by Roger Kaza
Today, re-inventing...wheels. The University of Houston presents this program about the machines that make our civilization run, and the people whose ingenuity created them.
For the non-mechanically-inclined, peering under the hood of a car is a mildly distressing prospect. All those hoses, pumps, pistons, fans, filters, cranks, cams. All that metal: transmission, driveshaft, differential, engine...all just to rotate a few tires. Isn't there a simpler way? Couldn't you just power the wheels directly with some sort of small motor? And why hasn't it been done?
Well, like most bright ideas, it has been done, and a long time ago. Hub motors, also called in-wheel motors, were first patented in the 1880s. In 1899 the Viennese firm of Lohner & Co. built a car powered by four battery-driven hub motors. It also had a gas engine, thus making it the world's first hybrid car. The mind that dreamed up such a mongrel belonged to none other than the legendary engineer Ferdinand Porsche. (And if you are a lucky driver of one of his later models, please don't tell me.)
Lohner-Porsche Mixte Hybrid car, circa 1900
A hub motor is exactly as it sounds: a wheel with a motor inside of it. The motor works like any other electric motor, except in reverse. Instead of the axle turning, like on your electric drill, the body of the motor turns, just as your drill would try to spin in circles if you clamped the bit into a vice. Hub motors, like all motors, are as powerful as their copper-coiled windings allow them to be, and most have gears to increase torque.
Hub (in-wheel) motor by Honda
A car powered by hub motors is much lighter than even the smallest, most efficient gas-powered vehicle. That translates to tremendous fuel efficiency...the equivalent of hundreds of miles per gallon. And having no engine or transmission, there's all kinds of extra space under the hood. But a few challenges remain. One is the issue of so-called unsprung weight. An ordinary car supports its load on a suspension of leaf springs, coils and shocks. Cars with hub motors, however, put considerable weight below the suspension, since the drivetrain and wheels are one and the same. On smooth pavement it's not an issue, but imagine that heavy hub motor diving into a pothole at 50 mph. Not a kind way to treat an electric device.
Another challenge is the same one that plagues all electric vehicles: batteries. Storing electricity is much harder than producing it. For example, the $100,000 Tesla roadster runs on something like 6,000 AA-sized lithium batteries, all of them temperamental. Most electric cars can't go much beyond 100 miles on a charge, though hub motors could theoretically double that. EV cars are best suited now for fleet vehicles or for commuting.
When it comes to technology, less is often more. Hub motors have already found widespread use in smaller, greener machines like bicycles or scooters. A small motor in the hub of a bike wheel is like having a phantom Lance Armstrong on board. The bike pedals like any normal bike, but the extra electric oomph erases hills and headwinds. Porsche's amazing hub-powered motorcar, so far ahead of its time, came factory-equipped with perhaps two wheels too many.
I'm Roger Kaza, at the University of Houston, where we're interested in the way inventive minds work.
Front wheel bicycle hub motor
Wikipedia entry on Lohner-Porsche Mixte Hybrid
Wikipedia entry on hub motors with list of concept cars using the technology.
YouTube video of Mitsubishi’s MIEV electric car powered by hub motors.
For more on hybrid cars, see Engines 2560.
Photographs courtesy Wiki commons. Bike hub photo by R. Kaza