1. Optimisation of transmission ratios and shift quality of two-speed automatic transmissions for pure electric vehicles
Summary:
The transmission is a key component of the vehicle drive train, which directly affects the performance of the vehicle. In order to improve the efficiency of the electric vehicle drive motor, the fixed speed ratio electric vehicle is modified and a two-speed transmission ratio scheme is adopted to improve the efficiency of the drive motor, which in turn improves the overall vehicle power performance and economic performance. The study focuses on the optimisation of the transmission ratio and shift quality of a two-speed automatic transmission for pure electric vehicles.
1.The basic parameters of the vehicle
The electric vehicle was studied based on a traditional microcar, retaining the original suspension system, using lithium manganese acid batteries for the power battery and permanent magnet synchronous motors for the drive motor. After comprehensive research, the vehicle parameters are: full load mass 1 350 m/kg, mechanical transmission efficiency 0.9, tyre rolling radius 0.258 r/min, wind area 1.868 A/m2, air resistance coefficient 0.31. According to the national standard GB/T 28382-2012 standards and market positioning, the vehicle dynamics indicators are as follows: 30 min maximum speed ≥ 80 km / h. Maximum climbing speed ≥ 20%, climbing speed of 4% slope ≥ 60 km/h, climbing speed of 12% slope ≥ 30 km/h, working condition method driving mileage ≥ 100 km.
2.Driving motor parameters are determined
When selecting the motor, it is important to ensure that the motor works at maximum efficiency and also to consider the peak discharge rate of the battery pack.
2.1 Calculation of the power of the drive motor at maximum speed
At the highest speed on a horizontal road, ignoring the acceleration resistance, let the wind speed be 0, then the output power of the motor is
P1 is the drive power at maximum speed;
ηt is the mechanical transmission efficiency;
mg is the fully loaded mass of the vehicle;
f(u) is the rolling resistance coefficient;
umax is the maximum vehicle speed;
Cd is the air resistance coefficient;
A is the windward area.
where
f (u) = 1.2 (0.009 8 + 0.002 5 [u/(100 km/h)] + 0.0004 [u/(100 km/h)]4).
According to the actual demand and international standards, choose 100 km/h speed, according to the formula (2), the calculation result is 0.015 24, substitute into the formula (1), the calculation result is P1 = 13.2 kW. if the speed of the vehicle in line with the national standard of not less than 85 km/h, then the motor power can also choose a smaller..
2.2 Calculation of the power of the drive motor at maximum climbing
The power required for hill climbing is calculated by ignoring the air resistance power and acceleration resistance power, then the motor output power can be calculated as f (u) = 0.012 7, according to the formula (3) can be calculated as P2 = 26 kW.
P2 is the maximum climbing driving power.
i is the degree of climbing;
ua is the minimum vehicle speed when climbing.
2.3 Acceleration performance calculation of the peak power of the drive motor
Assuming a wind speed of 0, the maximum power output of the electric vehicle on a horizontal road is located at the end of the acceleration process of the whole vehicle.
P3 is the maximum power required at the end moment of uniform acceleration;
ta is the uniform acceleration time;
ua is the speed at the end of uniform acceleration.
According to GB/T 28382-2012 standard, ta is 10 s, and P3=21.3 kW can be calculated according to equation (2) and (4). According to equation (1), the rated power of the motor is 15 kW, and the peak power of the motor is 30 kW according to equation (3) and (4). In order to meet the cost factor and the actual demand, the motor is finally selected with a rated power of 15 kW and a peak power of 30 kW.
3. The traditional ratio of the driveline is determined by comparing the power performance of the transmission using the following ratios without changes in driving conditions and motor characteristics, to achieve optimisation of the transmission ratio and to improve the shift quality.
3.1 Single ratio power performance
In order to take into account the maximum climbing degree and the maximum speed, the fixed transmission ratio is chosen to be 6.963, then its resistance and power balance, 85 km/h is the maximum speed achieved, 12% slope is the maximum slope, in order to make the climbing performance to be satisfied, the peak power of the motor is increased to 45 kW and the speed is increased to 9 000 r/min in order to achieve.
The main problems in this case are the need to increase the battery discharge power, the lubricity of the gearbox and the impact on the reversal of the gearbox input shaft in reverse gear.
3.2 Power performance of the two gear ratios If the power input of the motor is the same, the high gear ratio and the low gear ratio of the two gear transmissions are 6.5 and 10 respectively.
90 km/h is the maximum speed that can be achieved, while the maximum climbing gradient does not reach 20% and can only be approached. Therefore, a higher power output from the drive motor is required to achieve higher speeds and climbing degrees, which requires the performance of the battery to be improved as well.
3.3 Power performance of a five-speed transmission ratio
With a 15 kW power rating, the maximum and minimum ratios of the five-speed transmission are 3.538 and 0.78 respectively, with a main reduction ratio of 3.765 and a reverse gear ratio of 3.454. 96 km/h is the maximum speed that can be achieved with the five-speed transmission at the 15 kW power rating, and the maximum climbing gradient is more than 20%, so the power performance is effectively met. If the minimum standard speed of 85 km/h is required, the maximum and minimum ratios of the five-speed transmission are 5.494 and 1.033 respectively, with a main reduction ratio of 4.314 and a reverse gear ratio of 3.583. At 11 kW rated power, the vehicle can reach a maximum speed of 85 km/h and a maximum gradient of 20%. With two gears, the battery discharge power requirement is 30 kW, with a discharge multiplier of 1.28; with five gears, the battery only needs to provide 15 kW of discharge power to meet the power performance, with a discharge multiplier of 0.64. Therefore, the battery performance requirements are significantly reduced when using a five-speed transmission.
3. 4 Comparison of 3 types of transmission
Based on the above analysis, the maximum speed and the maximum hill climb for the three transmissions are shown in Table 1 if the motor is selected with a 15 kW power rating. With a 15 kW motor and a five-speed transmission, the maximum speed and the maximum gradient can be achieved.
In terms of energy consumption, under the same conditions, the minimum power output of the five-speed transmission is 11 kW, the minimum output of the two-speed transmission is 15 kW and the single-speed transmission is 45 kW.
In terms of energy consumption, the five-speed transmission is the lowest.
3. Conclusion
This study shows that the two-speed automatic transmission ratio of pure electric vehicles is better than the single-speed transmission ratio, but slightly worse than the five-speed transmission ratio. Therefore, for pure electric vehicles with two-speed transmission, in order to improve the traditional ratio and achieve the maximum speed and the maximum climbing degree, the transmission can be improved, using five-speed transmission, which can achieve the improvement of vehicle performance. At this stage, five-speed transmissions have already achieved industrial development, while the results of two-speed transmission development is obviously not obvious, so five-speed transmissions can be directly applied to existing technologies and achievements, to achieve a reduction in research and development costs, while five-speed transmissions on the battery, motor requirements are not high, is the main direction of future electric vehicle development.