What is a mild hybrid?

You've no doubt heard of hybrid cars, but what exactly is 'mild hybrid' technology? Allow us to explain...

It wasn’t long ago that a hybrid car meant a single thing: a car that’s powered by two different sources. Most conventionally, this was an internal-combustion engine working in conjunction with an electric motor connected to a lithium-ion battery.

However, recent years have seen the introduction of a new type of hybrid car, known as a ‘mild hybrid’. The most important distinction between a normal hybrid and a mild hybrid is that while the electric motor in a conventional hybrid can drive the car by itself, the electric motors in a mild hybrid cannot do so. Instead, the electric motor in a mild hybrid simply assists the petrol or diesel unit in powering the car forward.

This doesn’t sound impressive, but mild hybrids are a key development in the industry, and a crucial stepping stone towards fully electric cars. According to market research, 50 per cent of all hybrids sold by 2025 will be a mild hybrid.

How does a mild hybrid car work?

Mild hybrids come in various shapes and sizes, but most commonly they feature a larger battery pack that works with the conventional 12V unit. 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.

One of the 48V systems uses a lithium-ion battery (although some make do with lead-acid batteries). Instead of replacing the 12V unit that’s found on board cars, the 48V unit works with the standard battery. It’s connected to a hybrid motor and an electric supercharger, and takes over duties from the 12V unit 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 more quickly and smoothly. In some models, such as the new Audi A8 and A7 Sportback, the 48V system can turn off the car’s engine for up to 40 seconds when coasting.

When, say, approaching a red traffic light or a 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 any delay.

A different example is Suzuki’s SHVS (Smart Hybrid Vehicle by Suzuki) system, available in the Baleno, Swift and Ignis models. It incorporates a ‘starter generator’ and a relatively small 0.37kWh battery pack.

The generator’s built-in motor can be called upon 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.

Benefits of a 48-volt 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 a 48V mild hybrid system can offer the following benefits over a standard 12V stop-start system found in many cars today:

  • Increase low-end torque by 25 percent

  • Boost fuel efficiency some 10 to 15 percent

  • Deliver four times the power without increasing engine size

  • Reduce carbon-dioxide emissions by 25 percent

  • Provide up to 70 percent of the overall fuel savings at 30 percent of the cost

There are further benefits from a mild hybrid system, too. Tyre and tech company Continental has been working on ‘super-diesel technology’ that uses a 48V system to cut nitrogen oxide (NOx) emissions from a diesel car by 60 per cent.

Continental has developed a new, electrically heated catalytic converter that relies on the 48V battery to rapidly heat it up to a maximum operating temperature. This has been tested under real-world driving conditions. A catalytic converter is used to absorb and convert much of the harmful exhaust gases from diesels when starting the engine.

Normally, a catalytic converter relies on the engine to bring it up to temperature. Only once it’s up to temperature does it work properly. Using an engine to heat it up takes time. Thanks to the 48V battery system, this can now be done much more quickly, which reduces tailpipe emissions.

Mild hybrids compared with full hybrids

A full hybrid system comes with a larger and heavier battery pack, which increases the car’s overall weight. Mild hybrid systems by default are smaller and lighter, which makes them cheaper to manufacture.

However, because mild hybrids are lighter and smaller, it also means they can’t run on electric power for as long as regular hybrids. This means they will pollute more than a regular hybrid.

Are mild hybrids road tax exempt?

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

Because mild hybrids rely on a petrol or diesel unit to power them, they will emit carbon dioxide – which is used to measure which VED band your vehicle falls under.

The Audi A8, for example, pollutes 155g/km CO2 per km – meaning the owner pays £515 for the first year, and £140 each year thereafter. New owners will also be hit with a £310 supplement for the first five years, as the A8 comes with a list price above £40,000.

Will a mild hybrid feel different to drive than a normal car?

Not drastically. Most systems will 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 will still do the driving bit, although in some cars the battery may provide additional assistance when accelerating. You might also feel slightly different when braking, as some systems will use regenerative braking to recharge the mild hybrid batteries. This means that when lifting off the throttle, for example, the car will slow down as if brake pressure is already applied.