Mild Hybrid Electric Vehicles (MHEVs) are a type of hybrid vehicle that combines a traditional internal combustion engine (ICE) with a small electric motor and battery system. Unlike full hybrid vehicles or plug-in hybrid vehicles, MHEVs do not run solely on electric power. Instead, the electric motor assists the engine during specific driving conditions such as acceleration, start-stop operation, and regenerative braking, to improve fuel efficiency and reduce emissions.

Mild Hybrid Electric Vehicles (MHEVs) differ from other hybrids primarily in the degree of electric assistance and their ability to operate on electric power alone. MHEVs use a small electric motor and a compact battery (a 48 V battery) to support the internal combustion engine during acceleration. This enables smoother start-stop operation and recaptures energy through regenerative braking. MHEVs cannot drive using electricity alone; the electric motor only supplements the engine and never replaces it. In contrast, full hybrid vehicles (HEVs) have larger batteries and more powerful electric motors that allow for short-distance, low-speed driving in pure electric mode, resulting in greater fuel savings and lower emissions. MHEVs are generally simpler, less costly, and provide more modest improvements in efficiency and emissions compared to full hybrids, which offer more substantial benefits but at a higher cost and complexity.

Working of Mild Hybrid Electric Vehicles (MHEV)

A mild hybrid electric vehicle (MHEV) works by combining a traditional internal combustion engine (ICE) with a small electric motor and a compact battery, usually 48 V, to improve efficiency and reduce emissions. The electric motor in an MHEV cannot drive the vehicle on its own; instead, it assists the engine during specific situations:

• Acceleration Assistance: The electric motor provides extra torque during acceleration, reducing the load on the engine and improving fuel economy.
• Start-Stop System: When the vehicle comes to a stop (such as at traffic lights), the engine will automatically shut off. The electric motor quickly and smoothly restarts the engine when needed, minimizing idling and saving fuel
• Regenerative Braking: During braking or deceleration, the electric motor acts as a generator, capturing kinetic energy that would otherwise be lost as heat and storing it in the battery for later use.
• Energy Management: An onboard computer manages when to use or store energy, ensuring the system operates efficiently and supports vehicle systems as needed.

MHEVs do not require external charging; the battery is recharged automatically through regenerative braking and by the engine itself. This system results in fuel savings and lower emissions, especially in stop-and-go urban driving.

Regenerative braking plays a crucial role in boosting the efficiency of mild hybrid electric vehicles (MHEVs) by capturing and reusing energy that would otherwise be wasted during braking. In a conventional vehicle, braking converts kinetic energy into heat, which is lost. In contrast, MHEVs use an electric motor that acts as a generator when slowing down, converting kinetic energy into electrical energy. This energy is stored in a small battery within the vehicle and later used to assist the engine during acceleration, power vehicle systems, and enable smoother start-stop operation. By continuously topping up the battery, especially in city driving with frequent stops, regenerative braking improves overall fuel efficiency, reduces emissions, and lessens wear on traditional brake components, thereby lowering maintenance costs. This energy recovery is a key reason why MHEVs achieve better mileage and lower running costs compared to conventional vehicles.

Mild hybrids do not propel the vehicle independently like full hybrids or electric vehicles because their electric motor is much smaller and less powerful. The battery is significantly smaller compared to those found in full hybrids or EVs. The electric motor in a mild hybrid is designed only to assist the internal combustion engine, providing extra torque during acceleration, enabling smoother start-stop operation, and powering accessories, not to drive the wheels by itself. This limited capability is due to cost, weight, and packaging considerations, which keep mild hybrids simpler and less expensive but restrict them from offering electric-only driving. In contrast, full hybrids have larger batteries and more robust electric motors that can move the vehicle short distances on electricity alone, while mild hybrids always require the combustion engine for propulsion.

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