What is a hybrid? Everything you need to know

Both these set-ups are known as parallel hybrids, meaning the motor can assist the engine when accelerating but, because it has no connection with the wheels, can’t propel the car on its own.

As an aside, you might also come across micro-hybrids, which use similar starter-generator technology for their engine-stop-start systems but don’t have ability to deliver electrical assistance to the ICE. Essentially their use of the ‘hybrid’ title is a marketing ploy designed to cash in on the current trend of significantly electrified machines.

Full hybrids

When we think of the term hybrid, it will inevitably be the Toyota Prius that springs to mind. While not the first petrol-electric vehicle, the Japanese pioneer popularised the technology, which today is known in engineering terms as a series-parallel-hybrid or, more recently, a self-charging hybrid.

More expensive to develop and build than the mild hybrids we’ve already looked at, these models can work as an ICE car, an EV or a combination of both. As a result, they require more sophisticated transmissions, larger batteries and more complex control systems.

If we take the Prius as an example, it essentially acts like an EV at low speeds, using its battery to power an electric motor. Go faster or put greater demands on the battery and the engine fires into life – but rather than driving the car it’s being used as a generator to keep the cells at a minimum state of charge. This is known as series mode. Finally, the system has the ability to run in parallel mode, with the engine and motor working in tandem to accelerate the car. It can also disconnect both motors and run purely with the engine.

Yet it’s arguably even more complex than that. If we stick with the Prius as an example, you will discover that it effectively has two electric motors - MG1 is a starter-generator similar to those in mild hybrids and MG2 is the drive motor, which also acts as a generator (each channels power back into the battery through regenerative braking, while MG1 can also charge the cells by using the engine as a generator). Both of these have to work in unison with the engine. To achieve this requires something known as a power-splitter – a device that’s at the heart of every full hybrid. In simple terms, it's a gearbox, but it has to do much more than the transmission of an ICE car.

The Prius remains a good example here, because it pioneered the system that's used in similar forms in other full hybrids. Its power-splitter is often referred to as a continuously variable transmission (CVT), but while it’s capable of performing the same task and maintaining the ideal ratio for maximum efficiency, it’s actually mechanically different. Using a planetary gearset rather than the belts and pulleys of a traditional CVT, it’s packed together with the two electric motors and onto the engine. Computers (there’s no physical connection to the engine via the throttle or gear selector) then decide where best to send power depending on the prevailing conditions.

In many respects, the 'self-charging’ hybrid is designed to maximize the efficiency of the traditional ICE. Because of the relatively limited electricity generating potential of the various electric motors, these cars tend to have a small battery, which usually means only around a mile or so of pure EV motoring. However, because of the power-splitter, it can run the ICE at its most efficient revs regardless of road speed and use the electric motors for propulsion.

A slightly different form of series-parallel hybrid drive can be found in electrified cars with four-wheel drive, such as Volvo’s T8 Recharge models. Here the engine and traditional torque-converter automatic gearbox drive the front wheels in combination with a mild-hybrid-style starter-generator, while a rear-mounted electric motor powers the back axle. In EV mode, the electric motor drives the car, while at higher speeds the engine takes over. When more power is needed, both electric motors and the petrol engine work together as a parallel system, the starter-generator effectively acting as the power-splitter.

Plug-in hybrids

In essence, plug-in hybrids – also known as PHEVs – are an extension of full hybrids but with the added bonus of a much greater EV range. This is made possible by a much larger battery that requires plugging into the mains, as you would a battery electric vehicle (BEV).

Depending on the driving mode you chose, the car runs on electricity until the cells reach a minimum state of charge, at which point the engine chimes in and acts as generator to maintain enough energy to keep the batteries topped up enough to keep the electric motors running (some systems, such as Volkswagen's, will let you use the engine to deliver a full charge, although fuel economy slumps as a result).

While almost all full hybrids are petrol-electric, there are a handful of diesel-electric PHEVs. Mercedes-Benz is the biggest exponent of this technology, offering it on a range of saloon, estate and SUV models. On paper, they deliver remarkable fuel economy and CO2 emissions figures, thanks largely to the inherent efficiency of their diesel engines. That said, the continuing fallout of the Dieselgate scandal means that they're likely to remain a very niche choice, with few other manufacturers committing to delivering similarly powered vehicles.