Drivetrain design: FWD and RWD
| 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 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.
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.
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.
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