Introduction to How GM's Hy-wire Works

Cars are immensely complicated machines, but when you get down to it, they do an incredibly simple job. Most of the complex stuff in a car is dedicated to turning wheels, which grip the road to pull the car body and passengers along. The steering system tilts the wheels side to side to turn the car, and brake and acceleration systems control the speed of the wheels.

Given that the overall function of a car is so basic (it just needs to provide rotary motion to wheels), it seems a little strange that almost all cars have the same collection of complex devices crammed under the hood and the same general mass of mechanical and hydraulic linkages running throughout. Why do cars necessarily need a steering column, brake and acceleration pedals, a combustion engine, a catalytic convertor and the rest of it?

According to many leading automotive engineers, they don't; and more to the point, in the near future, they won't. Most likely, a lot of us will be driving radically different cars within 20 years. And the difference won't just be under the hood -- owning and driving cars will change significantly, too.

In this article, we'll look at one interesting vision of the future, General Motor's remarkable concept car, the Hy-wire. GM may never actually sell the Hy-wire to the public, but it is certainly a good illustration of various ways cars might evolve in the near future.

The original AUTOnomy concept car The drivable update of the AUTOnomy, the Hy-wire

Photo courtesy General Motors Photo courtesy General Motors

Hy-wire Basics

The original AUTOnomy concept car The drivable update of the AUTOnomy, the Hy-wire

Photo courtesy General Motors Photo courtesy General Motors

Two basic elements largely dictate car design today: the internal combustion engine and mechanical and hydraulic linkages.

If you've ever looked under the hood of a car, you know an internal combustion engine requires a lot of additional equipment to function correctly. No matter what else they do with a car, designers always have to make room for this equipment.

The same goes for mechanical and hydraulic linkages. The basic idea of this system is that the driver maneuvers the various actuators in the car (the wheels, brakes, etc.) more or less directly, by manipulating driving controls connected to those actuators by shafts, gears and hydraulics. In a rack-and-pinion steering system, for example, turning the steering wheel rotates a shaft connected to a pinion gear, which moves a rack gear connected to the car's front wheels. In addition to restricting how the car is built, the linkage concept also dictates how we drive: The steering wheel, pedal and gear-shift system were all designed around the linkage idea.

The Hy-wire has wheels, seats and windows like a conventional car, but the similarity pretty much ends there. There is no engine under the hood and no steering wheel or pedals inside.

Photo courtesy General Motors

The defining characteristic of the Hy-wire (and its conceptual predecessor, the AUTOnomy) 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.

Photo courtesy General Motors

Illustration of the AUTOnomy's body attachment concept

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 (28 cm) aluminum chassis -- also known as the skateboard -- 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.

But the coolest thing about this design is that it lets you remove the entire passenger compartment and replace it with a different one. If you want to switch from a van to a sports car, you don't need an entirely new car; you just need a new body (which is a lot cheaper).

You can always switch back when you need the van again. The logistics of switching is still unclear -- if the idea catches on, there might be special switching stations where you can keep your different car bodies, or there might be a way for drivers to switch the car bodies themselves in their own garage.

The hydrogen tanks and fuel-cell stack in the Hy-wire

Photo courtesy General Motors

Hy-Wire's Hydrogen Power

In a hydrogen fuel cell, a catalyst breaks hydrogen molecules in the anode into protons and electrons. The protons move through the exchange membrane, toward the oxygen on the cathode side, and the electrons make their way through a wire between the anode and cathode. On the cathode side, the hydrogen and oxygen combine to form water. Many cells are connected in series to move substantial charge through a circuit.

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.

The basic idea is to use a catalyst to split a hydrogen molecule (H2) into two H protons (H+, positively charged single hydrogen atoms) and two electrons (e-). Oxygen on the cathode (positively charged) side of the fuel cell draws H+ ions from the anode side through a proton exchange membrane, but blocks the flow of electrons. The electrons (which have a negative charge) are attracted to the protons (which have a positive charge) on the other side of the membrane, but they have to move through the electrical circuit to get there. The moving electrons make up the electrical current that powers the various loads in the circuit, such as motors and the computer system. On the cathode side of the cell, the hydrogen, oxygen and free electrons combine to form water (H2O), the system's only emission product. (See How Fuel Cells Work for more information.)

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.

Another view of the Hy-wire

Photo courtesy General Motors

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.

GM's diagram of the AUTOnomy design

Courtesy General Motors

Hy-Wire Computer Control

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 Hy-wire's X-drive The X-drive can slide to either side of the vehicle.

Photo courtesy General Motors Photo courtesy General Motors

The Hy-wire's X-drive The X-drive can slide to either side of the vehicle.

Photo courtesy General Motors Photo courtesy General Motors

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 the 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 from video 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.

GM concept of the AUTOnomy with and without a body attached

Photo courtesy General Motors Photo courtesy General Motors

GM concept of the AUTOnomy with and without a body attached

Photo courtesy General Motors Photo courtesy General Motors

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.

The other major hurdle for this type of car is figuring out energy-efficient methods for producing, transporting and storing hydrogen for the onboard fuel-cell stacks. With the current state of technology, actually producing the hydrogen fuel can generate about as much pollution as using gasoline engines, and storage and distribution systems still have a long way to go (see How the Hydrogen Economy Works for more information).

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.

For more information about the Hy-wire and other emerging automotive technologies, check out the links on the next page.