5 Jul 2011

BMW's big, bold battery scooter

BMW recognises that political and social conditions are changing as the urban landcape becomes more crowded, increasing pressure on transport systems. One solution is to get commuters out of their cars and on to two wheels - but that requires an acceptable level of comfort, an upmarket image and “green” credentials.

And that means zero-emission battery power but, so far, electric scooters have all been short-range, inner-city shuttles due to their performance and range limitations. Now BMW has taken the concept a step further with its E-Scooter.

The Blue Propeller boffins started with a big ask: since the E-Scooter was intended to be used between the suburbs and the city centre, it had to be stable at freeway speeds - and able to maintain those speeds, two up, over a relatively long range by battery-bike standards.

Let's put numbers on that, OK? The bike had to be as quick as a 600cc Honda Silver Wing maxi-scooter up to 60km/h (not difficult - electric vehicles accelerate hardest off a standing start), with a range of at least 100km in real-world conditions; that's not so easy.

The batteries would also have to be charged from a 220-volt household socket rather than a three-phase, 380V industrial outlet, and preferably in three hours or less from completely flat. Like we said, a big ask.

The E-Scooter doesn't have a conventional frame. Instead, the aluminium battery casing - which also contains the battery cell-monitoring electronics - forms a monocoque tub with the steering head attached at the front and a single-sided swing-arm with laydown hydraulic damper at the rear.

The electric motor is too big to be hub-mounted (although BMW doesn't giver power figures), so it's at the back of the battery casing, driving the rear wheel via a toothed belt to a pulley mounted co-axially on the swing-arm pivot and thence via conventional roller chain to the rear-wheel sprocket.

When the E-Scooter decelerates or brakes, kinetic energy is recovered, increasing its range by 1-0-20 percent depending on driving profile.

All the electrical components are on top of the battery casing where they're easy to get to; an independent system also monitors data such as the temperature and voltage of the battery cells, both during travel and while charging.

The charging system includes a cable to connect to household supply, and an isolation monitor (in a house you'd call it an earth leakage breaker), a high-voltage indicator and a high-voltage distributor. There's also a DC-DC converter to run the scooter's 12-volt systems off the high-voltage main batteries.

The electric mote and power electronics are liquid-cooled while the scooter is being ridden, as is the 60V transformer during charging, with an electric coolant pump pushing the coolant through the radiator. Most electric cars also use liquid cooling for their batteries, but the E-Scooter concept uses air cooling to save space.

Battery bikes aren't new, but BMW is hoping to make them more viable in the real world with developments such as the big E-Scooter. It has also used its experience with battery-powered cars to set new safety standards for high-voltage (more than 60 volts) power systems in electric two-wheelers.

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