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What is a mild hybrid?

Car companies are rushing to launch mild hybrid models as quickly possible. But what is a mild hybrid and should you bother with one?

The fastest-growing type of engine in the new car market at the moment is the mild hybrid. Most car manufacturers are replacing regular petrol or diesel engines with mild hybrids, with new announcements being made every week. But what exactly is a mild hybrid and what are its benefits for car owners?

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

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.

Type of carInternal combustionMild hybridRegular hybridPlug-in hybridElectric car
PowertrainPetrol or diesel engine onlyPetrol or diesel engine with electrical assistancePetrol or diesel engine with separate electric motorPetrol or diesel engine with separate electric motorElectric motor(s) only
Battery sizeNoneSmallMediumLargeVery large
Electric-only rangeNoneNoneMinor (a few miles)Good (20-50 miles)Very good (100-300 miles)
Charge from plugNoNoNoYesYes
Battery charging timeN/AN/AN/AShort (a few hours)Long (several hours)
Eligible for plug-in car grantNoNoNoNoYes

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

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.

Kia Sportage mild hybrid system

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.

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. As of mid-2020, mild hybrids only make up less than 10% of the new car market, but this is going to increase quickly as more car brands fit them to more of their models. Below are simply a few new models that have launched in the last few months.

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.

Stuart Masson
Stuart Massonhttps://www.thecarexpert.co.uk/
Stuart is the Editorial Director of our suite of sites: The Car Expert, The Van Expert and The Truck Expert. Originally from Australia, Stuart has had a passion for cars and the automotive industry for over thirty years. He spent a decade in automotive retail, and now works tirelessly to help car buyers by providing independent and impartial advice.
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