Understanding the Functionality of GM's Hy-wire

Automobiles are highly intricate devices, yet their primary function is remarkably straightforward. The majority of their complexity revolves around rotating wheels, which grip the road to propel the vehicle and its occupants forward. The steering mechanism adjusts the wheels' direction to navigate turns, while the braking and acceleration systems manage the wheels' speed.

Considering that a car's fundamental purpose is so simple (it merely needs to generate rotary motion for the wheels), it's somewhat surprising that nearly all vehicles feature the same array of intricate components packed under the hood and the same network of mechanical and hydraulic connections throughout. Why must cars include a steering column, brake and accelerator pedals, a combustion engine, a catalytic converter, and other such elements?

Many top automotive engineers argue that they don't; and more importantly, in the near future, they won't. It's highly probable that within two decades, many of us will be driving cars that are fundamentally different. These changes won't be limited to what's under the hood -- the experience of owning and operating vehicles will also undergo significant transformations.

In this article, we explore an intriguing vision of the future through General Motors' innovative concept car, the Hy-wire. While GM may never release the Hy-wire to consumers, it serves as an excellent example of how automobiles could transform in the coming years.

Hy-wire Basics

Modern car design is primarily influenced by two fundamental components: the internal combustion engine and mechanical and hydraulic linkages.

When you peek under a car's hood, you'll notice that an internal combustion engine relies on numerous additional components to operate efficiently. Designers must always allocate space for these elements, regardless of the car's other features.

Mechanical and hydraulic linkages follow a similar principle. This system operates on the concept that the driver directly controls the car's actuators (such as the wheels and brakes) by using driving controls connected via shafts, gears, and hydraulics. For instance, in a rack-and-pinion steering system, rotating the steering wheel turns a shaft linked to a pinion gear, which then moves a rack gear attached to the front wheels. This linkage system not only influences the car's construction but also shapes how we drive, with the steering wheel, pedals, and gear-shift system all designed around this concept.

The Hy-wire's standout feature (shared with its predecessor, the AUTOnomy) is the absence of these traditional components. Instead of an engine, it uses a fuel cell stack to power an electric motor connected to the wheels. Replacing mechanical and hydraulic linkages is a drive-by-wire system, where a computer controls the components that move the wheels, engage the brakes, and more, based on input from an electronic controller. This system is also used in modern fighter jets and many commercial planes.

Photo courtesy General Motors

Visualization of the AUTOnomy's body attachment concept

These two changes lead to a completely new kind of vehicle and a unique driving experience. The car lacks a steering wheel, pedals, and an engine compartment. Instead, all the components that propel the car are contained within an 11-inch-thick (28 cm) aluminum chassis, referred to as the skateboard, located at the car's base. Everything above the chassis is focused solely on driver control and passenger comfort.

This design eliminates the need for passengers and the driver to sit behind a cluster of machinery. Instead, the Hy-wire features a large front windshield, offering everyone an unobstructed view of the road. The floor of the fiberglass-and-steel cabin is entirely flat, providing ample legroom for all seats. Placing most of the vehicle's weight in the lower section also enhances safety by reducing the risk of tipping over.

The most impressive aspect of this design is the ability to detach the entire passenger compartment and replace it with a different one. Switching from a van to a sports car doesn't require a whole new vehicle; you only need a new body, which is significantly more affordable.

You can easily revert to the van configuration when needed. The logistics of swapping bodies remain unclear, but if the concept gains popularity, specialized stations might emerge for storing different car bodies, or drivers might be able to switch bodies themselves in their garages.

Hy-Wire's Hydrogen Power

In a hydrogen fuel cell, a catalyst splits hydrogen molecules at the anode into protons and electrons. Protons pass through the exchange membrane toward the oxygen on the cathode side, while electrons travel through a wire connecting the anode and cathode. On the cathode side, hydrogen and oxygen combine to form water. Multiple cells linked in series generate enough charge to power a circuit.

The "Hy" in Hy-wire represents hydrogen, the primary fuel for a fuel cell system. Similar to batteries, fuel cells have positive and negative terminals that drive electrical charge through a connected circuit. They also produce electricity through a chemical reaction. However, unlike batteries, fuel cells can be continuously recharged by supplying chemical fuel—hydrogen from an onboard tank and oxygen from the air.

The core concept involves using 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 pulls H+ ions from the anode side through a proton exchange membrane, while blocking electrons. The negatively charged electrons are drawn to the positively charged protons on the other side of the membrane but must travel through the electrical circuit to reach them. This movement of electrons creates the electrical current that powers components like motors and the computer system. On the cathode side, hydrogen, oxygen, and electrons combine to form water (H2O), the system's only byproduct. (For more details, see How Fuel Cells Work.)

A single fuel cell generates minimal power, so multiple cells are combined into a stack to produce usable energy. The Hy-wire's fuel-cell stack consists of 200 cells connected in series, delivering 94 kilowatts of continuous power and 129 kilowatts at peak output. The stack, roughly the size of a PC tower, is cooled by a traditional radiator system powered by the fuel cells.

This system provides DC voltage ranging from 125 to 200 volts, depending on the circuit's load. The motor controller increases this to 250 to 380 volts and converts it to AC current to drive the three-phase electric motor that turns the wheels (similar to systems in conventional electric cars).

The electric motor applies torque to the front wheel axle to rotate the front wheels. The control unit adjusts the car's speed by varying the power supplied to the motor. At full power, the motor's rotor spins at 12,000 revolutions per minute, generating 159 pound-feet of torque. A single-stage planetary gear with an 8.67:1 ratio amplifies the torque, delivering up to 1,375 pound-feet to each wheel. This enables the 4,200-pound (1,905-kg) car to reach 100 miles per hour (161 kph) on flat terrain. Smaller electric motors handle steering, while electrically controlled brake calipers stop the car.

The hydrogen fuel required to power this system is stored in three cylindrical tanks, weighing approximately 165 pounds (75 kilograms) combined. These tanks are constructed from a specialized carbon composite material, designed to withstand the high pressure of hydrogen gas. The current model's tanks hold about 4.5 pounds (2 kg) of hydrogen at 5,000 pounds per square inch (350 bars). Future models aim to increase the pressure to 10,000 pounds per square inch (700 bars), enhancing fuel capacity and extending the vehicle's range.

GM's long-term goal is to reduce the size of the fuel-cell stack, motors, and hydrogen-storage tanks, shrinking the chassis thickness from 11 inches to 6 inches (15 cm). This more streamlined "skateboard" design would offer even greater flexibility in body customization.

Hy-Wire Computer Control

The Hy-wire's central computer, located in the middle of the chassis, acts as its "brain." It sends electronic commands to the motor control unit to adjust speed, the steering mechanism to navigate, and the braking system to slow or stop the vehicle.

At the chassis level, the computer manages all driving and power functions. However, it receives instructions from the driver via the car body's electronics. The connection is made through a single universal docking port, functioning similarly to a USB port on a computer. This port transmits command signals from the driver's controller to the computer and feedback signals back to the controller. It also supplies power to the body's electronics. Ten physical linkages secure the body to the chassis.

The driver's control unit, known as the X-drive, resembles a video game controller more than a traditional steering wheel and pedal setup. It features two ergonomic grips on either side of a small LCD screen. Steering involves gliding the grips up and down gently, eliminating the need for continuous wheel rotation. Acceleration is achieved by twisting either grip, similar to a motorcycle throttle, while braking is done by squeezing either grip.

Advanced motion sensors, akin to those in high-quality computer joysticks, convert the driver's movements into digital signals for the central computer. The controller includes buttons for shifting between neutral, drive, and reverse, along with a starter button to power the car. With everything controlled by hand, your feet are free for other activities (picture using a foot massager during your daily commute).

A 5.8-inch (14.7-cm) color monitor in the controller's center displays standard dashboard information like speed, mileage, and fuel level. It also provides rear-view images from video cameras mounted on the car's sides and back, replacing traditional mirrors. A second monitor on the driver's console shows stereo, climate control, and navigation details.

Since the X-drive doesn't directly control any car components, it can be positioned anywhere in the passenger area. In the current Hy-wire sedan, the X-drive can pivot to either front seat, allowing for driver changes without leaving the car. It can also be adjusted vertically for comfort or moved aside when not in use.

One of the most impressive features of the drive-by-wire system is its ability to fine-tune vehicle handling without altering mechanical parts. Adjusting steering, acceleration, or brake sensitivity simply requires updating the computer software. In future drive-by-wire cars, drivers will likely customize controls to their preferences with a few button presses, similar to adjusting a car seat today. This system could also store individual control settings for each family member.

The primary concern with drive-by-wire vehicles is safety. Without a physical link between the driver and the car's mechanical systems, an electrical failure could result in complete loss of control. To make this system practical, drive-by-wire cars will need backup power supplies and redundant electronic connections. With proper safety measures, these cars could be as safe as, or even safer than, traditional vehicles, as the central computer can monitor driver inputs. Another challenge is integrating sufficient crash protection.

Another significant challenge is developing energy-efficient methods for producing, transporting, and storing hydrogen for the onboard fuel-cell stacks. Current technology often results in hydrogen production generating pollution levels comparable to gasoline engines, and storage and distribution systems are still in their infancy (see How the Hydrogen Economy Works for more details).

Will the Hy-wire ever be available for purchase? General Motors aims to release a production model by 2010, provided they resolve key fuel and safety concerns. Even if this deadline isn't met, GM and other automakers are committed to transitioning from conventional cars to computerized, eco-friendly alternatives. Major changes to highway travel are expected within the next few decades.