Automotive Systems/Transmission System
Engines deliver different power and torque levels at different speeds, range of driving speeds varies greatly. An engine turning at a very low speed needs gear reduction to move a car from a stop. However, the vehicle speed would be limited by the engine's top speed. The transmission allows the vehicle to operate over a larger speed range, while keeping the engine speed within operating range.
In most automobiles, engine power depends solely on the accelerator position. Different conditions demand different power requirements. While starting, for example, one needs some amount of power to overcome the static rolling resistance. However, very little power is required while cruising at constant speeds.
It is up to the driver to make maximum use of the power made available by pressing the accelerator pedal. This he does by assessing the driving conditions regarding torque and speed requirements. Taking the case of starting the vehicle, since a large amount of torque is required, the engine power(engine torque*engine rpm)is reconditioned by the transmission to the wheel power (wheel torque*wheel rpm). In this case, the high engine rpm is sacrificed for a high wheel torque by the transmission (first gear). Thus, the driver directs the power developed by the engine to produce more torque after assessing the driving condition. He aptly selects the first gear as it provides the highest reduction of engine rpm. Conversely, while moving on a smooth road at a high speed, there is very little rolling resistance (lower coefficient of kinetic friction) and hence very little torque requirement. Hence the engine torque is magnified only by the final drive (differential/transaxle) while retaining most of the engine's high speed. Again, it was made possible by the driver who shifted to a higher gear after assessing the driving conditions.