With urban air quality frequently in the news and pollution a real political hot potato, the word 'hybrid' is being heard more and more often these days. And it's likely to stay that way for a long time.

A decade or two ago, there was only one kind of hybrid – the petrolelectric. Exemplified by the first-generation Toyota Prius, this used an electric motor and battery pack for motive power at low speeds, reducing reliance on its petrol engine in congested towns and cities. Since those simple times, though, 'hybrid' has become an all-encompassing term for an increasing number of different types of car.

Plug-in hybrids are becoming increasingly popular and range-extender hybrids turn the tables on the petrol engine completely, allowing the latter to perform a supporting role to the electric motor. There's another type of hybrid, though, that we're sure to see more of in the coming years – the 'mild' hybrid.

As emissions legislation grows ever-stricter, culminating with a UK government announcement that sales of conventional petrol and diesel powered cars will be banned from 2040, manufacturers are finding new ways of using electricity to future-proof their cars.

With Volvo's announcement that all its cars will feature electric or hybrid technology from 2019, it won’t be long before mild-hybrid cars become more commonplace. Further evidence came with the launch of the latest Audi A7 Sportback and Audi A8 saloon, both of which employ mild hybrid technology throughout the line-up.

So, what exactly is a mild hybrid? Read on to learn what mild hybrid technology is and why it's a term you're likely to hear much more of in the future.

 

Mild hybrid: definition

The key difference between a traditional hybrid and a mild hybrid is that while a traditional hybrid’s electric motor is able to power the car on its own, a mild hybrid’s motor is only able to assist the engine; it isn’t potent enough to drive the car independently, hence the word ‘mild’.

Different mild-hybrid setups work in different ways. One example, Suzuki’s SHVS (Smart Hybrid Vehicle by Suzuki) system, available in the Suzuki Baleno, Swift and Ignis models, incorporates a 'starter generator' and a relatively small 0.37kWh (kilowatt hour) battery pack. The generator's built-in motor can be called on to assist the engine during hard acceleration, as well as allowing the car's stop-start system to bring the engine back to life more smoothly, thanks to a belt-drive system.

At the other end of the scale, all versions of the latest Audi A8 and A7 Sportback feature a mild-hybrid setup, although its operating effect is more far-reaching than that of Suzuki's system. Dubbed MHEV (mild hybrid electric vehicle), the Audi system is underpinned by a 48-volt electrical system and the greater power this provides the starter generator enables the car's engine to be turned off for up to 40 seconds when coasting, automatically restarting when acceleration is called for. This is said to offer greater fuel-economy savings than the conventional stop-start of previous models.

Not every mild-hybrid system is focused on fuel-efficiency, though. Ferrari’s flagship hypercar, the LaFerrari, uses its mild-hybrid system to boost the engine’s prodigious power, as part of an electrical network that supports a number of the car's auxiliary systems.

 

Mild hybrid vs. full hybrid

Although few would call the two biggest Audis and the LaFerrari affordable, a mild-hybrid setup is cheaper to manufacture than a full hybrid system. It’s also lighter, as a mild hybrid’s batteries are smaller. Mild hybrids also tend to recharge their batteries from regenerative braking – something some but not all conventional hybrids can do – making a mild-hybrid setup more efficient.

There are downsides, though: because mild-hybrid cars aren’t able to run on electric power alone, they tend to have higher CO2 emissions than conventional hybrids and are therefore less attractive for company-car users. Those after the ability to cruise through town on electric power alone must also look elsewhere.

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