Because electric vehicle motors have very high starting torque, sufficient to start and accelerate a stationary car, electric vehicles do not have the mechanical transmissions of traditional cars. They do not need gear mechanisms to amplify the motor's output torque; simply controlling the motor's speed is enough to achieve gear shifting. In other words, gear shifting in electric vehicles can be achieved using only an electronic control system. However, electric vehicles generally do have a reduction gear.
The reducer is mounted under the powertrain bracket in the front engine compartment and is connected to the drive motor.
The reducer is located between the drive motor and the drive half-shaft. The power output shaft of the drive motor is directly connected to the input shaft gear of the reducer via a spline. On one hand, the reducer transmits the power of the drive motor to the drive half-shaft, reducing the speed and increasing the torque. On the other hand, it ensures that the left and right wheels rotate at different speeds when the car is turning or driving on uneven roads, thus ensuring the smooth operation of the vehicle.
The power transmission mechanism of a speed reducer relies on a two-stage gear pair to achieve speed reduction and torque increase. It is divided into four main components based on function and location: the housing, the input shaft assembly, the intermediate shaft assembly, and the differential assembly.
Common reduction ratios, or transmission ratios, of speed reducers include 8.28:1, 7.793:1, and 8.1938:1. Taking the BMW i3's speed reducer as an example, its transmission ratio is 9.7:1, meaning the speed at the input end of the speed reducer is 9.7 times that at the output end. The input end of the speed reducer is connected to the motor output shaft, and the output end is fixedly connected to the differential housing, driving the differential. The differential distributes torque to the two output ends and provides speed compensation between them.
When the driver presses the accelerator pedal, sensors detect the depth at which the pedal is depressed. It's important to note that accelerator pedals are typically equipped with two identical sensors to prevent accidental operation. Only when the data from both sensors are completely consistent will the next step of signal processing proceed. Based on the sensor information, the electronic control system adjusts the frequency of the three-phase AC power supply to the motor. The motor's speed changes with the frequency of the three-phase AC power supply. Then, after passing through the differential, half-shafts, and other transmission systems, the change in motor power output is transmitted to the drive wheels, ultimately causing a change in the car's speed.
When reversing, simply reverse the phase sequence of the three-phase AC power supply UVW to the motor, and the motor will reverse, thus driving the car backward.
The power transmission route is as follows: drive motor → reducer input shaft → input shaft gear → intermediate shaft gear → intermediate shaft gear → differential half shaft gear → left and right half shafts → left and right wheels.
