October 1, 2003
by Jim Kerr
Hybrid vehicles in several configurations are being developed and tested by automobile manufacturers and aftermarket suppliers. Most of the designs use electric motors that are placed in series or parallel with the conventional gasoline engine to provide better fuel economy with lower emissions. These systems then drive through the transmission and the rest of the powertrain. Recently, General Motor’s Advanced Technology Center in Torrance, California showed off another way of building a hybrid vehicle. They refer to the technology used as “wheel hub motors”.
Instead of a central electric motor as used on some other designs, the wheel hub motors are individual motors placed within the circumference of the vehicle’s wheel. This concept has been used on the world’s biggest ore trucks, where a central generator supplies the electric power and one motor is used at each wheel to move the vehicle, but downsizing this concept to fit the needs of a standard passenger vehicle has been the challenge. General Motors has done this and has built a prototype electric S-10 pickup for evaluation. “This technology will lead to the industry’s first practical application of wheel hub motors for consumers” said Larry Burns, vice president of GM research and development and planning.
On this test vehicle, the hybrid electric system generates electrical power, which is sent through a controller to the motors at the wheels. On more conventional hybrids, up to 10% of the power created by an engine is lost as the energy is transferred through the gears and shafts of the powertrain to the wheels. The wheel hub motors eliminate much of the drivetrain for more efficient use of power.
There are several other advantages of wheel hub motors. Electric motors provide maximum torque immediately upon starting. With up to 60% more torque available from a stop, a vehicle can have sports car-like acceleration yet still provide the economy of a compact car.
While the GM experimental pickup utilises rear-wheel-drive only, four-wheel-drive is a natural extension of the technology. Wheel hub motors at each corner of the vehicle can be controlled to provide ultimate traction. Even towing the vehicle might be easier, because there is no mechanical drivetrain to disconnect.
On the S-10, each wheel hub motor is controlled by a Power Inverter Module (PIM) which converts the DC battery voltage to three-phase AC and a digital motor controller, also contained within the PIM. The two 25 kilowatt permanent magnet electric motors produce about 60% more torque than a conventional four cylinder gasoline S-10 and about the same torque as a V6 engine. Currently, the system has only motoring and regenerative braking designed into the controls but in the future ABS, traction control and stability control could all be programmed into the computer operating parameters for maximum vehicle stability.
Packaging the wheel hub motors was one challenge. For the experimental S-10 pickup, GM’s 25-kilowatt motors will fit within an 18 inch wheel. These sound like large wheels, but with current production vehicles using up to 20 inch wheels, an 18 inch wheel might be a common size in the future.
Having the motors in the wheels does open some other interesting packaging opportunities. Without the need for differentials, axle shafts, gearboxes and transmissions, the body can be designed for passenger or payload space. It also allows for more suspension travel, as there is no connection to other drivetrain parts. Imagine a vehicle where the wheels can be turned further, up to 90 degrees, for better manoeuvrability. Parking in tight spots becomes a simple task!
One small disadvantage of wheel hub motors is the unsprung weight they add to the wheels. Unsprung weight is any weight that is not supported by the vehicle’s springs such as wheels, tires, spindles, axles and brakes. Higher unsprung weight makes suspension control more difficult so handling typically suffers. GM’s wheel hub motors add only about 15 kilograms (33 lb.) to each wheel, which can be controlled relatively easily by changes in spring rates and suspension damping.
In the future, we will be driving many different types of vehicles than we now find currently on the market. Wheel hub motors might be one answer to having economical, low emission vehicles that still produce the performance we want.