91 Ev Transmission Configurations

In the case of front-wheel drive, the electric motor drives the gearbox, which is mounted on the front axle, as shown in Figure 9.1. This configuration is for an EV using a single propulsion motor. The single motor drives the transaxle on a common axis, delivering power to the two wheels differentially through a hollow motor shaft.1

The use of two motors driving two front wheels simplifies the transmission and eliminates the differential. Several configurations are possible with two propulsion motors driving two wheels. In one arrangement, the motors, mounted to the chassis, can be connected to the wheels through two short half-shafts. The suspension system of the vehicle isolates the wheels and its associated parts from the rest of the components of the vehicle for easier handling of the vehicle, depending on roadway conditions. The wheels are able to move freely without the weight of the motors when they are mounted on the chassis. In an alternate arrangement, the motors are mounted on the half-shafts with the motor driveshaft being part of the half-shaft. The half-shafts connect the wheels on one side and the chassis through a pivot on the other side. In-wheel mounting of motors is another arrangement possible in EVs. The difficulty in this case is that the unsprung weight of the vehicle increases due to motors inside the wheels, making traction control more complex. To minimize

FIGURE 9.1 Typical front-wheel drive.

FIGURE 9.2 Typical rear-wheel drive.

the unsprung weight of the vehicle and because of the limited space available, the in-wheel motors must be of high-power density. As mentioned at the beginning, the use of a speed reducer is desirable, which adds to the constraint of limited space. The cost of a high-power, high-torque motor is the primary impediment in using in-wheel motors for EVs. Another problem with in-wheel motors is the heating due to braking compounded by the limited cooling capability in the restricted space. Nevertheless, the transmission simplicity has led to several proj ects for the development of in-wheel motors for EVs.

The transmission is more complex in the case of a rear-wheel drive, which requires a differential to accommodate unequal speeds of the inside and outside wheels of the rear axle during vehicle cornering. A typical rear-wheel drive transmission configuration is shown in Figure 9.2.

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