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.
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.
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.
E-Traction’s key innovation, Heinen says, is in the design of its wheel motor. Typically, electric motors are designed to spin much faster than the rate of the wheels in order to generate the desired power. But such motors require gears to step down the revolutions per minute, which adds complexity and decreases efficiency. The company has eliminated the need for these gears by designing a large-diameter motor that can deliver the needed torque at low RPMs.
In-wheel motors have met with limited success in the past. In part, that’s because it’s been difficult to coordinate motors that have no mechanical connection to each other, a problem that the company says it’s solved by developing a proprietary electronic control system. But there could be remaining issues. Putting the motors in the wheels places larger demands on the suspension (it has to be stronger to hold on to the much heavier wheels) and can make the motor and electronics more vulnerable to damage, both of which can reduce reliability. “There’s little between the wheel and potholes,” says Bill Van Amburg, senior vice president at Calstart. Dan Pederson, a researcher at the National Renewable Energy Laboratory, says that the large in-wheel motor is likely very expensive, which could make the system hard to justify without government subsidies.
Heinen says that four years of testing of a prototype system have convinced the company that the reliability issues have been addressed, and that costs for the motor may come down with larger-scale production. In addition to hybrid buses, e-Traction is working on hybrid garbage trucks and is retrofitting a Mercedes G SUV in an effort to move into the passenger-car market.