Faced with the opportunities presented by electric vehicles, a new trend is emerging: challenging existing electric vehicle architecture to create multi-motor, multi-gear automatic transmission systems. In the overall vehicle structure, the motor and reduction gearbox hold a fundamental position. The multi-motor, multi-gear automatic transmission system in electric vehicles changes the structure of the motor and reduction gearbox, inevitably requiring a transformation of the three major control systems.
First, it's essential to understand the principles and structure of a multi-motor, multi-gear automatic transmission system in an electric vehicle. One possible mode is as follows: A motor (A) is connected to the sun gear, and the outer gear is activated to connect to another motor (B). ① When only motor A rotates, motor B is braked and stationary, and the inner and outer gear rings remain stationary. Driven by the sun gear, the satellite gear rotates within the inner and outer gear rings, transmitting power to the output shaft. ② Alternatively, if motor B rotates, motor A is braked and stationary. The gear shaft of motor B drives the inner and outer gear rings to rotate, causing the satellite gear to rotate around the stationary sun gear, transmitting power to the output shaft. ③ When both motors A and B rotate simultaneously, the rotation of the sun gear and the inner and outer gear rings drives the satellite gear to rotate, transmitting power to the output shaft. In each of these three states, there is a fixed transmission ratio; therefore, selecting the appropriate motor is equivalent to shifting gears. Since it can operate in all three states, and the transitions between them are independent of each other, and the transition process is controlled by the electrical controller, it can smoothly shift gears without a clutch. This design is simple, small in size, lightweight, has a high load-bearing capacity, long service life, stable operation, low noise, high output torque, high efficiency, and safe performance, and its technological content is not high. However, its function and effect are very ideal, and its emergence will bring a revolutionary change to the architecture of electric vehicles.
Analyzing the feasibility of this mechanism based on the diagram, we find that: the sun gear of motor A has 18 teeth, the gear on the shaft of motor B has 35 teeth, the internal gears of the inner and outer gear rings have 48 teeth, and the external gears of the inner and outer gear rings have 60 teeth. This means the transmission ratio between motor A and the output shaft is 3.67, and the transmission ratio between motor B and the output shaft is 2.35. If motor A operates at 3000 rpm, the output shaft rotates at 817 rpm, and motor B operates at 3000 rpm, the output shaft rotates at 1276 rpm. If a single motor is used to drive the output shaft at 2093 rpm from 3000 rpm, the transmission ratio would be 1.43. Numerically, the difference is clear: if a single motor is 100kW, then in a dual-motor structure, the output torque of motor A (50kW) is 1.28 times that of a single motor at 100kW. Similarly, a single motor at 100kW operates at 3000rpm with an output shaft of 2093rpm, while a 50kW motor B operates at 4920rpm with an output shaft of 2093rpm, yet the power consumption differs by 1.22 times. Numerically speaking, a dual-motor configuration could potentially save 1.2 times the energy.
The second-generation Tesla Roadster may achieve a 0-60 mph acceleration time of 2 seconds, which is entirely possible with its dual-motor configuration. If the 0-100 mph acceleration time is reduced to under 3 seconds, the power of the dual motors will be adjusted, with motor A accounting for 45/100 of the power and motor B accounting for 55/100. A 0-100 mph acceleration time of under 3 seconds is definitely achievable.
If the acceleration performance is to be maintained at 100KW, then when the power of motor A is adjusted to 40KW and motor B to 60KW, the power consumption will differ by 1.46 times.
For a car, the most important requirements for its powertrain are ease of use and driving, and low energy consumption. Performance metrics like 0-100 km/h acceleration and top speed are less significant. This is where the value of the matching system becomes apparent, and that's where the biggest highlight lies.
SUVs, sports cars, MPVs, and logistics vehicles use high-power motors, while smaller cars can use motors A (12kW) and B (18kW). If the high speed is set to 120km/h, motor A reaches 47km/h at high speed, and motor B reaches 73km/h at high speed. When motors A and B are running at high speed simultaneously, the combined speed can reach 120km/h.
Regarding range extension, with the same battery capacity, lower power consumption results in greater range. Motor B achieves an output shaft speed of 1276 rpm at its rated speed, while motors A and B rotating simultaneously achieve 2093 rpm. This means that while a single motor requires 3000 rpm to drive the output shaft to 2093 rpm, motor B only needs 4920 rpm. Furthermore, motor B's power consumption is only 0.8 times that of a single motor. Based on a maximum range of 300 km, this increases the range by 75 km, although the results will vary depending on the gear ratio and vehicle load. With motor A's power adjusted to 40 kW, the total power is 90 kW, while motor B's power consumption is only 0.72 times that of a single motor. Based on a maximum range of 300 km, this increases the range by 116 km, significantly improving range. In other words, with a fixed total motor power, using dual motors greatly improves both acceleration and range. The ideal is appealing, but what about in practice? First, we need to understand that in a planetary gear system, if the sun gear, satellite gears, and external ring gear are not fixed, they will form a coupled transmission. That is, when motor A has a certain speed, motor B can join and leave the system at any time, and the output speeds of motors A and B are superimposed. The torque at the same maximum speed is the average of the two. (Experimental conclusion)
The above examples demonstrate the effectiveness of multi-motor, multi-gear automatic transmission systems in multi-electric vehicles. What if we used a two-stage planetary gear system plus another motor? We'll use a 3, 3.5, 3.5 configuration: Motor A is 30kW, Motor B is 35kW, and Motor C is 35kW. The first-stage planetary gears are configured with the same number of teeth as the planetary gears mentioned above. The second-stage sun gear has 30 teeth. The inner teeth of the inner and outer gear rings are 48 teeth, and the outer teeth are set to 60 teeth. Motor C has 20 teeth. This results in a transmission ratio of 9.54 between Motor A and the output shaft, 6.1 between Motor B and the output shaft, and 4.87 between Motor C and the output shaft. At 3000 rpm, the output shaft speed is 314 rpm for Motor A, 491 rpm for Motor B, and 616 rpm for Motor C. If a single motor is used to drive the output shaft at 1421 rpm at 3000 rpm, the transmission ratio is 2.11. Numerically, the output torque of motor A (33 kW) is 1.35 times that of a single motor at 100 kW, meaning that the starting torque of motor A (30 kW) is stronger than that of a single motor at 100 kW. In terms of energy saving, motor C achieves 616 rpm of output shaft rotation at its rated speed of 3000 rpm, while a single motor at 100 kW achieves 1421 rpm of output shaft rotation at the same rated speed. This means that motor C only needs 6920 rpm to achieve 1421 rpm of output shaft rotation, and the power consumption of motor C (35 kW) is only 0.8 times that of a single motor at 100 kW. Based on a maximum travel of 300 km, this increases the travel distance by 75 km. If all motors are 30kW, then the power consumption of motor C (30kW) is only 0.69 times that of a single 100kW motor. Based on a maximum range of 300km, this increases the range by 134km. If the high-speed setting is 160km/h, motor A operates at 35km/h, motor B at 55km/h, and motor C at 70km/h. When all three motors (A, B, and C) operate simultaneously at high speed, their combined speed reaches 160km/h. In actual operation, when the vehicle is heavily loaded, motor A starts the engine. Once the vehicle reaches its maximum speed or sufficient torque, motor B engages, while motor A gradually decelerates and eventually disengages. Motor C then continuously accelerates, and if motor C has sufficient power, it can travel up to 70km/h on its own. When the vehicle is unloaded, motor B can be used for starting, or motor C can be used for acceleration when overtaking. As the speed of vehicles constantly changes, the interaction, substitution, and superposition of motors A, B, C, AB, BC, and ABC result in different functions and effects when used individually or in combination. This requires motor controllers to keep pace with the times and to be further improved, especially intelligent control, so as to flexibly control the individual or combined functions and switching functions of each motor.
In multi-motor, multi-gear automatic transmission systems for electric vehicles, the transmission mechanism itself does not generate torque. However, it can transmit torque or increase torque while decreasing speed, or increase speed while decreasing torque. Torque generation originates from the electric motor, and electric motors of the same power but different types will have different torques. Induction motors, brushless motors, and switched reluctance motors also have different torques. Torque can only be discussed in the context of specific design requirements. This article only clarifies the application of the relationship between speed ratio and power in multi-motor, multi-gear automatic transmission systems for electric vehicles.
Of course, these are only theoretical calculations. The actual results will vary depending on factors such as wind resistance, vehicle weight, vehicle type, purpose, power output, and the number of planetary gear teeth. The final outcome will depend on the specific vehicle conditions. During the vehicle development phase, the goal is to implement the most cost-effective, efficient, and optimal configuration design approach and measures into a multi-electric, multi-motor, multi-gear automatic transmission system, ultimately forming a highly feasible forward development design scheme.
Achieving a "leapfrog development" in China's electric vehicle industry is a requirement of the times. The application technology of electric vehicles cannot simply follow foreign technology; it must forge its own path of innovation, break through bottlenecks in electric vehicle transmission, and establish its own creative milestones. Becoming a leader in the electric vehicle industry will provide the impetus for China's leapfrog development, paving the way for a Chinese-style development path for pure electric vehicles and propelling China's electric vehicle industry to new heights.
