Power performance of pure electric vehicle

The difference between the power of a pure electric vehicle and the power of an internal combustion engine vehicle is the power source that generates the driving force. The power source of an internal combustion engine vehicle comes from the engine, and the power source of a pure electric vehicle comes from the motor.

1. External characteristics of the motor
The external characteristic curve of the drive motor output in an electric vehicle is shown in Figure 1. The characteristic curve is divided into two areas, constant torque and constant power working area. The constant torque region is from zero speed to the rated speed, the output torque of the motor is constant, and the power increases linearly with the increase of the speed: the constant power region is from the rated speed to the maximum speed, the output power of the motor is constant, and the torque varies with the speed The increase in the curve gradually decreases.

Power performance of pure electric vehicle
Figure 1 External characteristic curve of drive motor output

The output torque of the drive motor is

Pure electric vehicle power
(1)

In the formula, Ts (N·m) is the output torque of the drive motor.

In order to establish a mathematical model of the external characteristics of the drive motor, it is necessary to test the external characteristics of the drive motor on a special pure electric vehicle dynamometer platform, and then use the principle of least squares to fit the test data of the external characteristics of the motor to establish a mathematical model of the external characteristics of the motor . The mathematical model of the external characteristics of the drive motor is an important basis for the simulation calculation of the vehicle dynamics. It regards the motor torque tested on the power platform of a pure electric vehicle as a function of the motor speed. The mathematical model is

Pure electric vehicle power
(2)

In the formula, Ai is the coefficients to be fitted; k is the order of the polynomial, generally 3~5.

2. The driving force and driving resistance of pure electric vehicles
During the driving process of a pure electric vehicle, the electric energy stored in the power battery is output to the driving motor through the controller, and the motor outputs power, and the torque generated by the driving motor is transmitted to the driving wheels through the transmission system.

The driving force of pure electric vehicles is

Pure electric vehicle power
(3)

In the formula, Ft (N) is the driving force of the car.

In the constant power area, the driving force of a pure electric vehicle is a function of the motor speed.

The driving resistance of a pure electric vehicle also includes rolling resistance, air resistance, gradient resistance and acceleration resistance, and its expression is the same as that of a fuel vehicle, namely

Pure electric vehicle power
(4)

In the formula, Ff (N) is the automobile rolling resistance; Fw (N) is the automobile air resistance; Fi (N) is the automobile slope resistance; Fj (N) is the automobile acceleration resistance.

3. Power performance evaluation indicators of pure electric vehicles
The power performance evaluation index of pure electric vehicle is the same as that of fuel vehicle, it is also the maximum speed, acceleration ability and climbing ability. When the pure electric vehicle reaches the maximum speed, the motor is in the constant power area, and the driving force of the vehicle is in a balanced state with rolling resistance and air resistance. Find the intersection point of the pure electric vehicle driving force and driving resistance curve to obtain the maximum speed; at the same time, Know the maximum speed that the motor speed can reach, determine the maximum speed that can be reached, and take the smaller of the two.

Figure 2 shows the driving force-driving resistance balance diagram of a pure electric vehicle with two-speed transmission. It can be seen that the intersection of the first and second-speed driving force curves appears at a speed of about 38km/h. When starting to accelerate to this speed, in order to obtain the maximum driving force, the vehicle should shift from the first gear to the second gear; the driving resistance curve and the second gear driving force curve have an intersection, and the maximum speed of the car is close to 110km/h.

Pure electric vehicle power
Figure 2 The driving force-driving resistance balance diagram of a pure electric vehicle with a two-speed transmission

The acceleration of a pure electric vehicle is

Pure electric vehicle power
(5)

In the formula, aj (m/s²) is the acceleration of the pure electric vehicle.

The acceleration time of a pure electric vehicle from a standstill to full acceleration to speed νa is

Pure electric vehicle power
(6)

Figure 3 shows the relationship curve between the driving speed and time of a pure electric vehicle with a two-speed transmission. It can be seen that the acceleration time of 0→50km/h is about 7.3s, which is within 10s specified by the national standard; 50→80km/h The acceleration time is about 7.5s, which is within 15s stipulated by the state.

Pure electric vehicle power
Figure 3 The relationship curve between driving speed and time of a pure electric vehicle with a two-speed transmission

The climbing ability of a pure electric vehicle is closely related to the external characteristics of the drive motor. The maximum power factor of a pure electric vehicle is

Pure electric vehicle power
(7)

In the formula, Tsmax (N·m) is the maximum output torque of the drive motor: Dmax is the maximum power factor.

Pure electric vehicle power
(8)

In the formula, imax is the maximum gradeability.

As shown in Figure 4, it is a climbing degree diagram of a pure electric vehicle with a two-speed transmission. It can be seen from the diagram that the maximum degree of climbing of the vehicle is more than 20%, which meets the national standard. The driving speed of a car through 4% gradient is close to 70km/h, which is higher than the minimum speed of 60km/h specified by the national standard; the driving speed through 12% gradient is 36km/h, which is higher than the minimum speed of 30km/h specified by the national standard.

Pure electric vehicle power
Figure 4 Gradient diagram of a pure electric vehicle with a two-speed transmission

The external characteristics of pure electric vehicle drive motors are constant torque output in the low-speed area and constant power output in the high-speed area. The motor itself has a wide working range, and basically can provide the power performance required for the normal driving of the car without going through a transmission mechanism or a multi-speed transmission mechanism. Therefore, the drawn pure electric vehicle driving force-driving resistance balance diagram, acceleration diagram and gradeability are simpler than those of fuel vehicles.