Electric light truck matching electric drive axle solution and simulation analysis

1 Introduction

According to the "2020 New Energy Vehicle Promotion Subsidy Plan and Product Technical Requirements", the subsidy requirements for pure electric trucks are that the energy consumption per unit load (Ekg) is no more than 0.29 W·h/km·kg, the energy density of the power battery system is no less than 125 W·h/kg, and the pure electric driving range is no less than 80 km. According to the stricter trend of Ekg in subsidy requirements in recent years, as shown in Figure 1. Combined with the current level of development of new energy technologies, it is predicted that in order to meet the subsidy requirements for 2021-2022, the target of Ekg is set to be no more than 0.27 W·h/km·kg.

Figure 1 Trend of energy consumption per unit load (Ekg)

The calculation formula is as follows (1):

In the formula, E represents the electric energy consumption rate, which is measured under the constant speed method in accordance with GB/T 18386-2017 "Test Method for Energy Consumption Rate and Driving Range of Electric Vehicles" [1]; M represents the additional mass, which is measured in accordance with the additional mass provisions in the GB/T 18386 test.

Reducing the value of Ekg can be achieved by reducing the E value or increasing the M value, that is, reducing the energy consumption rate or increasing the load capacity (or reducing the curb weight).

Improving the efficiency of electric drive is one of the ways to reduce the rate of electric energy consumption. At present, the space for improving the comprehensive efficiency of motors and motor controllers is very limited, but there is still room for improving the efficiency of electric drive systems. At present, there are three main routes for the mainstream electric drive system of light trucks: motor direct drive, motor reducer and electric drive axle solution. The characteristics of the motor direct drive solution are high transmission efficiency, low failure rate, and high torque demand, so the motor cost is high. The torque of the motor in the motor reducer solution is low, but the transmission efficiency is not as high as that of the direct drive. The electric drive axle solution has the characteristics of high transmission efficiency, low quality and low cost. The electric drive axle solution is almost suitable for pure electric truck models of 2.5 to 18 t.

This paper mainly studies the electric drive axle matching solution and simulation analysis for electric light trucks.

2 Electric drive axle parameter matching

2.1 Basic parameters and technical indicators of the vehicle

The target model M-EB is based on the M-2019 model and is optimized. The electric drive axle solution replaces the motor direct drive solution. The Ekg of the M-2019 base vehicle is 0.29 W·h/km·kg. The curb weight of the M-EB modified product is reduced to 2,800 kg. The Ekg design target is no more than 0.27 W·h/km·kg, and it meets the requirements of power and economy. The specific basic parameters of the vehicle, the main technical indicators and the reference standards are shown in Tables 1 and 2 respectively.

Table 1 Basic parameters of the vehicle

Table 2 Main technical indicator requirements and reference standards

2.2 Drive motor matching design

Vehicle dynamics is an important indicator for measuring vehicle performance and is mainly evaluated by three aspects: maximum speed, maximum climbing grade and acceleration performance[3].

According to automobile theory, the power balance relationship of a car is as follows (2):

Where Pv is vehicle power, kw, ηt is transmission efficiency, m is curb mass, f is rolling resistance coefficient, i is road slope, Cd is wind resistance coefficient, A is frontal area, δ is rotational mass conversion coefficient, and ua is vehicle speed.

The maximum vehicle speed corresponds to the vehicle power demand calculation formula (3):

Where umax is the maximum speed of the vehicle, 90 km/h.

The maximum climbing grade corresponds to the vehicle power demand calculation formula (4):

Where αm is the climbing angle.

The acceleration time is calculated using the constant power acceleration method, based on the deduced power P3 required for the acceleration process [4], that is, formula (5):

Where tm is the acceleration time, which is 16 s; δ is the rotation mass conversion coefficient, which is 1.15.

Using the above formula, the vehicle power demand corresponding to each dynamic index can be obtained, as shown in Table 3.

Table 3 Analysis of drive motor parameter requirements

From the vehicle power balance relationship, it can be seen that the peak power of the motor must also meet the power requirements of the vehicle's dynamic indicators. Therefore, the peak power must be at least 93.4 kW and the rated power must be at least 47.8 kW.

In consideration of the characteristics of light trucks and the ability to cope with adverse working conditions, the overload factor is set to 1.2, that is, the rated power is not less than 57.4 kW and the peak power is not less than 112.1 kW. In consideration of the maturity of system resources and the configuration information of similar models of competitors, the drive motor power parameters are finally selected: rated power 65.0 kW and peak power 120.0 kW.

A larger constant power area of ​​the drive motor can improve the dynamics of the vehicle, while taking into account the low-speed climbing ability and increasing the maximum speed. The higher the rated speed of the motor with the same rated power, the smaller the volume. The speed of ordinary high-speed motors is generally 10,000 to 15,000 r/min, and the maximum speed is initially set to 12,000 r/min, as shown in formula (6).

In formula (6), β is the constant power coefficient, which is generally 2 to 3, and 3 is taken; nmax is the maximum speed; ne is the rated speed. Then ne is 4 000 r/min.

The peak torque calculated by formula (7) is 286.5 N·m and the rated torque is 155.2 N·m.

Currently, electric vehicles on the market mainly use permanent magnet synchronous motors and AC asynchronous motors. Permanent magnet synchronous motors have higher efficiency (about 95%) in transient conditions and higher power density, so they are suitable for frequent start-stop conditions; while induction motors are more suitable for use under high-speed conditions[5].

2.3 Design of electric drive axle transmission ratio

The transmission ratio of the electric drive axle must simultaneously meet the vehicle's maximum speed, maximum climbing grade, and acceleration time requirements[6].

The upper limit of the transmission ratio is determined by the maximum speed of the motor and the maximum driving speed, see formula (8).

The lower limit of the transmission ratio is determined by the output torque corresponding to the maximum speed of the motor to meet the requirement of the maximum vehicle speed, formula (9) and the peak torque of the motor to meet the requirement of the maximum climbing, formula (10).

The larger the transmission ratio, the larger the transmission gear radius of the rear axle of the same tonnage, the heavier the rear axle weight, and the worse the passability. Considering the resource situation, the initial transmission ratio is selected as 16.19. When the maximum climbing grade requirement is met, the peak torque demand is calculated as shown in formula (11):

From formula (11), we get Tm=283.0 N·m, and the peak torque is 286.5 N·m, which meets the requirement. However, considering the adaptability of light trucks to harsh working conditions, a 20% reserve torque is reserved, that is, the peak torque is set to no less than 340.0 N·m.

Finally, based on the actual operating conditions of light trucks, vehicle structure, system resource maturity and the above performance requirement parameters, the drive motor parameters are preliminarily selected as shown in Table 4.

Table 4 Drive motor parameters

2.4 Power battery matching design

Pure electric vehicles rely entirely on the energy of power batteries. The basic model is an 81.14 kW·h lithium iron phosphate battery, which has the characteristics of high specific energy, high power charging and discharging, and long cycle life. According to the design requirements, the power battery must meet the vehicle's 40 km/h constant speed driving range of more than 250 km. The power demand can be calculated by formula (12):

Without considering the loss of low-voltage electrical appliances, EB is not less than 72.8 kW·h. The basic model has a power of 81.14 kW·h, which meets the mileage and power requirements of existing models, so the power battery can be directly borrowed. The specific parameters are shown in Table 5.

Table 5 Power battery parameters

3 Simulation Analysis

3.1 System Modeling

AVL CRUISE software is an advanced simulation analysis tool used to study vehicle dynamics, fuel economy, emission performance, and braking performance. Based on the CRUISE platform, the vehicle model is established with reference to the vehicle parameters and the selected electric drive axle system and power battery. After the vehicle model is completed, the mechanical connection between the component modules and the electrical connection between the data signals are established [7]. The final pure electric light truck vehicle system simulation model is shown in Figure 2.

Figure 2 Pure electric light truck system simulation model

3.2 Simulation Analysis

3.2.1 Simulation analysis of electric drive axle transmission ratio

First, the CRUISE model is used for simulation and calculation to analyze the power and economy of the vehicle model under different electric drive axle transmission ratios and determine the optimal transmission ratio. According to the drive motor parameters and the electric drive axle transmission ratio selection calculation formula, the value range is determined and the interval value is selected for simulation analysis. The specific results are shown in Table 6.