What is a mild hybrid?

We look beyond the marketing speak to reveal all about mild hybrid technology.

If you're looking to buy a new car, then it's likely that you will have heard the phrase 'mild hybrid' mentioned. But what is a mild hybrid car? Well, the simple answer is that it's a car with extra electric running gear that's designed to help save fuel. It's becoming a more common feature on new cars, too, because manufacturers are becoming reliant on it to meet ever-stricter exhaust emissions legislation.

Electric assistance

In basic terms a mild hybrid car features an electric motor and a battery that provide extra power to the engine. However, unlike a full hybrid (cars like the Toyota Prius), a mild hybrid cannot run on electric power alone because the electrical system is too small to cope on its own.

Helping hand

The most common type of mild hybrid system features a belt-driven alternator starter - which essentially replaces the car's starter motor and alternator – which can give assistance or recoup energy from the engine. The alternator then stores the energy in a battery, which will be independent from the car's conventional 12-volt battery.

Weight for it

The main benefit of a mild-hybrid system is that it can help lower an engine's emissions without having to engineer a costly new drivetrain. The system can be inserted between the engine and driven wheels without too much of a weight penalty when compared with a full hybrid drivetrain, thus helping to boost economy and mitigate the extra cost of the hybrid system

Feel the power

On the road, you'll probably only really notice the benefits of a mild-hybrid drive system in slow moving stop-start traffic. While conventional stop-start systems will only cut the engine when a car is motionless (whether you're in neutral in a car with a manual gearbox, or with the brake pedal fully depressed in an automatic), a mild-hybrid set-up will cut the engine when the car is travelling at walking pace. If you need to accelerate again, the mild-hybrid system will offer instant response while the engine is firing up, making the transition smoother in the process.

Smooth progress

On some cars, the mild-hybrid system can smooth out the transmission's power delivery - cars with jerky automatic gearboxes can be made to feel a lot smoother if the mild-hybrid system fills in the power curve when accelerating.

Hushed refinement

Another good use of a mild-hybrid system is when travelling at speed using light throttle inputs - again, some systems can shut the engine off completely to help save fuel.

Backup supply

Another way mild-hybrid tech can help save fuel is by using stored electricity to power electrical items in the car. If stored energy is used to power kit such as the air conditioning, electric windows and in-car entertainment, then that can take the strain off the engine, which will help to save even more fuel.