Pros: Improves fuel consumption of petrol engines by up to 30%, available now.
Cons: Adds a huge amount of complexity to a car - plus a lot of environmentally-unfriendly batteries.
Hybrids are sometimes seen as the greenest car you can buy - and the CO2 figure of 104 g/km for the Toyota Prius is certainly impressive. However, a petrol hybrid is little more efficient than a modern diesel in terms of carbon dioxide emissions - VW has just launched a diesel Polo with 99g/km (although it should be said that a Polo is a lot smaller than a Prius).
Full hybrids feature an electric motor in addition to the conventional engine (petrol so far, but diesel hybrids are coming) to provide zero-emission electric running at very low speeds, and to assist the main engine during hard acceleration. The electric motor is driven by batteries charged both by the petrol engine and through regenerative braking (see separate entry). A hybrid is particularly good around town, both in terms of fuel economy and emission generation, having the potential to generate no emissions at all. On motorways, however the hybrid powertrain contributes almost nothing; indeed the extra weight of the motor and batteries will actually harm fuel consumption slightly.
