting price that's expected to be half the Roadster's. Look for it in 2010 as Tesla Motors moves into the mainstream.

Hydrogen And Hybrids
Fully electric vehicles have been around for some time, but they've had less commercial success than energy-efficient vehicles that incorporate fossil fuels. Ethanol may get a lot of press as a replacement for fossil fuels, but some companies are pushing hydrogen instead, especially in electric vehicles.

Unfortunately, the use of hydrogen is comparable to a storage system like a battery. Oil and coal have energy that can be released through oxidation. It takes energy to mine and process these substances, usually at a fraction of the energy they can deliver.

On the other hand, hydrogen must be obtained from other means, like electrolyzing water. This takes more energy than is released when the hydrogen is utilized in a fuel cell. The hydrogen model may only be a more efficient storage system than some batteries.

Still, many prototypes are being built around the assumption that hydrogen will be readily available. For example, Honda's FCX concept car uses a fuel cell that runs on hydrogen (Fig. 6). The hydrogen is stored in a high-pressure tank, and the system uses an ultracapacitor to even out the power requirements as the engine accelerates or when regenerative braking generates power.

Hybrids, however, are still the fuel-efficient vehicle-of-choice since they can meet buyers' range and performance requirements, as is evidenced by the Toyota Prius. But unlike Honda's Civic and Accord hybrids, the Prius starts with its electric motor and uses the gas engine as backup, essentially making it an electric vehicle. In fact, some Prius owners have hacked their rides to run exclusively on batteries.

The Volt is another interesting hybrid because it is strictly an electric vehicle for short hauls under 40 miles, but uses its fossil-fuel engine to charge the batteries for longer hauls. The engine can utilize a range of fuels.

Design Challenges
A number of challenges remain for both hybrids and fully electric vehicles. Capacity alone is not enough. The technology should require minimal maintenance. Additionally, it must safely survive accidents. These systems need to be rugged and able to endure a wide range of temperatures as well.

Hybrid vehicles tend to have an intricate mechanical design that's more complex than fully electric solutions like the Roadster. Their system redundancy, energy storage systems, and electrical engine control are only somewhat similar. While cars don't balance on two wheels, a comparable solution can be found in the fully electric single-passenger vehicle, Dean Kamen's Segway Human Transporter (Fig. 7; see "Smart Motion Makes For A Smarter Design,").

Since it's also a robotics platform (see "Segway's Concept Centaur: Computer Controlled Mobility Leads To New Concepts,"), its design addresses similar challenges faced by electric and hybrid vehicle designers.

For example, the Segway platforms employ numerous processors for control (Fig. 8). A number of these processors, though, simply monitor the system. While this leads to better efficiency, safety, and reliability, the design can get complex because of the number of subsystems involved.

Multiprocessing programming challenges can be as difficult to solve as improving the performance and reliability of the energy storage system. Low-cost 8- and 16-bit microcontrollers are meeting the needs at this point, but the falling prices of 32-bit platforms will change transportation's architectural landscape over the next few years.

SEGWAY TRANSPORTER (see figure)

Style: one person standing

Range: 16 to 24 miles

Speed: 12.5 mph

Carrying capacity: 100 to 260 lb

Motors: two brushless dc servomotors

Battery: two Saphion lithium-ion battery packs

Ground clearance: 3 in.

Turning radius: 0 in.

Weight: 105 lb

Footprint: 19 by 25 in.

Tires: 19-in. puncture-resistant tire

Price: starting at $5000


TESLA MOTORS ROADSTER (see figure)

Style: two-seat, open-top, rear-drive roadster

Top speed: more than 130 mph

Acceleration: 0-60mph in about 4 seconds

Range: approximately 250 miles/charge

Drivetrain: electric motor with two-speed electric-shift manual transmission with integral differential, traction control

Transmission:

Gear

Ratio

Overall

First

4.20:1

14.3:1

Second

2.17:1

7.4:1

Final

3.41:1

Motor: three-phase, four-pole electric motor; 248-hp peak (185 kW); redline at 13,500 rpm; regenerative "engine braking"

Front suspension: telescopic damper unit; anti-roll bar

Rear suspension: independent; upper and lower wishbone; co-axial coil spring/telescopic damper

Brakes: four-wheel disc brakes with ABS

Chassis: bonded extruded aluminum with four-wheel wishbone suspension

Dimensions: overall length 155.4 in.; width 73.7 in.; height 44.4 in.

Wheelbase: 92.6 in.

Curb weight: around 2690 lb

Lights: halogen low- and high-beam headlamp assemblies, LED tail lights

Energy storage system: lithium-ion batteries, 3.5-hour minimum recharge time, expected lifetime in excess of 100,000 miles, microprocessor controlled, heater for cold weather charging to 20°C

Security: PIN for operating the vehicle; valet mode restricts speed, acceleration, and distance

Price: starting at $92,000

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