The difference between mild hybrid, hybrid and plug-in hybrid

But those figures need to be understood carefully. Indeed, it has now become a market habit to include into the “hybrid” category two different technologies: HEV (hybrid electric vehicles) and MHEV (mild hybrid electric vehicles), the latter weighting not less than 60% of that whole category.

Though commonly grouped in communications, let’s take a step back to look again at the definition of the different hybrid powertrains and their acronyms to help avoid confusion.

Also called in a more self-explanatory way “full hybrid”, equipped with a battery up to 3kWh, a HEV operates on a blend of an electric motor and an internal combustion engine. It primarily relies on the electric motor for acceleration and low speed, making it a good choice for urban environments by avoiding fuel consumption and therefore CO₂ emissions. When higher speeds are required or when the battery is out of power, the internal combustion engine seamlessly takes over. Notably, the electric motor recharges each time you brake, enhancing efficiency.

Equipped with a small electric auxiliary motor powered by a 48V battery that offers extra torque, the MHEV primarily runs on petrol or diesel. It briefly engages the electric motor during acceleration to boost torque. This type of hybrid slightly reduces CO₂ emissions as its fuel economy is up to 10% better compared to conventional petrol and diesel cars. But we understand that, by design, MHEVs are not as efficient as HEVs.

Let’s also talk about PHEVs. Though commonly considered as electric vehicles alongside BEVs (battery electric vehicle) and not as “hybrids”, PHEVs still do have hybrid in their name! PHEVs feature both an internal combustion engine (ICE) and a larger battery. What sets them apart of HEV is the size of the battery, up to 30 kWh and the ability to charge via an external charging station, hence the name "plug-in” hybrid. This capability grants PHEVs a significantly extended electric range compared to hybrids, even up to 200km for the best one. In theory, as we can see in their WLTP CO₂ emission level, they are less CO₂ emitting than HEV thanks to the electric energy they get through the grid. But their global efficiency in reducing CO₂ emission requires regular charging, and the combination of a battery and an internal combustion engine increases the vehicle’s weight and reduces fuel efficiency when the battery runs out of remaining power. Therefore their efficiency performance is directly linked to the driver’s charging behaviour.

As we introduced in our previous 2023 article Is mild the new ICE: A deep dive to mild hybrid vehicles, OEMs have enhanced their traditional ICE vehicles by integrating the lightest level of hybrid technology: the mild hybrid technology. This means they increase the efficiency of their ICE vehicles without drastically changing the powertrain technology. This may also be one of the explanations in the decrease of classic petrol and diesel powertrains vs hybrids in the new registration mix. Their success also comes from the fact they imply no change of behaviours for users vs the historical ICE technology.

In situations where electrification can’t be yet a viable solution, hybrids can be an alternative. And now you know that not all hybrids have the same level of electrification in their power delivery and therefore the same impact in CO₂ reduction. PHEV could be the most effective hybrid but only under maximised usage electric mode, which leads to questioning the benefit in their usage compared to pure BEV. MHEVs are only an evolution of ICE to cope with regulations by reducing their polluting emissions. Thus, the only “real” hybrids are then the HEVs and could be an alternative to lower CO₂ emissions for situations where electrification is not straightforward, like in regions without a proper charging network.

*source: ACEA