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GM's Hy-wire

The defining characteristic of the Hy-wire is that it doesn't have either of these two things. Instead of an engine, it has a fuel cell stack, which powers an electric motor connected to the wheels. Instead of mechanical and hydraulic linkages, it has a drive by wire system -- a computer actually operates the components that move the wheels, activate the brakes and so on, based on input from an electronic controller. This is the same control system employed in modern fighter jets as well as many commercial planes. 

The result of these two substitutions is a very different type of car -- and a very different driving experience. There is no steering wheel, there are no pedals and there is no engine compartment. In fact, every piece of equipment that actually moves the car along the road is housed in an 11-inch-thick aluminum chassis at the base of the car. Everything above the chassis is dedicated solely to driver control and passenger comfort.

This means the driver and passengers don't have to sit behind a mass of machinery. Instead, the Hy-wire has a huge front windshield, which gives everybody a clear view of the road. The floor of the fiberglass-and-steel passenger compartment can be totally flat, and it's easy to give every seat lots of leg room. Concentrating the bulk of the vehicle in the bottom section of the car also improves safety because it makes the car much less likely to tip over.

The "Hy" in Hy-wire stands for hydrogen, the standard fuel for a fuel cell system. Like batteries, fuel cells have a negatively charged terminal and a positively charged terminal that propel electrical charge through a circuit connected to each end. They are also similar to batteries in that they generate electricity from a chemical reaction. But unlike a battery, you can continually recharge a fuel cell by adding chemical fuel -- in this case, hydrogen from an onboard storage tank and oxygen from the atmosphere.

One fuel cell only puts out a little bit of power, so you need to combine many cells into a stack to get much use out of the process. The fuel-cell stack in the Hy-wire is made up of 200 individual cells connected in series, which collectively provide 94 kilowatts of continuous power and 129 kilowatts at peak power. The compact cell stack (it's about the size of a PC tower) is kept cool by a conventional radiator system that's powered by the fuel cells themselves.

This system delivers DC voltage ranging from 125 to 200 volts, depending on the load in the circuit. The motor controller boosts this up to 250 to 380 volts and converts it to AC current to drive the three-phase electric motor that rotates the wheels (this is similar to the system used in conventional electric cars).

The electric motor's job is to apply torque to the front wheel axle to spin the two front wheels. The control unit varies the speed of the car by increasing or decreasing the power applied to the motor. When the controller applies maximum power from the fuel-cell stack, the motor's rotor spins at 12,000 revolutions per minute, delivering a torque of 159 pound-feet. A single-stage planetary gear, with a ratio of 8.67:1, steps up the torque to apply a maximum of 1,375 pound-feet to each wheel. That's enough torque to move the 4,200-pound (1,905-kg) car 100 miles per hour (161 kph) on a level road. Smaller electric motors maneuver the wheels to steer the car, and electrically controlled brake calipers bring the car to a stop.

The gaseous hydrogen fuel needed to power this system is stored in three cylindrical tanks, weighing about 165 pounds (75 kilograms) total. The tanks are made of a special carbon composite material with the high structural strength needed to contain high-pressure hydrogen gas. The tanks in the current model hold about 4.5 pounds (2 kg) of hydrogen at about 5,000 pounds per square inch (350 bars). In future models, the Hy-wire engineers hope to increase the pressure threshold to 10,000 pounds per square inch (700 bars), which would boost the car's fuel capacity to extend the driving range.

Ultimately, GM hopes to get the fuel-cell stack, motors and hydrogen-storage tanks small enough that they can reduce the chassis thickness from 11 inches to 6 inches (15 cm). This more compact "skateboard" would allow for even more flexibility in the body design.

The Hy-wire's "brain" is a central computer housed in the middle of the chassis. It sends electronic signals to the motor control unit to vary the speed, the steering mechanism to maneuver the car, and the braking system to slow the car down.

At the chassis level, the computer controls all aspects of driving and power use. But it takes its orders from a higher power -- namely, the driver in the car body. The computer connects to the body's electronics through a single universal docking port. This central port works the same basic way as a USB port on a personal computer: It transmits a constant stream of electronic command signals from the car controller to the central computer, as well as feedback signals from the computer to the controller. Additionally, it provides the electric power needed to operate all of the body's onboard electronics. Ten physical linkages lock the body to the chassis structure.

The driver's control unit, dubbed the X-drive, is a lot closer to a video game controller than a conventional steering wheel and pedal arrangement. The controller has two ergonomic grips, positioned to left and right of a small LCD monitor. To steer the car, you glide the grips up and down lightly -- you don't have to keep rotating a wheel to turn, you just have to hold the grip in the turning position. To accelerate, you turn either grip, in the same way you would turn the throttle on a motorcycle; and to brake, you squeeze either grip. 

Electronic motion sensors, similar to the ones in high-end computer joysticks, translate this motion into a digital signal the central computer can recognize. Buttons on the controller let you switch easily from neutral to drive to reverse, and a starter button turns the car on. Since absolutely everything is hand-controlled, you can do whatever you want with your feet (imagine sticking them in a massager during the drive to and from work every day).

The 5.8-inch (14.7-cm) color monitor in the center of the controller displays all the stuff you'd normally find on the dashboard (speed, mileage, fuel level). It also gives you rear-view images fromvideo cameras on the sides and back of the car, in place of conventional mirrors. A second monitor, on a console beside the driver, shows you stereo, climate control and navigation information.

Since it doesn't directly drive any part of the car, the X-drive could really go anywhere in the passenger compartment. In the current Hy-wire sedan model, the X-drive swings around to either of the front two seats, so you can switch drivers without even getting up. It's also easy to adjust the X-drive up or down to improve driver comfort, or to move it out of the way completely when you're not driving.

One of the coolest things about the drive-by-wire system is that you can fine-tune vehicle handling without changing anything in the car's mechanical components -- all it takes to adjust the steering, accelerator or brake sensitivity is some new computer software. In future drive-by-wire vehicles, you will most likely be able to configure the controls exactly to your liking by pressing a few buttons, just like you might adjust the seat position in a car today. It would also be possible in this sort of system to store distinct control preferences for each driver in the family.

The big concern with drive-by-wire vehicles is safety. Since there is no physical connection between the driver and the car's mechanical elements, an electrical failure would mean total loss of control. In order to make this sort of system viable in the real world, drive-by-wire cars will need back-up power supplies and redundant electronic linkages. With adequate safety measures like this, there's no reason why drive-by-wire cars would be any more dangerous than conventional cars. In fact, a lot of designers think they'll be much safer, because the central computer will be able to monitor driver input. Another problem is adding adequate crash protection to the car.

So will we ever get the chance to buy a Hy-wire? General Motors says it fully intends to release a production version of the car in 2010, assuming it can resolve the major fuel and safety issues. But even if the Hy-wire team doesn't meet this goal, GM and other automakers are definitely planning to move beyond the conventional car sometime soon, toward a computerized, environmentally friendly alternative. In all likelihood, life on the highway will see some major changes within the next few decades.

Hy-wire Numbers

* Top speed: 100 miles per hour (161 kph) -----* Weight: 4,185 pounds (1,898 kg) -----* Chassis length: 14 feet, 3 inches (4.3 meters) -----* Chassis width: 5 feet, 5.7 inches (1.67 meters)

* Chassis thickness: 11 inches (28 cm) -----* Wheels: eight-spoke, light alloy wheels. -----* Tires: 20-inch (51-cm) in front and 22-inch (56-cm) in back

* Fuel-cell power: 94 kilowatts continuous, 129 kilowatts peak ------* Fuel-cell-stack voltage: 125 to 200 volts -----* Motor: 250- to 380-volt three-phase asynchronous electric motor

* Crash protection: front and rear "crush zones" (or "crash boxes") to absorb impact energy -----* Related GM patents in progress: 30 -----* GM team members involved in design: 500+