(1) To alter the transmission ratio, thereby satisfying the traction requirements dictated by varying driving conditions, enabling the engine to operate-as much as possible-within its most efficient range, and meeting the demands for various driving speeds. This allows for significant variation in both the vehicle's driving speed and the torque delivered to the drive wheels. Since driving conditions vary widely, the vehicle's speed and drive torque must be capable of changing over a broad spectrum. For instance, on a highway, the vehicle should be able to reach speeds of 100 km/h, whereas in urban areas, speeds typically hover around 50 km/h. While an unloaded vehicle traveling on a flat, straight road encounters minimal driving resistance, a fully loaded vehicle ascending a slope faces significantly higher resistance. However, the inherent characteristics of an automobile engine-specifically its limited operational speed range-mean that its torque output range is insufficient to meet the diverse demands of actual road conditions on its own.
(2) To enable reverse motion, thereby fulfilling the requirement for the vehicle to travel backward. Since an automobile engine's crankshaft typically rotates in only one direction, yet the vehicle occasionally needs to move in reverse, a dedicated reverse gear-integrated within the transmission-is utilized to facilitate backward movement.
(3) To interrupt power transmission; specifically, to temporarily cut off the flow of power to the drive wheels during engine starting, idling, gear shifting, or when the vehicle needs to be brought to a halt while maintaining engine operation.
(4) To provide a neutral position; this allows the transmission to cease power output even while the clutch remains engaged. For example, this ensures that a driver can release the clutch pedal and leave the driver's seat without stalling the engine.