SMG stands for Sequential Manual Gearbox. It's essentially a transmission capable of functioning as an automatic, or functioning as a manual transmission. The innovation is that, when functioning as an automatic, the driver does not need to operate a clutch pedal, simply designate desired gear. This is done by a traditional stick, or a set of paddle shifters attached to the steering column or steering wheel, depending on the model BMW. While automatic mode is more fuel efficient, manual mode provides greater engine performance while removing the human component and its capacity for error. This helps to ensure a longer life to the transmission and all its related parts.
The transmission design itself is not all that different from a normal automatic transmission. There's a number of planetary gear sets including a number of brakes and clutches, connected to valves, which are in turn connected to the body valve. The back of the planetary gear assembly is connected to the flywheel, with a clutch set in between the two to allow the power coming from the flywheel to be disengaged from the gear set at will. The other end of the planetary gear set is connected to a universal joint, which connects to a driveshaft. The driveshaft connects with the vehicle's drive axle to turn the wheels.
How Does a BMW SMG Transmission Work?
When in automatic mode, the transmission functions normally. Power passes through the flywheel to the planetary gear sets. The gear sets spin in a certain configuration based on what clutches and brakes connected to them have engaged. The body valve controls these clutches and brakes based on the engine's RPM, oil pressure, and other factors. When these factors raise beyond a certain threshold, the body valve changes the configuration of the brakes and clutches. This changes the configuration in which the planetary gears spin, altering the amount of power that's reaching the drive axle. This is how any automatic transmission normally shifts gears. When set in manual, the driver presses a button or operates the paddle that indicates a shift in gears is desired. This signal is interpreted by an on-board computer which overrides the body valve and forces the desired change in gears.