Hybrid Electric Vehicle (HEV) Types and Working Principles

This page explains how different types of Hybrid Electric Vehicles (HEVs) work. We’ll cover the common types: Mild Hybrid Electric Vehicle, Series Hybrid Electric Vehicle, Parallel Hybrid Electric Vehicle, Series-Parallel Hybrid Electric Vehicle, and Plug-in Hybrid Electric Vehicle.

We’ll also highlight the key differences between these HEV types. As you probably know, a Battery Electric Vehicle (BEV) relies on a large electric motor and a substantial battery pack. The vehicle is powered entirely by electrical energy in a BEV. There are variations in BEV designs, depending on the number of battery packs, motors, and the use of clutches, gearboxes, differentials, and fixed gearing.

Hybrid Electric Vehicles (HEVs), unlike BEVs, utilize two energy sources: an internal combustion engine and an electric battery with a motor/generator.

Let’s delve into the workings of different HEV types.

Mild Hybrid Electric Vehicle

Image alt: Mild Hybrid Electric Vehicle

This is the least electrified type of HEV. It’s essentially a conventional internal combustion engine vehicle with an oversized starter motor that can also function as a generator.

In this type, the engine must always be ON while the vehicle is moving. However, when the vehicle is idling, the motor/generator can enable an “idle stop” feature, which turns the engine OFF. At high loads, the motor/generator can assist the engine to boost performance. At low loads, the motor/generator increases the load on the engine and recharges the electric battery.

Series Hybrid Electric Vehicle

Image alt: Series Hybrid Electric Vehicle

This HEV type uses a single path to power the vehicle but relies on two energy sources. As shown in the figure, the fuel tank feeds the engine, which is coupled to a generator to charge the battery.

The battery then provides electrical energy to a motor/generator pair, which powers the wheels. The motor/generator can also recharge the battery during deceleration and braking.

A series hybrid can also be designed using a fuel cell powered by hydrogen. These hybrids can operate in seven modes:

• Engine-only traction
• Electric-only traction
• Hybrid traction
• Engine traction and battery charging
• Battery charging with no traction
• Regenerative braking
• Hybrid battery charging

Parallel Hybrid Electric Vehicle

Image alt: Parallel Hybrid Electric Vehicle

This HEV type uses two parallel paths to power the vehicle, hence the name. A transmission module couples both the motor/generator and the engine.

Parallel hybrids can operate in five different modes:

• Engine-only traction
• Electric-only traction
• Hybrid traction
• Regenerative braking
• Battery charging from the engine

Series-Parallel Hybrid Electric Vehicle

Image alt: Series-Parallel Hybrid Electric Vehicle

This type of HEV utilizes both series and parallel power paths. The modules used in this hybrid electric vehicle type are illustrated in the figure.

Another type of HEV is the plug-in hybrid electric vehicle, which can be plugged in and recharged from a wall outlet.