An early twentieth century advertisement from the Electrical Development Association (United Kingdom) promotes the use of electricity for housekeeping. (Corbis Corporation)

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...