How do plug-in hybrid cars work?

Like hybrid cars, plug-in hybrids are part battery-electric and part conventional cars. The underlying principle of all hybrid vehicles is the use of a temporary energy storage device (usually a battery), which enables the main engine to be operated at close to its maximum efficiency.

Unlike conventional hybrids, plug-in hybrids also have the ability to be charged directly from an external electricity supply – usually the mains. Plug-in hybrids, therefore, can be charged and driven like an electric car, with the added advantage of haveing an on-board engine that can be used when the battery is depleted.

Series and parallel plug-in hybrids

Two types of conventional hybrid drive have been developed. In 'parallel hybrids', the most common type of hybrid car, the wheels can be either directly powered by the combustion engine or using a battery-powered electric drive-train, or both can be used simultaneously to provide power to the wheels. The standard Toyota Prius is probably the best known full parallel hybrid.

A plug-in parallel hybrid is equipped with a larger battery than a conventional hybrid and can also be directly connected to an external electricity supply. The new Toyota Plug-in Prius will be one of the first plug-in parallel hybrids to be launched in the UK.

'Series hybrids', however, use a combustion engine to generate electricity, which then powers an electric motor to provide motive power. In its purest form, the combustion engine is unable to drive the wheel directly. In cases where the engine is relatively small, series hybrids are also known as 'range extended electric vehicles' and the car behaves like an electric vehicle, the battery being charged by the on-board power unit. A plug-in series hybrid can also be recharged using an external electricity supply.

In both types of hybrid, when the engine loading is low, the excess energy is stored for later use. When more energy is required, the main engine and the energy storage device work together to deliver the required power. Battery storage also enables the use of regenerative braking which tops up the battery during braking, further reducing overall fuel consumption by around 20%.In this way, hybrids provide improved fuel economy and reduced emissions.

Driving a plug-in hybrid car

The driving performance of plug-in hybrids is not unlike that of a conventional car, road handling is very similar and acceleration is broadly comparable or even slightly improved. From the outside, most plug-in hybrids look similar to conventional models – other than having an electric connector as well as a conventional fuelling point.

With their additional dashboard information, its from the driving seat you are more likely to be aware of the differences of plug-in hybrid car design. Most plug-in hybrids, for example, will offer at least two driving modes including: 'eco-mode', where the car decides how to most efficiently use conventional and electric power; and 'zero-emission-mode', where the car runs purely on electricity. Plug-in hybrids can therefore be used a pure electric cars – although the electric-only driving range will be less than for a fully electric car.

As all plug-in hybrid cars can use conventional petrol or diesel, fuel is dispensed from fuel pumps in exactly the same way as for conventional non-hybrid models. Indeed, the great advantage of petrol- and diesel-fuelled plug-in hybrids is that they require no change in fuel and so use the existing fuelling infrastructure. However, plug-in hybrids can also be charged directly using any suitable source of electricity – most owners will rely most on home overnight-charging using a standard 13 Amp supply.

While electric-only driving is generally limited to 10-40 miles maximum, as the overall driving range of plug-in hybrids is better than their conventional counterparts there are no restrictions on the applications for which they can be used. With no technical barriers to their use, and with the added option of being used as a zero-emission electric car, plug-in hybrid vehicles therefore possess great potential to become one of the new standard automotive technology of the next decade.

Next: Environmental benefits of plug-in hybrid cars >>

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