What is a mild hybrid?
The term ‘mild hybrid’ is increasingly prevalent in the car industry but what does it mean? We explain what it is and why you might want to consider buying a car with mild-hybrid technology
Hybrid cars have become key players in the effort to reduce our carbon footprint, especially since the UK government announced that from 2030, manufacturers will no longer be able to sell pure petrol or diesel-powered cars. Figures released by the Society of Motor Manufacturers and Traders (SMMT) showed that more than 191,000 hybrids were sold in 2019 - up from 139,000 the year before - and, despite the effects of the COVID-19 pandemic in 2020, these sales continued to increase, reaching 465,331.
The reason this figure is notably larger is because the figure includes mild-hybrid electric vehicles (MHEVs), battery-electric vehicles (BEVs), plug-in electric vehicles (PHEVs) and hybrid-electric vehicles (HEVs). The HEV used to be the only type of hybrid that there was, with an electric motor and battery pack running alongside the petrol engine. This type of car can be driven on electric power alone for short distances but it can't be plugged in. In more recent years, other types of hybrid have emerged, such as PHEVs, which can drive further on electric power and be charged via a plug. But it’s the mild hybrid that we'll look at in detail here.
The key to understanding mild hybrids is to look closely at the name. As it's mild, you can guess - rightly - that there's not as much electrical assistance going on here. In a mild hybrid, the electrified parts of the powertrain can't drive the wheels - the engine does all the work, just like in a normal car. We've driven normal petrol models and mild hybrids back-to-back and only noticed the tiniest differences. But manufacturers have managed to improve fuel efficiency using these systems, making them a clever addition to standard petrol and diesel cars.
We expect the sales of mild-hybrid cars to continue to increase in the coming years, since they are effectively existing petrol and diesel models with mild-hybrid tech added to reduce their emissions. They will generally remain more affordable than PHEVs and BEVs. It’s quite possible that in a few years, before we even reach 2030, it will be very difficult to buy a petrol and diesel car that doesn’t have mild-hybrid assistance.
Cars currently available with this technology include all the latest Audi ‘S’ models, and popular family SUVs like the Kia Sportage and Hyundai Tucson, and smaller models like the Ford Puma, Ford Fiesta, Fiat 500 and Fiat Panda. Car makers including Volkswagen and BMW are starting to offer this technology in their family cars as well.
So, what exactly is a mild hybrid? Read on to learn what mild-hybrid technology is and why it's a term you're likely to hear much more of in the future.
Mild hybrid: definition
The key difference between a traditional hybrid and a mild hybrid is that while a traditional hybrid’s electric motor is able to power the car on its own, a mild hybrid’s motor is only able to assist the engine; it isn’t potent enough to drive the car independently, hence the word ‘mild’.
Different mild-hybrid setups work in different ways. One example, Suzuki’s SHVS (Smart Hybrid Vehicle by Suzuki) system, available in the Suzuki Swift and Suzuki Ignis models, incorporates a 'starter generator' and a relatively small 0.37kWh (kilowatt hour) battery pack. The generator's built-in motor can be called on to assist the engine during hard acceleration, as well as allowing the car's stop-start system to bring the engine back to life more smoothly, thanks to a belt-drive system.
At the other end of the scale, all versions of the latest Audi A8 and Audi A7 Sportback feature a mild-hybrid setup, although its operating effect is more far-reaching than that of Suzuki's system. Dubbed MHEV, the Audi system is underpinned by a 48-volt electrical system and the greater power this provides the starter generator enables the car's engine to be turned off for up to 40 seconds when coasting, automatically restarting when acceleration is called for. This is said to offer greater fuel-economy savings than the conventional stop-start of previous models.
Not every mild-hybrid system is focused on fuel-efficiency, though. Ferrari’s previous flagship hypercar, the LaFerrari, used its mild-hybrid system to boost the engine’s prodigious power, as part of an electrical network that supports a number of the car's auxiliary systems. It’s an innovation that was adapted from the company’s Formula 1 cars.
Mild hybrid vs. full hybrid
Although few would call range-topping Audis and the LaFerrari affordable, a mild-hybrid setup is cheaper to manufacture than a full hybrid system. It’s also lighter, as a mild hybrid’s batteries are smaller. Mild hybrids also tend to recharge their batteries from regenerative braking – something some but not all conventional hybrids can do – making a mild-hybrid setup more efficient.
There are downsides, though: because mild-hybrid cars aren’t able to run on electric power alone, they tend to have higher CO2 emissions than conventional hybrids and are therefore less attractive for company-car users. Those after the ability to cruise through town on electric power alone must also look elsewhere.