(1) Change the transmission ratio to meet the needs of different driving conditions for traction, so that the engine can work under favorable conditions as far as possible to meet the possible speed requirements. Change the size of the car's travel speed and the torque on the car's drive wheels over a wide range. Due to different driving conditions of the car, the required driving speed and driving torque of the car can be varied within a wide range. For example, the speed on a highway should be able to reach 100km/h, while in urban areas, the speed is often around 50km/h. When an empty car is driving on a straight road, the driving resistance is very small.
When fully loaded uphill, the driving resistance will be great. The characteristic of the automobile engine is that the speed change range is small, and the torque variation range cannot meet the actual road conditions.
(2) to achieve reverse driving, used to meet the needs of the car's reverse travel. To achieve reverse driving cars, the engine crankshaft generally can only turn in one direction, and the car sometimes need to be able to reverse the driving, therefore, often using the reverse gear set in the gearbox to achieve the reverse car driving.
(3) The transmission of power is interrupted, and the power transmission to the drive wheels is interrupted when the engine is started, idling, or when the vehicle shifts gears or needs to stop for power output.
(4) To achieve neutral, the transmission may not output power when the clutch is engaged. For example, it may be ensured that the driver releases the clutch pedal from the driver's seat when the engine is not stalled.