Parallel Hybrid Drivetrains with Torque Coupling

Parallel Hybrid Drivetrains with Torque CouplingSeries hybrid drivetrain couples primary and secondary power sources together electrically. However, a parallel hybrid drivetrain couples them together mechanically, in which the engine supplies its power mechanically to the wheels like in a conventional IC engine-powered vehicle. It is assisted by an electric motor that is mechanically coupled to the transmission. 

Parallel Hybrid Drivetrains with Torque Coupling

The powers of the engine and electric motor are coupled together by mechanical coupling, as shown in Figure 1. The mechanical coupling of the engine and electric motor power leaves room for several different configurations.

Parallel Hybrid Drivetrains with Torque Coupling
Figure 1. Configuration of a parallel hybrid drivetrain.

Torque Couples

The mechanical coupling in Figure 1 may be torque or speed coupling. In the torque coupling configuration, the torques of the engine and electric motor are added together in a mechanical torque coupler. Figure 2 conceptually shows a mechanical torque coupler, which has two inputs (one is from the engine, one is from the electric motor) and one output to a mechanical transmission. 

Parallel Hybrid Drivetrains with Torque Coupling
Figure 2 Torque-coupling device.

If loss is ignored, the output torque and speed can be described by (Eq. 1 & 2):

    \[T_{out}=k_1T_{in1}+k_2T_{in2},\]

and

    \[\omega_{out}=\frac{\omega_{in1}}{k_1}=\frac{\omega_{in2}}{k_2},\]

where k_1 and k_2 are the constants determined by the parameters of torque-coupling device. Figure 3 lists some typically used mechanical torque-coupling devices.

Drivetrain Configuration and Operating Characteristics

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There are several configurations in torque-coupling hybrid drivetrains. These are mainly classified into two-shaft and one-shaft designs. In each category, the transmission can be placed in different positions and designed with different gears, resulting in different tractive characteristics. An optimum design will depend mostly on the tractive requirements, engine size and engine characteristics, motor size and motor characteristics, and so on.

Parallel Hybrid Drivetrains with Torque Coupling
Figure 3 Commonly used mechanical torque-coupling devices.
Parallel Hybrid Drivetrains with Torque Coupling
Figure 4 Two-axle configuration.

Figure 4 shows a two-shaft configuration design, in which two transmissions are used; one is placed between the engine and torque coupling and other between the motor and torque coupling. Both transmissions may be single or multigears. Figure 5 shows the speed-tractive effort profiles with different transmission parameters. It is clear that the design with two multigear transmissions results in rich speed-tractive effort profiles. The performance and overall efficiency of the drivetrain is certainly superior to other designs, because two multigear transmissions provide more opportunities for both the engine and electric traction system (electric machine and batteries) to operate in their optimum region. This design also provides great flexibility in the design the of engine and electric motor characteristics. However, two multigear transmissions will significantly complicate the drivetrain. 

Parallel Hybrid Drivetrains with Torque Coupling
Figure 5 Tractive effort along with vehicle speed with different transmission schemes.

In Figure 4, a multigear transmission 1 and a single-gear transmission 2 may be employed. The speed-tractive effort profiles are shown in Figure 5(b). Employing a single-gear transmission inherently takes the advantage of the high torque characteristic of electric machine at low speeds. The multigear transmission 2 is used to overcome the disadvantages of the IC engine speed-torque characteristics (flat torque output along speed). The multispeed transmission 1 also tends to improve the efficiency of the engine and reduces the speed range of the vehicle, in which an electric machine must alone propel the vehicle, consequently reducing the battery discharging energy.

In contrast to the above design, Figure 5(c) shows the speed-tractive effort profiles of the drivetrain, which has a single transmission 1 for the engine and a multispeed transmission 2 for the electric motor. This configuration is considered to be an unfavorable design, because it does not use the advantages of both powerplants.

Figure 5(d) shows the speed-tractive effort profile of the drivetrain, which has two single-gear transmissions. This arrangement results in a simple configuration and control. The limitation to the application of this drivetrain is the requirement of the maximum tractive effort. When powers of the engine, electric motor, batteries, and transmission parameters are properly designed, this drivetrain would serve the vehicle with satisfactory performance and efficiency.

Another configuration of the two-shaft parallel hybrid drivetrain is shown in Figure 6, in which the transmission is located between the torque coupler and drive shaft. The transmission functions to enhance the torques of both engine and electric motor with the same scale. Design of constant k1 and k2 in the torque coupling allows the electric motor to have a different speed range than the engine; therefore, a high speed motor can be used. This configuration would be suitable in the case when a relative small engine and electric motor are used, where a multigear transmission is needed to enhance the tractive effort at low speeds. 

Parallel Hybrid Drivetrains with Torque Coupling
Figure 6 Two-shaft configuration.

The simple and compact architecture of the torque-coupling parallel hybrid drivetrain may be the single-shaft configuration, where the rotor of the electric motor functions as the torque coupler (k1 = 1 and k2 = 1 in Equation (1) and Equation (2)), as shown in Figures 7 and 8. A transmission may be placed between either the electric motor and drive shaft, or the engine and the electric motor. The former configuration is referred to as a “pre-transmission” (the motor is in ahead of the transmission, Figure 7), and the latter is referred to as “post-transmission” (the motor is in behind the transmission, Figure 8).

Parallel Hybrid Drivetrains with Torque Coupling
Figure 7 Pretransmission single-shaft torque combination parallel hybrid drivetrain.
Parallel Hybrid Drivetrains with Torque Coupling
Figure 8 Post-transmission single-shaft torque combination parallel hybrid drivetrain.

In the pre-transmission configuration, both the engine torque and motor torque are modified by the transmission. The engine and motor must have the same speed range. This configuration is usually used in the case with a small motor, referred to as a mild hybrid drivetrain, in which the electric motor functions as an engine starter, electrical generator, engine power assistant, and regenerative braking.

However, in the post-transmission configuration as shown Figure 8, the transmission only modifies the engine torque while the motor torque is directly delivered to the driven wheels. This configuration may be used in the drivetrain where a large electric motor with a long constant power region is employed. The transmission is only used to change the engine operating points to improve the vehicle performance and engine operating efficiency. It should be noted that the batteries cannot be charged from the engine by running the electric motor as a generator when the vehicle is on standstill because the motor is rigidly connected to the driven wheels.

Another torque-coupling parallel hybrid drivetrain is the separated axle architecture, in which one axle is powered by the engine and another powered by the electric motor (Figure 9). The tractive efforts from the two powertrains are added through the vehicle chassis and the road. The operating principle is similar to the two-shaft configuration shown in Figure 4. Both transmissions for the engine and electric motor may be singlegear or multigear. This configuration has similar tractive effort characteristics as shown in Figure 5.

Parallel Hybrid Drivetrains with Torque Coupling
Figure 9 Separated axle torque combination parallel hybrid drivetrain.

The separated axle architecture offers some of the advantages of a conventional vehicle. It keeps the original engine and transmission unaltered and adds an electrical traction system on the other axle. It also is four-wheel-drive, which optimizes traction on slippery roads and reduces the tractive effort on a single tire.

However, the electric machines and the eventual differential gear system occupy a lot of space and may reduce the available passenger and luggage space. This problem may be solved if the motor transmission is single-gear and the electric motor is replaced by two small-size electric motors that can be placed within two driven wheels. It should be noted that the batteries cannot be charged from the engine when the vehicle is at standstill.

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