What is a mild hybrid?

Mild-hybrid systems improve fuel economy and reduce exhaust emissions. We explain how

With urban air quality regularly 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 - at least until fully electric cars are easier to charge and cheaper. Figures released by the Society of Motor Manufacturers and Traders (SMMT) in January 2020 revealed that, collectively, over 191,000 hybrids were sold in 2019 - up from 139,000 the year before.

A decade or two ago, there was only one kind of hybrid – the petrol-electric. 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. All use a combination of an engine, electric motor and battery, in slightly different ways.

Best hybrids to buy this year

Plug-in hybrids are becoming increasingly popular, while range-extender hybrids turn the tables on the petrol engine completely, allowing the latter to perform a supporting role to the electric motor (although these are becoming less common). There's another type of hybrid that we're now seeing more of – the 'mild' hybrid.

Mild-hybrid technology helps manufacturers reduce their cars’ emissions by pairing an existing petrol or diesel engine with a small electric motor. It’s a cheaper and easier way to reduce consumption than design and build a whole new hybrid, and essentially works as an advanced stop-start system. The electrical assistance allows the engine to switch off below a certain speed when you’re slowing down and all the auxiliary functions like the radio will continue to work.

The same SMMT statistics mentioned above also show that sales of mild-hybrid petrol and diesel models grew by 172% and 740% respectively in 2019, and we expect the next few years to follow a similar pattern. This is because ‘mild-hybrid cars’ are, more often than not, existing petrol and diesel models that have had mild-hybrid tech added to reduce their emissions. It’s quite possible that in a few years, it will be very difficult to buy a petrol and diesel car that isn’t a mild hybrid.

Cars currently available with this technology include all the latest Audi ‘S’ models, and popular family SUVs like the Kia Sportage and Hyundai Tucson.

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 Swift and Suzuki 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 Audi 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 previous flagship hypercar, the LaFerrari, used 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. It’s an innovation that was adapted from the company’s Formula 1 cars.

Mild hybrid vs. full hybrid

Although few would call range-topping 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.

For more, see our guides to hybrid cars, plug-in hybrid cars and electric cars.

Get the latest electric and hybrid car news, reviews and analysis on DrivingElectric.com

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