Automatic Transmissions

An automatic transmission does away with all the clutch and gearshift operations that are normally carried out by the driver of a manual transmission vehicle. As well as automatic shifts, the transmission has a torque converter which acts as a fluid coupling. it also acts as a form of variable gear ratio that automatically adjusts to the operating conditions of a vehicle. These transmissions are now fitted to  most modern vehicles. They consist of three basic sections: A torque converter, Gearing, Hydraulic system.
Torque converter:
The torque  converter can transmit different torque ratios. Three main parts in the converter are: impeller, turbine, stator. With gears, speed reduction means torque multiplication. This principle also applies to a torque converter. Speed reduction with in the converter results in an increase in torque. Torque multiplication is achieved by the action of the stator.
When the gear selector lever is in D, and the engine idling, the fluid in the converter does not have enough force to turn the turbine, there for the vehicle stays stationary. when the engine speed picks up the oil will be thrown into the turbine with great force. at this point there will be maximum torque multiplication. as the speed increases the speed between the impeller and the turbine is quickly reduced. a point is reached where there speeds are about the same this is called coupling point. at this point there is no torque multiplication and the converter acts as a fluid coupling. coupling point occurs when the turbine speed reaches approximately 90 percent of the impeller speed. Below coupling point, the force of fluid against the stator blades cause the one way clutch to lock and hold the stator to  prevent it from rotating backwards. The function of the one way clutch is to prevent torque loss.

Fluid flow in a torque converter

Gears:

Most automatic transmissions use a planetary gear set up. they have three main parts: the ring gear, the planet carrier and pinions and the sun gear. all three parts are concentric. The pinions connect the ring gear to the sun gear. The pinions not only rotate on their shafts, but they can also walk around the teeth of both the sun gear and ring gear

 

All the gears are in constant mesh and do not slide. the gear ratios are obtained by holding one of the parts stationary with a brake. one part then acts as a driver and the other part is the driven. The two ways  the gear set operates is called non-planetary and planetary depending on which part is being held and which part is the driver.