Automatic Transmission

Automotive illustrated glossary

Drivetrain design: FWD and RWD

Rear-wheel drive automatic transmission Cutaway view. Click for larger view

Planetary gear set

Friction disks sandwiched between the steel disks inside the clutch pack

Friction disks and the steel disks of the clutch pack. Click for larger photo

Automatic transmission oil pump Click for larger photo

Torque converter

Part of the valve body

A transmission is a part of the drive train. The transmission is used to transfer the engine power to the drive wheels. It also allows the driver to choose more power (1-st gear) or more speed (4-th or 5-th gear) by changing the gear ratio.

In an automatic transmission, gears are changed automatically, according to the engine RPM, vehicle speed, load and other factors. A typical automatic transmission has four or five forward gears or speeds. The highest gear is sometimes called an Overdrive or O/D.

Some automatic transmissions also have a manual shift mode allowing the driver to select gears manually (e.g. Porsche's Tiptronic ®).

To change the gear ratio, most automatic transmissions use planetary gears. A typical planetary gear consists of three main parts: outer Ring Gear, inner gear, called Sun Gear and the Planetary Carrier with three or four small gears that are placed between the Inner Sun Gear and the Outer Ring Gear.

At any time, one of the parts of the planetary gear can be connected to the input shaft leading to the engine, a second part can be held stationary and the third part is connected to the output shaft. Depending on which part is stationary, which part is the input and which part is the output, different gear ratios will result.

Hydraulic clutch packs, brake bands and one-way clutches are used to control power flow through the planetary gear. Each gear corresponds to a certain combination of clutch packs and brake bands being engaged or disengaged.

Each clutch pack contains a few friction disks that are sandwiched between the steel disks in an alternate arrangement. The steel disks have external teeth that mesh with the outer clutch pack housing. The friction disks have inner teeth that mesh with the clutch pack hub. Each clutch pack also has a hydraulic piston. When the clutch pack is not engaged, the friction plates rotate between the steel plates almost freely, lubricated by the transmission fluid. This means that the clutch pack housing can rotate separately from the clutch pack hub. When the clutch pack engages, the hydraulic pressure pushes the clutch pack piston and it holds the friction disks and the steel disks against each other, so the clutch pack housing and the clutch pack hub are mechanically connected.

Almost all components inside an automatic transmission are operated by hydraulic pressure. For this purpose, a specially-formulated oil, called Automatic Transmission Fluid or ATF is used. Automatic Transmission Fluid has multiple functions: it transfers hydraulic pressure, it lubricates moving parts and it cools down the transmission.

An oil pump in the Automatic Transmission supplies necessary fluid pressure to operate all transmission components and lubricate rotating parts. The pump works only when the engine is running. For this reason some manufacturers do not recommend having a vehicle with an Automatic Transmission towed with the drive wheels on the ground. You can find this information in your owner's manual. It's important to use only the recommended type of transmission fluid as different transmissions require different types of transmission fluid and wrong fluid can cause problems.

An automatic transmission is connected to the engine via a torque converter. A torque converter acts in a similar fashion to the clutch in a manual transmission. However, unlike a clutch which relies on mechanical friction, a torque converter uses fluid flow to transfer the engine torque to the transmission.

To understand how a torque converter works without going too deep into details, picture a sealed container, filled with transmission fluid, with two propellers inside, one in front of the other. One propeller is connected to the engine and the other is connected to the transmission. When you start the engine, the propeller connected to the engine pushes transmission fluid through to the second propeller. At low engine speed, when the engine is idling, the flow of the fluid is not strong enough to spin the second propeller. If you press on the gas, the engine speed increases. This creates stronger fluid flow inside the torque converter, forcing the propeller connected to the transmission to spin and a vehicle starts moving.

Modern Automatic Transmissions are electronically controlled by a computer which could be a part of the Powertrain Control Module (PCM) or a separate unit called the Transmission Control Module or TCM. Analyzing various inputs, such as vehicle speed, engine rpm, throttle position, etc., the TCM decides which gear to shift to and when. To shift a gear the TCM sends a command to the solenoid valves located in the transmission valve body. Solenoid valves redirect the transmission fluid flow, engaging or disengaging certain clutch packs and brakes, which results in gear shifting.

Overdrive

Generally speaking, overdrive (O/D) is the highest gear in the transmission. On most cars the automatic transmission has three or four speeds and overdrive (which is fourth or fifth speed accordingly). Overdrive allows the engine to have less rpm with higher speed for better fuel efficiency. When you switch it ON, you allow the transmission to shift into overdrive mode after a certain speed has been reached (usually 30-40 mph depending on the load). When it's OFF, you limit transmission shifting to third or fourth speed, not allowing the transmission to shift into overdrive.
In normal driving conditions the overdrive should be always ON. You may need to switch it OFF if you drive in a mountainous area.
[The automatic transmission automatically shifts from O/D to the lower gear when it feels more load. When it feels less load it shifts back to O/D, but under certain conditions, e.g. driving uphill or towing a trailer, the transmission cannot decide whether to stay in O/D or shift into 3rd speed and it starts shifting back and forth. That's the time when you may switch the O/D off and help the transmission to decide].
You also may need to switch the Overdrive OFF when you want to slow down using the engine braking, for example, when driving downhill. For more details, check your owner's manual.