Drivetrains
The principal role of the automotive drivetrain is to transfer power from the engine output shaft to the drive wheels of the vehicle. Among its other functions are to multiply engine torque for improved vehicle performance, to operate the engine at a point offering good fuel economy, to enable operation of the vehicle in reverse, and to allow the engine to continue operation at idle speed while the vehicle is stationary. The transmission makes all of these functions possible. The drivetrain also incorporates a differential, so that when turning the vehicle, the outside drive wheel can rotate faster than the inside wheel. Finally, the drivetrain must accommodate the relative motion that occurs between the engine/transmission and the drive wheels. This requirement is fulfilled by the universal joint.
Transmission
In the absence of drivetrain losses, the power available to the drive wheels is equal to the power delivered by the engine. Power is proportional to the product of torque (the turning effort around the center of rotation) and rotational speed (typically measured in revolutions per minute, or rpm). The inevitable drivetrain losses are accounted for by a drivetrain efficiency. Losses in the transmission, and also the differential, arise from the friction of gears, bearings, and seals, and from churning of the gear lubricant.
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