1. Series HEV
2. Parallel HEV
3. Series-parallel HEV
4. Power-split HEV

Series-parallel HEVs and Power-split HEVs are essentially variations of parallel HEVs,
but they are nevertheless classified separately according to their particular features. The development of deceleration and braking technologies and systems for regenerating energy are being carried out to still further reduce the emission gases and increase the fuel economy of these vehicles.

a. Series HEV
The Series HEV is designed to power a vehicle with an electric motor through the electricity generated by an engine dynamo in the EV. The engine is only used to generate power and does not directly transmit a driving force to the drive axle. There are two types of Series HEV. One is a range extender and the other one is independent system.

b. Parallel HEV
The Parallel HEV has two sources of energy for driving the axle. The combustion engine and the electric motor can drive the axle independently or together.
For example, the zero-emission EV system is run when driving in small city areas or when going on short trips. Large power outputs are generally not required in these situations. The conventional combustion engine is required when running at high-speed between cities.

c. Series-parallel HEV
Series-parallel HEVs have the functions of both the Series HEV and the Parallel HEV. Figure 3.31 shows a Series-parallel HEV's changeover controller.
The drive line is composed of an engine, a first clutch, a second clutch, a transmission, an electric motor in the rear transmission, a battery, an inverter, and a rear motor for the rear-wheel-drive system.
A) Series mode: the engine will power the drive system only when the first clutch is on and the second clutch is off.
B) Series-parallel mode: both clutches are on. Because the second clutch is on, engine torque is applied to drive the front wheels through the transmission.
C) High-speed engine drive system mode: the first and second clutches are both on, but the electric motor in the vehicle's engine drive will only drive the front wheels mechanically. This system is more efficient than the hybrid system and is used when traveling at high-speed or when carrying heavy loads.

d. Power-split HEV
Power-split HEVs have functions similar to those of Series-parallel HEVs. However, Power-split HEVs differ in that is branch engine output uses planet gear. Toyota has been marketing this system in its Prius since 1997.

Figure 3.31�@MItsubishi Motors NEDO-ACE HEV changeover controller