What is a mild hybrid?

When the first hybrid cars (HEVs) hit our roads nearly 30 years ago, there was only one type of hybrid technology that buyers needed to worry about. That is no longer the case, as mild hybrids (MHEVs) and plug-in hybrids (PHEVs) have grown in popularity, offering different applications of hybrid technology.

All hybrid systems use a combination of petrol or diesel combustion engine with one or more electric motors to help reduce fuel consumption, lower emissions or, in some cases, improve performance. Out of the three systems, mild hybrids use the smallest battery and electric motor combination, which together provide the lowest level of assistance to the combustion engine.

What does mild hybrid mean?

As the name suggests, mild hybrids only offer a mild form of assistance to the combustion engine. The battery and electric motor are not powerful enough to move the car under electric power alone for any considerable distance, although they can still be used for low-speed manoeuvring in some circumstances. Instead, the electric motor is used to alleviate some of the strain on the combustion engine, helping to reduce fuel consumption. The combustion engine runs most of the time and provides the majority of power needed to move the car.

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A key benefit of a mild hybrid system is that the average person would not notice a significant difference from behind the wheel compared to regular petrol or diesel cars. This contrasts other HEVs and PHEVs, both of which can drive using pure electric power for some short journeys. 

Mild hybrids have grown in popularity in recent times, as they usually provide a cheaper alternative to HEVs and PHEVs, while still offering improved fuel efficiency over a regular combustion-engined car. The technology can now be found in a wide range of vehicles including humble city cars like the petrol-powered Fiat 500, up to larger SUVs such as the Kia Sportage or Hyundai Tucson. 

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, the Audi’s technology is underpinned by a 48-volt electrical system and the greater power of this system 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.

Some manufacturers refer to mild hybrids as 48-volt hybrids which can muddy the waters, causing buyers to confuse them with regular hybrid systems (HEVs), so bear this in mind if you’re in the market for a fully-fledged hybrid.

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.

Just like other types of hybrid models, mild-hybrid cars are liable for a slightly discounted rate of VED tax compared to a conventional petrol or diesel car.

Can mild hybrids drive on electric power?

The general answer is no. If you’re looking for a hybrid car that can transition from combustion power to pure electricity once you reach a town centre, then you’ll have to look at pricier HEVs or PHEVs. 

However, some modern mild hybrids do offer limited electric-only driving. The latest mild hybrids manufactured by Stellantis brands, including Peugeot and Jeep, can be driven using just the electric motor for limited ranges and up to a certain speed. For example, the Jeep Avenger eHybrid can use its 28bhp electric motor to drive for up to 0.6 miles and up to 18mph, after which the petrol engine kicks in. This can save quite a bit of fuel in heavy stop-and-start traffic

Mild hybrids vs other hybrids

The main difference between mild hybrids and HEVs and PHEVs, is the level of electrical assistance. Mild hybrids offer the least electrical assistance, and therefore cannot be driven on electric power alone in most circumstances. Other hybrids can make use of their bigger batteries and electric motors to drive on pure-electric power for short journeys. 

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.

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