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.