What is a mild hybrid car?

A boost to fuel economy without the cost of a full hybrid car: why mild-hybrid technology could well be coming to your next car.

BuyaCar team
Nov 12, 2018

As emissions regulations become stricter, carmakers are looking for new ways to make cars greener - without making them too expensive.

For increasing numbers of manufacturers the answer is hybrid technology, which combines batteries and an electric motor with a petrol or diesel engine.

Hybrid cars recover energy during braking and use this to turn an electric motor. This provides extra power during acceleration and can drive the car at slow speeds, boosting efficiency. 

The most efficient type of hybrid is the plug-in variety, which can be charged up to provide several miles of electric-only motoring. Less expensive, but not as economical, are standard hybrids - like the Toyota Prius, with smaller batteries that can't drive the car very far or fast on electrical power, but provide significant assistance during acceleration.

At the lowest end of the scale are mild hybrid cars. These are designed to bring smaller fuel economy benefits at a lower cost - improving the efficiency of petrol and diesel engines in everyday driving.

What is a mild-hybrid car?

Mild hybrids are cars with slightly bigger batteries than normal. This might not sound very exciting, but their extra capacity enables them to drive a motor that boost power during acceleration, reducing the load on the engine and improving fuel economy.

The best systems also allow the engine to be switched off for short periods while the car is travelling at a steady speed.

 

Mild hybrid cars: the good

✔  Cheaper than other hybrid technology
✔  Some fuel economy benefits
✔  Performance improvements

Mild hybrid cars: the not-so-good

Limited improvement in efficiency
Still more expensive than non-hybrid cars
Small number of models so far

 

How mild hybrid cars work

Mild hybrid cars usually have a 48 volt battery that can deliver more power than the standard 12 volt battery that’s under the bonnet of most cars. The example of the Kia Sportage, above, shows the battery at the back of the car.

These batteries can store more energy than a conventional car but they are much smaller than other hybrid cars. A Toyota Prius, for example, can store three times as much electricity as an Audi A7 or Kia Sportage mild hybrid.

The extra capability still comes in useful to drive a small motor next to the car’s engine (labelled mild hybrid starter and generator, above). In some systems, such as those used by Kia, the battery powers the motor under acceleration, reducing the need to rev the engine, which is particularly inefficient.

In others, the motor can keep the car going at a steady speed, which doesn’t require a great deal of energy, allowing the engine to be switched off - for up to 40 seconds in mild hybrid-powered Audis.

The batteries recover energy as the car slows down and brakes, shown by the yellow arrows in the diagram above. They can also be recharged during steady driving, when the car calculates that it is efficient to do so.

 

What mild hybrid cars are available to buy?

The number of mild hybrid cars are increasing quickly, as manufacturers introduce the technology as part of their strategy to meet lower emission targets.

Audi has led the way: every one of its latest A6, A7 and A8 models use mild hybrid technology. Kia and Hyundai - which are sister companies - have recently launched their mild hybrid system in updated versions of the Kia Sportage and Hyundai Tucson. It’s only available with the largest 2-litre diesel engine, and costs significantly more than the standard diesel - which is similarly efficient in real-world driving.

 

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