What is a hybrid vehicle?

In the animal world, a hybrid is a cross between two different species or varieties. In the motoring world, the term has evolved to define an electric car that is also powered by an internal combustion engine. The electric motor does much of the work, but in the original concept of hybrid car the petrol or diesel engine also provides propulsion and keeps the electric motor's battery charged.

The best-known hybrid vehicle is the petrol-electric Toyota Prius (pictured), which hit the road in 1997 as the solution to emerging challenges with pollution, environmental damage and fuel economy. At a time when battery technology couldn't meet the demands of consumers in the market for an electric car, hybrid models like the Prius could overcome the lack of range that plagues electric vehicles to this day.

The original Prius combined a 1.5-litre petrol engine with a high-capacity electric motor, an on-board generator and a big battery behind the back seats. Even the air-conditioning and power steering, normally parasitic loads on the combustion engine, were electric.

The biggest surprise from driving Toyota's first hybrid car was the lack of engine noise at start-up; then at (very) low speeds and in low-demand situations, it could run on electric power alone. The petrol engine only kicked in to drive the wheels for extra go and at higher speeds.

This type of vehicle recharged the battery using the braking action to generate electricity via the drive motor, which became a generator when the hybrid car was slowing. A hybrid vehicle like the Prius could also generate electric power from the engine and pump it directly into the battery when the charge ran low.

By relying on the electric motor to run at low speeds and from standing starts, the hybrid car runs its internal-combustion engine for shorter periods than a conventional car, reducing tailpipe emissions and enhancing fuel economy. A hybrid car like the Prius is at its most efficient in an urban environment.

Hybrid vehicles have evolved massively since the original Prius. The hybrid car is a stepping stone on the path to the development of a practical electric vehicle that won't be hamstrung by current battery technology and short touring range.

Already in production, there are other types of hybrid vehicle in which the engine (petrol or diesel) is not actually connected to the drivetrain but acts entirely as a means of powering a generator and charging the battery. The BMW i3 REx is one example of this kind of 'range-extended' electric vehicle - essentially a 'plug-in' hybrid car - as is Nissan’s upcoming range of e-Power models.

Mild hybrid is a more recent advent that clouds the waters even more. Closer in nature to a conventional vehicle than an electric car, the mild-hybrid car uses its internal-combustion more often than not, but can power down at traffic lights and stop signs, and the electric motor can supplement the torque from the petrol or diesel engine.

In the case of Mercedes-Benz' new C-Class, for example, the 48V electric motor is used to augment acceleration and/or for starting, coasting and stop-start operation in traffic.

Click here for more on Plug-in Hybrids.