# Transmission ratio parameter design of transmission system

Under the premise of certain motor output performance, the choice of transmission ratio mainly depends on the power of the car. For traditional internal combustion engine vehicles, the increase in the number of gears increases the possibility of the engine working in the high-performance area, thereby improving the fuel economy of the vehicle. In contrast, the power of a pure electric vehicle comes from the drive motor, which has a wide working range. The characteristics of the motor are low-speed constant torque, high-speed constant power, suitable for the operation of pure electric vehicles, and do not need too many gears. Too many gears will increase the structural complexity of the transmission. A transmission with a fixed speed ratio cannot make the motor often work in a high-efficiency area. The two-speed transmission has the characteristics of simple structure, low cost and easy control, and at the same time can meet the requirements of automobile power and economy.
The selection of the transmission ratio of the transmission system should meet the principle that the maximum transmission ratio should meet the climbing performance of the car, while taking into account the working efficiency of the motor in the low-speed area; the minimum transmission ratio should meet the maximum speed of the car, while reducing the motor input as much as possible The speed of the shaft takes into account the efficiency of the motor under high-speed conditions.
When the motor output characteristics are constant, the selection of the transmission ratio of the transmission system depends on the power index of the whole vehicle, that is, the selection of the transmission ratio of a pure electric vehicle should meet the requirements of the vehicle’s maximum speed, maximum gradeability and acceleration time.

1.The minimum transmission ratio of the transmission system
The minimum transmission ratio of a pure electric vehicle transmission system is determined by the maximum speed of the motor and the maximum speed of the vehicle.

In the formula, itmin is the minimum transmission ratio of the transmission system.

2.The maximum transmission ratio of the transmission system
The maximum transmission ratio of a pure electric vehicle is determined by the maximum value calculated by the following two methods.
The maximum transmission ratio of the transmission system is determined by the output torque corresponding to the maximum motor speed and the driving resistance corresponding to the maximum vehicle speed.

In the formula, Tumax (N·m) is the output torque corresponding to the highest speed of the motor.
The maximum transmission ratio of the transmission system is determined by the driving resistance corresponding to the peak torque of the motor and the maximum grade

The maximum transmission ratio of the transmission system is

In the formula, itmax is the maximum transmission ratio of the transmission system.