What is a mild hybrid?

Over the last couple of years, many conventional petrol and diesel engines have been upgraded to mild hybrid units, in an attempt to meet tougher EU (and UK) legislation on exhaust emissions. But what exactly is a mild hybrid and what are its benefits for car owners?

There are various kinds of hybrid engines for new cars, which is not helped by marketing departments all trying to jump on the eco-bandwagon with confusing names that make their hybrids sound more impressive than they really are.

In this guide, we will explain how a mild hybrid engine works and how it compares to other types of hybrid cars.

You may also like: What is an electric car like to drive?

Levels of electrification

A mild hybrid engine is the simplest and smallest way to get an electrical boost for a petrol or diesel engine. However, it’s also the lowest level of electrification and offers the least benefits compared to a regular hybrid or plug-in hybrid. As you can see on the table below, it offers few real differences to a regular petrol or diesel engine.

*some mild hybrids have the ability to creep in stop-start traffic on electric power, but it’s literally only for a few metres

How does a mild hybrid work?

Modern cars can have a lot of electrical systems, like the stereo, electric windows, power steering, headlights, heated seats and many other creature comforts. All of these systems require electrical power to function. In a regular petrol or diesel car, an alternator draws energy from the engine and converts it to electricity to power these. However, this means that a certain percentage of your engine’s workload is always required to power all these systems. This is called parasitic accessory load, and it reduces your car’s fuel economy and performance.

A mild hybrid replaces the alternator and starter motor with a small electric motor (called the MHSG in the image below) and a battery. This unit can start the engine and powers the various ‘parasitic’ electrical systems, which means the engine spends more of its energy driving the wheels. As a result, it uses less fuel and may provide slightly better performance.

A small battery stores energy for the electric motor, which is charged whenever you coast or brake. The battery is significantly smaller and lighter than found on a regular or plug-in hybrid, as it doesn’t need to store anywhere near as much electricity.

Some systems can provide a power boost to the engine under acceleration, however a mild hybrid cannot drive the car on electrical power alone. (Some can allow the car to creep for a few metres in stop-start traffic, but it’s literally a handful of metres and only up to about 5mph, so it’s basically nothing.)

What’s the difference between a mild hybrid and other hybrid cars?

A small electric motor assists the petrol or diesel engine by reducing the amount of work that the engine has to do. However, unlike a regular or plug-in hybrid vehicle the electric motor on a mild hybrid cannot drive the vehicle itself – the petrol or diesel engine will always drive the wheels. On a regular or plug-in hybrid, the petrol engine can shut down while the car runs purely on electric power – even up to motorway speeds – as long as there is electricity in the battery.

This also means that mild hybrid works perfectly happily with a manual gearbox as there is no electric motor driving the wheels. Full hybrid or plug-in hybrid cars are only available with an automatic transmission.

Like a regular hybrid, and unlike a plug-in hybrid or fully-electric car, you can’t connect a mild hybrid vehicle to an outside electrical source to charge the battery – it only charges while driving. Some car companies refer to this as “self-charging”, but that’s just marketing bollocks and rather misleading.

What’s the benefit of a mild hybrid system over a regular petrol car?

A mild hybrid will feel almost exactly the same as a regular petrol or diesel car to drive. If you have a car with a start-stop system that cuts the engine when the car is stopped, you’ve basically got most of a mild hybrid system anyway.

By cutting the engine whenever it’s not needed, and by reducing the need for the engine to power all the accessories, a mild hybrid system can improve fuel economy by up to about 10%.

For car companies, a mild hybrid system is relatively cheap compared to a full hybrid setup, and it’s fairly easy way to help them achieve their legally-binding emissions targets. Most of their existing petrol or diesel models can be adapted to a mild hybrid setup at a much lower cost than alternative ways of improving their efficiency.

What are the disadvantages of a mild hybrid?

There are no real disadvantages to a mild hybrid system compared to a normal petrol car, although they’re generally a bit more expensive. It drives in exactly the same fashion – especially if you’re used to a car with a start-stop system.

Compared to a full hybrid, you get much less electrical power and the car can’t be powered by electricity alone, so the regular internal combustion engine must always be running to turn the wheels.

Which cars are available as mild hybrids?

Most manufacturers are gradually replacing their regular petrol and diesel engines with mild hybrid versions. You can already find mild hybrid versions of many of the UK’s most popular models, and more are going on sale every month. For 2022, mild hybrids only made up about 14% of the new car market, but this will keep growing as more car brands fit them to more of their models.

Within a couple of years, almost every new petrol or diesel car on sale will have a mild hybrid system and there will be almost no new cars around without at least some form of electrification, from the cheapest superminis to the most expensive supercars.

Ultimately, it’s only a stop-gap solution as petrol and diesel engines will disappear from the UK’s new car market by 2030 – apart from plug-in hybrids, which will run until 2035.