Saturday, June 11, 2011

Hub-motors: how a hybrid can make sense.

Electric drive is often reported as better because, despite the use of heavy batteries, can be applied as a weight-saving feature eliminating the components associated to a mechanical transmission when hub-motors are used, and also allowing a smaller (then lighter) internal-combustion engine to provide all the kinetic energy needed to move a generator to provide electric power to the drivelines, like the Audi A1 e-tron that uses a 250cc 20HP Wankel engine to move the generator and 4 hub-motors delivering 61hp to the wheels like a 1.0L average Otto engine would do, but the 145Nm torque range is closer to a 1.6L.
Another benefit is the flexibility to the floorpan arrangement due to the absence of a gearbox (this is due to their reversibility, not requiring a reverse gear) and a driveshaft tunnel in the middle of the vehicle, allowing a lower boarding platform, extremely useful when handling heavy loads or there is an elder or a wheelchair user getting inside the vehicle. These advantages are well-known by the hybrid/electric vehicles enthusiasts but we don't see it so widespread as it's supposed to be. For example, would be the best setup for a serial-hybrid such as the Chevrolet Volt. Even the acclaimed parallel-hybrid Toyota Prius, cult-followed by a lot of tree-huggers just for being the first mass-produced hybrid car, could have some benefits from a quite simple change in its current driveline, replacing the electric motor bolted to the front transaxle for a pair of hub-motors in the rear wheels. We must also consider that usually a parallel-hybrid relies on the electric drive only in low speed, such as city traffic, while in highway the electric driveline means just a dead weight. The hub-motors setup, by the other side, while providing traction would even allow the internal combustion engine to have a lighter work, then needing less fuel.
When a front-wheel drive vehicle accelerates, there is always a weight transference to the rear axle due to gravitational acceleration, then, plus the advantage of the friction losses reduction, the rear electric drive would compensate for the weight distribution unfavorable to the traction, improving the drag and then reducing the energy amount needed for the vehicle to start moving. Driving uphill the traction enhancement from this system is far more sensible. Also, with all-wheel drive the stability in higher speeds is enhanced...

In partnership with Protean Electric, GM Europe is developing a hybrid version of the Opel/Vauxhall Vivaro van, a major developer of the hub-motors technology.

While the current non-hybrid versions are fitted with an average FWD driveline, some of the prototypes when fitted with a parallel-hybrid setup have hub-motors added to the rear axle. This system can even be retrofitted into existing vehicles. Considering again the benefits of the auxiliary rear-wheel drive, would also be helpful either in snow days or unpaved roads.

1 comment:

  1. This auxiliary hub-motors scheme seems to be great. I don't know what's wrong with people from the auto industry who believe the electric motor must be bolted to the same transmission while it doesn't need it.


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