A hybrid vehicle is one that has at least two sources of power which can function independently or together in an effort to boost efficiency.
Typically, a hybrid will use both a petrol engine and an electric motor; doing so allows for reductions in emissions and fuel consumption and can also grant silent, zero local-emissions pure electric motoring for short periods.
Hybrids need not necessarily rely on petrol engines as their alternative source of power, though; diesel engines, compressed air and fuel cells can also be used to provide the required motive power.
The systems available have different degrees of hybridisation, though; they range from ‘micro hybrids’ that offer a simple stop-start function through to ‘full hybrids’ that offer significant emissions and economy benefits.
There are also several different configurations of hybrid powertrain, which have varying degrees of complexity and capability.
The degrees of hybridisation
Micro hybrid: A vehicle only requires a start/stop system to be classified as a ‘micro hybrid’, despite the fact the integrated starter/alternator system often used provides no drive assistance or pure EV mode. Some micro hybrids offer regenerative braking, though, which can serve to improve efficiency.
Mild hybrid: In this configuration, a more powerful integrated starter/alternator system is usually fitted. This can sometimes propel the vehicle at low speeds - the engine otherwise runs most of the time - or, more commonly, be used to assist the vehicle’s engine when accelerating. These set-ups also feature regenerative braking, in order to help improve efficiency. A Honda Insight, for example, would be classified as a mild hybrid.
Full hybrid: A full hybrid, sometimes called a ‘strong hybrid’, is one that can be driven on the electric motor alone - or solely on its combustion engine, or a combination of the both. When its batteries are depleted, the engine will be brought online to either supply power or drive in order to keep the vehicle going. The Prius is a classic example of a full hybrid.
Plug-in hybrid: This configuration denotes a full hybrid which can also be charged from the mains. Because its batteries will be charged before setting off, it can subsequently operate in pure EV mode more often and won’t need to call on the engine as frequently for additional charge - reducing its fuel consumption and emissions further.
Different types of hybrid powertrain
Series hybrid: This increasingly common form of hybrid set-up features an electric motor and an internal combustion engine that drives a generator. Only the electric motor drives the wheels; when the battery is depleted, or more power is required, the engine fires up to generate the required power to propel the car. This set-up is often referred to as a ‘range extender’ or ‘extended range’ configuration. Examples of series hybrids include the range-extending version of the BMW i3.
Parallel hybrid: These feature both an internal combustion engine and an electric motor, both of which can send drive through the vehicle’s transmission; in many cases, the electric motor is integrated into the transmission. The engine is often the primary source of power but the electric motor can often be used alone or to assist acceleration. There are also numerous ‘mild’ parallel set-ups, including those that use an integrated starter/generator to provide regenerative braking and a limited amount of assistance.
‘Through the road’ parallel hybrid: This configuration is used to define a car that has two drivetrains. The internal combustion engine will drive one axle, while the electric motor will drive the other - and they can work together or in isolation. This set-up can be seen in cars such as the Peugeot 508 RXH and BMW i8.
Series-parallel hybrid: These are, as the name suggests, a combination of the series and parallel configuration. This set-up, sometimes called ‘combined hybrid’, consists of both an internal combustion engine and an electric motor - but with mechanical and electrical connections between both. This means that both the electric motor and engine can drive the wheels, or the engine can be disconnected and used solely to charge the batteries. Examples of such systems include the ‘Hybrid Synergy Drive’ found in the Toyota Prius. In some cases, this configuration is called a ‘power-split’ hybrid.
Plug-in hybrid: This term indicates that the hybrid in question can run entirely on battery power alone and that, when required, it can be plugged in to charge. The range-extending petrol-electric Vauxhall Ampera is a good example of a plug-in configuration; it can run in pure electric mode and then plugged in to charge - meaning that, when the owner sets off the next day, it should have a substantial amount of pure electric range at its disposal. This cuts down the need to run the engine.
A brief history of hybrid vehicles
Hybrid vehicles were introduced not long after the invention of the automobile itself; the petrol-electric ‘La Cuadra’ range extender, for example, was introduced in 1899. Electric cars were popular because they were easy to operate, quieter and cleaner - but range restrictions and a lack of charging infrastructure made them unsuitable for many buyers.
Consequently, manufacturers began introducing hybrids that offered the best of both worlds. For short trips, you could rely on unobtrusive and refined electric power. If you wanted to travel further afield, or got stuck with flat batteries, you could then fire up the petrol engine to generate the required power to charge the batteries and drive the motors; this tackled any range concerns while simultaneously freeing buyers from relying solely on complex, unreliable and noisy early petrol engines.
More advanced hybrids followed, including the hub motor-equipped Lohner-Porsche ‘Semper Vivus’ of 1900 and the Woods Motor Vehicle Company ‘Dual Power’ of 1911 - a car which featured regenerative braking.
By this point, though, internal combustion engines were advancing at a vast rate of knots. They were getting quieter and more reliable and, as the road network improved, people were travelling further afield. This quelled enthusiasm for costly, heavy electric and hybrid vehicles and, as a result, internal combustion quickly became the dominant motive power.
As pollution in urban areas increased, manufacturers would later begin investigating hybrid power as a means to tackle rising environmental concerns. Cost and complexity, however, kept the idea at bay for many years. Ultimately, it took until 1997 for the first production hybrid to arrive - in the form of the petrol-electric Toyota Prius; Audi’s diesel-electric Duo also arrived in 1997.
Nowadays, a plethora of models benefit from hybrid technology. As emissions regulations continue to tighten, and customer preferences continue to shift towards more efficient vehicles, expect hybrid cars to become increasingly prevalent.