An automatic transmission in an automobile gearbox that can change gear ratios automatically as the automobile moves under varying conditions, thus freeing the driver from having to shift gears manually.



Principal :-

The main components of an automatic transmission are the converter housing case, oil pane and the extension housing. The converter housing encloses the torque converter and maybe integral with the case or separatly bolted to the case. The case contains the epicyclic gear train while the extension housing encloses the output shaft. The oil pane is bolted to the case. The entire transmission unit is attached to the engine block by means of bolts through holes in the converter housing flange.

The case is usually made of aluminium while the oil pane is ordinarily made of stamped steel or aluminium. The oil pane contains the transmission fluid.

The selection of the particular gear and application of corresponding clutch and brake is done hydraulically. The hydraulic pressure is regulated by two factors (i) car speed that controls oil pressure on one side of the shift valve and (ii) the throttle opening controlled by the driver through accelerator pedal, which controls oil pressure on other side of the shift valve. This is made clear by means of diagram. The working of the shift valve to operate at two speed transmission is shown in figure. At low speeds, the governor pressure is less due to less centrifugal force. The throttle pressure is thus more than the governor pressure, which causes the shift valve to move left. This connects the oil under pressure to go to the brake drum for applying low gears. Similarly at high speeds, the increase the governor pressure moves the shift valve towards right, connecting the oil pressure supply to the system engaging direct gear. Automatic transmissions with more number of torque ratios are also operated on the same principal.

All the oil pressure required in the transmission is produced and maintained by the transmission oil pump, which is mounted to the front of the transmission case and is directly connected to a flange on the torque converter housing, which itself is connected to the engine crankshaft. Thus the pump will run whenever the engine is running. The oil enters the pump through a filter at the bottom of the transmission oil pan and travels up a tube to the oil pump. When the pump is running, it sends oil under pressure to the pressure regulator, the valve body and other components as required. The regulator is usually a speed governor controlled valve, the opening of which is controlled by the vehicle speed. The oil from the pump is fed to the generator. The faster the vehicle goes, the more the regulator valve would open and higher the pressure of the oil it permits to pass.

The selector lever is in fact, directly connected to a manual valve. Depending upon the position of the selector lever, the manual valve covers and uncovers various oil passages for activating the clutch packs and the brake bands for the desired gear. In computer control transmissions, there is a pack of electrical solenoids mounted in the valve body to direct oil to the appropriate clutch packs or brake bands as per directions from ECV which gets signals from various sensors on the engine and transmission, such as vehicle speed, engine load, throttle position, brake pedal position, etc. This results in more precise control of shift gears. Moreover, the computer control makes it possible to have a self diagnostic mode for the transmission, which enables it to detect a problem early and warn the driver by means of an indicator light on the dashboard.

Modern trend is towards electronic control of automatic transmissions, which results in better drivability, being more precise. Such a transmission utilizes an electronic control module which signals when to shift. The module also decides when to lock or unlock the torque converter. In these transmissions, computer-controlled solenoids are employed to control primary functions (internal pressure, converter lock up, shift and clutch operation) through the  transmission hydraulic system.