What is a mild hybrid?

Just what is a mild hybrid car, and how do they work? Our handy guide explains it all

'Hybrid car' used to mean a car that was driven by two distinct power sources: most commonly a petrol engine and electric motor. They could work separately or in tandem to drive the wheels and thus make the car move.

More recently, a new type of hybrid car has started to appear, as part of efforts by carmakers to get traditional internal-combustion petrol and diesel engines to be as efficient as possible. This new type is known as a 'mild hybrid'. What sets one of these apart from a regular hybrid is the fact that the electric part of the drivetrain never powers the car on its own. Instead, it serves to help the diesel or petrol engine to move the car.

Although that sounds low-key, it can make quite a difference to a car's CO2 emissions, and mild hybrids are beginning to appear across the car industry, in everything from small superminis to large luxury SUVs. According to the latest market research, 50% of all hybrids sold by 2025 will be mild hybrids.

How does a mild hybrid car work?

Mild hybrids come in several different configurations, but most commonly they feature a larger battery pack that works with the conventional 12v battery found in every combustion-engined car. Often, this is a 48v system that features an integrated starter-generator, which acts as both a starter motor and a power bank to assist the engine.

Some 48v systems use a lithium-ion battery, while some make do with more traditional lead-acid batteries. Either way, instead of replacing the 12v unit, the 48v unit works with the regular battery. It’s connected to a hybrid motor and an electric supercharger, and takes over from the 12v unit duties such as powering the air-conditioning, catalytic converter and engine fan.

The 48v unit also supplies power to the hybrid motor and supercharger, allowing the car to accelerate slightly faster and smoother. In some models, such as the latest high-end Audis, the 48v system can turn off the car’s engine for up to 40 seconds when coasting.

Approaching a red light or roundabout, if the signal turns green or a gap appears, and you release the brake, the combustion engine starts immediately. The car can then accelerate without delay.

A different example is Suzuki’s SHVS (Smart Hybrid Vehicle by Suzuki) system, available in the Swift and Ignis models. It incorporates a ‘starter generator’ and a relatively small battery pack. The generator’s built-in motor can be called on to boost the engine under 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.

Benefits of a 48v mild-hybrid system

One of the key benefits mild-hybrid systems offer is improved fuel efficiency and reduced pollution. Engineering firm Delphi, which has developed a 48v mild-hybrid system, says that it can offer a 25% increase in low-end torque, a 10-15% boost in fuel efficiency and a 25% reduction in CO2 emissions compared to a traditional 12v system.

There are further benefits to a mild-hybrid system. Tyre and automotive technology company Continental has been working on 'super-diesel technology’ that uses a 48v system to cut nitrogen oxide (NOx) emissions from diesel cars by 60%. Continental has developed an electrically heated catalytic converter that relies on the 48v battery to rapidly heat it to maximum operating temperature.

Normally, a catalytic converter relies on the engine to bring it up to temperature and it only works properly once this has happened. Using an engine to heat it up takes time, but thanks to the 48v system, this can now be done faster, which reduces tailpipe emissions.

Mild hybrids vs full hybrids

A 'full hybrid' system (also known as a 'parallel hybrid' or 'self-charging hybrid') has a larger and heavier battery pack, which increases the car’s overall weight. Mild-hybrid systems are smaller and lighter, which makes them cheaper to build. However, because mild hybrids are lighter and smaller, they also can’t run the car on electric power alone – even for the very short distances managed by regular hybrids. This means they pollute more than a regular hybrid in most driving conditions.

Are mild hybrids road-tax-exempt?

Unfortunately not. Since new vehicle excise duty (VED) bands came into effect in April 2017, only zero-emissions vehicles are exempt from paying road tax. Because mild hybrids emit carbon dioxide, owners do have to pay road tax, although the cars' official classification as 'alternatively fuelled vehicles' (AFVs) by the UK government means they're liable for £10 less a year than regular cars: £140 compared to £150.

Does a mild hybrid feel different than a normal car?

Not drastically. Most systems improve the car’s start-stop feature, meaning you may coast to a stop with no engine power, rather than have the engine cut out at the last minute. The internal-combustion engine still does all driving, although in some cars the battery may provide additional assistance when accelerating.

You might also feel a slight difference when braking, as some systems use regenerative braking to recharge the mild-hybrid batteries. This means when you lift off the throttle, for example, the car can slow down as if brake pressure has already been applied.