A Hybrid Electric Vehicle (HEV) refers to a vehicle featuring a powertrain composed of two or more independent drive systems operating simultaneously, with motive power delivered by an individual system or jointly depending on actual driving conditions. In general context, hybrid vehicles usually refer to petrol-electric hybrids, which are equipped with both a conventional internal combustion engine (ICE) and an electric motor as power sources.
The core principle of a hybrid electric vehicle is utilising the electric motor to assist the internal combustion engine, keeping it within its most efficient operating band to achieve superior fuel efficiency and lower emissions. Its operating modes include: driving purely on the electric motor during move-off or low-speed crawling to prevent inefficient engine operation; delivering combined output from both the engine and electric motor during hard acceleration or hill climbs for robust performance; cruising at steady speeds powered efficiently by the engine while simultaneously recharging the traction battery; and switching the electric motor into a generator during deceleration or braking to capture kinetic energy via regenerative braking and store it as electricity in the battery.
Based on powertrain architecture and layout, hybrid vehicles are categorised into three main types:
Series Hybrid: The internal combustion engine functions solely to run a generator, producing electricity to power the traction motor driving the wheels. As there is no mechanical link between the engine and the driven wheels, the structure is straightforward but exhibits lower energy transmission efficiency, making it most common in commercial buses.
Parallel Hybrid: Both the internal combustion engine and electric motor can drive the wheels independently or in tandem. Featuring a comparatively simpler design and lower production cost, this setup has been utilised in models such as the Honda Accord and Civic.
Series-Parallel Hybrid: Combining the advantages of both series and parallel layouts, the engine and electric motor dynamically split and blend power via a power split device. Although mechanically complex, it delivers optimal fuel economy, with the Toyota Prius standing as the benchmark example.
Based on the degree of hybridisation (motor-to-engine power ratio), hybrid systems are further classified into micro-hybrids (BSG systems, under 10% fuel savings), mild hybrids (ISG systems, around 20% fuel savings), medium hybrids (around 30% fuel savings), and full hybrids (fuel savings exceeding 40%).
In today's passenger car market, prevailing hybrid variants include:
HEV (Full / Self-Charging Hybrid): Fitted with a compact battery pack that requires no plug-in charging, automatically regenerating power through the engine and regenerative braking. It operates on pure electric power at low speeds to cut fuel consumption, represented by models like the Toyota Camry Hybrid and Honda Accord Hybrid.
MHEV (Mild Hybrid): Typically powered by a 48V electrical architecture with a small battery. Beyond powering start-stop functions and energy recuperation, it can run ancillary components like electric superchargers to enhance fuel economy, widely used by brands like Mercedes-Benz and Audi.
PHEV (Plug-in Hybrid Electric Vehicle): Features a higher-capacity battery pack that can be plugged in to charge, typically offering a pure electric range of over 50 km. It delivers zero-emission city commutes alongside the long-distance touring flexibility of a petrol engine, exemplified by models like the BYD Tang DM-i and BMW 5 Series PHEV.
REEV (Range-Extended Electric Vehicle): The internal combustion engine serves exclusively as an onboard generator to top up the battery pack and never drives the wheels directly; propulsion remains 100% electric. It offers an authentic EV driving experience without range anxiety, adopted by marques such as Li Auto (Li ONE) and Voyah.
Hybrids command a substantial market share within the new energy vehicle (NEV) landscape. Following August 2024, NEV market penetration surpassed 50%, with hybrid powertrains accounting for over half of total NEV sales, even hitting 67.7% in peak months. In August 2024, wholesale volume of range-extended electric vehicles (EREVs) surged by 109% year-on-year, while plug-in hybrids (PHEVs) jumped by 84% year-on-year—both substantially outgrowing battery electric vehicles (BEVs).
Hybrid technology serves as a vital bridging solution during the transition from ICE vehicles to full BEVs, striking a pragmatic balance between real-world fuel economy and lower emissions under prevailing market and infrastructural conditions. While hybrids may eventually complete their transitional mission as charging networks mature, they are set to play a pivotal role for an extended period before full electrification takes over.
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