Automatic Transmission

Function

The automatic transmission is supposed to choose the transmission ratio in accordance with the position of the accelerator pedal, the speed and the position of the automatic transmission selector lever and others. In addition, it should allow the driver manual interference.

How it works

With the very most automatic transmissions the transmission drive shaft is connected to the engine by a torque converter (left above), not perfectly removable from the crankshaft. Elements of the planetary gearbox (above to the right) are put into the power flow through multi-disc clutches and at the same time other elements are being stopped by multiple-disc brakes and band brakes (in older versions). This technique enables fast changes of the transmission ratio, even more than one gear at a time, without interruption of the power flow. A pump at the drive shaft products the pressure for the clutches and brakes in the automatic transmission. The control of the oil flows is performed hydraulically, or nowadays electronically. Special ATF – oil (Automatic Transmission Fluid) is used for the automatic transmission. If the transmission is equipped with a hydraulic control (picture above) the oil pressure operated pilot valve, and subsequently the chosen ratio, is steered by the intake pipe pressure (depends on the position of the accelerator pedal) and the centrifugal governor (depends on the speed). If the transmission is equipped with an electronic control, the pilot valves of the control unit are electronically actuated with respective sensors. Different selector lever positions allow back up driving, idle-speed, locking the car for parking, and blocking one or more higher gears. Widespread is the possibility for the driver to influence the gear with a second gear-box. The driver may either choose between an early economical or sportive late changing of the gears, or the software realizes the style of driving with the help of FUZZY LOGIC. Newer models allow the adjustment of the speed in which the gears are changed.

Planetary sets:
Sun wheel, annulus and three planet wheels (1),Sun wheel, annulus and six planet wheels (2),the combined planetary set (3) .The planet set (1) makes a duplication of the gears possible together with two multiple disk clutches or with one multiple disk clutch and a freewheel the reverse gear. Planet set (2) with multiple disk clutch(es) only produces forward gears and planet set of (3) allows together with several multiple disk clutches and a freewheel three forward of and a reverse gear. With a freewheel usually only one multiple disk clutch for changing gears is necessary. With the full mechanism a fast change of the speed ratio is possible also over more than one drill step, without interrupting the power flow.
The electronic control in the automatic transmission has a number of advantages. Mistakes may be recognized and corrected; a kind of communication via CAN – bus (controller area network) is possible between the automatic transmission and the engine management. This communication makes sensors for the transmission management obsolete (number of revolutions, temperature, etc.) and the engine management can be influenced. Nowadays while shifting the gears the load is decreased by changing the ignition towards ‘late’.