Automobiles of the future must increase both environmental and safety performance, while significantly increasing the all-important motor vehicle characteristic of being fun to drive. To achieve superior driving performance, which is the basis for driving enjoyment, the conventional approach has been to increase output and torque by increasing engine displacement or using supercharging. However, this approach decreases fuel efficiency, making it difficult to achieve compatibility of environmental performance and power.



In other words, fuel efficiency and power are in a trade-off relationship. By using the Toyota Hybrid System (THS), the Prius was able to escape the inevitability of this relationship in a paradigm shift. The goal of the Hybrid Synergy Drive concept is to achieve compatibility of high levels of both environmental performance and power.

THS, which is a series parallel hybrid, contains a power split device that splits power into two paths. In one path, the power from the gasoline engine is directly transmitted to the vehicle's wheels. In the other path (electrical path), the power from the engine is converted into electricity by a generator to drive an electric motor or to charge the battery. This unique configuration achieves idling stop, stopping of the gasoline engine while the vehicle is running, running of the vehicle using the electric motor, motor assist at any speed, and highly efficient energy regeneration, without using a clutch or transmission. This is achieved through the use of a motor having large low-speed torque and large output.

The newly developed hybrid system, THS II, targets both greater power and improved motor power transmission efficiency, advancing energy management control for the entire vehicle. As a result, Hybrid Synergy Drive has been developed, which markedly increases power performance, improves acceleration performance, and at the same time achieves the highest degree of environmental performance in the world.

The new hybrid system THS II is based essentially on THS. In a bid for even higher efficiency, the new system adopts a high-voltage power circuit between the motor and generator, and greatly reduces energy loss during energy transmission to deliver optimal energy efficiency. THS II significantly increases the use of the electric motor, and under conditions in which the engine experiences poor efficiency, the engine is stopped and the vehicle runs using only power from the electric motor. Under conditions in which engine efficiency is high, THS II operates the engine at optimal fuel efficiency and generates optimum electricity.

It also achieves greater energy regeneration during deceleration and braking, thereby increasing the electricity input/output efficiency, in pursuit of the world's highest fuel efficiency. In terms of environmental performance, THS II aims to meet the ATPZEV (Advanced Technology Partial Zero Emission Vehicle) Regulations in California, U.S.A, which are proposed to go into effect in 2005, the Ultra-Low Emissions Level in Japan, as well as the EURO IV Regulations, scheduled to go into effect in 2004, thus realizing the world's highest level of clean emissions.

THS II boasts by 1.5 times more power from the motor thanks to a higher rpm of the engine, motor, and generator. It adopts a high-voltage power circuit and a higher-performance battery for increased power supply. As a result, the motor power and engine power together provide a more powerful yet smoother running performance.

All of the major components of THS II have been developed by Toyota on its own. The high-voltage power circuits, the motor, the generator and the battery have all been designed anew, enabling further evolution of the hybrid system.

The system consists of two kinds of motive power sources, i.e., a high-efficiency gasoline engine that utilizes the Atkinson Cycle, which is a high-expansion ratio cycle, as well as a permanent magnet AC synchronous motor with 1.5 times more output, a generator, high-performance nickel-metal hydride (Ni-MH) battery and a power control unit. This power control unit contains a high-voltage power circuit for raising the voltage of the power supply system for the motor and the generator to a high voltage of 500 V, in addition to an AC-DC inverter for converting between the AC current from the motor and the generator and the DC current from the hybrid battery.

Other key components include a power split device, which transmits the mechanical motive forces from the engine, the motor and the generator by allocating and combining them. The power control unit precisely controls these components at high speeds to enable them to cooperatively work at high efficiency.