The Hidden Genius Inside Your Automatic Gearbox

Most modern automatic gearboxes have a set of gears called a planetary or epicyclic gear train. This ingenious arrangement of gears, combined with a fluid coupling called a torque converter, is what allows your car to change gears smoothly without any input from the driver. Understanding how these systems work reveals the remarkable engineering that goes into every automatic transmission.

Understanding the Heart of an Automatic Transmission

A planetary gear set consists of a central gear called the sun gear, an outer ring with internal gear teeth (also known as the annulus or ring gear), and two or three gears known as planet gears that rotate between the sun and ring gears. The planet gears are mounted on a carrier, which allows them to orbit around the sun gear just as planets orbit the sun—hence the name.

The drive train is coupled to a mechanism known as a torque converter, which acts as a fluid drive between the engine and transmission. Unlike a manual clutch, which uses friction plates to connect and disconnect the engine from the gearbox, a torque converter uses hydraulic fluid to transmit power. This allows for smooth acceleration from a standstill and eliminates the need for a clutch pedal.

How Planetary Gears Create Different Ratios

The beauty of planetary gear sets is that they can produce different gear ratios simply by locking different components. Here's how the basic configurations work:

• Speed Increase: If the sun gear is locked and the planets are driven by the planet carrier, the output is taken from the ring gear, achieving a speed increase.

• Speed Reduction: If the ring gear is locked and the sun gear is driven, the planet gears transmit drive through the planet carrier and speed is reduced.

• Reverse: With power input going to the sun gear and with the planet carrier locked, the ring gear is driven, but transmits drive in reverse.

• Direct Drive: To achieve direct drive without change of speed or direction of rotation, the sun is locked to the ring gear and the whole unit turns as one. This is equivalent to a 1:1 ratio.

The same effect can also be achieved by locking the planet gears to the planet carrier.

How a Torque Converter Works

A torque converter is a fluid coupling that acts like a clutch, except that drive is by hydraulic pressure rather than mechanical friction. The converter has three main components:

• The impeller, bolted to the flywheel.

• The turbine, connected to the gearbox input shaft

• The central reactor between the two, which has a one-way clutch called a freewheel.

As the engine speed is increased, the centrifugal force acting on the hydraulic fluid via the impeller vanes transmits the torque, or turning effort, to the turbine. The central reactor converts this turning effort by redirecting the flow of fluid back to the impeller to give higher torque at low speeds.

At low revs, the reactor is stationary and deflects the oil back to the impeller, increasing the torque applied to the turbine. At high revs, the reactor starts to turn. When turbine, impeller and reactor are running at the same speed, oil is not deflected, and the converter acts as a fluid flywheel, directly connecting the engine to the gearbox.

The Role of Hydraulic Controls

Most automatic gearboxes have three forward speeds and use two sets of epicyclic gears. The locking sequences of the epicyclic gear train are achieved by hydraulic pressure operating brake bands or multi-plate clutches. The bands are tightened around the ring gear to prevent it turning, and the clutches are used to lock the sun gear and planets.

The correct sequence of pressure build-up and release is controlled by a complex arrangement of hydraulic valves in conjunction with sensors that respond to engine load, road speed and throttle opening. This is why automatic transmissions feel so responsive; they are constantly monitoring driving conditions and adjusting gear selection accordingly.

A mechanism linked to the throttle known as a kickdown is used to effect a change-down for rapid acceleration. When you press down the accelerator suddenly to its full extent, a lower gear is selected almost instantly, providing the sudden surge of power needed for overtaking or climbing steep hills.

Most automatic gearboxes also have an override system so that the driver can hold a low gear as required, giving the driver some control over the transmission's behavior.

Putting It All Together: The Gear Selection Process

When you select "Drive" (first gear), the forward-drive clutch is engaged and the engine turns the first ring gear. This causes the planet gears to drive the common sun gear in the opposite direction. The second planet carrier is held by a brake band, so the planets drive the second ring gear and output shaft, producing two reductions in the speed of the engine.

In second gear, the forward-drive clutch remains engaged so the engine drives the first ring gear. The sun gear is braked and the planet gears revolve around it, causing the carrier to be driven in the same direction. The carrier shaft is the output shaft, so one reduction is used while the second planet gears and their carrier freewheel.

In top gear, the forward-drive clutch is engaged and the engine drives the first ring gear. The reverse clutch is also engaged, which locks the ring gear to the sun gear so that both turn at the same speed. The planet gears cannot turn, so the carrier also turns at engine speed. Output is taken from the carrier shaft, which turns at engine speed; this is known as direct drive.

When reverse is selected, the forward-drive clutch is disengaged and the first ring gear freewheels. The reverse clutch engages, and the engine drives the sun gear. The second planet carrier is braked and the sun gear causes the planet gears to drive the second ring gear in the opposite direction, giving reverse drive.

The automatic transmission is a masterpiece of mechanical engineering, combining planetary gear sets and hydraulic controls to provide seamless, effortless driving. While modern transmissions have become increasingly sophisticated with eight, nine, or even ten speeds the basic principles remain the same. The next time you accelerate smoothly away from a stoplight, take a moment to appreciate the remarkable system working under your car.