Wheel Motors to Drive Dutch Buses

The technology moves out of the lab and into commercial vehicles.

A company based in the Netherlands called e-Traction has developed a new kind of hybrid bus that uses in-wheel electric motors to improve efficiency and a GPS system to reduce pollution in congested areas of a city. The bus is a series hybrid: a diesel generator charges a battery, which in turn supplies electricity for two motors, one in each rear wheel. Thanks largely to its in-wheel motors, the bus can travel twice as far as a conventional bus on a liter of diesel, says Arend Heinen, who is both an engineer and spokesperson for the company. That translates into a reduction in fuel consumption of 50 percent. The company has been awarded contracts to retrofit seven commercial buses with its technology, with the first to be completed next month.

Hybrid commuter: One of the buses that e-Traction has retrofitted with its prototype hybrid system is shown above (top). The company is now retrofitting buses for commercial use. Behind the hubcap (bottom) is the motor that makes the new hybrid possible. Credit: E-Traction

In-wheel motors have been around for some time: they have been used in several concept cars and experimental, low-production vehicles. But with the exception of electric bicycles, the idea has never found its way into a mass-production vehicle, says John Boesel, the president and CEO of Calstart, a nonprofit based in Pasedena, CA. The use of e-Traction's system in commercial buses would be a step toward more widespread use.

As with other hybrid buses, thousands of which are already in use in the United States, e-Traction's design saves fuel by capturing energy from braking, using it to generate electricity that can later be employed for acceleration. The in-wheel motors confer additional savings by eliminating the need for a transmission, differential, and related mechanical parts. That reduces both the overall weight of the bus and energy losses due to friction. Hybrid buses typically see fuel-cosumption reductions of about 25 to 30 percent compared with conventional buses, but e-Traction's design offers 50 percent reduction. In certain conditions--at low speeds in frequent stop-and-go traffic--some other hybrid buses have seen similar fuel-economy improvements. The in-wheel motors can also improve traction by allowing precise control over each wheel, and they allow for greater flexibility in vehicle design since there is no need to mechanically link the wheels to an engine.

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The bus also incorporates a GPS-based system that changes the way that the bus operates in congested areas. In ordinary operation, the generator cycles on and off, keeping the battery at an optimal state of charge. But when the GPS system senses that the bus has entered an area of the city that usually sees a lot of traffic, the generator switches off to reduce emissions. The battery stores enough power to propel the bus for an hour without the generator running to recharge it.