Hybrid Electric Vehicle

Automotive technology that uses an internal combustion engine and an electric motor together to improve fuel efficiency and emissions

Hybrid Electric Vehicle

Overview

A hybrid electric vehicle (HEV) is an automobile equipped with both an internal combustion engine (gasoline or diesel) and an electric motor, along with a battery, and drives using two or more power sources. It charges the battery through regenerative braking and surplus engine output without external charging, and improves fuel efficiency by controlling the engine to operate only in its most efficient range. Since the 1997 Toyota Prius signaled the start of popularization, it has established itself as a core technology in the transition period toward electrification.

Main Content

Operating Principle

The core of a hybrid system is the division of roles between the engine and motor. During startup and low-speed sections, it drives on the motor alone (EV mode), while during acceleration and high-speed sections, the engine and motor provide power together. During deceleration, the motor is switched to a generator to recover kinetic energy as electricity through regenerative braking, charging the battery. The power control unit (PCU) and power distribution device oversee this process.

Series, Parallel, and Series-Parallel Systems

Series: The engine only turns a generator, and the motor always drives the wheels. The structure is simple and the engine can be fixed at an optimal operating point, so it has been adopted by e-POWER, BMW i3 REx, etc.
Parallel: Both the engine and motor can directly drive the wheels. It is advantageous for high-speed driving, but control is complex.
Series-Parallel (Power-split): Toyota Hybrid System (THS), which distributes power using a planetary gear, is representative, and it is currently the most widely used.

Mild Hybrid and Plug-in Hybrid

The technology spectrum is divided according to the degree of electrification. Mild hybrid (MHEV) uses a small 48V battery to assist only with starting and re-acceleration, and the fuel efficiency improvement is small. Full hybrid (HEV) can drive a significant distance on the motor alone, and plug-in hybrid (PHEV) allows external charging and can run tens of kilometers on pure electric power. PHEV is a compromise that operates as an electric vehicle for short distances and as an internal combustion engine for long distances.

Advantages and Disadvantages

The advantages are that it can achieve a 20–40% level of fuel efficiency improvement and emissions reduction without charging infrastructure, and there is no range anxiety compared with electric vehicles. On the other hand, because it carries both an engine and a motor and a battery, vehicle weight and cost increase, and the structure is complex, making maintenance difficult. Also, during high-speed constant-speed driving, engine efficiency is already high, so the benefits of a hybrid decrease.

Representative Models and Market

Representative examples include the Toyota Prius, Camry Hybrid, Honda Accord Hybrid, Hyundai Grandeur and Sonata Hybrid, and Kia Niro. In the Korean market, since the 2020s, hybrids have been rapidly replacing internal combustion engines and account for a large share of new car sales.

Latest Trends

In 2024–2025, the hybrid market is growing rapidly again in conjunction with the slowdown in electric vehicle growth (the chasm). As consumers return to HEVs and PHEVs due to the burden of charging infrastructure and reduced subsidies, major automakers such as Toyota, Hyundai, and Ford have greatly expanded their hybrid lineups. Technologically, the expansion of 48V mild hybrids, the extension of PHEV electric driving range to more than 100 km, and the trend of lowering costs by applying lithium iron phosphate (LFP) batteries to hybrid systems are prominent. In addition, modularization strategies that share hybrid powertrains with electric vehicle platforms and research on next-generation hybrids combined with hydrogen and synthetic fuels are underway. On the regulatory side, with the strengthening of European emissions regulations, verification of real-driving emissions from PHEVs is becoming stricter.