hide announcement WIN your very own meteorite! Subscribe, gift or renew a subscription to Cosmos and automatically go into the draw – Shop now!

A cure for electric car range anxiety


Sales should rise rapidly thanks to the game-changing series hybrid design recently released to the market. Alan Finkel explains how it works.


iBWM xxxxxx
BMW

1. The car’s body panels are made from a carbon fibre plastic composite, a lightweight alternative to metal that offsets the weight of the battery pack.

2. Tall, skinny tyres have the same sized contact patch with the road as conventional tyres, but lower drag through the air.

3. Lithium batteries under the car’s floor hold 22 kilowatt hours of energy, typically enough to power the car on electricity alone for 150 km, BMW claims.

4. The i3’s 125 kilowatt electric motor, which revs at up to 11,400 rpm, drives the car’s rear wheels. Lifting off the accelerator pedal instantly switches the motor into generator mode, feeding energy back into the battery.

5. The tiny petrol-powered 25 kilowatt range-extender engine is not connected to the wheels. Its sole job is to charge the battery.

In 1900, America’s roads resonated with the sound of whirring electrical cars, only occasionally jolted by the roar of a petrol engine. Fifteen years later the orchestration reversed: electric cars had vanished. A conspiracy by the petroleum trade? No. It was the buyers. They suffered from range anxiety: a fear of being stranded with a flat battery.

But this August, BMW offered the world a cure. The i3 is the first of its kind. It is a fully electric car but can be equipped with a small petrol tank – not for running the motor but for running an electric generator that cuts in as needed to top up a flattening battery.

Today’s electric cars are a dramatic improvement on their forebears – more responsive and faster and, if powered by low-emissions electricity, much greener than petrol or diesel cars. But range anxiety has inhibited sales the same way as it did at the turn of last century.

High-powered, quick-charge recharging stations are an alternative.
But the process still takes around an hour.

Numerous cures have been attempted. The now defunct Israeli company “Better Place” came up with the idea of battery swap stations. Three minutes after driving in, your empty battery would be swapped for a fully charged one. Sounds perfect, but carmakers did not come to the party to fit their vehicles with the standard battery required to make this viable. High-powered, quick-charge recharging stations are an alternative. But the process still takes around an hour, far too long for consumers used to two-minute petrol stops. Massive batteries are another approach. Tesla’s Model S can drive 450 km on a single charge. But that still won’t get you from Melbourne to Sydney, and big batteries are heavy and expensive.

Toyota’s Prius and GM’s Volt solved range anxiety but they are not 100% electric cars: they have a hybrid of a petrol engine and an electric motor known as a “parallel hybrid”. A computer toggles between them. At normal cruising speed it’s petrol, at low speeds it’s electric. But because parallel hybrids carry a full-strength petrol engine in addition to the electric motor, plus a gearing system to control which one drives the wheels, this approach is heavy, complex and expensive, and still uses a lot of petrol. Good as it is, the Prius uses 3.9 litres of fuel per 100 km.

By contrast BMW’s i3 is much simpler with only the electric motor ever driving the wheels. This saves weight, complexity and cost.

Because the petrol generator that tops up the battery, the battery itself and the motor that drives the wheels are all connected in line, it is dubbed a “series hybrid”. Most of the time the i3 uses electricity alone. In six months you might drive 10,000 km purely on the energy flowing out of your home charger. But when the open road beckons, with petrol in the tank the battery never runs flat.

The petrol generator, known as an auxiliary power unit (APU), does not need to be big. Although the 125 kilowatt electric motor can launch the i3 from 0 to 100 kph in seven seconds, for most of the time it operates on a fraction of that power, using not more than 20 kilowatts on average during the course of an hour. Since the APU only has to supply the average power, it can be small, quiet and highly efficient.

Series hybrids look like the stuff of revolution. It may not be long until the symphony on our roads returns once more to a resonant whir.

Alan finkel 2014b.png?ixlib=rails 2.1
Alan Finkel is an electrical engineer, neuroscientist and Chief Scientist of Australia.
Latest Stories
MoreMore Articles