What Ever Happened to the Hub Motor BEV?
Back in 1900, Porsche introduced a hybrid vehicle that featured two gas engines and a wheel hub electric motor. Since then, the hub motor has been largely relegated to low-powered, two-wheel electric vehicles. In the mid-2000’s, when battery electric and fuel cell vehicles were starting to show up on the radar (again), the wheel hub motor was seen again, but was clearly not headed for prime time. While shown in numerous car prototypes, wheel hub motors have never really got off the ground in passenger cars. In 2008, Michelin introduced a concept called the WILL with the bold claim that their hub motors would be on the road by 2010. With the introduction of the hand-built Venturi Volage electric sports car (to launch in 2012 for an unspecified price), the Michelin system has found an on-ramp to the road. In 2009, Daimler announced that hub motors were powering their Citaro fuel cell bus. And most recently, in Japan, SIM-Drive Corporation claims to have developed a BEV that has a range of 207 miles using hub motors that they will launch in 2013.
One of the major challenges facing hub motors is the increase in the mass of the wheel. The increased mass at the wheel causes increased inertia which can wreck havoc with vehicle stability, particularly on corners. Additionally, any shock to the wheel (like the pothole that gave me a flat tire not too long ago) could be transferred to the hub mounted motor and is substantially more likely to cause catastrophic damage. Finally, the complexity of the motor controls is substantially higher in hub motors than in chassis mounted motors. Some of this complexity is because turning wheels rotate at different speeds (the wheel inside the turn rotates slower than the outside wheel). While this problem is tacitly solved by the differential gear in vehicles with axles, it must be computerized for hub motor vehicles.
Hub motors do have the advantage of being able to act much more independently to changing road conditions. Plus, removing the motor from the potential passenger compartment will increase vehicle package flexibility (more storage or passenger room). The aforementioned complexity of the system can be overcome, as well, albeit at a cost. Perhaps most importantly to the BEV industry, the elimination of the axle or transmission makes the hub motors more efficient than a chassis mounted electric motor.
When I have asked about the hub motor design, most say the design remains too expensive, potentially lower powered, and unreliable for mainstream vehicles. Hub motors are smaller than typical chassis motors (30-50kW vs. 80+kW), which has raised concerns with the amount of power available to the vehicle. The cost of using two 40 kW wheel hub motors is higher than using one 80 kW motor (plus the resulting power is not a simple additive equation as implied by that statement). Add to that a more expensive control unit for the hub motors and you have a drivetrain that pushes the boundaries of cost effectiveness.
That said, hub motors aren’t dead yet. As the SIM-Drive and Michelin Active Wheel shows, there is work that is still being done with new lighter weight and smaller motors. These advances are solving many of the challenges facing hub motors in passenger cars and will likely result in a mainstream hub motor vehicle, though I don’t expect you will see it in your neighborhood showroom this decade.
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