Under the skin, though, it is very different. 'Hybrid' refers to a car that runs on both electricity and petrol, in this case as part of Toyota's Hybrid System (THS). The battery pack itself - which consists of advanced nickel-metal hydride (Ni-MH) cells located behind the rear seat - is smaller than that found in a conventional electric vehicle (EV) as they are not subjected to the continual deep-cycling operation of 'pure' EV batteries. Under the Prius's bonnet is a 1.5-litre four-cylinder 16-valve petrol engine developing 57bhp at 4,000rpm - the engine's rev limit. This is linked through the THS to a high-torque electric motor and separate generator and hence to a constantly variable automatic transmission.
A power-splitting device automatically divides the engine output between the driving wheels and the generator, thus ensuring the batteries are always charged. Additionally, a regenerative braking system is used, which returns energy to the battery pack as the vehicle slows. The petrol engine's main function is to drive the wheels, but any excess output recharges the batteries. The electronics ensure that when the engine is being used inefficiently - for example when at a standstill or while motoring downhill - it is switched off altogether. Thus, stop-start town work is usually accomplished in electric mode, the petrol engine cutting in only when the car gathers speed.
The ECU also compensates for dynamic changes in driving conditions such as ambient temperature, driving style and traffic density. If, for example, hot weather means the air conditioning system is in use, the engine will 'stay alight' longer to recharge the batteries. And likewise, if the car is used in hilly terrain, the engine cuts in more often. Conversely, light usage or unchallenging road conditions results in increasing use of battery power, and thus zero emissions.