Mild Hybrid Electric Vehicle (MHEV)

What is a mild hybrid eletric vehicle (MHEV) ?

A mild hybrid electric vehicle (MHEV) is a variety of hybrid vehicle. It differs from full hybrids (HEV) and plug-in hybrids (PHEV). This is also known as a light hybrid or micro-hybrid car.

And for good reason, the hybridization system used in MHEVs is much less sophisticated and energy-efficient than in full-hybrid cars or plug-in hybrids.

Lightweight hybrid cars are an interesting first level of hybridization to reduce the carbon emissions of thermal engines (gasoline or diesel) and thus meet increasingly stringent European standards.

The specificities of a MHEV

In micro-hybrid cars, the vehicle's internal combustion engine (ICE) is slightly assisted by an electric starter-alternator. The combustion engine (gasoline or diesel) and the starter-alternator are connected by a timing belt.

A battery with a voltage limit of 48 volts (V) completes the device which allows:

• to manage a Start & Stop system;
• to complement the combustion engine with engine power during the acceleration phases.

In traffic, the vehicle's deceleration energy is recovered during braking to recharge the battery. The captured energy is immediately reused to relieve the combustion engine, thus consuming less fuel.

This hybridization system does not require major changes to the vehicle's architecture. Relatively simple to install, it is therefore inexpensive compared to the hybridization systems of full hybrid or plug-in hybrid vehicles.

However, the fuel savings of a mild hybrid car are much more limited. Therefore, light hybrid cars cannot really be considered as clean vehicles. This is why they are not eligible for the eco rebate.

The use of the starter-alternator nevertheless makes it possible to reduce the fuel consumption of MHEV vehicles, and consequently to limit their carbon emissions. The fuel savings are greater in urban areas due to the use of the Start & Stop system.

Examples and practical application

Advantages of mild-hybrid electric vehicles (MHEVs)

• Simple and very light, the vehicle's hybridization system requires few modifications compared to a conventional car.
• The battery is small and takes up little space. The ensemble causes a very slight overweight.
• Thanks to the starter-alternator, the engine ignites smoothly, making starting the vehicle very smooth and comfortable.
• In terms of cost, mild-hybrid vehicles are much more affordable than full-hybrids and plug-in hybrids.
• They reduce fuel consumption compared to an internal combustion engine.

Disadvantages of mild-hybrid electric vehicles (MHEVs)

• The batteries connected to the starter-alternator are not rechargeable from an electrical outlet.
• The vehicle is also not capable of running 100% on electric.
• In fact, fuel savings are much lower for mild-hybrid cars than for full-hybrids or plug-in hybrids.

Mild hybrid electric vehicles in numbers

Depending on the vehicle model, the battery voltage on mild hybrid cars is 12, 24 or 48 V. Today, a majority of MHEVs have a 48 V hybrid system.

In February 2021¹, sales of non-plug-in hybrids accounted for 16.1 percent of vehicles sold compared to 8.4 percent in February 2020.

According to an AAA Data study, 21,162 non-plug-in hybrid cars were put into service in February 2021. This volume represents about 70% of the electrified vehicles sold during this period.

Of all non-plug-in hybrids put on the road in February 2021:

• 11,086 were mild-hybrid cars ;
• compared to 10,076 full-hybrids.

In a study carried out in France, Germany, the Netherlands and Portugal, Inovev estimates that MHEVs (Mild Hybrid Electric Vehicles) will account for nearly 15% of the European car market in 2021, twice as much as in 2020.

Over the ten months of fiscal 2021, Audi, BMW, Ford, Suzuki, Hyundai, Fiat, Mazda, Kia, Volvo and Mercedes-Benz were the brands that sold the most mild hybrid cars in these countries.

At the end of October 2021, the Fiat 500, Ford Puma and BMW 3 Series were the three best-selling mild hybrid vehicles in the four countries that were considered.

Regulatory framework

At the European level, the reference text for hybrid electric vehicles is Regulation No. 100 of the United Nations Economic Commission for Europe (UNECE). Applied to all vehicles since January 1^st, 2013, it establishes uniform requirements for the approval of battery electric vehicles.

The regulations related to hybrid electric cars mainly concern safety issues related to:

• electrical risks (protection against direct and indirect contact) ;
• the battery (reliability tests, gas accumulation, crash resistance, short-circuit resistance, overload and underload ;
• the operation of the vehicle (impossibility of moving a loaded vehicle, switching from forward to reverse gear, signalling these silent cars...).

It can refer to existing ISO standards² :

• ISO 6469-1: Electrically propelled road vehicles — Safety specifications — Part 1: Rechargeable energy storage system (RESS)
• ISO 6469-3: Electrically propelled road vehicles — Safety specifications — Part 3: Electrical safety
• ISO 12405-4: Electrically propelled road vehicles — Test specification for lithium-ion traction battery packs and systems — Part 4: Performance testing

Thanks to the fuel savings achieved on thermal engines, mild-hybrid cars can meet the so-called Euro standards set for carbon emissions. Over the years and with each new version, however, these standards have become more and more restrictive in order to meet the carbon neutrality objectives to be reached by 2050.

Emerging in 2015, mild hybrid technology has therefore really taken off after the tightening of European carbon emission rules. It allows manufacturers to comply with regulatory requirements.

Sources :

1. https://www.statistiques.developpement-durable.gouv.fr/publicationweb/347 

2. https://www.expert-ve.fr/norme-securite-electrique-hybrid.html