What is a mild hybrid (MHEV)?

If you're looking at buying a new car these days, whether it's an SUV or a supermini, you're likely to find it offered in hybrid form. But while the word 'hybrid' can be used to describe any car with some combination of an engine and electric motor(s) to help it reduce emissions and increase efficiency, there are several different types that work in different ways.

These include plug-in hybrids and full hybrids – both of which can run for some distance (anything from less than a mile up to 40 miles more) on electric power alone. Where mild hybrid electric vehicles (MHEVs) differ is that the electric motor always works in tandem with the petrol or diesel engine, never driving the car on its own for any distance. Mild hybrids often feel little different to a standard petrol or diesel engine, but typically offer small fuel economy gains.

Strict emissions regulations mean nearly every manufacturer is having to incorporate mild-hybrid technology into its cars these days, so it's important to understand exactly what this technology is if you're in the market for a new motor. Read on to find out more.

How does a mild hybrid car, or MHEV, work?

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

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

The 48-volt battery also supplies power to the electric motor and supercharger, allowing the car to accelerate slightly faster and smoother than a purely petrol-engined example. In some models, such as the latest high-end Audis, the 48-volt 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 48-volt mild-hybrid system

One of the key benefits mild-hybrid systems offer is improved fuel efficiency and reduced pollution compared to non-hybrid engines. Engineering firm Delphi, which has developed a 48-volt 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 12-volt 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 48-volt mild-hybrid system to cut nitrogen oxide (NOx) emissions from diesel cars by 60%. Continental has developed an electrically heated catalytic converter that relies on the 48-volt 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 48-volt system, this can now be done faster, which reduces tailpipe emissions.

Mild hybrids (MHEVs) vs full hybrids (FHEVs)

A 'full hybrid' system (also known as a 'parallel hybrid' or 'self-charging hybrid') has a larger and heavier battery pack than a mild hybrid, 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 full hybrids. This means they pollute more than a regular hybrid in most driving conditions.

Are mild hybrids (MHEVs) road-tax-exempt?

Unfortunately not. Since new vehicle excise duty (VED, aka road tax) bands came into effect in April 2017, only zero-emissions vehicles are exempt from paying. 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.

Does a mild hybrid feel different to 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.