Different Kinds of Electric Cars

Electric motors powered by batteries are inherently capable of higher fuel efficiencies than those achieved by internal combustion engines. That's because more than half of the energy stored in a liter of petrol or diesel gets lost as heat when the fuel is burned in a combustion engine. Electric motors, by contrast, can turn the majority of the energy stored in a battery into movement.

There are three kinds of cars with electric motors: electric hybrids, which have motors powered by petrol; fully electric vehicles that are charged up exclusively by mains electricity; and plug-in hybrids that can be plugged in and filled up with petrol. Let's take a brief look at each.

Non-plug-in hybrid cars, such as the Toyota Prius and Honda Insight, look and drive just like normal cars yet under the bonnet they contain a battery-powered electric motor as well as a regular combustion engine. All the energy is provided by petrol, but the electric part of the system enables the car to achieve significantly higher fuel efficiency.

The car charges its own battery when the brakes are applied - convert­ing otherwise wasted kinetic energy into electrical energy - and also when the petrol engine is powering the vehicle along at high speeds. The bat­tery's energy is then automatically used when lower speeds are required. The result is that hybrid cars can achieve over 60 miles per gallon - and exceptionally low levels of local air pollutants.

There are two main down sides with this kind of hybrid. First, they're ultimately powered by petrol, so their emissions remain higher than those of a car that can be plugged in and charged up via the mains. Second, they're currently quite expensive. If buying new, you can expect to pay around 10-20% more than you would for an equivalent non-hybrid model. For less money, you could get a super-efficient diesel - such as the Toyota Yaris or the Volvo S40 DRIVe - with roughly similar emissions. If you need a larger car, though, a hybrid may well be the best choice.

As some commentators have pointed out, the extra components in a hybrid car will add to the carbon footprint of its manufacture. This is likely to be true (it's hard to say for sure as good data is not readily avail­able), but most experts agree that these extra emissions will be comfort­ably outweighed by the reduced fuel consumption. One much-publicized report claimed that the Toyota Prius had a bigger carbon footprint than an SUV if you took its manufacturing emissions into account. This conclusion, still sometimes repeated as if it were fact, seems to have been the result of a flawed calculation: the entire emissions involved in creating a new Prius factory were allocated to the small number of cars it had produced so far.

Though not widely available at the time of writing, plug-in hybrid electric vehicles (PHEVs) are seen by many experts as the cars of the future. The idea is that you get all the convenience of a regular hybrid (you can fuel it with petrol when necessary) as well as the environmental and economic benefits of an electric car charged up by mains electricity.

The most promising PHEVs are those which have only a small petrol engine that exists solely to recharge the battery rather than actually power the car directly. In this arrangement, the engine is basically just an on­board generator that enables on-the-go charging.

Plug-in hybrids should start to appear on US forecourts in 2011 with the launch of the Chevrolet Volt, and in the UK in 2012 with the British version of the same car, the Vauxhall Ampera. Plug-in hybrid versions of the VW Golf and various other cars should also appear in 2012 or shortly thereafter.