(1) Change the transmission ratio to meet the traction requirements of different driving conditions, so that the engine can work in a favorable working condition as much as possible and meet the possible driving speed requirements. Change the speed of the car and the torque on the car's drive wheels within a large range. Due to different driving conditions, the car's driving speed and driving torque are required to vary within a large range. For example, the speed on the highway should be able to reach 100km/h, while in the city, the speed is usually around 50km/h. When an empty car is driving on a straight road, the driving resistance is very small, but when it is fully loaded and going uphill, the driving resistance is very large. The characteristics of the car engine are that the speed change range is small, and the torque change range cannot meet the actual road conditions.
(2) Realize reverse driving to meet the needs of the car to drive backward. To achieve reverse driving of the car, the engine crankshaft can generally only rotate in one direction, and the car sometimes needs to be able to drive backward. Therefore, the reverse gear set in the gearbox is often used to achieve reverse driving of the car.
(3) Interrupt power transmission. When the engine is started, idling, shifting gears, or stopping to take power out, the power transmission to the drive wheels is interrupted.
(4) Achieve neutral. When the clutch is engaged, the transmission does not take power out. For example, this allows the driver to release the clutch pedal and leave the driver's seat without shutting down the engine.