You’ll see plenty of manufacturers offering ‘mild-hybrid’ petrol or diesel engines, but what are they, and can they drive on electric power alone?
Mild-hybrid electric vehicles (MHEVs) are a relatively recent invention, designed to reduce exhaust emissions and improve fuel economy of petrol and diesel cars. They’re also more affordable than conventional hybrids and plug-in hybrids. So what makes them whirr?
But the MHEV tag implies that mild hybrids offer more electrification than is actually the case. Mild-hybrid cars should be thought of as normal petrol or diesel engines with some clever tech to save a little bit of fuel.
How does a mild hybrid work?
A mild-hybrid car works by using a small battery pack and electric motor, which replaces the car’s traditional starter motor and alternator. This motor is often called an integrated starter-generator, or ISG.
The starter-generator charges the battery as you coast or brake. This electricity is then used to help the engine when you accelerate, and to power the rest of the car’s electrical components – thus saving the engine a job and leading to slightly improved fuel economy.
One trick mild-hybrids can often perform is to shut the engine down when you’re coasting, saving you fuel. Don’t worry – the ISG can restart the engine far more quickly than a regular starter motor, so power will come back near-instantly when you accelerate.
What’s the difference between a hybrid and a mild hybrid?
The main difference is that hybrid cars – such as the Toyota Prius – can drive on electric power for an appreciable amount of time. In fact, both Toyota and Renault say that their hybrid Yaris and Clio models can run on electric power for around 50% of driving around town. Many hybrid cars can use the motor’s power to get the car up to around 30mph.
A mild-hybrid system cannot do these things, and is generally only there to power the ancilliaries of the car and take a little bit of stress off the engine.
Different names for mild hybrids
If the description of the car says MHEV or 48V, you know that it’s a mild hybrid. But sometimes you’ll come across other names for the same technology.
- BMW – MHT
- Nissan – MH
- Mazda – e-Skyactiv
- Subaru – e-Boxer
- Volkswagen, Skoda – eTSI
- Volvo – B engine codes (e.g. B4 or B4D)
How much more efficient are mild hybrids?
Several car companies claim mild hybrids can boost fuel efficiency by a noticeable amount. In reality, though, you’re unlikely to notice a difference at the pumps. When the mild-hybrid engine launched on the Fiesta, for example, it only promised a 1mpg increase over the non-hybrid version of the same engine.
Pros and cons of mild hybrids
| Pros | Cons |
| Can offer slightly better economy and/or performance over non-MHEV engines | Won’t really save you money versus a non-hybrid car |
| Manual or automatic gearbox options | No electric-only driving |
Is a mild hybrid worth buying?
Yes, as long as you realise that mild hybrids don’t really feel any different to a normal petrol or diesel engine. MHEV engines will save you a marginal amount of money versus a non-MHEV engine and, in most cases, they’re not available side-by-side – if you want a Ford Puma, for example, the 1.0-litre EcoBoost engine comes with mild-hybrid tech as standard.
Popular mild-hybrid models
- Alfa Romeo Tonale MHEV
- Audi A4 petrol and diesel engines, post 2019
- BMW 3 Series diesels, post 2020
- Fiat 500 Mild Hybrid
- Ford Fiesta EcoBoost mHEV
- Ford Puma EcoBoost mHEV
- Hyundai Bayon 48V MHEV
- Kia Sportage 48V
- Nissan Qashqai MH
- Mazda 2 e-Skyactiv (note, not the Yaris-based Mazda 2 Hybrid)
- Mazda CX-30 e-Skyactiv
- Suzuki Swift 12V Hybrid
- Suzuki S-Cross Boosterjet 48V
- Volkswagen Golf eTSI
- Volvo XC60 B4D
