Hybrid and plug-in hybrid

Examples: Toyota Prius and Honda Civic hybrids, Chevrolet Volt

In a hybrid vehicle, a smaller-than-normal petrol engine and a battery-powered electric motor power the car. The electric motor assists the petrol engine when required. It also acts as a generator when the car is going down hill or decelerating: it captures energy that would otherwise be wasted on heating the brakes and uses this to recharge the battery. And because the electric motor provides reserve power, the petrol engine can be tuned for economy rather than outright power.

When a hybrid car goes down a hill or comes to a halt, the petrol engine stops – which saves fuel. (Hybrid vehicles save most fuel when used for stop-start city and commuter driving, rather than long-distance motorway cruising.)

A further development is the plug-in hybrid. Here a slightly larger battery can be recharged through a mains power outlet or by the moving vehicle. This means the first battery charge of the day (possibly enough to power the vehicle for a daily commute) can be supplied from the electricity mains. If the battery charge gets low, the petrol engine kicks in.

Toyota Prius

The vehicles

Now on its third generation, the Toyota Prius is the world’s most successful hybrid vehicle. Power for the latest version comes from a 1.8L petrol engine and a 60kW electric motor supplied from a nickel-metal-hydride battery pack. Toyota has announced its intention of producing a plug-in version equipped with lithium-ion batteries.

The Prius doesn’t have it all its own way: Honda is challenging with its Civic and Insight hybrids. Now in its second generation, the Honda hybrid system is simpler mechanically than Toyota’s and so it’s cheaper to make. The electric motor isn’t as powerful as the Toyota version – so the car doesn’t save quite as much fuel.

The Chevrolet Volt keeps alive the legacy of General Motors’ EV1. This plug-in hybrid prototype seems to have survived General Motors’ financial problems and its US launch is expected in late 2010.

Unlike the Toyota and Honda hybrids, the Volt is always solely driven by its 111kW electric motor; its petrol engine is connected only to an electric generator. The lithium-ion battery pack can be recharged from mains power and gives the car a battery-only range of around 60km. From then on, the petrol motor generates electricity to power the electric motor and recharge the battery. The battery-only range is sufficient for most daily commutes.