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How does an automatic transmission work?

Taking a step back, there are several "types" of automatic transmissions. The step ratio automatic transmission in modern automobiles is the most popular with the general public and will be the topic of discussion. Other types of automatic transmissions include belt and toroidal continuously variable transmissions (CVTs) and hydrostatic transmissions.

So, why do we need a transmission in the first place? The automatic transmission serves several functions:

  • Provides automated control of vehicle launch (starting the vehicle from a stop)

  • Selects the desired gear ratio

  • Shifts to the desired gear ratio

  • Modifies the engine's speed/torque

  • Transmits power efficiently (helps provide good fuel economy)

The automatic transmission is one of the more complex assemblies of the modern automobile. The transmission is bolted directly behind the engine on rear-wheel-drive vehicles and is typically mounted on the driver's side of the vehicle on front-wheel-drive vehicles. The engine's power is transmitted from the crankshaft, through the flywheel, and into the transmission assembly via the torque converter.

The torque converter is an "automatic" clutching device that allows the engine to rotate independent of the transmission at low engine speeds. The torque converter internal components function similar to two fans, each facing the other. One fan (the torque converter's impeller) is driven by the engine and provides air or in the transmission's case, automatic transmission fluid (ATF) flow. The second fan (the torque converter's turbine) is mounted to a shaft but has no source of power. The ATF flow from the first fan (impeller) hits the second fan (turbine) and causes it to turn.

If the first fan's speed is slow enough, you can grab the second one and stop it. This is called a "stalled" condition, and occurs when your car is in drive or reverse, and you are stepping on the brake pedal. When you release the brake and step on the accelerator (throttle), the torque converter's impeller (first fan) speed increases, which gradually brings up the turbine's (second fan) speed, resulting in movement of your car.

The turbine provides the input to the transmission's "gearbox," the guts of the transmission. This area contains gears, shafts, bearings, hydraulic clutches, and a whole lot more. In manual transmissions the driver moves the gearshift to engage and disengage the different gear ratios. In an automatic transmission, the gear ratios are selected "automatically" by engaging or disengaging clutches via the transmission control unit (TCU).

The TCU is a computer that determines when the transmission needs to shift gears, and what clutches to engage/disengage to obtain the desired gear ratio. The TCU looks at both the vehicle's speed and the throttle (accelerator pedal) position when selecting the transmission ratio or "gear." The driver of a manual transmission vehicle must decide when to shift gears and what gear to shift into. In an automatic transmission vehicle, the TCU does all of this for the driver.

This month's Whizard is Doug Fussner, a research engineer in the Engine and Vehicle Research Division (03).

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April 15, 2014