Toyota THS, one of the most valuable hybrid technologies

20 years ago, when other car companies were focusing on traditional automobile technology, Toyota had already started researching and improving hybrid technology. In 1997, Toyota launched the first generation of Prius in the Japanese market , and the first generation of THS system was launched at the same time. The first generation of THS power frame diagram is shown in Figure 3-2-1 . The launch of the first generation of Prius also opened the prelude to the development of modern oil-electric hybrid.

The THS-Ⅰ structure installed in the first generation Prius is very simple. The structure diagram of the first generation THS is shown in Figure 3-2-2. The power of the engine and the motor are coupled through the planetary gear mechanism, so that the three components can work together efficiently and improve fuel economy.

MG1 AC permanent magnet synchronous motor is used as a motor when starting the ICE and as a generator (AC generator) when charging the high-voltage battery; MG2 AC permanent magnet synchronous motor is used as the main drive motor and generator (AC generator) and as a generator to charge the battery during brake energy recovery.

In 2003, Toyota released the second-generation Prius equipped with THS-Ⅱ. The second-generation THS system further improved efficiency and modified the control algorithm. The second-generation system only made minor structural adjustments compared to the first-generation THS. Compared with the first generation, the second generation was changed to a high-voltage electronic air conditioner, and the battery bus was increased from 201V to 500V.

The first two generations of THS systems have made great achievements in fuel economy, but their power and acceleration capabilities are slightly insufficient. In 2009, Toyota released the third-generation Prius equipped with THS-Ⅲ to make up for this shortcoming. The third-generation system has undergone major changes compared to the first two generations of THS, and has made many optimizations and improvements. The structure diagram is shown in Figure 3-2-4.

The hybrid transmission adopts a double planetary gear structure, as shown in Figure 3-2-5. The two planetary gears share a ring gear. Compared with the previous two generations, a set of reduction planetary gears is added to reduce the speed difference between MG1 and MG2. The driving speed in pure electric mode can be higher.

The size of MG1 and MG2 has been reduced, and the size of the entire transmission has also been reduced, as shown in Figure 3-2-6; the original chain reduction transmission has been changed to gear reduction transmission, and the transmission loss has been reduced.

In addition to the structural changes, the THS-III system has a 1.8L engine compared to the THS-II system, and the engine output power has been increased from 57kW to 73kW; the THS-III system uses lithium-ion batteries compared to the previous two generations that used nickel-metal hydride batteries; MG1 uses concentrated windings, and the operating voltage has been increased from 500V to 650V; the maximum speed of MG2 has been increased from 6500 to 13000r/min, and the power has been increased from 50kW to 60kW.

In 2016, Toyota launched the fourth-generation hybrid system, the framework diagram of which is shown in Figure 3-2-7. The transmission structure and traction motor have been redesigned to reduce the total weight. The traction motor itself is more compact and has a higher specific power. Mechanical losses due to friction have been reduced by 20% compared to before.

The motor reduction device directly connects the traction motor to the power distribution device, as shown in Figure 3-2-8, and then connects to the wheels with parallel gears. The original motor was a series mechanism, but now it has become a balance shaft structure, which makes the transmission shorter, as shown in Figure 3-2-9, and reduces the axial dimension requirements. A series of improvements have increased the pure electric driving speed of the fourth-generation Prius from 70km/h to 110km/h.

Taking the Toyota Camry Hybrid equipped with the THS system, which is the most familiar to everyone, as an example, its structure is analyzed as shown in Figure 3-2-10.

The Camry's hybrid system is mainly composed of a 2.0L Atkinson engine (Figure 3-2-11), E-CVT transmission, dual motors, and PCU. The system is called THS-Ⅱ and is a series-parallel hybrid system.

PS: Atkinson Engine

In the early days of the Atkinson cycle engine, this working method was achieved through complex connecting rod cooperation. Today, the overly complex structure is obviously no longer suitable for the development trend of the industry. Therefore, most cars now use the method of extending the valve closing time to discharge part of the inhaled air, so as to achieve the effect of expansion ratio greater than compression ratio. Avoid negative effects such as deflagration caused by too high compression ratio, while increasing the expansion ratio, extending the engine's power stroke, and achieving the ultimate goal of fuel saving.

Working Principle of THS-Ⅱ

The core component of the THS-Ⅱ system is the power distribution system composed of two permanent magnet synchronous motors and a planetary gear mechanism. The THS-Ⅱ system has two motors - MG1 and MG2.

MG1 is mainly used for power generation and as a starter motor. MG2 is mainly used to drive the car. MG1, MG2 and the engine output shaft are connected to the sun gear, ring gear and planetary carrier of a set of planetary gear mechanisms. Power distribution is controlled by the power control unit to control the MG1 and MG2 motors and distribute them through the planetary gear mechanism. Under this structure, the engine output is decelerated by the fixed reduction mechanism and then directly drives the wheels. The THS-Ⅱ system drive mode is as follows.

1. Pure electric mode

When driving at low speed, the motor works more efficiently. When driving at low speed, the system will use pure electric power to drive the vehicle. Of course, this can only be achieved when the battery pack is fully charged. In this mode, the power of the battery pack drives MG2 to operate, thereby driving the wheels, as shown in Figure 3-2-13.

2. Hybrid mode

In hybrid mode, the engine starts and intervenes, using part of the power to drive the vehicle, while the other part is transmitted to MG1, which becomes a generator to drive MG2 to assist in driving the vehicle. When the power is sufficient, the battery pack will also use the remaining power to drive MG2, as shown in Figure 3-2-14.

3. Energy recovery

When the vehicle decelerates or coasts, it drives MG2, which is then converted from a drive motor to a generator, converting the energy generated by the vehicle's deceleration into electrical energy and storing it in the battery. In addition, when the engine output is excessive, the engine will also store the excess power in the battery through MG1 for use when needed, as shown in Figure 3-2-15.

Finally, I want to say that although Toyota's THS hybrid system has certain achievements in system maturity and technical reliability, the statement that "there are two kinds of hybrid in the world, one Toyota and one other" is obviously a bit "exaggerated". From the perspective of adapting to the domestic market, BYD dmi, Geely Thor Hybrid, Chery Kunpeng e+, Great Wall Lemon Hybrid and other DHT dedicated hybrid transmission plug-in hybrid systems are obviously more in line with the needs of Chinese people. Since entering the new energy era, domestic new energy vehicle companies have also achieved quite good results. It is really hard to say who will win in the future.