A brief analysis of Toyota THS 1/2 generation and GM Voltech 2 generation power split hybrid transmissions

At present, the most mainstream hybrid systems for passenger cars around the world mainly include: single motor parallel, dual motor series parallel (hybrid), and power split. This article starts with the structure of Toyota THS 1/2 generation and GM Voltech 2 generation, and analyzes the power flow of these two systems under their respective driving modes.

European manufacturers are relatively more inclined to use a single-motor parallel hybrid structure. According to the different layout positions of the motor in the entire transmission system, it can be divided into P0, P1, P2, P3, P4 and other structures (see Figure 1). For example, the German Volkswagen Glof GTE (DQ400E) arranges the motor and motor clutch between the engine and the transmission, which is a P2 structure; the BYD Qin, which is popular in the domestic market, arranges the motor in front of the differential, which is a P3 structure; BYD Tang adds another motor on the rear axle based on the Qin, forming a P3+P4 dual-motor parallel structure, and realizing electric four-wheel drive at the same time; in addition, the current popular 48V mild hybrid system mostly arranges a single motor in the original generator position, which is a P0 structure.

Figure 1 Single motor parallel structure

Fig.1 Single-EM Parallel Structure

Shanghai Auto's Roewe E550 (Figure 2), Mitsubishi Outlander PHEV and Honda's i-MMD hybrid system all use a dual-motor hybrid structure, which can achieve hybrid modes such as series, parallel and engine direct drive. This article will not introduce them in detail.

The hybrid structure that occupies the absolute majority in the hybrid passenger car market in the United States and Japan is the E-CVT power-split hybrid system represented by Toyota THS (TOYOTA Hybrid System) and General Motors Voltech. The following will give a detailed and in-depth introduction and analysis of these two systems.

Figure 2 Shanghai Automotive Roewe e550 EDU structure diagram

Fig.2 SAIC Rowea E550 Twin-EM Series/Parallel Structure

1 Toyota THS 1/2 generation system

Hybrid models such as Toyota Prius, Corolla Hybrid, Levin Hybrid, and Lexus CT200h all use the THS (TOYOTA Hybrid System) hybrid system [4-6].

1.1 Structural principle

The structure of the THS 1/2 generation hybrid system is shown in Figure 3, which consists of an Atkinson engine (ICE), two motors (EM1, EM2) and a simple planetary gear.

ICE is connected to the planet carrier (C), EM1 is connected to the sun gear (S), EM2 is connected to the ring gear (R) and connected to the differential to output power. The system can achieve two driving modes: pure electric (EV Mode) and power split (Power-Split Mode).

In order to facilitate the subsequent calculation and analysis, the ratio of the number of teeth Zr of the ring gear of the simple planetary gear to the number of teeth Zs of the sun gear is defined as:

(1)

Figure 3 TOYOTA THS Gen1/2 structural principle

Fig.3 Structure of THS Gen1/2

1.2 Power flow analysis of different driving modes

1) EV Mode:

In pure electric mode, as shown in Figure 4, the ICE is in a stopped state (locked), and EM2 directly drives the vehicle through the ring gear. At this time, EM1 is in a reverse idling state due to the motion relationship of the simple planetary gear. Assuming that the ring gear rotates clockwise when the vehicle moves forward (the same below), the motion relationship between EM1 and EM2 is shown in Figure 5.

Figure 4 Power flow in pure electric mode

Fig.4 EV Mode Power Flow

Figure 5 EV Mode planetary gear components motion relationship

Fig.5 EV Mode kinematic relation

By using the lever method (The Level Analogy[7]), we can analyze the speed and torque of each moving element of the planetary gear under this operating mode, as shown in Figure 6.

At this time, EM2 is rotating forward to output power, EM1 is rotating backward and idling, and the speed is α times that of EM2 (Formula 2). The output torque Tsum of the entire system is TEM2 (Formula 3):

(2)

(3)

Figure 6 EV Mode lever method speed analysis

Fig.6 EV Mode Level Analogy Analysis

Since the speed of EM1 is α times that of EM2 and is limited by the maximum speed of EM1, the vehicle speed in pure electric mode can only reach a maximum of 40MPH (about 64KPH, based on Prius α=2.6), after which the engine must be started to reduce the speed of EM1.

2) Power-Split Mode:

In the power split mode, as shown in Figure 7, the ICE is in operation and inputs power into the planetary gear train through the planet carrier, EM1 is in a negative power generation state, and EM2 is in a power output state. The motion relationship of each element of the planetary gear train is shown in Figure 8.

Figure 7 Power split mode

Fig.7 Power-Split Mode Power Flow

Figure 8 Motion relationship of various components of Power-Split Mode planetary gear

Fig.8 Power-Split Mode kinematic relation

The speed and torque of each moving element of the planetary gear under this operating mode are also analyzed by the lever method (see Figure 9).

Figure 9 Power-Split Mode lever method speed torque analysis

Fig.9 Power-Split Mode Level Analogy Analysis

From Figure 9, we can conclude that the speed relationship between the engine (ICE), EM1, and EM2 satisfies equation 4:

(4)

The torques of ICE, EM1 and the output to the differential are balanced, satisfying Equation 5:

(5)

Substituting α=2.6 into equation 4 and equation 5, we can obtain equation 6 and equation 7, that is, 72% of the torque input by the engine to the planetary gear train is output to the differential drive vehicle, and 28% is used to generate electricity through EM1.

(6)

(7)

The total torque output from the entire system to the differential to drive the vehicle is shown in the following formula:

(8)

In summary, when the instantaneous driving state of the vehicle is certain (that is, the output speed nEM2/OUT and the driving resistance torque TOUT are determined):

● From Equation 6, we can see that by controlling the speed of EM1, the engine speed can be adjusted;

● From formula 8, we can see that by controlling the torque of EM2, the output torque of the engine can be adjusted.

Figure 10 shows the fuel consumption characteristic curve of a traditional gasoline engine. For this engine, the instantaneous fuel consumption of the engine is the lowest when the speed is 2600 rpm and the torque is around 130 Nm (light green area). Therefore, for a hybrid system with dual motor power split, in the power split driving mode, the engine torque and speed can be decoupled and adjusted, so that the engine can operate in the optimal fuel consumption area as much as possible. This conclusion also applies to the GM Voltech 2nd generation hybrid system to be introduced later. [8-9]

1.3 Power Split Hybrid Efficiency Analysis

In order to consider the hybrid efficiency in power split mode, we put forward the following two assumptions:

1) The vehicle is a HEV, i.e. a non-plug-in hybrid;

2) The battery SOC is not considered, that is, the power generated by the generator motor is completely supplied to the drive motor for driving the vehicle, that is, PEM1=-PEM2.

Under the above two assumptions, the more power output by the engine in hybrid mode is used to drive the vehicle, and the less is used to generate electricity and then drive the vehicle through another motor (multiple energy conversions), the smaller the loss of energy conversion will be, which can be understood as the higher the theoretical hybrid efficiency.

Through the analysis of the lever method in the previous section, we can obtain the relationship between the two motor powers PEM and the engine power PICE9:

(9)

The relationship between the two motor speeds nEM and the engine speed nICE is shown in equations 10 and 11:

Figure 10 Fuel consumption characteristic curve of a traditional gasoline engine

Fig.10 Gasoline Engine Fuel Consumption Diagram (Example)

(10)

(11)

The relationship between the two motor torques TEM and the engine torque TICE satisfies equations 12 and 13:

(12)

(13)

According to formula 9-13, the curves of power ratio, torque ratio and speed ratio are drawn respectively, as shown in Figure 11:

Figure 11 Motor and engine power ratio, torque ratio, and speed ratio curves

Fig.11 EM,ICE Power Ratio, Torque Ratio, Rotation speed Ratio

As can be seen from the figure, there is a node P (Knot) in the system, and its corresponding speed ratio K is 0.72 (as mentioned in the previous section, assuming α=2.6). At this node, the power of EM1 and EM2 are both zero, where the torque TEM2 of EM2=0, and the speed nEM1 of EM1=0. At this time, all the power output by the engine is used to drive the vehicle.

When the speed ratio is greater than the node K=0.72 (left side of 0.72), EM1 generates electricity (negative power), and all the power generated is used for EM2 to output power to drive the vehicle. When the speed ratio is ∞, the power ratio of the two motors to the engine is ±1, that is, all the engine power is used for EM2 to generate electricity and then supplied to EM1 to drive the vehicle.

When the speed ratio is less than the node K=0.72 (right side of 0.72), EM2 starts to generate power (negative power) and EM1 starts to output positive power. At this time, the engine power is divided at the system output end (ring gear), part of which is used to drive the vehicle, and part of which is generated by EM2 and then provided to EM1 to drive the sun gear, thus forming a power circulation with relatively low efficiency.

Therefore, in actual operation, the speed ratio K is generally controlled above the node 0.72 (to the left of 0.72 in the figure).

In order to compensate for the low maximum speed in pure electric mode, Toyota has added a set of simple planetary gears as a reduction mechanism in the new generation THS system, as shown in Figure 12. The addition of the second set of simple planetary gears can solve the problem of high idling speed of EM1 in pure electric mode, which is more meaningful for PHEV, and can reduce the system's power and torque requirements for EM2.