How do electric vehicles work?
Electric cars use an on-board battery to store electrical energy, which is recharged by connecting it to an electricity supply (usually the 'mains'). When required, energy is drawn from the electric-cells and converted to motive power by the use of one or more electric motors.
Battery and motor technology
A battery is constructed from stacking individual electro-chemical 'cells', each of which produces a voltage (typically 2V) that is the result of a chemical reaction within the cell.
Although the lead-acid battery was the most common electric vehicle battery until the late 1990s, the latest generation of rechargeable cells includes lithium-ion (Li-Ion) and lithium-polymer (Li-Poly) cells. These provide a significant improvement in performance and vehicle range and are now preferred by most electric vehicle manufacturers.
First generation electric vehicles used direct current (dc) motors. More recent models tend to convert the direct current to alternating current (ac) using an inverter, which then drives an induction motor. These have increased efficiency, a higher specific power (per kg) and require less maintenance. However, the disadvantages include higher costs and increased complexity of the controller, which needs to both act as an inverter and regulate the motor's speed.
Some electric vehicles also use 'regenerative braking', which tops up the battery when the brakes are applied – this can increase vehicle range by as much as 20%.
FAQ: Am I right in thinking that it's all about efficiency rather than the fuel source? I believe power plants are more efficient than a car engine. Also, a petrol engine is consuming petrol if the engine is on whether you are moving or not.
Electric drive trains are generally more efficient that combustion engines, which being heat engines are limited by the second law of thermodynamics. Typically petrol and diesel engines utilise 25-30% of the fuel's energy at best. Just for an idea of scale, in a recent (not too scientific) RAC Brighton to London run, measured in megajoules (MJ) per kilometre on a tank-to-wheel basis, on average: Electric vehicles used 0.61MJ/km; Hybrids used 1.16 MJ/km; and Diesels used 1.74 MJ/km.
Driving an electric car
Driving an electric car is a very different experience to using a conventional (combustion engine) vehicle. Forward drive is usually selected in much the same way as in an automatic – and another similarity is that there is no clutch pedal.
On depressing the accelerator, an electric car initially moves in almost total silence, which can be a little disconcerting. As the speed picks up, the small amount of 'engine' noise that can be heard is drowned out by wind and tyre noise, which become more noticeable as the speed increases.
Most electric vehicles have excellent acceleration and high torque (especially at lower speeds) and are more than capable of holding their own in city-driving conditions. Although some models are designed as city cars and are speed limited to around 40-50 mph, most of the newer high quality models can be easily reach 60-70 mph on a motorway.
Electric cars can also be high performance vehicles – the Tesla roadster is capable of 130 mph and goes from 0 to 60 mph in 4 seconds!
Current electric cars have a range and performance that is adequate for many driving applications including: city driving, commuting, regular drive cycles (such as delivery routes), short range trips (up to 100 miles per day) and where only zero or low emission vehicles are allowed access. As a result, electric cars are most suited for use as private cars for city use, for commuting trips, in commercial fleets (for small loads), and as company 'pool' cars.
FAQ: I would be happy to accept a shorter range and slower top speed in exchange for low-tech batteries at a lower cost. Is that possible? If so how much cheaper would the low-tech option be?
Some manufacturers are likely to offer battery electric cars with short, medium or long range with prices to match – a little like the different types of mobile tariffs on offer. In fact Tesla is planning to do just this with their Sedan S which will be available by 2013.
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