How do plug-in hybrid cars work?

plug-in hybrid technology

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. Plug-in hybrids, therefore, can be charged and driven like an electric car, with the added advantage of having an on-board engine that can be used when the battery is depleted.

Series and parallel plug-in hybrids

Plug-in hybrids are most often based on one of two power-train architectures. In a 'parallel hybrid', the wheels can be either directly powered by the combustion engine or the electric drive-train, or both can be used simultaneously to provide power to the wheels.

A plug-in parallel hybrid is equipped with a larger battery than a conventional parallel hybrid and can also be directly connected to an external electricity supply. The Toyota Prius PHEV is a popular example of a plug-in based on a parallel hybrid.

'Series hybrids', however, solely 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 plug-in 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.

Plug-in hybrids with relatively short electric-only range tend to use a parallel configuration. In contrast, range extended EVs are based on a series power-train – an example being the BMW i3 battery electric car which has the option of adding a small petrol engine under the rear seat to provide 'extended range' capability.

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 as 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 domestic supply.

While electric-only driving is generally limited to 15-40 miles, 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 technologies of the next decade.

Ben Lane

Author:Ben Lane
Date Updated:10th Mar 2015

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